stunthanger.com
Design => Stunt design => Topic started by: Mark wood on October 05, 2020, 08:52:19 AM
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I would like to open a dialog about propeller precession and it's impact on the flying qualities of the model. Suppose the impact of precession could be virtually eliminated, hypothetically speaking... Before you jump up and say it is impossible take a step back and do the "what if" pondering. Suppose, if it could be done, how much weight penalty would be worth paying to achieve it? An ounce? Four?
The first steps to doing the impossible are understanding the obstacles. Impossible is that which your mind prevents you from doing.
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I would like to open a dialog about propeller precession and it's impact on the flying qualities of the model. Suppose the impact of precession could be virtually eliminated, hypothetically speaking... Before you jump up and say it is impossible take a step back and do the "what if" pondering. Suppose, if it could be done, how much weight penalty would be worth paying to achieve it? An ounce? Four?
The first steps to doing the impossible are understanding the obstacles. Impossible is that which your mind prevents you from doing.
D>K :! :o :'( #^
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I know, right... Crickets. I wanted to spark the conversation because I was playing around with an idea and it produced some results I wasn't expecting although with my background I should have. I'm guessing from looking around that there have been some interesting things happen like coaxial systems and such which had some positive and more negative reactions. So, I'm guessing there isn't much interest in discussion about how far would we go and how open minded we might be.
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I know, right... Crickets. I wanted to spark the conversation because I was playing around with an idea and it produced some results I wasn't expecting although with my background I should have. I'm guessing from looking around that there have been some interesting things happen like coaxial systems and such which had some positive and more negative reactions. So, I'm guessing there isn't much interest in discussion about how far would we go and how open minded we might be.
So let's beat your idea around, what is it?
Ken
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I know, right... Crickets. I wanted to spark the conversation because I was playing around with an idea and it produced some results I wasn't expecting although with my background I should have. I'm guessing from looking around that there have been some interesting things happen like coaxial systems and such which had some positive and more negative reactions. So, I'm guessing there isn't much interest in discussion about how far would we go and how open minded we might be.
Alternately, you posted something at 8 in the morning and were expecting learned responses by 3 the same afternoon, and then claimed the "delay" as people being "closed-minded" - when you haven't even bothered to show your "solution". It's would easy to see that and conclude you are spoiling for an argument.
It would be a lot better if you posted your idea with enough detail to evaluate it, then wait for a day or two, before you start calling people out.
Brett
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Is your "propeller precession" quest something different than solving gyroscopic precession issues? If so, disregard all that folows.
The idea to use counter rotating propellers for CL stunt is not new.
Some time ago, Randy Smith set up a system where the rear propeller free wheeled and only the front propeller was powered. I do not know that there were any negative aspects to the system. I recall that torque problems were at least reduced if not eliminated and that gyroscopic precession problems were at least reduced, if not eliminated. I am not aware if any further work was done by him or anyone else using this approach. I do not know if any improvements were experienced that offset the additional weight/complexity of the system compared to just a single tractor propeller approach. Hopefully, Randy can shed some light on this.
Their is a single electric motor system driving a gear box for counter-rotating propellers used on CLPA models that has been available from Poland. For me, I saw this the first time in Poland at the World CL Championships in 2016. There did not appear to be any problem with the performance of this system. At the time, copies of the system were available and they still might be. It is/was a very neat integrated motor/gearbox system easily packaged in the nose of a model for CLPA.
The RC crowd has various systems available from integrated gear boxes driven by a single electric motor to systems that use two electric motors using co-axial shafts directly driving the counter rotating propellers without gear boxes. Most of these systems on the RC market are for the larger pattern ships and do not appear to be practical for our CLPA models. Certainly, these approaches are doable if the equipment can be sized to adapt to our CLPA environment.
Twin electrics for CL stunt are not new. Bob Hunt has a great flying twin where he uses contra rotating propellers and reports no problem with gyroscopic precession. With the advent of electrics, there are other twins out there doing very nicely with contra rotating propellers.
Another way to reduce gyroscopic precession is to use the Rabe Rudder approach. I do not know if this approach to use the rudder will completely negate gyroscopic precession, but it certainly can be used to minimize it to the point that it is not a detectable problem. In fact, it can be used to improve line tension in most inside and outside maneuvers through careful selection/trimming of the rudder position with any given elevator setting.
It should be remembered that for models used in the AMA CL Precision Aerobatic events, all control functions on the model "shall be through the flying lines" except for the use of the 2.4 GHZ spread spectrum radio signals to operate a retracting landing gear and/or a one-time irreversible engine or motor stop function. (Of course, the passive exhaustion of fuel to end the flight is inherent to the operation of these models.) Having some sort of electronic system to sense and then provide correction for gyroscopic precession would not be allowed under the current rules.
(Let's not get hung up here on the definition of counter-rotating props and contra-rotating props.)
So, what are your ideas?
Keith
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Alternately, you posted something at 8 in the morning and were expecting learned responses by 3 the same afternoon, and then claimed the "delay" as people being "closed-minded" - when you haven't even bothered to show your "solution". It's would easy to see that and conclude you are spoiling for an argument.
It would be a lot better if you posted your idea with enough detail to evaluate it, then wait for a day or two, before you start calling people out.
Brett
Good point Brett. Sorry if came off that way. I'm not after an argument I'm interested in a dialog. I've been working on an idea for a long time and sometimes it's better to open up a discussion first. Often times outsiders are not received well within a community and ideas brought in from outside are shut down because the new guys doesn't speak the local language. This is natural as the not invented here is typical of all communities. My question was what if and what would it take to demonstrate it as in what would be a good test? My inclination is to get to a flight test but I haven't an airplane which truly exhibits issues clearly precession related enough to do a good A - B test. I'm on a bigger path than just this particular incidental observation and a divergence involving sophisticated test equipment isn't in my plans. Also several people here have done a lot of experimenting which I am very much interested and opening a dialog first is a good way in my mind to get folks talking. Trust me, if I managed to have an original idea, it would be a miracle. Now plagiarizing and making things work or applying them to something different... I can do that.
Sorry again if I came off wrong, wasn't meant that way.
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Good point Brett. Sorry if came off that way. I'm not after an argument I'm interested in a dialog. I've been working on an idea for a long time and sometimes it's better to open up a discussion first. Often times outsiders are not received well within a community and ideas brought in from outside are shut down because the new guys doesn't speak the local language. This is natural as the not invented here is typical of all communities. My question was what if and what would it take to demonstrate it as in what would be a good test? My inclination is to get to a flight test but I haven't an airplane which truly exhibits issues clearly precession related enough to do a good A - B test. I'm on a bigger path than just this particular incidental observation and a divergence involving sophisticated test equipment isn't in my plans. Also several people here have done a lot of experimenting which I am very much interested and opening a dialog first is a good way in my mind to get folks talking. Trust me, if I managed to have an original idea, it would be a miracle. Now plagiarizing and making things work or applying them to something different... I can do that.
Certainly, a big part of being an engineer is knowing where to rip off good ideas.
How much have you studied the (extraordinarily extensive) literature on how precession affects stunt planes? Do you have specific questions beyond that? Or is it a more general discussion of the effects?
Brett
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It would be a lot better if you posted your idea with enough detail to evaluate it, then wait for a day or two, before you start calling people out.
Brett
y1
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I haven't done any calculation of precession of stunt planes. I also don't know what you guys have done in the treatment of the propeller in that regard. I'm going to assume that it has been treated as rigid rotating body such as a gyroscope. While that is a good first order approximation it is only roughly valid under certain constraints such as blades which are rigidly attached to the hub or rather as rigid as blades can be. My background is in rotary wing aerodynamics, helicopters. Flying rotor systems.
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Certainly, a big part of being an engineer is knowing where to rip off good ideas.
Seems like it isn't me looking for a fight. I was actually simply trying to start a conversation, however it seems I started a fight. And I apologized. The new guy has been put in his place.
Have a nice day.
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I haven't done any calculation of precession of stunt planes. I also don't know what you guys have done in the treatment of the propeller in that regard. I'm going to assume that it has been treated as rigid rotating body such as a gyroscope. While that is a good first order approximation it is only roughly valid under certain constraints such as blades which are rigidly attached to the hub or rather as rigid as blades can be. My background is in rotary wing aerodynamics, helicopters. Flying rotor systems.
For our purposes, it has always been treated as a rigid body, which for any maneuvering purposes, is almost certainly true, the natural frequency of the blades flexing is much higher than the maneuvering rate, as it the rotation rate of the prop. Sometimes, the fact that the inertia changes between the axes as it rotates can cause or permit some vibration (which a big difference between two- and three-blade props) matters, but has no direct effect on the maneuvering, assuming the inertia is the otherwise the same.
Almost all the inertia is in the propellor, you can ignore the crankshaft, spinner, rotor, or nose weights on the shaft as far as that goes. Figure that the radius of gyration around the spin axis is approximately .225*diameter (an approximation, but quite good for a surprising number of cases, regardless of number of blades). Ixx= mr2.
Precession routinely works in two dimensions. For a conventional rotation engines, the continuous left yaw rate caused by going around the circle causes a "nose up" pitch torque in the airplane's body frame, that is, nose up from the ground in level upright flight, and nose down in level inverted flight, as if the pilot was continually pulling a small amount of "up" elevator. That is generally dealt with in trimming, either positive stab incidence, downthrust, or most of the time, "down" elevator with neutral flap.
As I recall, the example in my SN article was about 33 in-oz* of nose-up for my particular case. Pete Soule' came up with a value for a team racer on his website, but he had a unit conversion error, which he may have corrected since the last time I looked.
The other direction is in maneuvers, "inside" corners inducing right/nose-out yaw, and outside corners inducing left/nose-in yaw. It's about 5x as large at the pitch torque for very tight corners, since the pitch rate for an inside turn is about 5x the rate for going around in a circle. So it's not nothing. At the same time, in most cases, there is also some p-factor that operates in the other direction, which is hard to calculate, but appears to be something like 1/10th the magnitude of the precession for most cases.
This can be dealt with a number of ways, but for the most part, people use passive stabilization (positive yaw stability), and Al Rabe invented a movable rudder that has the capability to compensate for it. It amounts to a feed-forward. The rudder moves to "nose-right" yaw on outside maneuvers, and "nose-left" yaw on inside maneuvers, although you need less inboard movement than outboard movement, since the restoring force from the line goes up nose-out and down nose-in.
In practice, you can trim it out to NATs-level quality without any active control and virtually all of the Rabe-rudder systems end up *grossly maladjusted*, although there is nothing wrong with the idea. Igor also uses his Rabe rudder system to, to first approximation, to rotate the airplane around an axis displaced from the principle axes, so you can hold a more-or-less constant outboard yaw angle, which would otherwise cause wild roll and yaw motion due to the dynamics and kinematics.
I would strongly suggest reviewing Al Rabe's Bearcat or Mustunt article, my SN discussion from about 2005-early 2006s (about "positive incidence"). I have lost the link to Pete Soule's old site, it is somewhere, but I have lost track of it.
Note that the inertia of the prop matters in another way, unrelated to precession - the more inertia, the more torque it takes to spin it up or down, or for a fixed amount of torque, how fast it accelerates or decelerates in RPM. I don't know and haven't carefully considered how much it matters for IC engine systems, but Igor told us it matters a lot in how well his feedback system works, the key problem being a rate slew limit that limits the response enough to inhibit the maximum permissible gain to maintain stability.
Certainly, a big part of being an engineer is knowing where to rip off good ideas.
That's a joke. I got every neat trick I know from various big names, having ripped off Paul Walker more than is seemly.
People are not out to try to get you, me least of all. If they were, you would know. But you can't really expect someone to write a 5000-word report summarizing all the work done to date over the last 70 years, in a few hours, when you fly off the handle at the slightest setback, and haven't done any research. Calm down.
Brett
*12.5" prop, 34 grams, 11000 rpm, 5.4 second laps
r=.225*12.5" = 2.81"=.234 feet
m=34 grams=.075 lb = .00233 slugs
Ixx=.00233 slugs*(.234feet)2=0.000128 slug-ft2
omega= 11000 rpm=1151 rad/sec
H=Ixx omega=0.000128 slug-ft2*1151 rad/sec=.147 ft-lb-sec
yaw rate = 6.28 rad/5.4 seconds = 1.185 rad/sec (5.4 seconds/lap)
Nose-up pitch torque = H*pitch rate=.147 ft-lb-sec * 1.185 rad/sec=0.1742 ft-lb= 33.6 in-oz
pitch rate (hard corner) ~ 6.28 rad/sec (360 degrees/second)
Nose-out yaw torque = .147 ft-lb-sec * 6.28 rad/sec = .923 ft-lb = 177 in-ounces
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Suppose, if it could be done, how much weight penalty would be worth paying to achieve it? An ounce? Four?
I think this is a good way to look at the problem. In my case, my airplane is too heavy and has ballast in the nose. Any weight in the tail gets multiplied by 3. Weight in the nose, though, is free.
The gyroscopic effect is one of many moments we try to sum to zero about the x and z axes during a turn. Folks like Brett do a good job doing this without extra doodads, but more tricks are welcome.
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Some time ago, Randy Smith set up a system where the rear propeller free wheeled and only the front propeller was powered. I do not know that there were any negative aspects to the system. I recall that torque problems were at least reduced if not eliminated and that gyroscopic precession problems were at least reduced, if not eliminated. I am not aware if any further work was done by him or anyone else using this approach. I do not know if any improvements were experienced that offset the additional weight/complexity of the system compared to just a single tractor propeller approach. Hopefully, Randy can shed some light on this.
So that's why he did it. Somehow I missed the point of that until now, or maybe forgot it. Pretty clever.
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Brett:
In the spirit of promoting a discussion, I want to thank you for explaining some things that I observe but never knew the cause. Even though math, as it relates to physics, is my short suit, cause and effect on the other hand is trump.
You have just explained to me why changing props and/or basic RPM's from a pitch change can change my elevator trim. One thing that sparked my curiosity was the difference in the torque values between axis. I would have thought them to be similar. Does nose length (from the tether point) have any thing to do with that?
Ken
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During I/C times, the inertia moment of the crankshaft was substantially lower than the one of the propeller, so it could be discarded. The gyroscopic effect would be compensated with a Rabe Rudder.
Going electric, the inertia moment of the rotor is about a fifth of the one of the propeller, as Peter Germann and I showed here four years ago.
Therefore the Polish solution has a drawback, because their solution gets rid of the precession effect of the propeller, but the gyroscopic effect of the rotor increases (runs at a higher speed than the props!).
So, the mathematically suggested solution is to use two props and two motors, running in opposite direction, at the same speed....
No Rabe Rudder needed.
Regards,
Wolfgang.
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With electric systems, proper design and modern materials precession is not a problem.
Motorman 8)
Perhaps a lesser problem since we can use lighter props, but I can still feel the difference if I change diameters. Electrics did not repeal the laws of physics. Even twins will have a very small amount due to the different distances to center. The power spikes per revolution probably has a calming effect. It certainly reduces vibration! Ironically, my last ship flew best on a wooden 11-6 three blade which should be the last thing you would pick but using a CAM rudder I was able to eliminate the precession effects and enjoy the smooth power that wood gives you. It amazes me how much of trimming is a balancing act.
Ken
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Brett:
In the spirit of promoting a discussion, I want to thank you for explaining some things that I observe but never knew the cause. Even though math, as it relates to physics, is my short suit, cause and effect on the other hand is trump.
You have just explained to me why changing props and/or basic RPM's from a pitch change can change my elevator trim. One thing that sparked my curiosity was the difference in the torque values between axis. I would have thought them to be similar. Does nose length (from the tether point) have any thing to do with that?
No, torque is torque, where it applied is moot. "Where" would matter of there is a force, but not a torque.
Torque arises because torque is required to reorient the angular momentum vector of the prop. The magnitudes are different because the imposed rate, that is, the rate at which you are trying to reorient the angular moment, is different. The lap rate, the 5.4 seconds a lap in my example, is much slower than the pitch rate in a hard corner, which goes 90 degrees in about a 1/4 second.
There is *much more* than just gyroscopic precession going on, of course. The moments of inertia of the airplane, which is also rotating, is one source of additional angular momentum. But simply having more inertia also makes it harder to turn. Plenty of people worry about aerodynamics, to a fault, and ignore plain old rigid body dynamics. Even when they are talking about "starting and stopping" corners, they worry about the air a lot, and never bother to consider the rest of it F=ma and for rotations, T=Iomegadot, plenty of people worry about F, m, and T, they don't ever consider I.
Even the plain old kinematics are generally ignored, and the fact that most people don't even know what "kinematics" means (except for Howard and Frank..) is a significant problem in trying to explain it.
Note also that this is why it is so difficult to have real discussion about design, because you have to start from first principles every single time. People have little snippets of things they know, but for the most part, not where it comes from or what the relevance might be, so every discussion, you have to derive everything from first principles.
Brett
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With electric systems, proper design and modern materials precession is not a problem.
I am skeptical, let's see some numbers.
Brett
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I can't link to it right now, but for Pete Soule's excellent and very interesting site with all sorts of amazing nuggets, search for "microaeronautics" and then look for his name. It was definitely hosted elsewhere when Geocities imploded, so it is well worth the effort to find ,for both real technical information (that's where LINEII came from in the first place) and for fascinating historical articles.
Brett
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As noted above, here is an example of some work by Pete Soule that is the basis of "LINEII" and "LINEIII" programs. There were great little nuggets like this all over his MicroAeronautics website, plus excellent action photos from the ancient times. I can't find the website now.
(https://stunthanger.com/smf/index.php?action=dlattach;topic=57666.0;attach=316676;image)
http://www.fesselflug.ch/download/pdf/2011/Leinezug/linedrag2003.pdf
Brett
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So that's why he did it. Somehow I missed the point of that until now, or maybe forgot it. Pretty clever.
And *vastly lighter* than actually putting in a gearbox to drive it. I am at least suspicious about the effects on propulsion, but having a piped 75 will resolve any power problems with good old fashioned brute force.
Brett
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Hello Mr. Wood! Let me extend greetings from a fellow newbie in this place!
It appears that, save for Howard's tentative response, none of the posts so far have actually answered the OP's hypothetical question; instead, y'all ignored the new guy's question, drove him away, then followed up with irrelevant -- and at times esoteric -- drivel.
Let me give it a shot.
Mark, I don't think it's easy to answer in terms of a specific weight. Perhaps a better answer would be in terms of some ratio that stays roughly equivalent between aircraft of similar sizes and/or desired performance. Let's assume that you aim to maintain equivalent performance with your solution as compared to the same aircraft without it. In that case I'd say that if the additional weight were within a range that maintained the aircraft's power-to-weight ratio or the wing loading within acceptable limits, your solution would be acceptable. The actual acceptable range is an exercise left for field experiments. If that metric fails, then adjust the powerplant and/or geometry of the aircraft to suit the new weight, if reasonable (don't rip the lines off, eh?). I'm choosing power/weight and wing loading as starting points here since they are typical rough indicators of stunt performance. Choose any other constant characteristic related to weight as necessary.
Cheers,
-Andrey
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Hello Mr. Wood! Let me extend greetings from a fellow newbie in this place!
It appears that, save for Howard's tentative response, none of the posts so far have actually answered the OP's hypothetical question; instead, y'all ignored the new guy's question, drove him away, then followed up with irrelevant -- and at times esoteric -- drivel.
The OP never asked a question, or clarified what he wanted, came in with a chip on his shoulder,, and stormed off mad over a joke (that I have used repeatedly over the years). Since he never actually specified what he wanted to talk about, I spent several hours summarizing the state of the art to date, and that is drivel? Amazing.
Brett
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...Suppose, if it could be done, how much weight penalty would be worth paying to achieve it? An ounce? Four?...
Did everyone except me miss the question? It looks quite clear.
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Did everyone except me miss the question?
OK, so what is Mark's question. He wanted to "open a dialog about propeller precession and it's [sic] impact on the flying qualities of the model." So, what is the problem he is trying to solve? Is it gyroscopic precession that tends to yaw the model inboard or outboard during maneuvers on our hemisphere, depending on propeller rotation? Or is it something else that he recognizes in his world as a problem (a thing he calls "propeller precession") that he does not explain what he is looking for it to be "virtually eliminated" but is willing to explore some system that might weigh "An ounce? Four?"
Keith
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Did everyone except me miss the question? It looks quite clear.
I think the original question was lost in the second post. He is looking for parameters in order to explore solutions to gyroscopic precession. I wrote a rather lengthily response that got zapped by a sticky mouse (now there is something worth eliminating) so I will summarize.
I am not convinced that our current trim techniques and flying experience at the upper levels make eliminating precession really necessary. I shot some off angle (not the typical judges position) video on one of my PA ships looking for trim issues in maneuvers. Even though I could distinctly feel the precession, there was no visible evidence of it that a judge could detect and that ship does not have a Rabe (sorry Al).
So my position is simple. If I can deal with it without any visible degradation to the pattern, how would eliminating it do anything but make me learn a whole lot of new trim norms since gyroscopic precession is really a part of everything. If you want to fight the prop and motor following the laws of physics, go after torque roll.
Ken
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..Suppose, if it could be done, how much weight penalty would be worth paying to achieve it? An ounce? Four?Did everyone except me miss the question? It looks quite clear.
Suppose "what" could be done? I can think of 0-ounce "solutions" and I can think of half-a-pound "solutions" - but to what? Precession affecting the yaw axis in cornering? Precession affecting the pitch response inside VS outside? Precession + varying moment of inertia transfer to the airplane axes? I did get that it was about the spinning prop, rather than the other precessional forces, outside that.
Fairly obvious "solutions" include
Prop into yaw - put on a big fin and lots of fuselage side area
Rabe rudder
CMGs
contra-rotating props (driven or not)
cancel momentum using a flywheel
using a lighter prop
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Prop into pitch - downthrust
dropped elevator
positive stab incidence
asymmetrical stab airfoil
practice more
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If it means "a broad discussion of precession" then you have to consider the precessional forces and other rotational forces on the airplane. I can't figure out what was meant, but I did try to give an overview.
I am not trying "chase anyone off" or insult anyone. I spend a lot of time on some of these responses, and gave some references to provide guidiance, and it's not like I am getting paid (Customers are charged ~$300 an hour for very similar sort of work by my employer), I am doing it to try to help.
I have something like 30,000 message board posts over the last 25 years of hobby internet forums, and I have started maybe 100 of those. The rest are attempts to address other people's topics.
I don't appreciate having it called "drivel". You are not compelled to read it, and you are not compelled to comment on its value, or complain about not getting your questions answered fast enough or not in the way you wanted.
If Mark or anyone else wants to talk about dynamics of model airplanes, I am still more than willing to work with you and share whatever information I might have. But I am not going to tolerate getting abused for it.
Brett
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Edit: Wow, I thought I was reading this on the main forum. The question was indeed posted in the right place. My comment relative to the use of the term "drivel" still stands. It's inappropriate. Using weight won't correct for precession forces, which are dependent on orientation.
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... plenty of people worry about F, m, and T, they don't ever consider I.
Especially when the subscripts don’t match.
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Brett,
Well, you started out well. The background on the effects of gyroscopic precession you provided was a useful start, Iomegadots and Whatnots. I have to admit, that background actually makes it easy enough for the average newcomer to grasp what's involved here.
Your mention of Pete Soule's site is noteworthy. I enjoyed reading his articles, especially the one on gyroscopic propeller effects:
(https://i.imgur.com/eIoGByk.jpg)
That's the website you've been searching for, right? ;) It's difficult to find these days.
But then things went downhill. It's so difficult not to be compelled to read an imposing screenshot with pretty formulas! I can see that math with lots of squiggly lines excites you, and Mr. Soule's treatment of the aerodynamic drag of control lines is nothing short of beautiful work. However, with all due respect to the original author, line rake and line shape are irrelevant to this discussion. It just looks cool and shiny in this thread. Furthermore, all that math can only be understood by a select few and so it adds very little value here. And yes, I dare call that lonely screenshot you posted drivel because you ripped it from Page 9 of the document out of its context. Without including the definitions of all the variables from Page 3, it's total nonsense. Thanks for including the full PDF though.
Note also that this is why it is so difficult to have real discussion about design, because you have to start from first principles every single time. People have little snippets of things they know, but for the most part, not where it comes from or what the relevance might be, so every discussion, you have to derive everything from first principles.
I understand this difficulty, I really do. Let me propose a constructive solution. Why not aggregate all that fundamental knowledge in one place in a modern blog-like format and refer to it as needed? I'm not talking about the most basic principles. General knowledge of physics and aerodynamics is plentiful out there. However, its applications to microaeronautics are highly specialized, sometimes counterintuitive, and not as widely known. It appears that Mr. Soule was able to express its implications in a magnificently practical way for the average modeler. And he did the best logical thing: he captured it in one place for easy reference. Why can't we do the same in 2020? I think it would be a great resource. It would be a great reference of all the things that have been tried and tested over the decades in this hobby so that newcomers don't go reinventing the wheel. One would expect that an experienced scholar such as yourself would create a neat corner of knowledge like that on the web after 25 years instead of repeating yourself 30,000 times to feed your inner masochist. Seriously, what do you think?
Now I'm extra curious to hear Mr. Wood's idea. Maybe it's worthless, maybe it has merit. Sadly, we might never find out.
-Andrey
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Why not aggregate all that fundamental knowledge in one place...
I’m working on it. Stuff keeps coming up.
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Well, you started out well. The background on the effects of gyroscopic precession you provided was a useful start, Iomegadots and Whatnots. I have to admit, that background actually makes it easy enough for the average newcomer to grasp what's involved here.
But then things went downhill. It's so difficult not to be compelled to read an imposing screenshot with pretty formulas! I can see that math with lots of squiggly lines excites you, and Mr. Soule's treatment of the aerodynamic drag of control lines is nothing short of beautiful work. However, with all due respect to the original author, line rake and line shape are irrelevant to this discussion. It just looks cool and shiny in this thread. Furthermore, all that math can only be understood by a select few and so it adds very little value here. And yes, I dare call that lonely screenshot you posted drivel because you ripped it from Page 9 of the document out of its context. Without including the definitions of all the variables from Page 3, it's total nonsense. Thanks for including the full PDF though.
Nowhere to "Full Avaiojet" in 35 posts, that may be a new record.
Brett
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Now I'm extra curious to hear Mr. Wood's idea. Maybe it's worthless, maybe it has merit. Sadly, we might never find out.
-Andrey
If you read his posts carefully he doesn't have an idea. He is looking for parameters around which to start brainstorming an idea and thanks to your insults sadly we *may* never find out. It is one thing to attack an idea, we do that regularly here. That is how you grow, but to attack the person is something that adds nothing to our chosen hobby/sport.
Ken
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I actually read his posts carefully. He has not only an idea but also some results that may be of interest:
...I was playing around with an idea and it produced some results I wasn't expecting although with my background I should have...
::)
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I actually read his posts carefully. He has not only an idea but also some results that may be of interest: ::)
What is it then? I read his posts entirely differently. I read that he has an idea where to look for an idea based on his observations of the same issues in his chosen profession and would like to know what the finished product parameters such as weight, and I would assume cost, are before spending a lot of time developing it.
Ken
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(https://media.giphy.com/media/l41YsG7Y1U5X0ysw0/giphy.gif)
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Actually, yes I have a solution that looks extremely promising at this point. My intent was to discuss exactly how much impact precession does have on the models, weight, trim flying, handling qualities etc. Having been an engineer working for a helicopter company in my early career, precession is something I have had to deal with and have a solid understanding of. I promise to publish once I have achieved some flight testing which requires some exotic materials (aluminum filled epoxy in transit) to work with. I am hesitant to discuss this because without a good flight or more, the understanding basis would be simply conjecture. As one of my bosses told me, one good test is worth 10,000 opinions. On the other hand if I fail, well, at this point, I look like a typical moron. Not the first time. I will say that I have had a couple very knowledgeable engineers look at what I have and I have made a few videos.
I missed the humor in Bretts reply and he missed the humor in mine. Yeah, I bowed out for a few days. Partly to cool down and more to spend some time in development and demonstration. I am currently at the point of needing a flight article to test. That I expect will take a week or two once I have materials all in the shop.
The reason I asked the question is if it is really worthwhile to pursue and to what magnitude would a typical. Certainly the contra rotating mega dollar powerplants have some curb appeal just like Ferrari's do. I don't have the resources for that nor do I think it is that necessary. They are heavy and I'm not sure the propulsion efficiency is easily achieved and my guess is that a standard propeller and trimming techniques have as good of results without the weight and complexity issues. Albeit lackluster comparative coolness factor.
