I’ve had a think overnight. Its surprising what the little grey cells can trawl up after sleeping on it. But I’m away from home at the moment so nothing to refer to. Sorry if this is obvious.
The fundamental difference between auto gyros and helis is that a heli’s rotor is powered whereas the gyro’s relies on airstream for rotation. Consequently air flows downwards through a heli’s rotor but passes upwards through a gyro’s. A heli’s rotor is tilted forwards in s&l flight but a gyro’s is tilted backwards. Lift is derived from the blade’s AoA to the airstream. The teetering head teeters, to adjust the blade’s AoA automatically throughout a revolution to equalise lift around the disc. If you drag the rotor blades through the air fast enough a lift component causes the forward facing blade to advance and a state of equilibrium can be achieved at circa 200 rpm.
Now any change to AoA caused by turbulence or mishandling will upset that equilibrium. The rotor head will teeter trying to maintain the appropriate AoA resulting in rotor speed fluctuations. There is no power drive to maintain rotor speed regardless as in a heli. In extreme cases, the teetering head reaches its stops. Not only does the inertia of the blade impart a shock to the mast but then insufficient AoA is generated. The distribution of lift around the disc is perturbed, exacerbating the problem. (OK SR?) This is the inherent problem of the teetering head. It appears difficult, if not impossible, to manufacture one with sufficient range for the more extreme flight conditions. In a heli, at least you have the engine to help overcome the perturbations. You’re not relying totally on inertia.
The other condition to cause the same problem is the push-over. When the rotor disc is unloaded in a bunt type manoeuvre, the AoA reduces and with it, the lift component that ensures rotation. The rotor slows. In extreme cases the AoA can become negative which positively and rapidly slows the rotor. It doesn’t need a lot to slow the rotor below a critical speed from which it does not recover its rotational speed.
What price an aircraft that shouldn’t be flown in turbulent conditions? I do think that it has been all too easy to blame the pilots’ mishandling for what appear to me to be inherent deficiencies of the concept. Certainly, not fully understood by their pilots but don’t we all rely to some extent on intrinsic good manners? I’m not saying don’t fly them but, if you choose to…… Do you have your answer now SSD? That’s enough from me. I would rather see the argument pursued by more technically knowledgeable types.
But finally,
Q. Have you ever wondered why these machines have such a low aspect ratio fin and rudder?
A. Well, sketch a blade in on a side elevation just missing the top of the rudder and see how far they are designed to go before de-capitating the rudder! One builder modified his with a taller rudder. Guess what happened?
WWW: You certainly get 12 out of 10 from me for stirring it! Can you do the same for the Instructors’ Forum?
[This message has been edited by Yogi-Bear (edited 22 June 2001).]