If the prop is turning  CCW as seen from the front, (normal IC engine) I would expect the reaction to the prop torque to twist the fuselage in the other direction. What does it look like with the engine not running?? I would think the GP force is but a small percentage of the prop torque. How stiff is the fuselage??

Flying level. The aft fuse is square to the wing as best as I could tell, its the nose that is twisting away. I've got plenty of in-flight pic's of ARF Pathfinders (which are notorious for the tail twist), going away from the camera, so I do know exactly the other situation you mean. I sincerely don't think that is what you are witnessing here on this plane. (Not that this plane's stab might not also be tilted a little for the same other reason, but I'm fairly certain it wouldn't be related to what you are witnessing multiplied by a fairly extream factor on the front end)

The fuse is typical RC ARF fare, built up, wimpy and covered with non-structural monokote. That's fine with me, could be made of flubber for all I care, heh heh. I'm not trying to fix the plane, just demonstrate the effect, which this plane seems taylor made to do.

EricV

Is it flying level or doing a trick?   Looks like the wing is warped and the fuselage is twisted, maybe in two axes.   My guess is that it's the aft fuselage that's twisted.  Profiles do that.  Scott Riese had one awhile back the stab of which would twist about 30 degrees relative to the wing.  It would be interesting to see a control photo with the airplane at rest.

It's demonstrating something.  I'm not sure what.  Maybe it's "centrifugal" force on the engine

  I agree, I don't see what this is demonstrating exactly.

   Brett

Igor, I understand what you are saying, and I wouldn't normally debate things with someone at your high level of expertise, but I'm still curious about the situation...so as long as I have your attention, I have to ask, have you ever held a bare running engine with your bare hands (wearing gloves of course) when you were a kid?

I have, and yes, it's pulling forward, and yes, when you wave the engine around you can feel the gyroscopic effect, but it's also trying to twist out of your hand the opposite direction of the prop. If you let go of the engine, depending on the weight/diameter of the prop and rpm, the engine case will counter rotate opposite direction of the prop as it flies away (for a very brief "flight" before it hits the ground). Not sure what you call that, GP or whatever, but it's there, and I thought that is some of what is seen in the pic, no? I would assume the effect is stronger during acceleration, and once an engine reaches equilibrium and static RPM, the effect lessens, but is still there, no? Since IC engine is single cyl pulse, it's constantly accelerating each revolution, or each "bang", even when static RPM, no? Just curious... Thanks for anything you can add to help me understand.

EricV

It is not precession, precession makes pitching moment up or down regarding prop spinning direction. It will make up moment for tractor prop (that is why we have thrust line over wing, drag of landing gears down and still we benefit on motor and elevator down offset). So it cannot make any effect on such type of nose. If elevator acts agains that pitching moment to keep model in level flight, it will make precession moment just opposite to te orriginal force, so the prop shaft tends to stay in the orriginal direction (that is how the gyroscope works) and that is force pulling moment out of the circle which acts against the line drag and in case of flexible nose, it also bents the flat construction. Besithat, we see also twisted nose because of reaction moment equal to motor moment.

  That's the reaction torque from the engine, not precession.

     Brett

Ok, I'll re-visit the theory to make sure I've got this straight. Assuming we are talking about Torque Induced procession or Gyroscopic procession, not Classic or Torque-free procession: If the engine is spinning along in a clockwise direction, as seen looking from the back of the plane, when the plane is turned to the right (and here is where I’m adding the supposition that the GeeBee is in a tethered turn to the right in constant yaw in relation to the tow location of leadouts, towards the outside of the circle, not straight ahead as we ideally trim for) forces are applied to the structure of the plane forcing the front end of the plane down and the back end up. If the plane is turned to the left (towards the pilot, not very practical, heh) then the front end of the plane is forced up and the back end forced down. If the engine is spinning in the opposite direction then the reverse applies. If the reverse applies, and you trim for a lot of yaw, you get something that feels like up-thrust, and this could make our e-power bretheren very unhappy in level flight and perhaps why some complain of something that resembles a hunt?

Then taking it further, if the plane was moved without change of angle, by say a thermal, or whatever, directly upwards, downwards, forwards, or side-to-side the gyroscopic effect would not apply any extra forces, right? OK, But… what I’m thinking here, is if the engine only applies extra forces when the plane is moved at an angle (rotational turning left/right) then it also applies forces when the front/back of the plane is rotationally turned up/down via elevators.

Even though the shot of the GeeBee is in level flight, it was in a mild climb as it went by, (probably at 10ft elevation when I snapped the picture, and at 25ft as it went by me in the span of less than a 1/4 lap) and so this mild climb would create a procession to the outside of the circle in addition to the torque induced by the engine and perhaps result in the twist seen in the front end?

I know this is all pretty basic, and why a Rabe Rudder exists for outside turns, but all I was trying to show in the pic in a humorous way was that not only does it effect the flight path, but puts stresses on the airframe as well, that some might not have been aware of, and that maybe the distortion of the airframe could add other trim issues to deal with as well.