Taking that a bit farther, I believe that there still are significant advancements in propulsion systems to be made. Perhaps I'm not the meat servo with enough talent to drive one of these models to the level of that performance but on the other hand I have a background suitable enough create my version of such a beast.
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Actually, yes I have a solution that looks extremely promising at this point. My intent was to discuss exactly how much impact precession does have on the models, weight, trim flying, handling qualities etc.
???
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Just because I feel like a trouble maker due to COVID-19 I thought I should ask the following.
Early in my flying "career" I flew lots and lots of single engine aircraft...although none of them aerobatic. Without exception also, they all had propellers spinning in the same direction as our classic (non electric) stunt ship, counter clockwise when in front of them but clockwise from the pilot's seat. None of the aircraft I flew had remotely the power to weight ratio of pretty much any modern stunt ship (or the old Fox .35 equipped Smoothie, Nobler veterans, for that matter).
Every such aircraft I flew when the nose was raised (and, generally, power increased for a climb, takeoff etc.) required "significant" right rudder to correct the left yaw due to P-factor (more thrust from the right half of the prop rotation due to the propeller's increased "angle of attack" with respect to the aircraft's flight path). However, this phenomenon has seldom been addressed with respect to stunt ships which have "significantly" greater thrust to weight ratios.
To add to my quandary, although I've flown out of numerous first loops of a four leaf clover entered from level flight (albeit at 45 --or so degrees (for Keith)--over my 65 or so years of flying stunt in troublesome wind conditions I've "never" flown out of the third (outside) loop performed at the same elevation after a vertical climb to get to the "do it now" location. Outside square loops have always felt more positive in the corners than insides as well. I had always harkened back to my youthful full scale single englne years to explain that as opposed to what is now...a stunt conundrum...the opposite need for potential correction than that "solved" by, for instance, Rabe rudders or other mechanism's like left had rotation props on Electrics.
I'm not looking for a fight...just for an explanation of why my experience over many years is so different from everyone so concerned about correcting precession issues.
Ted
Has P-factor been exorcised?
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Ted,
I'm glad you posted this because the first loop in the Clover is for me a lot of times loose as a goose. Glad to hear some other people have had the same and you being a top flyer with some explanation is helpful to me.
John L.
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Every such aircraft I flew when the nose was raised (and, generally, power increased for a climb, takeoff etc.) required "significant" right rudder to correct the left yaw due to P-factor (more thrust from the right half of the prop rotation due to the propeller's increased "angle of attack" with respect to the aircraft's flight path). However, this phenomenon has seldom been addressed with respect to stunt ships which have "significantly" greater thrust to weight ratios.
I presume we are just talking about precession in yaw from maneuvering, not precession in pitch from going around the circle, or IxIomega torque.
P-factor is a function of the angle the relative wind makes with the prop disk, the diameter, and how fast it spins (actually the effect of the Law of Sines based on the tangential velocity of the prop at any point VS the forward speed). Precession is a factor of the angular momentum and pitch rate.
During a climb in a light airplane, you have some pretty good, constant, angle of attack and a pitch rate of zero, so, some P-factor and no precession. During a stunt plane corner, you might have about the same relative wind, similar tangential velocities, and lets say for sake of argument the diameter scales so it is a wash. But you also have *extremely high* pitch rates, not zero, and probably faster than any maneuvering routinely possible for full-scale manned aircraft - something like 360 degrees/second. Try that in a full-scale airplane and you will stall it in the first few milliseconds, or pull the tail off trying to get it accelerated.
So, what are the relative sizes of the torques in our situation. I calculated the precession above, it's trivial. Estimating the P-factor is a lot more difficult, but I estimated it quite a while ago. To estimate it, you can break the prop into spanwise sections, calculate the lift from each section, and then add them all together. The basic geometry is in this post:
https://stunthanger.com/smf/engineering-board/effect-of-wind-on-maneuvers/msg569458/#msg569458
In the example case, just for illustration, I only did 3 separate points, but you can break it up into as many as you want (say, 100, that's what computers are good for). Assume some forward speed and RPM, lift/AoA curve (i.e. cl= AoA/10), and an angle of the relative wind to the prop disk. Instead of the right triangles from the illustrations, you have to use the Law of Sines to figure out the resulting AoA of each section, you know the chord, you know the width of each section (since you chose it) you know the velocity, so you can calculate the lift from each section. The sum is the thrust. The torque from each section is the force on the section x the radius of the section, one blade will have it's AoA increased by the relative wind, the other will have it reduced, so you will get a net torque. That's the *peak* of the P-Factor torque for each revolution, but it goes to zero when the blade is "vertical" instead of "horizontal", so figure the average is 71% of the peak (RM). That's the effective P-factor torque.
Do that, and you will find that the value is in the range of around 10 in-ounces depending on what you assume, and ignoring things like the lift distribution changes near the tip, which is conservative (the estimate calculates a higher value for the torque because of this simplification). For a similar situation in a hard loop, the torque from precession is 177 in-ounces - an order of magnitude or more larger.
No one has to believe me, they can do the same thing. You can imagine ways to test for it, but it's not wrong by a factor of 10 or more.
Reasoning by analogy is frequently useful, but can take you only so far, the numbers matter, too.
Brett
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Actually, yes I have a solution that looks extremely promising at this point. My intent was to discuss exactly how much impact precession does have on the models, weight, trim flying, handling qualities etc. Having been an engineer working for a helicopter company in my early career, precession is something I have had to deal with and have a solid understanding of. I promise to publish once I have achieved some flight testing which requires some exotic materials (aluminum filled epoxy in transit) to work with. I am hesitant to discuss this because without a good flight or more, the understanding basis would be simply conjecture. As one of my bosses told me, one good test is worth 10,000 opinions. On the other hand if I fail, well, at this point, I look like a typical moron. Not the first time. I will say that I have had a couple very knowledgeable engineers look at what I have and I have made a few videos.
No one is mad at you, but you have to remember that no one else knows what you have in mind, and there have been *many* people come in and claim some deep knowledge, and then are later found to have deeply flawed fundamental assumptions, on top of which they have build a complicated skein of reasoning, and are absolutely hell-bound to "show" everyone how right they are. That's probably not you, but when the posts starting playing it "coy", that tends to be a red flag for many people, because it looks a lot like how many of the other posts started.
Again, just tell us what you are thinking, there are multiple very experienced aerospace engineers here, don't beat around the bush. No one will treat you like a moron as long as you aren't trying to play games with us. By the same token, we all have finite time to spend on this, and are not here all the time like it was Skype, so give people time to think it through.
BTW, trying to sort through your various hints, it appears that you are talking about some sort of hinge/damper system. Frank Williams was experimenting with something like that (a folding prop) but as far as I can tell, it has some momentum in a horizontal plane in level flight, and 1/4 second later it has the same amount of angular momentum in the vertical plane, so there has to be some torque applied to reorient the angular momentum vector by 90 degrees in the 1/4 second, just from conservation of momentum reasons.
Since you know about helicopters, be aware that you could also use *cyclic* prop pitch control to try to mitigate or change how this works, or get more torque for maneuvering. I don't think anyone has successfully done this, if nothing else, merely making a variable-pitch prop that is light enough but doesn't fly apart seems to be enough of a challenge that no one has done anything more than experiment with it.
Brett
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No one is mad at you, but you have to remember that no one else knows what you have in mind, and there have been *many* people come in and claim some deep knowledge, and then are later found to have deeply flawed fundamental assumptions, on top of which they have build a complicated skein of reasoning, and are absolutely hell-bound to "show" everyone how right they are. That's probably not you, but when the posts starting playing it "coy", that tends to be a red flag for many people, because it looks a lot like how many of the other posts started.
Again, just tell us what you are thinking, there are multiple very experienced aerospace engineers here, don't beat around the bush. No one will treat you like a moron as long as you aren't trying to play games with us. By the same token, we all have finite time to spend on this, and are not here all the time like it was Skype, so give people time to think it through.
BTW, trying to sort through your various hints, it appears that you are talking about some sort of hinge/damper system. Frank Williams was experimenting with something like that (a folding prop) but as far as I can tell, it has some momentum in a horizontal plane in level flight, and 1/4 second later it has the same amount of angular momentum in the vertical plane, so there has to be some torque applied to reorient the angular momentum vector by 90 degrees in the 1/4 second, just from conservation of momentum reasons.
Since you know about helicopters, be aware that you could also use *cyclic* prop pitch control to try to mitigate or change how this works, or get more torque for maneuvering. I don't think anyone has successfully done this, if nothing else, merely making a variable-pitch prop that is light enough but doesn't fly apart seems to be enough of a challenge that no one has done anything more than experiment with it.
Brett
Since we are in this topic about variable pitch and cyclic pitch, I have a question: Is variable pitch prop allowed on the stunt model by the rules?
Jerry
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Since we are in this topic about variable pitch and cyclic pitch, I have a question: Is variable pitch prop allowed on the stunt model by the rules?
Sure, why not? Collective (that is, changing the pitch of both props the same amount, as traditionally used on full-scale airplanes) is the obvious solution to finite bandwidth/current saturation problem for feedback electric control. You only have to change the pitch slightly to have a huge effect, so you can do it very quickly. I had a brief discussion of this topic with several notables at the NATs in 2009, so don't count on me doing it anytime soon.
I sincerely doubt that cyclic (changing the pitch of one blades up and and one blade down down during a each revolution) would be of any value for enhanced maneuvering. I am reserving my opinion on doing anything else with it (like cancelling precession). It does get around the "no servo control of flying surface" aspect of Tim's proposal, because the most straightforward way to deal with it is with a gyro-controlled rudder, which will, for good or bad, be illegal.
At least part of the problem with any variable-pitch system is making it stand up to the huge vibrational forces from precession and P-factor while at the same time not flinging the blades off. Kind of like a helicopter.
Brett
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In this case it’s possible to make an electric stunter with collective pitch control of the prop, and the same servo operating the pitch will move the rudder, to compensate gyroscopic precession. In the twin model configuration with counter rotating props there will be zero gyroscopic precession. Variable or collective pitch control will be hard to synchronize, but it’s doable.
In my opinion, active timers and ESC in the electric setups are not quick enough to change RPM, because they are “fighting” themselves on the function of constant RPM in governing mode and variable RPM for maneuvers. If the timer for collective pitch will run constant RPM, and in the same time change pitch, then the model can really perform stunts with constant air speed. It’s more of a question for timer designers...
Jerry
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In this case it’s possible to make an electric stunter with collective pitch control of the prop, and the same servo operating the pitch will move the rudder, to compensate gyroscopic precession.
Until it gets outlawed in 2022.
In my opinion, active timers and ESC in the electric setups are not quick enough to change RPM, because they are “fighting” themselves on the function of constant RPM in governing mode and variable RPM for maneuvers.
It governs to the "rpm" that is commanded by the feedback system, that can change. But, the ability to change RPM certainly has a finite time response and can easily current-limit trying to follow the commands. That's why some controllers work and others don't, and why they can easily go unstable if you turn the gain up too much.
That's why we were talking about the possibility of traditional variable pitch (collective, in helicopter terms) a very long time ago. The problem in the IC era was that there was so much force/torque variation on the hub that I never got past the fear of having it come apart on me. With electric, the problem is greatly reduced and now you can at least imagine doing it. Note that even with the mechanism figured out, it's at least still a little tricky, because when change the pitch, you are also changing the load, so the governor response still matters.
Also - that 177 in-lb still has to come from somewhere, that somewhere is now your variable pitch mechanism.
Brett
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Has anyone tried using hinged blades like the Free Flight guys have on their supergalactic engines?
I'm gonna take a SWAG and imagine this is the direction Mark is looking.
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Single blade props come to my mind. They are used widely in speed models at one end of the RPM spectrum, and in rubber powered free flight models in the other end of the RPM spectrum. Stunt requirements would be some where in the middle to low end of the spectrum, I would guess. how does that affect P-factor if there is no blade on the other side? Or does it make it worse?? I made one for a 1939 Korda Wakefield model, and once I understood how to properly balance it, it was pretty easy to trim out and get it to do what I wanted. And how many times has this all gone around and around on the record player and there really isn't anything new?
Type at you later,
Dan McEntee
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Many moons have passed since I flew speed or racing but I did try to use a single bladed stunt prop *once*. In theory they should decrease the gyroscopic element of precession but I never got that far. I couldn't get sufficient thrust at an RPM that kept my Fox 35 from melting so I moved on. '' ''
On collectives, I am really curious now how we can have a rules legal device that is not a set of counter rotating props that can react quick enough to impact precession twice in the 1/4 second or so that we induce dramatic AOA changes during a corner 90 degree corner.
The other fallacy I see in the collective approach, and I may be completely wrong, is that a helicopter's rotor, it's lifting surface, attached parallel to it's body and controlling the AOA , partially through precession, is not the same thing as a prop attached 90 degrees from a different lifting surface whose AOA is controlled elsewhere fighting precession. You could sort of mirror that by vectoring the thrust of the engine by changing the up/down thrust. That has been tried too, don't think it worked very well.
Also, the RPM differential is huge. Our hobby helicopters operate at what, about 2500 rpm? We are at 10,000.
So let's say for the sake of argument that a fully mechanical (it's those nasty rules again) one could be produced to fit into a 2" spinner or even a 2 1/2" spinner, the cost is going to be pretty high and where is it going to be when you hit the ground?
That 's my $.02 on the feasibility of a mechanical device to counteract precession. I am not going to be tearing out my CAM rudders any time soon.
But, there is another side of this that should be extremely interesting, especially with our brighter engineering minds already engaged and that is the *possibility* #^
Sometimes in brainstorming the possibility, someone trips over the feasibility. Won't be me, I am not the brightest bulb on *this" tree. LL~
Ken
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I would mention that Dan Banjock has a stunt plane that eliminates any worries or effects of precession.
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On collectives, I am really curious now how we can have a rules legal device that is not a set of counter rotating props that can react quick enough to impact precession twice in the 1/4 second or so that we induce dramatic AOA changes during a corner 90 degree corner.
