Torsional Instabilities and Helicopters

[Schuyler]

Can someone please provide a simple explanation of torsional instabilities and how it relates to the operation of helicopters. I have also heard of helicopter related torsional instabilities associated with engine power demands. Please comment.

Also, any good web sites and or basic books that describe these issues.

 

[electricpete]

I don't have my copy here, but torsional resonance of helicopter blades is described in Den Hartog's Mechanical Vibrations

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[MikeHalloran]

In fixed wing aircraft, there's an oscillation where the aircraft can exchange airspeed and altitude in a cyclic manner with no control input. The oscillation may continue indefinitely unless the pilot intentionally damps it out.

In a rotary wing craft, there might be such a no- control- input oscillation mode, but we'll probably never know. Helicopters are not naturally stable; the pilot is _always_ busy.

In a helicopter with a single main rotor and a single rotary rudder, _all_ of the controls are coupled. E.g., say you're hovering and you want to correct a slight altitude loss; add collective, and the engine has to supply more power, so you must add throttle. The added torque on the main rotor means you have to move the rudder pedals to change the pitch on the rotary rudder's blades in order to stay pointed in one direction. When you do that, you've added a little lateral force too, so the machine wants to roll, and you need to correct it with some lateral cyclic. And that causes a gyroscopic precession so you need some fore/aft cyclic, too. Or something like that.

A very accessible description of the actual operation of a helicopter is in a book entitled "Chicken Hawk", by Robert Mason. It's not a textbook; it's about his experience, learning to fly an Army helicopter, the Army way. If that book doesn't cure you of wanting to fly a helicopter, you were born to it.

-Mike-