Prediction of air and ground resonance stability of soft-inplane tiltrotors using a semispan analytical model

A soft-inplane tiltrotor can be subject to the aeromechanical instabilities of ground and air resonance in addition to whirl flutter. An investigation of these tiltrotor aeromechanical instabilities was undertaken along with an assessment of some methods for improving the stability of a soft-inplane tiltrotor. A new semispan analytical model was developed consisting of a rigid blade rotor coupled to a rigid pylon and an elastic wing with bending-torsion couplings. The results from this model were validated using a 1972 wind tunnel test of the Boeing model 222 tiltrotor. Hover, transition, and cruise configurations were investigated. While the analysis did not predict a ground resonance instability or an air resonance instability in hover, air resonance instabilities are predicted for transition and cruise configurations as well as whirl flutter instabilities at high speeds. Aeroelastic couplings in the rotor blades and wing are shown to affect the air resonance stability in cruise and transition and to be useful in augmenting the inherent stability of the aircraft. Wing vertical bending coupled to wing torsion and rotor low-frequency lag coupled to blade torsion were helpful in avoiding air resonance. Positive blade pitch-lag coupling was quite detrimental to whirl flutter stability. Wing vertical bending-torsion coupling was unable to stabilize all the air resonance regions completely.