AIR RESONANCE OF HINGELESS ROTOR HELICOPTERS IN TRIMMED FORWARD FLIGHT

Air resonance of a soft inplane hingeless rotor helicopter is examined in hover and forward flight using a simple model combining a rigid bl ade flap-lag model with body pitch and roll motions, Stability is calc ulated about a coupled rotor/body trim condition obtained using a fini te element in-time approach for blade response and a force-summation m ethod for hub loads. The linearized rotor-body stability equations are expressed in tbe fixed system using a multiblade coordinate transform ation and solved via Floquet analysis, Predicted stability results cor relate well with experimental data, Both shaft fixed flap-lag stabilit y and air resonance stability improve in forward flight above an advan ce ratio of approximately 0.2. This is largely the result of changes i n the vehicle trim solution. The effects of variations of several desi gn parameters are investigated, For the configuration examined, air re sonance stability varied with body roll inertia but was insensitive to pitch inertia, Variations in blade lag frequency had a considerable e ffect on stability, while the blade flap frequency had relatively less influence. Increasing Lock number decreased hover stability but had a strong stabilizing effect in forward night.