But it's also spinning 200+ times a second and all it would take is a fraction of an inch of movement of the cyclic. There may well be some dynamics problem with the swashplate or the blade, but they are already pretty stiff. The idea, of course, would be to "fly" the blades to the new plane - again, like a helicopter - so you don't have to use the engine shaft to brute-force it, and get the reaction torque that will cause.
Probably obvious (after someone else figured it out in the 30's, everything is obvious...) is that you can't build a simple helicopter where the shaft is rigidly connected to the blades - the blades are hinged to permit some degree of up/down movement, so the blade plane in a hover remains essentially fixed, and the helicopter sort of hangs from it and is free to swing around. The blade plane is reoriented by use of the cyclic pitch control, not by pitching the body down and hoping the blades get forced to follow. If you don't deal with this, the coupling, the same effect we are concerned with, will cause it to be wildly unstable, with a pitch torque causing a roll motion - that also makes a great Youtube video.
That is the effect we are fighting, in Al's case, with a movable rudder, and in my case, "3 billboards in formation".
That was the key to early helicopter design, and only with great effort ( https://en.wikipedia.org/wiki/Lockheed_AH-56_Cheyenne ) can you make it rigid and stable at the same time - by using the cyclic in much the manner we are discussing here.
So let's say for the sake of argument that a fully mechanical (it's those nasty rules again) one could be produced to fit into a 2" spinner or even a 2 1/2" spinner, the cost is going to be pretty high and where is it going to be when you hit the ground?
That 's my $.02 on the feasibility of a mechanical device to counteract precession. I am not going to be tearing out my CAM rudders any time soon.
But, there is another side of this that should be extremely interesting, especially with our brighter engineering minds already engaged and that is the *possibility* #^
Sometimes in brainstorming the possibility, someone trips over the feasibility. Won't be me, I am not the brightest bulb on *this" tree.
First - it is not going to be outlawed, as far as I can tell, and electronic control of the propulsion end of the system will have no restrictions in any proposed rule. If nothing else, you definitely *want* to permit at least collective pitch changes because it has the potential to greatly simplify the problem of electric feedback control - or allow you to do it effectively with an IC engine.
That having been said - it's really hard to see how a cyclic pitch system to reduce/eliminate precession effects is going to end up lighter/simpler/higher performance than contra-rotation, and we already have contra-rotating systems that you can just go out and buy. Maybe, but years of work VS click a PayPal link is a pretty each choice.
I note again that we still *have not seen Mark's idea*, which may be different/smarter/better-thought-out than what we have discussed so far.
Brett
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First - it is not going to be outlawed, as far as I can tell, and electronic control of the propulsion end of the system will have no restrictions in any proposed rule. If nothing else, you definitely *want* to permit at least collective pitch changes because it has the potential to greatly simplify the problem of electric feedback control - or allow you to do it effectively with an IC engine.
Brett
You are right. I was seeing/thinking this as similar to radio which has limits but it is really not. It is 100 ESC/Timer or a new whatever box. Since there is no movement to outlaw electronic gain there should be none for this either. However, would changing the plane of rotation of the prop, which I cannot rule out since we don't know what the *idea* is yet be considered control since it will affect the AOA of the wing and not be coming from the bellcrank? Just a thought.
Thanks Brett for your input. New ideas rarely come from doing it the same way every time. Since I am sidelined till at least mid 2021, can't build, nothing to fly, I am getting my "fix" here and learning as much as I can about everything I can.
Ken
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You are right. I was seeing/thinking this as similar to radio which has limits but it is really not. It is 100 ESC/Timer or a new whatever box. Since there is no movement to outlaw electronic gain there should be none for this either. However, would changing the plane of rotation of the prop, which I cannot rule out since we don't know what the *idea* is yet be considered control since it will affect the AOA of the wing and not be coming from the bellcrank? Just a thought.
Thanks Brett for your input. New ideas rarely come from doing it the same way every time. Since I am sidelined till at least mid 2021, can't build, nothing to fly, I am getting my "fix" here and learning as much as I can about everything I can.
Ken
The next obvious problem is coming up with a mechanism that will fit in the spinner and immediately behind it. Scaling down a full-size system seems straightforward for a collective seems straight forward, and you need to have something like tiny ball bearing in a slot to transfer the position - and have it stay alive long enough because it will be spinning much faster than the engine.
Far smaller pitch changing mechanisms are used on FAI Wakefield models (including cams/springs that respond to the torque), so the actual pitch change part of it seems like the simple part . You should be able to get away with plain bearings for the bearings in the pitch rotation axis, presumably split front/back so you can glue/pin the blades into the (steel or titanium) hub, and the close the hub halves over it to retain it.
If you want cyclic, I am having a lot of trouble seeing how you could practically do it, because now the swashplate or other mechanism has significant force on it at 440 times/second trying to yank the blades up and down twice a revolution, everything in there is going to get pounded to dust in short order.
Brett
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The next obvious problem is coming up with a mechanism that will fit in the spinner and immediately behind it. Scaling down a full-size system seems straightforward for a collective seems straight forward, and you need to have something like tiny ball bearing in a slot to transfer the position - and have it stay alive long enough because it will be spinning much faster than the engine.
Brett
I can envision a mechanism that would do this including the spinner but where it falls apart is the cost. Making a prop with movable blades will be very difficult and making it cheap enough to afford will be more difficult than making the collective. However, I think the effort to make the prop might be worth playing with though since getting it to work and hold up gives you an adjustable prop. Screw the rest of it! LL~
How much of a pitch change do you think it would take to pull this off. My guess, and it really is a guess, is less than 1/4" of pitch given the speed it has to happen which would probably equate to the thickness of a couple of sheets of paper at the collective plate. If that is true then we move from impossible to impractical. D>K
No matter what the end result is there are 10,000 reasons it probably won't work - 1 RPM each. LL~
Ken
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I would mention that Dan Banjock has a stunt plane that eliminates any worries or effects of precession.
Nary a wiggle that I could hear.
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We’re still waiting for Mark’s “Idea”...
He may have already wanted to change his opinion a few times after reading our comments...
Here is one more suggestion, concerning gyroscopic precession on the single prop configuration - if using the electric motor and the gearbox. Set the direction of rotation of the motor and prop opposite to one another. The momentum of the motor rotating can and the momentum of the prop/shaft assembly are equal by dynamic balance - at certain RPM. This can be accomplished by gear ratios or by adding a flywheel to the prop shaft.
Now the issue of collective pitch. The prop shaft is hollow, allowing a control rod from a servo to go through the shaft and to the bar ad linkage to be in front of the prop. This way the operation is much smoother than a slider on the shaft behind the prop. More importantly, the front bearing supporting the shaft will be located as close as possible to propeller hub.
My vision of collective pitch on the stunt model is similar to the electric helicopter collective pitch control in “idle up” mode. No matter what the pitch value, the governor will keep the RPM constant. Of course, it will be different torture during the pitch change. That’s why it’s better to have a twin engine model configuration with counter-rotating props. My question to electric timer designers is still open. One timer has to keep/govern constant RPM, do the delay, track flight time - all passive timer functions. The second active timer controls the pitch, based on the sensor output for the position, acceleration, centrifugal force, and so on.
The most annoying thing about electric stunt models is the low line tension in overhead/vertical maneuvers. Or, if compensating for that, too much line tension in level flight.
Jerry
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Here is one more suggestion, concerning gyroscopic precession on the single prop configuration - if using the electric motor and the gearbox. Set the direction of rotation of the motor and prop opposite to one another. The momentum of the motor can and the momentum of the prop/shaft assembly are equal by dynamic balance - at certain RPM. This can be accomplished by gear ratios or by adding a flywheel to the prop shaft.
At any RPM, assuming you set the gear ratio between the prop and rotor equal to the ratio of the inertias. I think you will find that it will have to spin *much faster* than you think, at least 3x or more the prop RPM. That's not impossible, but in my example I figured it was more than a factor of 20, and thus negligible.
Brett
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The most annoying thing about electric stunt models is the low line tension in overhead/vertical maneuvers. Or, if compensating for that, too much line tension in level flight.
Jerry
What are you flying? All of my electrics have better overhead tension and pretty much normal tension in level flight.
Ken
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What are you flying? All of my electrics have better overhead tension and pretty much normal tension in level flight.
Ken
I fly TF ARF Nobler. Before I converted it to electric I was flying it with OS.35 FP.
Hubin , KR, and Igor’s timers are not as good as wet power 4-2-4 brake. Passive timers don’t “lean “, or accelerate at all.
Jerry
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I fly TF ARF Nobler. Before I converted it to electric I was flying it with OS.35 FP.
Hubin , KR, and Igor’s timers are not as good as wet power 4-2-4 brake. Passive timers don’t “lean “, or accelerate at all.
Your symptoms (large difference between level flight and overhead line tension) sound like you left the airfoiled rudder and rudder offset, with leadouts too far aft.
Straighten everything out, get it trimmed flying tangent, and I think you will find it performs *much much better* than any 4-2 break arrangement.
Brett
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I fly TF ARF Nobler. Before I converted it to electric I was flying it with OS.35 FP.
Hubin , KR, and Igor’s timers are not as good as wet power 4-2-4 brake. Passive timers don’t “lean “, or accelerate at all.
Jerry
Interesting. I had a TF ARC Nobler with an OS35s that I converted. Significantly better overhead after conversion. Maybe it is how we set our motors. I ran the OS at a fast 4. Never much liked a 4-2-4 so when I went electric I didn't miss it. Maybe it was all the years with the Fox Burp that soured me! LL~
Enough Off Thread - Ken
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Your symptoms (large difference between level flight and overhead line tension) sound like you left the airfoiled rudder and rudder offset, with leadouts too far aft.
Straighten everything out, get it trimmed flying tangent, and I think you will find it performs *much much better* than any 4-2 break arrangement.
Brett
TF ARF Nobler has no adjustable leadouts. Rudder also is not adjustable. I have chance to compare different power- glow and electric on the same model. Maybe electric is not as light at 52 oz compare to glow at 46 oz. Nobler is a test bed for different timers, it’s not the best flyer after two crashes.
Speaking of propeller precession: once I tried to fly Nobler with Rimfire .32 on 4 cell and gas prop APC tractor 13x4 . The sound of bearings during square corners was very dramatic, and torque was making top of outer wing very visible.
Jerry
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TF ARF Nobler has no adjustable leadouts. Rudder also is not adjustable.
Sure they are:
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Brett, it’s a nice tool! But I prefer hammer and chisel! R%%%%
Jerry
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Sure they are:
LL~ LL~ y1
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LL~ LL~ y1
Ken, you laughed so hard because you realise what tool you used to trim your TF ARF Nobler?
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Ken, you laughed so hard because you realise what tool you used to trim your TF ARF Nobler?
I wasn't entirely kidding. I have seen airplanes cut up, some, multiple times, *after* they had already won the Walker Trophy - for the same reason you need to.
The effect Ted was referring to earlier is quite apt - if you have some rudder offset, and trim it for tangent flight with the leadouts, the first thing that happens when you give it up elevator entering, say, a 4-leaf, is that the extra line tension from pulling on the controls is that the airplane *noses in*, because the airplane is nosed out to begin with from the rudder offset. The higher you get in the circle, the worse it gets, because the component of the weight opposing centrifugal force gets bigger.
In point of fact, both David's 1997 NATs winning airplane, and Ted's 1995 NATs-winning airplane, suffered from offset rudders, and both were cut loose and straightened afterwards with great improvement. David's, in particular, was cut with a Zona saw exactly like the one in the picture, at the field in Napa, cutting through from the inboard side from top to bottom, but not quite all the way through, bent over to close the kerf, and glued back with Hot Stuff. Flew a few flights, another cut, another few flights, another cut, until it was about straight. Differential between level flight and overhead tension went up with each cut.
Of course, you need to know what you are doing and evaluate the trim changes as you do them, and it certainly helps to have the participants all be NATs winners (David, Ted, and some other idiot they hang around with). But all those offsets that Aldrich needed to fly 5.5-second laps with a Fox 35 are there to "manufacture" line tension with very marginal power and power/weight. You don't need that with any modern system, and you don't have to compromise as much just to get line tension. That's one of the huge advantages of having modern propulsion system - which most people are STILL not taking full advantage of, even 30ish years on.
Brett
p.s. as you fix this, you might also run into the next problem - the flaps are way too big and/or move too much with respect to the elevator. But first things first...
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Ken, you laughed so hard because you realise what tool you used to trim your TF ARF Nobler?
Actually I used a #11. New rudder - no offset, adjustable elevetor horn and adjustable leadouts all before hitting the circle. I won't go into the warped wing and the God awful big flaps.
Ken
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Nowhere to "Full Avaiojet" in 35 posts, that may be a new record.
Brett
Now that is GOLD!
Funniest retort I have ever read on this forum - Brett in full afterburner mode!
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Hello
I have not gone entirely away. I have not been following this post for quite some time as yes, there were some harsh words that put me off and caused me not wish to share. It seems that some have trod closely to what I have surmised. Let me state this. A gyroscope precesses because it is a spinning rigid body and only because it is a rigid body. A propeller does not have to be a rigid body and as such, the classic notion of precession we expect would not be present. A propeller can be treated a rigid bar on a spinning axel and if it is spun in a vacuum and the spin axis is tilted the plane of rotation results in a change in momentum 90 degrees out of phase, it will precess. However if a frictionless hinge is placed in the attachment point between the spinning axis and the bar the result is very different. The cross bar will continue to spin in it's original plane.
I am not a master control line precision stunt pilot and all of the discussion about the impact of precession and all of the efforts are what begat my initial question which resulted from experimenting I was conducting for a different application. People are going as far as installing counterrotating systems to battle this beast. Which, although these do negate the precession resulting from one propeller, the combined rotating inertia still creates a significant amount of resistance to maneuver. This effect is, in fact, a not insignificant portion of what stabilizes quad rotor drones.
With my limited flying skill, I marginally used to notice the impacts of recession while flying my models. Having a more heightened awareness and practice, I can say I can truly tell. Previously I would do like most and just fly around it. For a pilot who is in the greater classes this is likely another story. Perhaps I'll get there depending on how well I get past some less than welcoming to the new guy engineer in the group. Yeah, I own some of that too as, like many I have my technical badges.
I've also had a quite the year. This time last year, we were running an ammunition manufacturing business and gun shop. I have to tell you, it was F'ing crazy and we decided to exit the business. Which, honestly this is the 85 percentile reason I haven't been active. We decided that it was time to sell / close the shop and that effort took until sometime close to trout fishing season in April. During that period I collected materials to make prototypes.
