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Author Topic: Propeller precession  (Read 13222 times)

Offline Brett Buck

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Re: Propeller precession
« Reply #50 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 #51 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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Offline Brett Buck

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Re: Propeller precession
« Reply #52 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 #53 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 #54 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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Offline jerry v

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Re: Propeller precession
« Reply #55 on: October 16, 2020, 05:08:39 PM »
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
« Last Edit: October 16, 2020, 05:26:53 PM by jerry v »
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Offline Brett Buck

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Re: Propeller precession
« Reply #56 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 #57 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 #58 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 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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Offline Brett Buck

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Re: Propeller precession
« Reply #59 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 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

Online Ken Culbertson

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Re: Propeller precession
« Reply #60 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 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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Offline jerry v

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Re: Propeller precession
« Reply #61 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, 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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Offline Brett Buck

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Re: Propeller precession
« Reply #62 on: October 17, 2020, 07:47:14 AM »
TF ARF Nobler has no adjustable leadouts. Rudder also is not adjustable.

  Sure they are:

Offline jerry v

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Re: Propeller precession
« Reply #63 on: October 17, 2020, 09:08:39 AM »
Brett, it’s 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 #64 on: October 17, 2020, 11:38:21 AM »
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Offline jerry v

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Re: Propeller precession
« Reply #65 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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Offline Brett Buck

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Re: Propeller precession
« Reply #66 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 #67 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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Offline Chris Wilson

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Re: Propeller precession
« Reply #68 on: April 07, 2021, 10:25:41 PM »


    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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Offline Mark wood

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Re: Propeller precession
« Reply #69 on: August 27, 2021, 10:56:33 PM »
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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Offline Howard Rush

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Re: Propeller precession
« Reply #70 on: August 28, 2021, 01:40:59 AM »
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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Offline Brett Buck

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Re: Propeller precession
« Reply #71 on: August 28, 2021, 09:14:06 AM »
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.

Offline Howard Rush

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Re: Propeller precession
« Reply #72 on: August 28, 2021, 04:42:16 PM »
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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Offline Brett Buck

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Re: Propeller precession
« Reply #73 on: August 28, 2021, 05:00:34 PM »
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.

 
Quote
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

Offline Mark wood

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Re: Propeller precession
« Reply #74 on: August 29, 2021, 10:37:44 PM »
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:



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:



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:




Enjoy.





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Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
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Offline Igor Burger

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Re: Propeller precession
« Reply #75 on: August 30, 2021, 03:09:32 AM »
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.



 
   

Offline Brett Buck

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Re: Propeller precession
« Reply #76 on: August 30, 2021, 11:37:06 AM »

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

Offline Howard Rush

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Re: Propeller precession
« Reply #77 on: August 30, 2021, 12:40:59 PM »
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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Offline Brett Buck

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Re: Propeller precession
« Reply #78 on: August 30, 2021, 01:40:24 PM »
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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Re: Propeller precession
« Reply #79 on: August 30, 2021, 04:53:21 PM »
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

Offline jerry v

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Re: Propeller precession
« Reply #80 on: August 30, 2021, 09:07:55 PM »
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
« Last Edit: August 31, 2021, 10:26:58 AM by jerry v »
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Re: Propeller precession
« Reply #81 on: August 31, 2021, 10:16:40 AM »
...
The shafts of helicopters are diameter 8 -10 mm and there is some long distance from the fuselage bearing to the main rotor hub.
...

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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Offline Bob Hunt

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Re: Propeller precession
« Reply #82 on: August 31, 2021, 11:37:01 AM »
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

 

Offline Dan McEntee

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Re: Propeller precession
« Reply #83 on: August 31, 2021, 12:53:50 PM »
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,
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Offline Mark wood

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Re: Propeller precession
« Reply #84 on: August 31, 2021, 02:17:35 PM »
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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Offline Mark wood

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Re: Propeller precession
« Reply #85 on: September 01, 2021, 11:05:29 AM »
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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Offline Brett Buck

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Re: Propeller precession
« Reply #86 on: September 01, 2021, 03:15:09 PM »
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

Offline Mark wood

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Re: Propeller precession
« Reply #87 on: September 01, 2021, 05:58:45 PM »
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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Offline Howard Rush

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Re: Propeller precession
« Reply #88 on: September 01, 2021, 06:49:37 PM »
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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Offline jerry v

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Re: Propeller precession
« Reply #89 on: September 02, 2021, 10:39:29 AM »
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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Offline Brett Buck

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Re: Propeller precession
« Reply #90 on: September 02, 2021, 11:04:38 AM »
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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Re: Propeller precession
« Reply #91 on: September 02, 2021, 11:11:48 AM »
This is not a problem.
It's an opportunity.

-Andrey
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Offline Brett Buck

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Re: Propeller precession
« Reply #92 on: September 02, 2021, 11:23:53 AM »
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

Offline Brett Buck

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Re: Propeller precession
« Reply #93 on: September 02, 2021, 11:26:13 AM »
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

Offline jerry v

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Re: Propeller precession
« Reply #94 on: September 02, 2021, 11:49:42 AM »
     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
Variety is the spice of life.

Offline Brett Buck

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Re: Propeller precession
« Reply #95 on: September 02, 2021, 12:32:05 PM »
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

Offline jerry v

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Re: Propeller precession
« Reply #96 on: September 02, 2021, 03:39:33 PM »
  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
Variety is the spice of life.

Offline Mark wood

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Re: Propeller precession
« Reply #97 on: September 20, 2021, 02:46:25 PM »

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.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline jerry v

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Re: Propeller precession
« Reply #98 on: September 20, 2021, 07:09:08 PM »
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
Variety is the spice of life.

Online Ken Culbertson

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Re: Propeller precession
« Reply #99 on: September 20, 2021, 07:50:40 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
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If it is not broke you are not trying hard enough.
USAF 1968-1974 TAC


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