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
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.
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?
That's the reaction torque from the engine, not precession.
Brett
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 ;D ;D ;D ;D. 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.....
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?
...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?
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.
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 !! LL~
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?
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.