So, I am or was or perhaps still am working on a variable pitch propeller and I was concerned about the fatigue resulting from the precession of the propeller disk while flying in circles which could quite rapidly result in catastrophic failure. A propeller spinning 10k rpm goes through 880 ish bending cycles every second in flight which means every flight will have about 40k fatigues cycles. That's a problem. There is one method of removing bending in a structure, use a pinned joint. No bending can pass through a pinned joint. Great, easy fix for my variable pitch prop idea to address the bending induced fatigue issue, pin the blade at the hub.
That sounds easy enough to do. But then another concern came to mind, what about maneuvering with a propeller that is hinged at the hub? I am a retired powerplant engineer today but my early days I was a rotor and drive system guy for a helicopter company, we did things like make helicopters without tail rotors and such and in my design consideration for the propeller and I wasn't anywhere close to rotor zone.
For what it's worth I have almost a dozen F1C free flight planes in a few boxes and they all have propellers with hinges in them for folding. A boss of mine once said one good test is worth a thousand opinions and I had the bright idea of taking one of these F1C propellers and attaching it to a small electric plane I have and did so. I wanted to see if the propeller would follow the airplane in pitch and yaw without blades striking anything.
I pretty much kinda knew the answer but hadn't though it through totally. I knew if I pitched or yawed the airplane, the propeller would follow just like a tail rotor does on a helicopter and for the same reasons. BTW there are actually two kind of precession inertial and aerodynamic. When I conducted my test, the primary result was, as I expected, the propeller followed with no apparent issue or troubles. However, I noticed something unexpected to me at the instant but, like I said, I didn't have my helicopter hat on. There was no darting of the nose as result of precession of the propeller. I switched between the hinged propeller and a fixed propeller and the effects are clearly noticeable. I made a couple youtube videos showing the testing and some other quick discussions so I could come back to this and repeat with a more scripted manor.
I have flown this airplane and propeller combination but the motor / battery combination and propellers didn't work well. The prop the airplane works well with is a 7x5 while the F1C is more like 7x3.5 and the motor battery won't turn the hinged prop fast enough to fly anything meaningful. Upright level flight won't highlight precession. A more meaningful test is one of two propellers with as close as can be done base characteristics. This is where I am today. I have a 35 size model I have been flying for quite some time and it uses a 10x5 propeller. I have taken an XOAR wooden electric and flown the airplane quite a bit and the model works well with this combination. I took one of the these propellers and crafted a blade mold to make some carbon / glass blades from.
I have enough blades to make a test propeller and am finishing a few hubs. I also have a shop quoting me the price of making 25 hubs. My test plan is to run both the fixed propeller and the hinged propeller back to back to see how much impact there truly is. From my bench testing, rotor system knowledge and more better flying skills I should be able to determine the improvement. I have zero doubt there will be some noticeable improvement. So the question I opened for discussion was how much value to flight or aircraft performance does correcting or reducing the impact of precession have? I honestly didn't know at the time and yes, I started @#$% storm. Is it worth $100 and 10 grams? That's about what it will cost. My next version will grow to a 13 inch propeller and then a 11 inch.
Then again, I could flight test and find out I'm full of @#$%.
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That is very cool, Mark. One problem I can see is that it dashes my hope of someday understanding propellers.
Those of us with ballast in our plane's noses won't be offput by the 30 grams.
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I pretty much kinda knew the answer but hadn't though it through totally. I knew if I pitched or yawed the airplane, the propeller would follow just like a tail rotor does on a helicopter and for the same reasons. BTW there are actually two kind of precession inertial and aerodynamic. When I conducted my test, the primary result was, as I expected, the propeller followed with no apparent issue or troubles. However, I noticed something unexpected to me at the instant but, like I said, I didn't have my helicopter hat on. There was no darting of the nose as result of precession of the propeller. I switched between the hinged propeller and a fixed propeller and the effects are clearly noticeable. I made a couple youtube videos showing the testing and some other quick discussions so I could come back to this and repeat with a more scripted manor.
So, in this case, what *does* cause the angular momentum vector to change direction? You are flying along level, angular momentum vector parallel to the ground, and 1/4 second later, you are flying vertically, with the angular momentum vector vertical. What rotates it from beginning to end, if not torque applied at the shaft?
Brett
p.s. OK, how about this - I presume your hinge is in the usual direction, that is, a "flapping" hinge that lets the blades flop up and down out of the spin plane, with the pin perpendicular to the shaft. Do a corner, and what seems to happen is that when the span of the prop is "vertical", that is, the hinge pins are parallel to the pitch axis, and as you apply torque at the hub, one blade moves forward relative to the hub and the other back (above and below the nominal spin plane) and no torque precession torque is applied, because it in fact doesn't change the angular momentum vector. As the prop rotates to "horizontal", it is applying torque around the spanwise axis of the prop. So the torque applied to the prop is applied around the long axis of the prop, with a tiny inertia. So the torque precessional torque varies from zero to some maximum in one revolution. Still, the integral of the torque still has to equal to the change in the angular momentum, so it must be that *something* is getting astronomically more torque than it would have otherwise at some point, because the same angular momentum is applied over much less time.
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Good point, I think. I guess I’ll revert to my previous solution of counter-(contra-?)rotating props with axes separated by y>(prop diameter +fuselage width).
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Good point, I think.
I am not so sure. Something (a torque) has to reorient the H vector, that torque can only be applied by the shaft, which means that the reaction torque will effect the airplane. What is unclear (to me) is how this torque is applied to the shaft by the prop blades. They clearly orient themselves to the spin plane if disturbed, like when the spin plane changes.
It's clear that it is not the same through a rev, so it has to induce some vibration or variable loads somewhere.
Hopefully Mark has some insight on that one.
I guess I’ll revert to my previous solution of counter-(contra-?)rotating props with axes separated by y>(prop diameter +fuselage width).
Oh dear, not you, too? How far forward does the battery have to go to get the CG in the right spot? That's what scared me about the B-36 - ~ a *pound* of battery hung far out on a skinny nose.
Brett
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I made several flights today and yeah it works. The effect of precession on the airplane is not huge so the correction less than major change to performance. The good news is that the propeller does not crash in to the nose of the airplane causing total destruction as I had feared. I flew the propeller on two different airplanes and one responded to this mod better than the other did. The one which was less clear actually flies super well and the difference is subtle but it may also be from some other elements such as flight velocity. The baseline prop was an APC 10x5 and test prop also a 10x5 but from an XOAR modified blade. Is it worthwhile, I can't say as the impact is such that my skill level would benefit more from wearing out $100 worth of batteries flying than from spending the $100 on a device that makes the airplane a bit better. Having said that, my planes will likely get these props on them.
To answer your question of how the angular moment changes is very interesting. There are two interactions taking place and it is very difficult to write down clearly. I have a youtube video on this which will do a better job of presenting the concepts. I also did some motor ona stick videos with a motor allowed to pivot freely on the precessional axis to demonstrate the problem and the performance of the solution.
The first is a demo of a hinged prop on a model and discussion about how and why it works the way it does:
https://youtu.be/mO4zpLGfP9A
This video is the motor ona stick. I couldn't always keep the subject centered but there is some decent content and you can get the gist:
https://youtu.be/yj5bEluQnv0
Finally this video is a basic repeat of the second but the motor ona stick got a camera ona stick added to it so you can watch the motor tilt:
https://youtu.be/AoLCrNCO7TY
Enjoy.
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Nice explanation Mark. some years ago (15? ;D) when I started with electric, I did also such tests in hope it will solve precession, because the only usefull prop for my models was heavy plastic Graupner or APC. I found usefull folding Graupner prop and did face to face test with both of them. I found 2 things:
1/ that model was "somehow strange" I think because instead of expected precession and stabilization, model did other strange forces or effects or simply precession took longer time after model was already on straight flight, it was hard to "understand" and even harder to explain in words : - )))))))) I can little bit feel similar thing if I replace full cabon prop by plastic APC (I mean not hollowed carbon prop so that the weight is the same). The rigid prop is easier to trim than plastic APC. At least is I use Rabe rudder.
2/ there is technical problem with hub, I think there are forces on the hinge perpendicular to expected precession (which is eliminated as the blade is hinged) comming from transition of mass movement as result of pitching. That means that blades are oscillating right-left in corners (similar to compression-expansion load on IC engines at prop root). It was visible on root of plastic blade and sides of the "H". While I did normal static tests all was nice, but after few real flights the hinge was visibly abused. So I affarided continue tests because of safety. I had only props for R/C gliders and they are simply not prepared to withstand such forces, they are for straight flight.
You mentioned little angled hingeline. There are hubs available with offset (like you also mentioned) and they are also for 2 blade and also for 3 blade props.
Here is my old video reposted by Brent. It is flight with folding prop (initial picture is with Graupner prop). You can hear sound of prop especially in upper corner of houglass when model is closer to camera. Pictures show that model.
https://www.youtube.com/watch?v=IcszAfFy6_k&t=63s
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1/ that model was "somehow strange" I think because instead of expected precession and stabilization, model did other strange forces or effects or simply precession took longer time after model was already on straight flight, it was hard to "understand" and even harder to explain in words : - )))))))) I can little bit feel similar thing if I replace full cabon prop by plastic APC (I mean not hollowed carbon prop so that the weight is the same). The rigid prop is easier to trim than plastic APC. At least is I use Rabe rudder.
2/ there is technical problem with hub, I think there are forces on the hinge perpendicular to expected precession (which is eliminated as the blade is hinged) comming from transition of mass movement as result of pitching. That means that blades are oscillating right-left in corners (similar to compression-expansion load on IC engines at prop root). It was visible on root of plastic blade and sides of the "H". While I did normal static tests all was nice, but after few real flights the hinge was visibly abused. So I affarided continue tests because of safety. I had only props for R/C gliders and they are simply not prepared to withstand such forces, they are for straight flight.
I think at least one effect is that the hinge puts some different dynamics on the prop, "soften" it up, and the hinged prop depends on the forces keeping the blades perpendicular to the spin plane. I haven't been able to visualize how that affects the forces at the hinge, but it is very obvious from first principles that the force applied at the root is cyclic at the rotation frequency/RPM, whether it is hinged or not. Which means the peak torque is much higher than the average.
This also makes me start to wonder what that long, skinny, unsupported motor shaft, with a motor mounted in the middle of a flexible diaphragm, is doing with all these large, high frequency, oscillatory forces applied. Maybe we have just gotten lucky so far.
One thing I am 100% sure about is that conservation of angular momentum is true in this case, and, that the necessary torques are ultimately applied through the shaft.
Brett
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This also makes me start to wonder what that long, skinny, unsupported motor shaft, with a motor mounted in the middle of a flexible diaphragm, is doing with all these large, high frequency, oscillatory forces applied. Maybe we have just gotten lucky so far.
There have been motor mount stiffness mysteries, as in days of yore. Some motor brands have to be mounted differently than others or experience short bearing life.
I think Mark is on the right track with using prop blade angle of attack to apply a moment aerodynamically and with variable pitch. How about a mechanism like the one helicopters use to tilt fore and aft and sideways? You could experiment with axis and magnitude to trim the airplane. It would be immoral and should be illegal to do this electronically: it's an aerodynamic control. One oughta be able to do it mechanically, though.
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There have been motor mount stiffness mysteries, as in days of yore. Some motor brands have to be mounted differently than others or experience short bearing life.
Interesting, I was unaware of that, but I was rather alarmed at some of the arrangements I have seen, like a rear-mounted motor on a maybe 3/32 plywood firewall - "to save weight" - and the prop/drive hardware/spinner hung out there on a 3" x1/4" (or metric equivalent) shaft. Also, that despite the apparent 3/16" of clearance, it was rubbing the paint off the nose ring.
Brett
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OK, I think I get the gist of the differences, after doing some force diagrams. As Igor notes, it's not that the effect goes away, but it is applied differently and "softer" and has some dynamic effects that weren't there before.
Brett
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There have been motor mount stiffness mysteries, as in days of yore. Some motor brands have to be mounted differently than others or experience short bearing life.
I think Mark is on the right track with using prop blade angle of attack to apply a moment aerodynamically and with variable pitch. How about a mechanism like the one helicopters use to tilt fore and aft and sideways? You could experiment with axis and magnitude to trim the airplane. It would be immoral and should be illegal to do this electronically: it's an aerodynamic control. One oughta be able to do it mechanically, though.
My experience- broken components in electric RC: “front” motor mount in a glider with folding prop and Mega 4 turn direct drive. And 3D model with E-flite 25 870 kv on 4 cell 4000 mAh lipo. 3D was pulling 50 amps and was flying on the 15x6 APC E prop. In sharp maneuvers 3D broke the shaft twice and front mount once. After stiffening front mounts problem was solved . I preferred firewall mount, or rear mount. Less vibration, no wires next to rotating can, nose of the model doesn’t have to be strong in front of firewall, cowl can be used. Bearings of cheap electric motors take big abuse during CL stunt. And Cobra motor with big bearings runs much quieter compared to E-Flite or Rimfire.
It’s possible to make CL stunter with cyclic pitch control. Just like a sideways helicopter. Swash plate will be mechanically connected to the belcrank, no rules broken. This system will make corners effortless. And for pilot it will be a feeling like a “power steering “ . How do modern models helicopters are making quick changes of moving directions with huge momentum of 2000 rpm main rotor and not breaking anything? The shafts of helicopters are diameter 8 -10 mm and there is some significant distance from the fuselage bearing to the main rotor hub. If , hypothetically, helicopter will be controlled by the conventional tail of autogyro ( rudder and elevator), but not by the cyclic pitch , then everything will be broken on the first sharp turn. And none of the dampers, pivoting pins, flexible hubs will survive to help the problem.
Jerry
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The shafts of helicopters are diameter 8 -10 mm and there is some long distance from the fuselage bearing to the main rotor hub.
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And if you make that same helicopter with a 6mm main shaft, you will quickly find the system's "KAPUT" point in a fast turn. ;D
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Keith Trostle mentioned in an earlier reply here that I am now flying twins from time to time, and that I'm using counter-rotating props (actually Keith mistakenly called them contra-rotating props; that's something completely different...). The twins fly extremely clean in this manner and exhibit none of the normal single engine/motor 'bad" traits. My twins are setup to have the tops of the props turning towards the canopy. Perhaps someone with more knowledge on this can explain what's happening with a twin setup in this manner.
