News:



  • November 28, 2020, 01:10:57 AM

Login with username, password and session length

Author Topic: Propeller precession  (Read 1587 times)

Online Ken Culbertson

  • 21 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2918
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


 

AMA 15382
If it is not broke, don't fix it.
USAF 1968-1974 TAC


Advertise Here

Offline Dan Berry

  • Trade Count: (0)
  • Captain
  • *****
  • Posts: 730
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.

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 11267
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

  • 21 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2918
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

AMA 15382
If it is not broke, don't fix it.
USAF 1968-1974 TAC

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 11267
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

  • 21 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2918
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
AMA 15382
If it is not broke, don't fix it.
USAF 1968-1974 TAC

Offline Howard Rush

  • 2020 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7085
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.
The Jive Combat Team
Making combat and stunt great again

Offline jerry v

  • Trade Count: (0)
  • Lieutenant
  • ***
  • Posts: 64
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 »
Variety is the spice of life.

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 11267
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

  • 21 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2918
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
AMA 15382
If it is not broke, don't fix it.
USAF 1968-1974 TAC

Offline jerry v

  • Trade Count: (0)
  • Lieutenant
  • ***
  • Posts: 64
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
Variety is the spice of life.

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 11267
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

  • 21 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2918
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
AMA 15382
If it is not broke, don't fix it.
USAF 1968-1974 TAC

Offline jerry v

  • Trade Count: (0)
  • Lieutenant
  • ***
  • Posts: 64
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
Variety is the spice of life.

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 11267
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:

Offline jerry v

  • Trade Count: (0)
  • Lieutenant
  • ***
  • Posts: 64
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
Variety is the spice of life.

Online Ken Culbertson

  • 21 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2918
Re: Propeller precession
« Reply #66 on: October 17, 2020, 11:38:21 AM »
AMA 15382
If it is not broke, don't fix it.
USAF 1968-1974 TAC

Offline jerry v

  • Trade Count: (0)
  • Lieutenant
  • ***
  • Posts: 64
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?
Variety is the spice of life.

Online Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 11267
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

  • 21 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2918
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
AMA 15382
If it is not broke, don't fix it.
USAF 1968-1974 TAC


Advertise Here
Tags: