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Author Topic: Propeller precession  (Read 1596 times)

Offline Mark wood

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Propeller precession
« on: October 05, 2020, 08:52:19 AM »
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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Online Ken Culbertson

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Re: Propeller precession
« Reply #1 on: October 05, 2020, 03:21:33 PM »
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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Offline Mark wood

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Re: Propeller precession
« Reply #2 on: October 05, 2020, 03:51:11 PM »
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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Online Ken Culbertson

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Re: Propeller precession
« Reply #3 on: October 05, 2020, 05:01:44 PM »
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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Online Brett Buck

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Re: Propeller precession
« Reply #4 on: October 05, 2020, 05:30:27 PM »
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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Re: Propeller precession
« Reply #5 on: October 05, 2020, 05:49:11 PM »
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

Offline Mark wood

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Re: Propeller precession
« Reply #6 on: October 05, 2020, 06:42:56 PM »
  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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Online Brett Buck

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Re: Propeller precession
« Reply #7 on: October 05, 2020, 07:48:34 PM »
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

Online Ken Culbertson

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Re: Propeller precession
« Reply #8 on: October 05, 2020, 09:43:57 PM »
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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Offline Mark wood

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Re: Propeller precession
« Reply #9 on: October 05, 2020, 10:19:18 PM »
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.

Quote:
  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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Online Brett Buck

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Re: Propeller precession
« Reply #10 on: October 05, 2020, 11:26:20 PM »
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.


Quote
  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
« Last Edit: October 06, 2020, 01:38:29 PM by Brett Buck »

Offline Howard Rush

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Re: Propeller precession
« Reply #11 on: October 06, 2020, 01:03:19 AM »
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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Offline Howard Rush

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Re: Propeller precession
« Reply #12 on: October 06, 2020, 01:09:12 AM »
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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Online Ken Culbertson

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Re: Propeller precession
« Reply #13 on: October 06, 2020, 06:38:52 AM »
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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Re: Propeller precession
« Reply #14 on: October 06, 2020, 08:07:50 AM »
With electric systems, proper design and modern materials precession is not a problem.


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Offline Wolfgang Nieuwkamp

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Re: Propeller precession
« Reply #15 on: October 06, 2020, 09:23:38 AM »
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.

Online Ken Culbertson

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Re: Propeller precession
« Reply #16 on: October 06, 2020, 11:47:56 AM »
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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Online Brett Buck

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Re: Propeller precession
« Reply #17 on: October 06, 2020, 12:16:16 PM »
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

 

Online Brett Buck

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Re: Propeller precession
« Reply #18 on: October 06, 2020, 12:20:35 PM »
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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Microaeronautics
« Reply #19 on: October 06, 2020, 01:43:36 PM »
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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Re: Propeller precession
« Reply #20 on: October 06, 2020, 02:52:27 PM »
Perhaps a lesser problem since we can use lighter props, but I can still feel the difference if I change diameters.   
Ken

Your props aren't my props.


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Online Brett Buck

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Pete Soule nuggets - line drag paper
« Reply #21 on: October 06, 2020, 07:31:21 PM »
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.




  http://www.fesselflug.ch/download/pdf/2011/Leinezug/linedrag2003.pdf

   Brett

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Re: Propeller precession
« Reply #22 on: October 06, 2020, 11:28:07 PM »
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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Re: Propeller precession
« Reply #23 on: October 07, 2020, 11:24:40 AM »
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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Online Brett Buck

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Re: Propeller precession
« Reply #24 on: October 07, 2020, 02:27:06 PM »
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

Offline AMV

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Re: Propeller precession
« Reply #25 on: October 07, 2020, 02:30:48 PM »
...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.
« Last Edit: October 07, 2020, 02:51:34 PM by AMV »
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Re: Propeller precession
« Reply #26 on: October 07, 2020, 02:59:53 PM »
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

« Last Edit: October 08, 2020, 09:51:31 AM by Trostle »

Online Ken Culbertson

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Re: Propeller precession
« Reply #27 on: October 07, 2020, 04:22:40 PM »
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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Online Brett Buck

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Re: Propeller precession
« Reply #28 on: October 07, 2020, 07:11:35 PM »
Quote
..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
                              ...
       Prop into pitch  - downthrust
                                  dropped elevator
                                  positive stab incidence
                                 asymmetrical stab airfoil
                                 practice more
                                 ...

         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

   
                                 
                               

Offline Serge_Krauss

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Re: Propeller precession
« Reply #29 on: October 08, 2020, 08:22:54 PM »
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.
« Last Edit: October 09, 2020, 01:08:10 PM by Serge_Krauss »

Offline Howard Rush

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Re: Propeller precession
« Reply #30 on: October 08, 2020, 08:35:35 PM »
... plenty of people worry about F, m, and T, they don't ever consider I.

Especially when the subscripts donít match.
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Re: Propeller precession
« Reply #31 on: October 08, 2020, 08:40:08 PM »
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:


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
I know that you believe you understand what you think I said, but I am not sure you realize that what you thought you heard was not what I meant.

Offline Howard Rush

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Re: Propeller precession
« Reply #32 on: October 08, 2020, 09:23:51 PM »
Why not aggregate all that fundamental knowledge in one place...

Iím working on it.  Stuff keeps coming up.
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Re: Propeller precession
« Reply #33 on: October 08, 2020, 10:06:44 PM »

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.

Quote
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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Re: Propeller precession
« Reply #34 on: October 08, 2020, 11:23:14 PM »

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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Offline AMV

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Re: Propeller precession
« Reply #35 on: October 09, 2020, 07:09:05 AM »
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...
::)
I know that you believe you understand what you think I said, but I am not sure you realize that what you thought you heard was not what I meant.