Bob Hunt
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Keith Trostle mentioned in an earlier reply here that I am now flying twins from time to time, and that I'm using counter-rotating props (actually Keith mistakenly called them contra-rotating props; that's something completely different...). The twins fly extremely clean in this manner and exhibit none of the normal single engine/motor 'bad" traits. My twins are setup to have the tops of the props turning towards the canopy. Perhaps someone with more knowledge on this can explain what's happening with a twin setup in this manner.
Bob Hunt
Good Afternoon Bob;
As you probably know, Walt Brownell built a twin electric model many years ago called the Gemini , and I believe he had you fly it at a NATS for your input. This was probably one of the dominoes that fell that put you on the path to electrics? Walt has been dealing with advancing age and dementia the last few years and donated all his modeling stuff to our club. One of the things I looked for first when we started to clean out his basement was the Gemini. I remember seeing it fly a few times and was very impressed with how it flew, and Walt won a contest or two with it back then. I was working at the local hobby shop part time while he was building it and the subject of props came up several times. When finished, Walt did a lot of prop testing and there must have been a hundred or more old props scattered about his basement. Walt was a retired aerospace engineer who worked his entire career at McDonnell-Douglas. He told me that conventional theory was to have the props turn like you mentioned, tops in. Walt tried a lot of props in both direction and I remember him saying that he felt the model flew best with the tops turning out, and that the effect of having the prop blast working right against the flaps and having the motors that close to the balance point had a definite positive effect. ( The motor nacelles are not that long, and the two motors are powered by one battery and speed controller/timer.) Mark Hughes has the model now and has been working his way through figuring things out. I remembered that Walt was zeroing in on 9-6 as being the best prop for it but in all of the stuff we found we found very few left hand props. The battery he used was so old it eventually crapped out and Mark could only find ne near that size required and he had to actually cut a small opening in the fuselage sides of the nose for the battery to fit. With two right hand props, it flies very well and Mark may try to fly it at a contest if the opportunity comes up. I'm still trying to convince Mark to get a left hand prop or to and try that out, but he has had some hand surgery and that has slowed the process for a while. If you try the props turning the other way. I sure would like to hear what you think of it. This has been a pet project of mine for as long as I have been flying stunt since 1987, long before electric stuff came along. I wanted to try it with I/C set ups, and have been collecting engines, props and left handed cranks and such since then, all in order to just see what it feels like!! Not that retirement has been forced on me, once I get a few more fires put out around here, there is an RSM Sheeks Mosquito and a couple of Enya 30 engines sitting in the basement, along some other candidates for some smaller engines. It is still possible to experiment with the prop direction this way, you just have to swap engines from side to side instead of swapping wires!!
Type at you later,
Dan McEntee
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Thank You Igor
Wolfgang blew a blade on his model using the made for gliders propeller. I really don't think those props are configured correctly for our application. My prop is derived from F1C models and is molded from carbon fiber with an aluminum spindle with an inner lining of fiberglass to limit the contact of CF with the aluminum. Carbon fiber and aluminum will corrode if moisture gets in. There are definitely in plane dynamics involved. The RC helicopter rotor blades are hinged the way they are to reduce the fatigue and allow the blades to fly nicely with each other. The maneuvers we do certainly cause more and less drag forces cyclically which is part of the reason for how wide I made the yoke portion of my hubs and use a 3mm bolt. The F1C prop uses stainless steel for its hub and can run on an IC engine turning 30k rpm. In a long term idea progression this prop configuration could be done on IC by using stainless steel as opposed to aluminum.
In my design, I only accounted for the tension load of the blade and sized the CF fiber path and one yoke side to carry the load giving a minimum 200 load factor or better. The CF runs from tip around the bobbin back to the tip. Changing to SS would not make a huge weight penalty. To compare, the APC 10x5 weights 19.6 grams and the hinged prop weight 23.6 grams. Here's a couple photos.
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Interesting notion about using helicopter style cycle controls to change the thrust angle. However it isn't necessary to go that far. Case in point, gyrocopters such as the Benson use no cyclic control and steer using a simple tiller control. There are a few simple light helicopters using the same principal as well. These use a teetering hinge rotor which follows along happy dan as can be. Yes, there are some control moments but they are along the axis of input and don't change phase. The same would be and is true with a flapping hinge rotor / propeller. Simply mounting the motor on a hinge such as my motor ona stick would suffice.
If the intent of doing such a thing is to reduce the control moments at the handle, installing cyclic pitch and linkages and other claptrap is not really the way to do it. A much simpler method and lighter would be to offset the hinge of the flap portion of the wing and stabilizer. Many small jets and aerobatic airplanes do this now and it isn't an unknow region. There are lots of test reports available from the NACA days of exactly this. Most small jets use ailerons and elevators hinged between 20% and 25% of the surface chord. Some of the aerobatic airplanes are pushing the hinge line back toward 30% - 33% which results in very low stick force gradients. Doing this treatment to a stunt model could reduce the handle stick force gradient down to flying wing combat model levels. I don't think that far is very desirable, especially in my ham handed case, as the model would do lots of wobbling around following exactly my shaky inputs. Having said that, the eventual model I have been working on will have offset hinge surfaces and flap profiles to help.
In one of my videos you can see the motor wobble. This is a classic whirl event which is caused by the coupling between the way the motor mount is hinged with an elastic damper and the hinges of the propeller blades. Some input sparked a displacement which resulted in a tip path plane offset slightly from the nominal which in turn resulted in the inertial axis tilting slightly and the conservation of angular momentum was accomplished by the inertial axis spinning around the rotational axis. Think the spinning of a coin slowing down and as it stops it begins to wobble, same thing. The thrust of the propeller is perpendicular to the tip path plane which is basically aligned with the new inertial axis causing a displacing moment which drove the mount to move which then oscillated for a period and damped out. This was either caused by the mount coupling or drove the mount coupling regardless the two interacted. An early turboprop airplane design airplane, I can't recall which one, had this same problem and shed an engine catastrophically. The fix is simple, make the mount stiff enough it won't resonate. On the model tests indicate plenty of mount stiffness is present not to worry about the whirl.
If you are inclined to work the math, I'm not but did many years ago when I was in college and today I'm too lazy, there are some good resources Stepniewsky and Keys - Rotorywing Aerodynamics is the textbook we used and Ray Prouty - Helicopter Aerodynamics is a more simplified discussion. The former is a good development of the rotor system dynamics and will remove the guess work in attempting to derive all of the equations of motion yourself.
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Would it work in a different way if the blade hinge point would be at very center of motor shaft? Or, if you forget the folder and just hinge the one-piece prop with a hinge pin at center of shaft? L
It is definitely going to be different depending on how far the pivot is from the spin axis, but the blade will still end up perpendicular to the spin axis.
Brett
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Would it work in a different way if the blade hinge point would be at very center of motor shaft? Or, if you forget the folder and just hinge the one-piece prop with a hinge pin at center of shaft? L
Slightly yes it would. The minor affects that can be seen can be attributed to the hinge offset. To move the hinge to the center would be a challenge to maintain accuracy and create a good shear path for the pin(s). The other thing which would need to be done is that the pivot in the middle absolutely would need to be a delta hinge at an angle in order for the rotor / prop to fly properly in plane. Several helicopters use this type of hinge and they work just fine. I chose the configuration I did because it was a minor variation on parts I already have the skill to make and I am interested in using this for a variable pitch propeller so a centrally hinged rotor wasn't even contemplated.
The blades stay in plane due to the forces involved. The tension is around 450 pounds per blade which quickly restores any displacement at least enough for the application as demonstrated in my test. Actually this was why I was concerned in the first place as the in-plane restoration force is solely the tension force on the blade. Granted the ratio is about 100:1 but still... Without any angle in the hinge there would be no change in blade pitch angle with flapping and resulting restoring force. With an angle in the hinge as a blade flaps up the pitch angle decrease and flapping down would cause increase in the pitch angle generating restorative forces of the blade to in plane flight. Generally, in a good rotor system a disturbed blade will restore itself to in plane flight within one revolution. BTW, I repeated the violent twisting "maneuvers" with the 10 inch version before flying and it behaved like the smaller 7 inch one did. Followed along all happy Dan.
Whether you could do this with a delta three hinge rotor I honestly don't know the answer but probably.
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Here's how I'd look at this, not that anybody will read it. As Brett said sometime back, when you turn a stunt plane, you want it to rotate about a line between your hand and the CG of the airplane. You trim the plane to sum moments about axes perpendicular to this axis to zero. The components of the moment about those other axes caused by precessing (I think this is the correct usage of this word) the prop are contributors to those zero sums. If you look at videos of the world champs or US Nats finals, you will see little evidence of any nonzero moment about those other axes. If prop flailing works to trim a stunt plane, it's because it makes alternative contributions to these zero moments, not because it fixes a problem that existing well-trimmed stunt planes have. The same is true of the Rabe rudder. Igor uses it, US Nats finalists don't. Both have well trimmed stunt planes.
Brett also looked at basic principles regarding changing the rotational axis of the prop. Take away one of the axes about which the prop shaft can apply a moment to the prop and the prop will figure out some combination of moments about the other two axes (hence aluminum bushing wear) plus aerodynamic forces to get itself aligned with the new shaft axis. You can figure out how to put this into the trim mix if you want to. I think it would be easier to use cyclic prop pitch, but I am inclined to favor solutions I understand, rather than things that work.
We tend to look at this stuff as quasistatic (Did I use that term correctly? Where are Ted's quotation marks when you need them?), but flight data from my own, poorly trimmed dog look like the yaw problem is an oscillation. Most attempts to theorize stunt trim address stuff that would stir up the oscillation, but maybe we should be trying to damp it. I suspect it's nature's way of telling us to follow the trim chart and stop asking questions.
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Here's how I'd look at this, not that anybody will read it. As Brett said sometime back, when you turn a stunt plane, you want it to rotate about a line between your hand and the CG of the airplane. You trim the plane to sum moments about axes perpendicular to this axis to zero. I think it would be easier to use cyclic prop pitch, but I am inclined to favor solutions I understand, rather than things that work.
Howard,
I read! My saying about “power steering “ was to describe the difference in forces during the turn of the model. Model with conventional tail is “fighting “ the gyroscopic momentum of the propeller disk . If cyclic pitch is involved, than model will move trough the turn much easier. It’s like on one model there are two elevators in the same time turning conventional tail elevator and canard elevator on the nose.
Mark,
To make simple cyclic pitch control from existing folding prop hub - one more pivot point should be added. Perpendicular to the blade hinges, and perpendicular to the main shaft. And the stoppers, so hub will tilt only 4-5 degrees each way. It can be done as rotor hub of the fixed pitch helicopter with fly bar or flybarless .
Jerry
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Howard,
I read! My saying about “power steering “ was to describe the difference in forces during the turn of the model. Model with conventional tail is “fighting “ the gyroscopic momentum of the propeller disk . If cyclic pitch is involved, than model will move trough the turn much easier. It’s like on one model there are two elevators in the same time turning conventional tail elevator and canard elevator on the nose.
Mark,
To make simple cyclic pitch control from existing folding prop hub - one more pivot point should be added. Perpendicular to the blade hinges, and perpendicular to the main shaft. And the stoppers, so hub will tilt only 4-5 degrees each way. It can be done as rotor hub of the fixed pitch helicopter with fly bar or flybarless .
Jerry
You still need some sort of swashplate to vary the pitch of each balde up and down during each revolution, and gears to or something to actuate it. Not that any of that could not be done, but trying to make it strong enough and tough enough to work back and forth 200x a second doesn't sound all that simple to me.
Also, varying the pitch up and down during a rev wildly changes he drag. If Igor's hinges were falling apart after just trying to transfer the precessional torques, what the heck is going to happen when you run the blade pitch up and down from 2 to 8 in 1/400th of a second on top of it?
The other point is - how would you use it? To apply torque in pitch (and presumable cancel) the torque from precession? To cancel torque in yaw during corners? If so, what do you use to determine how far to move it - a pushrod from the controls? A gyro or MEMs rate sensor? t's a very interesting problem, even if you could work out the mechanism.
Brett
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This is not a problem.
It's an opportunity.
-Andrey
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Here's how I'd look at this, not that anybody will read it. As Brett said sometime back, when you turn a stunt plane, you want it to rotate about a line between your hand and the CG of the airplane. You trim the plane to sum moments about axes perpendicular to this axis to zero. The components of the moment about those other axes caused by precessing (I think this is the correct usage of this word) the prop are contributors to those zero sums. If you look at videos of the world champs or US Nats finals, you will see little evidence of any nonzero moment about those other axes. If prop flailing works to trim a stunt plane, it's because it makes alternative contributions to these zero moments, not because it fixes a problem that existing well-trimmed stunt planes have. The same is true of the Rabe rudder. Igor uses it, US Nats finalists don't. Both have well trimmed stunt planes.
I think the last is because the precession (from the prop) is small enough that it doesn't matter very much, at least, with the size/mass of props we like to use - in the noise. It you trim it so that your principle axes are lined up with the circle "R vector", all you have to generate is enough torque to handle the effects of the kinematics moving the angular momentum vectors around. Not nothing, but pretty small.
On the other hand, as far as I can piece together, I think Igor is also attempting to rotate his airplane around an R vector well off the principle axes by running an intentional yaw angle, and even a few degrees of that is definitely *not* negligible. Do that and you need to generate some sort of roll/yaw torque to make it stay that way, and I think it is *a lot* of torque. So, the Rabe rudder is there to cancel a lot more than just the precession of the prop - and it may well be that the precession helps, rather than hurts. In any case, his motor controller needs a very light/low inertia prop to make it respond fast enough, so it won't be *nearly* as much of it as, say, David and I would have.
You comment about "static" effects is apt, a lot of people who are traversing the steep part of the learning curve look at the *lines* as a static item, applying torques if it roll and yaws, and adjusting yaw angles with leadout adjustments. Which is true enough at DC, but very interesting at other frequencies.
Brett
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This is not a problem.
It's an opportunity.
Absolutely. I am applying the common American/english engineering use of the word "problem" - and interesting topic that we don't understand and should be worked out. Not a difficultly to be overcome.
Brett
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You still need some sort of swashplate to vary the pitch of each balde up and down during each revolution, and gears to or something to actuate it. Not that any of that could not be done, but trying to make it strong enough and tough enough to work back and forth 200x a second doesn't sound all that simple to me.
Also, varying the pitch up and down during a rev wildly changes he drag. If Igor's hinges were falling apart after just trying to transfer the precessional torques, what the heck is going to happen when you run the blade pitch up and down from 2 to 8 in 1/400th of a second on top of it?
The other point is - how would you use it? To apply torque in pitch (and presumable cancel) the torque from precession? To cancel torque in yaw during corners? If so, what do you use to determine how far to move it - a pushrod from the controls? A gyro or MEMs rate sensor? t's a very interesting problem, even if you could work out the mechanism.
Brett
Previously Mark sent us a list of books about helicopters. I went trough all of the reading material in 1981. And built fixed pitch RC model helicopter from scratch in that year. Just and unjust from existing materials available to a school kid. Homemade flat bottom main blades, tail boom from a ski pole, gears from mechanical calculator “Феникс». Housing of tail rotor gearbox was made from two MK-12B engines crankcases. And more. Helicopter flew, went to competition and shows.
And yes, cyclic pitch controls by the swash plate.
When I asked one of the helicopter design engineers what to do if any of helicopter components will break , he answered - “ make it thicker”.
Jerry
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And yes, cyclic pitch controls by the swash plate.
When I asked one of the helicopter design engineers what to do if any of helicopter components will break , he answered - “ make it thicker”.
I know that, people have known *in principal* how to do it since the 30s. That doesn't mean we know how to do it and make it practical with a 10,000 RPM stunt motor. As noted above, we don't even know which direction we want the torque to go, or in fact, how it works in detail even *without* a variable pitch mechanism.
Could I hack something together that more-or-less functions and stays together for a whole flight? Sure, probably. Could I make it practical enough and effective enough to stand up to several hundred stunt flights while improving the performance over a conventional system? Far from obvious. Even if we don't acheive that, it's still worth looking at, if nothing else, we will learn something in the process.
I think you took my comments as negative - not knowing something and having a problem to solve *is a very interesting and engaging issue*. It's very interesting.
If nothing else, as noted much earlier I still think starting with a variable pitch system (collective, in helicopter terms) is a simple enough dynamic problem that you can solve it, and has most of the mechanical elements you need to develop the cyclic system. And, has the great advantage of us *knowing exactly how we would use it* with a pretty good understanding of what we were trying to accomplish.
Brett
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I know that, people have known *in principal* how to do it since the 30s. That doesn't mean we know how to do it and make it practical with a 10,000 RPM stunt motor. As noted above, we don't even know which direction we want the torque to go, or in fact, how it works in detail even *without* a variable pitch mechanism.
Could I hack something together that more-or-less functions and stays together for a whole flight? Sure, probably. Could I make it practical enough and effective enough to stand up to several hundred stunt flights while improving the performance over a conventional system? Far from obvious. Even if we don't acheive that, it's still worth looking at, if nothing else, we will learn something in the process.
I think you took my comments as negative - not knowing something and having a problem to solve *is a very interesting and engaging issue*. It's very interesting.
If nothing else, as noted much earlier I still think starting with a variable pitch system (collective, in helicopter terms) is a simple enough dynamic problem that you can solve it, and has most of the mechanical elements you need to develop the cyclic system. And, has the great advantage of us *knowing exactly how we would use it* with a pretty good understanding of what we were trying to accomplish.
Brett
Brett, I don’t think I said any insults, and I don’t take any suggestions negatively. In the time of my helicopter scratch building I went trough a lot of comments, but all negative comments served me as motivation to complete the project.
As collective pitch control on the stunt model - we already tested it. I have a witness. System is proved by RC 3D indoor flyers. In CL this system works on the governed RPM of the motor and the servo, connected to the variable pitch prop. Servo is controlled by the real active timer, based on the G-force. Generally said, the collective/ or variable pitch prop is the tail rotor unit of any model helicopter. Two options- slider trough the hollow shaft or outside. Advantage of the pitch control vs rpm control - instant response. No matter how heavy propeller blades are.
Now, if add the pivot shaft, swash plate, and connect the swash plate to a belcrank- we have cyclic pitch control. The leverage is adjustable, like control horn on elevator.
And for Rabe rudder - connect the linkage from rudder to the swash plate. 90 degrees apart from the connecting point for the belcrank. (My personal opinion on the Robe rudder- it helps to have a less line tension during the level flight.) Rudder turns out more , and cyclic pitch turns nose out . Elevator “Up” and cyclic pitch pull the nose “Up”. Loosing tension- collective pitch “Higher “ . “All 9 yards “system will have a character of its own. To set up and adjust it will take a lot of patience.
So far, in this topic of propeller precession the best answer to it is a twin model with propellers rotating in opposite direction. But in the real life how many twin stunt models we see? At the 2021 NATS I saw model built by Bob Hunt and model built by Rick Huff. No wonder why my idea of cyclic/collective pitch is complicated, if stunt pilots consider a twin engine stunter to be complicated already.
Jerry
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Mark,
To make simple cyclic pitch control from existing folding prop hub - one more pivot point should be added. Perpendicular to the blade hinges, and perpendicular to the main shaft. And the stoppers, so hub will tilt only 4-5 degrees each way. It can be done as rotor hub of the fixed pitch helicopter with fly bar or flybarless .
Jerry
Jerry
This is, in essence where I began. My effort was to make a controllable pitch propeller. The problem is one of fatigue at the base of the propeller and hub. The primary source of the fatigue is the bending stresses resulting from pivoting the propeller disk during flight in a circle. Each blade undergoes a bending cycle for every rotation during flight which the number of cycles add up very quickly. My background in modeling includes flying the high powered F1C gas free models with the folding propellers. The engine on these airplanes used to turn 30,000 RPM and I have a modified Nelson .15 which topped out quite a bit more and produces 1.15 Hp. I also have flown RC helicopters since the mid 70's until just a few years ago when it became not so much fun as a result of the onboard stability augmentation.
So, my path to the controllable pitch propeller resulted in the idea of hinging the propeller blade to eliminate the fatigue. It wasn't intended to reduce or even address the effects of precession. That wasn't on my mind but it was a result of a bench test I did.
Here's the thing about angular momentum in this specific case. Because the hinged propeller is basically non rigid, precession like a gyroscope is almost non present. However the work is still being done within the system. What is not happening is that there is a significant change in phase of where the work is going. There still exists a moment from the input a torque which will always be there. The difference is that the change in angular moment remains in plane. You can't change the laws of physics that state energy is always conserved, the question is where did the work come from and where did it go?
So, the failures that have occurred by Igor and Wolfgang both involve plastic folding propellers. If you look at the root portion of the blade, you'll notice quite a bit of offset from the pin axis to the blade centerline. I would have flown one of these but I was afraid of the way the blade is constructed of plastic. If you do the math you'll soon figure out there is a huge bending load is apply along the "knee" at the root of the blade. The tension in the blade can be easily determined by measuring the thrust dividing that by two and then dividing that by the sine of 2 degrees. It's a big number probably on the order of 200 pounds or more.
As Igor pointed out, the blades see an in plane cyclic load as result of drag forces. Add this to the large tension force on the folding blade geometry and being made from plastic, in hind sight, it is easy to realize it was going to fail in the application. This was part of why I did the bench test. For the glider and free flight models the prop generally isn't going through wild gyrations. Therefore one good test will answer a thousand questions and I hammered that prop and it tracked the whole time, perfectly.
I've made dozens of carbon fiber props for my F1C models which an example of is the test subject in my video. The design of the prop is such that the tension forces stay in plane and all of the load is carried in tension by the fibers within the prop. I was very tempted to fly the plastic glider prop several times but better sense prevailed. Wear on the hub is another issue and that also can be addressed by reducing the stresses in the hub which means either materials or size. I've used a fairly large pin size in the blade and the hub is designed such that one side of the yoke can carry 150% of the load which should result in a good safety margin.
To date I have about 20 flights on one airplane with the hinged prop on it. I don't see any points of concern yet and all of the flight characteristics are good. This model has always been a good flying model and the difference between non hinged and hinged is subtle. I've had a few different pilots fly A-B tests and it 50/50 on makes a difference or not. On another model which isn't as well yaw damped, the difference is much more significant. The hinged prop does indeed make a noticeable change. One thing I can tell you is the propellers that I made are better performing that the APC that came on in terms of raw performance and battery drain.
Going forward, I have a 13x6 blade I working on to use on my SV11. I haven't yet done the loads analysis to begin working on a hub for it which I will do after I pull a blade out of the mold and weight it. I don't know how long that will take me as I'm currently working on a Smoothie which it's hinged propeller is waiting for completion. I do have intent on creating a three bladed version for the larger size airplanes. There's a lot to pack in under the spinner.
There is a question remaining and there are dialogs within this about doing something cyclic pitch wise in order to create a power steering and about doing "collective" pitch AKA controllable pitch propeller. I'm on design model version 6 and will pretty much scrap previous versions.
The question boils down to - "to what end?" which is to say, what is the mission statement and how does it improve on the current state of the art? Well, my notion is that a controllable pitch propeller should be better at regulating the speed of the model. To do that doesn't necessarily require very sophisticated controls. A propeller which can change it's pitch is actually just moving it's efficiency band around. This can be used in several ways for our models. It can help with climbing and diving by decreasing pitch. Going up it can make more power by flattening and speeding up and going down it can brake by also flattening and slowing down. Wolfgang had the same basic idea although I was unware of his idea when I began working on this idea.
Here's my experience fly with a Fiorroti timer. It works quite well. A controllable pitch propeller most likely could be made to work better. However that is a big leap in terms of cost. Again the question with which I intended this discussion initially to be is it worth the pursuit for other than intellectual curiosity? My guess is, maybe. Especially on that first corner of the outside square which is one place where a fixed pitch propeller looses. Doing a simple speed governor on the prop and pumping the power demand the way Fiorroti does would be kinda slick. There's a whole lot more that would be incorporated.
In regards to doing a cyclic pitch affair on the front of the airplane AKA V22 Osprey or XV-3. It's doable but to what end? Attaching a swashplate to the front of the airplane would add quite a bit of complexity and remember this one important detail, the swashplate stays where it is because there is a servo connected to it whether said servo is hydraulic, electric or meat which restrains it from moving. When a swashplate connected to a bellcrank connected to lines going to a handle held by the primary meat servo, looses tension it is free to move about at will. What then is the result? Scary thoughts go through my brain. Also, it takes torque to move the swashplate which will add to the control moments of the airplane. There's no free lunch.
So, I am on this road which includes some airplanes and the whole powerplant project is going to hold at this current state until I get a couple models done and a Citabria in the hangar started. Yeah, work. I gotta make some money. The current airplane is going to it's new owner and the next is in queue.
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Mark,
My answers may not be in order of your questions.
Safety first. In F2D combat now it’s mandatory to have an engine shut off. Few ways to activate it - mechanical , electrical or 2.4 GHz. If engine/motor shut off will be installed on the stunt ship , it will take care of all worries about the accidents. Here is the fun part: it’s ok to put 1 oz in the weight box of the outboard wing, it’s ok to put 1 oz of lead in the tail to balance the model. But to put 1 oz shut off system it is a dead weight!))
About F1C. I was making folding prop blades from the hardwood, no inserts or bushings. Steel yoke, M3 screws. Prop will survive 7 rounds plus attempts. Engine- Rossi R 15, 28 000 rpm.
Main reason for glider folding props having offset pivoting point - to minimise the spinner diameter and have blades folded along the fat fuselage.
If you remember the fixed pitch helicopter “ Cricket ” , or any helicopters with a fly bar - fly bar helps to control cyclic pitch change and any standard servo like Futaba s148 will move it.
Once we conducted experiment with helicopter. Heli was restrained, and when cyclic pitch was applied “full backwards “ blades struck the tail boom. In the real flight helicopter will move backwards and blades will not brake the tail boom.
In my opinion, the stiffer prop on the stunt ship is better. Igor is right. Carbon prop is better than plastic. The only weak point is motor/engine bearings. If folding prop is used, then it will be easier on the bearings, but engine/motor mount will suffer. If make completely flexible hub without cyclic pitch- then during stunt prop will disintegrate. If use a swash plate and cyclic pitch control- then system will perform flawlessly. It’s just to complicated!))
Nowadays common stunt is 70 ft lines, 5 to 5.5 seconds per lap. If motor runs 7 inch pitch prop, it can have a proper lap time by 7800 rpm. For years most common prop on Fox .35 was 10x6 . And now we see all kinds of engines, electric motors and prop sizes. It doesn’t have to be 10 000 rpm, it can be less.
The beauty of CL stunt is in the idea or prohibited engine/motor control by the pilot. But it also shows the problem - none of the traced squares or triangles of the best US pilots are flown to the shape, described by the rules. Nether PAMPA or F2B. Because models fly too fast. If models will fly slower, they will make all shapes, but they will drop down on overhead maneuvers with no line tension.
For now we have what we have.
Jerry
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Here is the fun part: it’s ok to put 1 oz in the weight box of the outboard wing, it’s ok to put 1 oz of lead in the tail to balance the model. But to put 1 oz shut off system it is a dead weight!))
What does the receiver end of 2.4gh weigh? The wires to the esc are thin. Why can't the receiver and a small battery to power it BE the tip weight? I have no clue if that could work but if.....
Ken
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Quote from: jerry v on September 20, 2021, 07:09:08 PM
Here is the fun part: it’s ok to put 1 oz in the weight box of the outboard wing, it’s ok to put 1 oz of lead in the tail to balance the model. But to put 1 oz shut off system it is a dead weight!))
What does the receiver end of 2.4gh weigh? The wires to the esc are thin. Why can't the receiver and a small battery to power it BE the tip weight? I have no clue if that could work but if.....
Ken
If I read what Jerry and Ken have written, they appear to be complaining that an electronic device cannot be used to stop a motor on a stunt ship in flight.
For the record, Rule 2.6 in the Control Line Precision Aerobatics rule book states:
"2.4 GHz spread spectrum radio control signals may be used to control ... a one-time irreversible engine or motor stop function."
My apology to those if I misunderstood what they wrote.
Keith
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If I read what Jerry and Ken have written, they appear to be complaining that an electronic device cannot be used to stop a motor on a stunt ship in flight.