EricV

Doug - No argument here... I also thought the procession and engine torque would sorta cancel each other out, but the pic made me believe otherwise. Since I was standing on the downwind side of the circle, the engine could have been backing off, I know on the downrev, the engine torque in my handheld experiment tried to twist the engine out of my hand in the other direction, rotating with the prop. If nothing else, the picture was a fun item for discussion. You can bet if the plane flys again and I have my camera handy, I will crank up the ISO and try to get some better shots with faster shutter speeds and various attitudes.

EricV

hmmmm, this is interesting and I tend to wonder, like Don, why it isn't twisting opposite the rotation of the prop like the engine in your hand experiment.  Which I would never condone but I have seen it first hand  .  Sorry couldn't resist.

Then I started wondering.  Maybe the nose is twisting back and forth following each compression stroke.  This would be verified easily by watching it on the ground.  Is it vibrating badly, or bad enough to see it a little bit?

Eric, I think you could be talking about two different things.  I am not an expert here so please if I get this wrong let me know.  You are stating the engine is causing the gyro procession and the yaw and pitch etc....  Well its the prop that is doing that not the engine.  The engine is supposed to be still and trapped and the procession of the prop blades one up one down causes the yaw rolling and pitching.  Thus the need for the Rabe rudder. 

The engine torque you felt when you held the running motor would be felt even if there were no prop on it while it was running. 

Three things are happining is this pic.  Vibrations out the wazoo..(love that word right now) Gyroscopic P from the prop, probably negated from the size of the plane to really be able to feel it or see it. And engine torque on the frame is beating the holy crap out of it and maybe the pic just caught it on a vibration to that side.....

  ...The twist in the fuselage in the picture is backwards from the expected engine torque reaction. I expect that the bend you see is the centrifugal force on the engine, and that it twisted because it's stiffer at the bottom of the fuselage than it is at the top because the wing is at the bottom....

    All of these forces/torques aside from the P-factor are easily estimated if you know the input parameters. I think the centrifugal force is going to be a lot bigger than any of the others.

    Brett

OK, did a quick calc, based on a 10 ounce engine, I come up with Centrifugal acceleration of 2.83940 G, and Centrifugal force of 1.77463 LBF.
Does that sound about right? Screen capture included to check method... just an online quickie calc I found. It doesn't seem enough force to be twisting the nose that much on it's own to me? What say you?

EricV

OK, did a quick calc, based on a 10 ounce engine, I come up with Centrifugal acceleration of 2.83940 G, and Centrifugal force of 1.77463 LBF.
Does that sound about right? Screen capture included to check method... just an online quickie calc I found. It doesn't seem enough force to be twisting the nose that much on it's own to me? What say you?

   The numbers sound about right, so yes, that's seem like a likely cause. It's definitely the wrong direction for engine torque (presuming the engine is spinning the normal way), it appears to not be in a hard maneuver, so not likely precession or p-factor.

    Brett

  The numbers sound about right, so yes, that's seem like a likely cause. It's definitely the wrong direction for engine torque (presuming the engine is spinning the normal way), it appears to not be in a hard maneuver, so not likely precession or p-factor.

    Brett

Ya, on further review, it would seem our intrepid reporter was full of crap. LOL! As it has been pointed out, the deal breaker here is the fact that it's not in a hard inside turn, at least not that I can remember for that shot so we'll have to blame CF.

Also, not out of the realm of possibility, is that it's an optical illusion because of the way the pic was taken. Going through my discard pile, I found another similar shot, not straight ahead, but it doesn't appear to be twisting. I had a conversation with the owner today and showed him the picture, who says the back end is built up but the front end is quite rigid. He's bringing it out again maybe next week so we can play some more.

EricV

   I have a bunch of in-flight photos and interpreting them is definitely not trivial, and prone to error. But once you get past that, you can see an amazing amount of detail if you catch everything just right.

    Brett

or... you could send em off to the  FBI and have them   "pixel analyzed"  ya can learn tons of info from that !!   

or... you could send em off to the  FBI and have them   "pixel analyzed"  ya can learn tons of info from that !!   

    I think the technical name is "pixelized boolean projection" or "pixillated bullshit", or something like that.

    Funny thing about that proud moment in stunt history. Someone on the old AOL forum started in on that story, and said they had the FBI do all the work which proved their point. I asked for the mailing address for the FBI field office in Atlanta, and speculated that this might not have been the best use of public resources, and that the district director might want to know about it. It got *real* quiet after that.

   The silliest part of the whole story was the notion that Billy needed to cheat to beat our hero in a stunt contest.

    Brett

The Enya SS50 has a optional reverse crank and Zinger has pusher props. I posted about this possibility a few years ago here. 

This engine could be running in reverse rotation with a pusher prop.

Martin

can't fool the GORT eye. Some will get the original  day the earth stood still.  1951  Klaatu barada nikto

Deleted...
noticed in text it was CCW rotation.

  I'll put my money on the fact that the GeeBee profile fuselage (which is flimsy -I have one - RC), and with a .50 ci engine mounted that high above the vertical CG with an outward acceleration of about 3 g's (assuming 5 sec laps at 60'), that's a lot of force on the fuselage. I'd wager that, if the wing were higher and the engine were in line with the wing, there's be no twist.

  Howard Rush, reply #5.

   Brett