Online Ken Culbertson

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Re: Propeller precession
« Reply #36 on: October 09, 2020, 10:34:21 AM »
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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Offline AMV

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Re: Propeller precession
« Reply #37 on: October 09, 2020, 10:41:19 AM »
I know that you believe you understand what you think I said, but I am not sure you realize that what you thought you heard was not what I meant.

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Re: Propeller precession
« Reply #38 on: October 12, 2020, 05:58:59 PM »
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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Re: Propeller precession
« Reply #39 on: October 12, 2020, 09:09:07 PM »
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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Offline Ted Fancher

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Re: Propeller precession
« Reply #40 on: October 13, 2020, 11:05:49 PM »
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?

Offline John Leidle

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Re: Propeller precession
« Reply #41 on: October 14, 2020, 08:35:16 AM »
   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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Re: Propeller precession
« Reply #42 on: October 14, 2020, 01:57:31 PM »
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
« Last Edit: October 14, 2020, 03:07:12 PM by Brett Buck »

Online Brett Buck

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Re: Propeller precession
« Reply #43 on: October 14, 2020, 02:19:41 PM »
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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Re: Propeller precession
« Reply #44 on: October 14, 2020, 06:26:33 PM »
    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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Online Brett Buck

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Re: Propeller precession
« Reply #45 on: October 14, 2020, 07:42:04 PM »
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

Offline jerry v

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Re: Propeller precession
« Reply #46 on: October 14, 2020, 08:39:10 PM »
In this case itís 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 itís doable.

In my opinion, active timers and ESC in the electric setups are not quick enough to change RPM, because they are ďfightingĒ 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. Itís more of a question for timer designers...

Jerry
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Online Brett Buck

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Re: Propeller precession
« Reply #47 on: October 14, 2020, 09:45:31 PM »
In this case itís 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.

   

Quote
In my opinion, active timers and ESC in the electric setups are not quick enough to change RPM, because they are ďfightingĒ 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
« Last Edit: October 15, 2020, 07:38:13 AM by Brett Buck »

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Re: Propeller precession
« Reply #48 on: October 15, 2020, 06:22:26 PM »
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.

Online Dan McEntee

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Re: Propeller precession
« Reply #49 on: October 15, 2020, 09:52:31 PM »
    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?
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Online Ken Culbertson

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Re: Propeller precession
« Reply #50 on: October 16, 2020, 10:36:55 AM »
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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Online Dan Berry

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Re: Propeller precession
« Reply #51 on: October 16, 2020, 10:52:26 AM »
I would mention that Dan Banjock has a stunt plane that eliminates any worries or effects of precession.

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Re: Propeller precession
« Reply #52 on: October 16, 2020, 11:08:57 AM »
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.


Quote
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

Online Ken Culbertson

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Re: Propeller precession
« Reply #53 on: October 16, 2020, 02:42:09 PM »
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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Online Brett Buck

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Re: Propeller precession
« Reply #54 on: October 16, 2020, 02:57:16 PM »
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

Online Ken Culbertson

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Re: Propeller precession
« Reply #55 on: October 16, 2020, 04:02:46 PM »
   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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Offline Howard Rush

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Re: Propeller precession
« Reply #56 on: October 16, 2020, 04:26:07 PM »
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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Re: Propeller precession
« Reply #57 on: October 16, 2020, 05:08:39 PM »
Weíre still waiting for Markís ďIdeaĒ...

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 ďidle upĒ 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. Thatís why itís 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
« Last Edit: October 16, 2020, 05:26:53 PM by jerry v »
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Online Brett Buck

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Re: Propeller precession
« Reply #58 on: October 16, 2020, 05:35:38 PM »
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
« Last Edit: October 16, 2020, 07:32:32 PM by Brett Buck »

Online Ken Culbertson

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Re: Propeller precession
« Reply #59 on: October 16, 2020, 07:55:44 PM »

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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Offline jerry v

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Re: Propeller precession
« Reply #60 on: October 16, 2020, 08:36:19 PM »
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 Igorís timers are not as good as wet power 4-2-4 brake. Passive timers donít ďlean ď, or accelerate at all.

Jerry
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Online Brett Buck

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Re: Propeller precession
« Reply #61 on: October 16, 2020, 09:54:38 PM »
I fly TF ARF Nobler. Before I converted it to electric I was flying it with OS.35 FP.
Hubin , KR, and Igorís timers are not as good as wet power 4-2-4 brake. Passive timers donít ďlean ď, 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

Online Ken Culbertson

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Re: Propeller precession
« Reply #62 on: October 16, 2020, 10:24:13 PM »
I fly TF ARF Nobler. Before I converted it to electric I was flying it with OS.35 FP.
Hubin , KR, and Igorís timers are not as good as wet power 4-2-4 brake. Passive timers donít ďlean ď, 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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Offline jerry v

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Re: Propeller precession
« Reply #63 on: October 17, 2020, 07:16:23 AM »
   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, itís 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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Online Brett Buck

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Re: Propeller precession
« Reply #64 on: October 17, 2020, 07:47:14 AM »
TF ARF Nobler has no adjustable leadouts. Rudder also is not adjustable.

  Sure they are:

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Re: Propeller precession
« Reply #65 on: October 17, 2020, 09:08:39 AM »
Brett, itís a nice tool! But I prefer hammer and chisel! R%%%%

Jerry
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Online Ken Culbertson

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Re: Propeller precession
« Reply #66 on: October 17, 2020, 11:38:21 AM »
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Re: Propeller precession
« Reply #67 on: October 17, 2020, 01:41:18 PM »
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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Re: Propeller precession
« Reply #68 on: October 17, 2020, 05:01:29 PM »
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...

Online Ken Culbertson

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Re: Propeller precession
« Reply #69 on: October 17, 2020, 07:35:13 PM »
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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