For the record, Rule 2.6 in the Control Line Precision Aerobatics rule book states:
"2.4 GHz spread spectrum radio control signals may be used to control ... a one-time irreversible engine or motor stop function."
My apology to those if I misunderstood what they wrote.
Keith
Keith,
Rules are good, reasonably weighted equipment is available , but how many pilots installed the motor shut off system on their PAMPA / F2B stunt ships? I don’t know anybody who has… Maybe it is a dead weight?))
Jerry
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If I read what Jerry and Ken have written, they appear to be complaining that an electronic device cannot be used to stop a motor on a stunt ship in flight.
My apology to those if I misunderstood what they wrote.
Keith
You did miss my point. I was considering the weight and where it could be placed and secondarily, not in my post, could a non 2.4ghz device be on the plane in competition but not used. If that were the case it could be used to stop a flight in a bad or dangerous situation resulting in a -0- score or a C/D ruling. When was the last time anyone aborted a flight for any reason and got a score that still placed?
Electric PA has no need for a cutoff during competition, the timer does that just fine. IC in general would LOVE one. It is the non-competition flights and the "emergency" use in competition that I am concerned with. If the penalty for using one is to DQ the flight, that is OK. If the penalty is to DQ the flier then that is not OK. If simply having the plane equipped is a violation then this is all moot which the PA rules seem to do. Simply having a non 2.4ghz electronic device on the plane is a violation whether you use it or not.
We are doing the same thing here we did with Spectra lines. Focusing on a single solution to a problem that has many.
Ken
Having said all of that there appears to be several options within the 2.4ghz that just might offer even better options. What if there was a Timer that could be programmed in flight. RPM, gain, cutoff to let you actually trim the powertrain while in the air in one long flight instead of many short ones then disabled for all but cutoff and gear retraction in competition. Now that is a pie I would look to the sky for! And, all of it could be mounted in the outboard tip. ZERO weight gain!
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Keith,
Rules are good, reasonably weighted equipment is available , but how many pilots installed the motor shut off system on their PAMPA / F2B stunt ships? I don’t know anybody who has… Maybe it is a dead weight?))
Ultimately, 8 minutes for a 5:45 flight is enough tolerance that there is very little motivation to do anything different. Even 7 is plenty if you can get the engine started quickly. I haven't flown a tremendous number of FAI-rules practice and contest flights, I can't think of a single time I overran it, even on practice flights where we timed it to check.
Brett
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"If that were the case" --- "If the penalty" --- "If the penalty" --- "If simply having"
Ken
"What if there" ---
So many "ifs". My head spins. I guess I am getting old and cannot keep up.
Keith
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I was talking about engine/electric motor shut off system for emergency situations. For “wet “ power to pinch the fuel line and for electric power to override the timer settings and turn the motor off.
Of course some pilots will see the opportunity to use the system for better score on landing.
Jerry
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Of course some pilots will see the opportunity to use the system for better score on landing.
One could do a cutoff loop. No radio required.
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I was talking about engine/electric motor shut off system for emergency situations. For “wet “ power to pinch the fuel line and for electric power to override the timer settings and turn the motor off.
Of course some pilots will see the opportunity to use the system for better score on landing.
Jerry
Howard is right. What is the difference in a kill loop and a kill switch. The Kill Loop would look silly if you are using electric but I do the same thing by positioning the plane at the right height and speed through whipping when I get the 10 second cutoff warning from the timer. As far as I am concerned, anything you do under the rules to gain an advantage is OK.
Ken
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Electric PA has no need for a cutoff during competition, the timer does that just fine. IC in general would LOVE one. It is the non-competition flights and the "emergency" use in competition that I am concerned with. If the penalty for using one is to DQ the flight, that is OK. If the penalty is to DQ the flier then that is not OK. If simply having the plane equipped is a violation then this is all moot which the PA rules seem to do. Simply having a non 2.4ghz electronic device on the plane is a violation whether you use it or not.
Having said all of that there appears to be several options within the 2.4ghz that just might offer even better options. What if there was a Timer that could be programmed in flight. RPM, gain, cutoff to let you actually trim the powertrain while in the air in one long flight instead of many short ones then disabled for all but cutoff and gear retraction in competition. Now that is a pie I would look to the sky for! And, all of it could be mounted in the outboard tip. ZERO weight gain!
There is an over reliance on the 2.4Ghz technology in the rules and it doesn't provide any better security than using a simple 833 Mhz Tx Rx device. The difference is that the 2.4 Ghz devices are intended for continuous data stream while most of the 833 Mhz devices are simply listening for a 128 bit code they have been bound with and only toggle an output on and off. This is inherently compliant with the intent of this rule. The rule truly should simply state a non reversible cut off device. I use one to operate my electronic "stooge" and I have a version using the other button to start / stop a timer. It's a simple interface to the timer. Of course this idea can be expanded to using the device additional buttons and associated bit toggling to do other things but again it is a simple non proportional control.
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You should never base the rules in a specific technology, like in this case.
That is correct. Alas, this is not the worst violation of this principle in American control line rules.
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I find it amusing that we always seem to come up with all of these rule change ideas just after the window for getting them passed is closed. I am 100% in agreement with Lauri. To me it is simple...."Any form of remote cutoff may be used as long as it is irreversible and cannot interfere with any other flier." So if we all agree then it will be two years before we can use one. Don't want to rush into anything that makes sense.
Ken
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I find it amusing that we always seem to come up with all of these rule change ideas just after the window for getting them passed is closed. I am 100% in agreement with Lauri. To me it is simple...."Any form of remote cutoff may be used as long as it is irreversible and cannot interfere with any other flier." So if we all agree then it will be two years before we can use one. Don't want to rush into anything that makes sense.
Ken
Ken I think we (people on the forum) have had this discussion previously on here and I was pretty much shut down. Ultimately, I'm not planning on worrying much about it. I'm not certain about my participation in competitions with any device which has a remote shut off on it. My current timer breadboard is far from readiness as it doesn't include any accelerometer input and I'm not sure I am ready to spend the effort to write the code and go through the development when I'm flying a Fiorotti device which works good. Should I happen to arrive with a device which is on my airplane and is in violation of the rules and I get disqualified for such device well, that speaks volumes and I'd probably never ever participate again. It's not likely that this will occur but it could. The point is that there has been intense opposition spoken here to such ideas and staunch vocal oppine that any device must be 2.4Ghz which is nonsense.
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Ken I think we (people on the forum) have had this discussion previously on here and I was pretty much shut down.
To a point I agree. It seems that "good" ideas need to come from the right source to have any traction but that is not why I am replying.
I just switched to the Fiorotti timer and I am hoping to get feedback from others using it as I try and figure out how to maximize it's features. I have kept my "Trifacta" thread going primarily to provide feedback on the timer to Fiorotti. I have flown the plane with both Nose Up and G-Force turned off and even with that it has been a pleasant surprise over the Hubin (which is great for what it does). I am also getting used to logarythmic flaps at the same time. Now that is an experience if you have never used them. Any "rookie" tips would be appreciated. I think enough people are using it that it would be nice to have a section here dedicated to it or maybe to active timers in general.
Ken
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Sorry, this is not about precession.
I have asked this before and I'll ask it again:
Why do you require specifically the 2,4gHz in your rules? You should never base the rules in a specific technology, like in this case.
Because that is the only form of RC permitted in CL General, and, they don't want to have to deal with impound. It's really not that hard to understand.
Brett
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Brett
There would be no need to impound the 833 Mhz devices as well. Once the transition to digital code format interference goes away. Once TX and RX have been "bound" there is no need to worry about other transmitters on the same frequency. It all falls under the open use by the FCC just the same as the 2.4 Ghz frequencies. There's others here more knowledgeable than I am on all of those regulations but basically it is unrestricted.
Here is the reason for being motivated. The 2.4 Ghz devices are intended to communicate digitally and pass data back and forth. This in turn creates a more complex requirement for the transmitter and receiver which drives the cost up. The lower frequencies can be used for this but typical. Generally the 833 devices transmits a single code which the paired receiver listens for. So a tx for 2.4 Ghz low end costs around $80 while a 833 Mhz TX can be purchased for $5. Likewise the receivers are $35 ish and $4 ish. There is no loss of security of the signal or so minor as to be insignificant.
If the rule simply said no proportional control allowed would cover enough of need.
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Because that is the only form of RC permitted in CL General, and, they don't want to have to deal with impound. It's really not that hard to understand.
Brett
To expand on what Brett just explained, the AMA Control Line General rules specifically states: "The use of radio control to accomplish any control functions on Control Line models is specifically prohibited except as follows. The use of 2.4 GHz (utilizing spread spectrum, 47 CFR Part 15) radio control to accomplish functions other than providing aerodynamic control of the model's elevation on Control Line models is allowed, but only to the extent and in the manner specifically allowed by the rules of the individual event." The CL Precision Aerobatic rule was adopted to agree with the CL General rule on this matter.
If other radio control technologies are available to provide non "aerodynamic control of the model's elevation on Control Line models" without the need for frequency control/impounding transmitters, then maybe a rules change proposal would be appropriate to the Control Line General rules as well as the CL Precision Aerobatics rules.
Keith
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Keith
First this divergence of the thread should move elsewhere. I think we recognize what the rule says and that is specifically what is objectionable to a number of us ( I thought I was alone). The fact that it restricts the technology to only the 2.4 Ghz devices. The other devices on other frequencies have just as adequate lack of interference and do not require impoundment. The specific rule for CLPA requires a non reversible function. In both cases the 833 Mhz devices meet the intent of the rule. Therefore, a rule change is in order. Unfortunately, this has come up outside of a rule cycle.
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...... lack of interference and do not require impoundment"."
That, put into AMAeze should be the extent of the CL General Rule. Let each branch deal with it from there. Here may be the rub. In order to fix the PA rule you are going to have to fix the General First. I don't know if there is a provision to pass a change contingent on another change passing. If not then there are 2 rules cycles. Maybe in this case our current rule is adequate given a change to the general. Personally I think remote motor kill is an extremely desirable thing to have, especially for those lost soles still flying IC. Not just for contests but for trimming and safety - you take off and something is wrong with the controls or you have a prop strike on takeoff and it snaps part of a blade off. How many planes would be saved if we could abort immediately. How many contests would run on time if we didn't have to wait for that guy that couldn't measure his fuel and was still flying when everyone else left for lunch or to simply take an attempt. It just makes sense.
Ken
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Personally I think remote motor kill is an extremely desirable thing to have, especially for those lost soles still flying IC. Not just for contests but for trimming and safety - you take off and something is wrong with the controls or you have a prop strike on takeoff and it snaps part of a blade off. How many planes would be saved if we could abort immediately. How many contests would run on time if we didn't have to wait for that guy that couldn't measure his fuel and was still flying when everyone else left for lunch or to simply take an attempt. It just makes sense.
Ken
EXACTLY...!!!
This is directly and specifically my number one drive to have a remote on / off feature in my timer. To restrict such a device to an expensive limitation is counter productive to safety.
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The only problem is that those who cannot measure fuel or set the needle will probably be the last to adopt the system. And they don't need extra complications in their power system.
As I wrote earlier, it's not really that important in contest use, but really nice when trimming and practicing.
But then, we could have it like in some f'ing X-Factor and let the judges stop the engine if they feel it's not worth watching.. That would make contests shorter :)
L
I have seen some world champions have an overrun. We had a contest this spring where the entire advanced class save one overran along with 2 experts. It is not the overruns that gum up the works it is the attempts when the weather baffles even the best of us and the smart ones take an attempt. I would support giving the judges a 4-10 shotgun if it would bring down some of those never ending attempts. VD~
This is mostly an electric issue anyway. Most of the weight issues would be eliminated by enhancements to timers. Some may already be considering it for the reason you champion - trimming. I am going to look into the RDT's available. I have an idea they could be made removable until we can get some sensible rules.
Ken
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Keith
First this divergence of the thread should move elsewhere. I think we recognize what the rule says and that is specifically what is objectionable to a number of us ( I thought I was alone). The fact that it restricts the technology to only the 2.4 Ghz devices. The other devices on other frequencies have just as adequate lack of interference and do not require impoundment. The specific rule for CLPA requires a non reversible function. In both cases the 833 Mhz devices meet the intent of the rule. Therefore, a rule change is in order. Unfortunately, this has come up outside of a rule cycle.
This has been in there for at least 8 years - 4 entire rules cycles - and there was a fairly extensive discussion of the entire topic here when I wrote the proposal about it for CL Stunt something like 6 years ago.
For goodness sake, this happens every two years - no one pays the slightest attention to the rules cycle, despite it being posted, for more than a year, and despite Keith and others keeping everyone informed here, and lots of discussion threads. Then the vote results are announced, and sometimes the very next day, someone starts complaining that "they changed the rules!" or "they didn't fix the rules!", "or that was stupid they should have said THIS...!" or some variant.
When the first proposal for Spectra lines was made, and failed (for perfectly obvious reasons), literally the next day someone - who admitted that he had never heard there was a proposal at all and had never made a single peep about the topic - said they "might just as well quit CL". Over the failure of a proposal they had never heard of until afterwards?
You know why the rules didn't change? Because no one made the slightest effort to change them, or even suggest they be changed, or even have a comment about it either way. If you don't do anything, then nothing is going to happen, or it will happen in a way you don't like.
I would also note - the alternative to 2.4 GHz is not some other frequency - it is *no RC at all*. To be honest with you, I would support a change to CL General to disallow any form of radio or other EM emissions in CL, as not conforming with the idea of "Control through Lines". I am seriously considering removing the current rule for CL Stunt and just disallowing it completely.
Brett
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I would also note - the alternative to 2.4 GHz is not some other frequency - it is *no RC at all*. To be honest with you, I would support a change to CL General to disallow any form of radio or other EM emissions in CL, as not conforming with the idea of "Control through Lines". I am seriously considering removing the current rule for CL Stunt and just disallowing it completely.
Brett
Well, some of us have not been around this forum and have been participating in other activities. So, give us a break and allow us to be ignorant. By all means disallow any kind of RC to be spiteful which is certainly a solution. Taking such a stance is not very becoming and I realize I am particularly good at soliciting such responces. However, it is much simpler than that, when the cycle comes around again, simply remove the reference to 2.4 Ghz and replace with transmitter and receiver compliant with FFC regulations for open use which will not cause interference with others. In the specific section for CLPA simply state no proportional control of engine or electric powerplant allowed.