An introduction to helicopter air resonance

Abstract

The phenomenon known as 'air resonance', peculiar to helicopters with hingeless rotors, has been analysed and presented in as simple a manner as possible. In the air resonance case the 'body mode' with frequency close to the whirling frequency of the rotor C.G. is much more heavily damped than in ground resonance and the damping of the modes of motion is apparently unaffected by coincidence of the body and whirl frequences. The slower of the two C.G. whirl modes is unstable if only the aerodynamic lag damping is present and it appears that at least 4 per cent of critical damping is required for all the modes to be stable. A resonant situation may occur if the lag stiffness is so high that the blade lag frequency coincides with the flapping frequency. With low damping large lag amplitudes may be excited by Coriolis forces when blade flapping occurs.