The aeromechanical stability of soft-inplane tiltrotors

A soft-inplane tiltrotor can be subject to the aeromechanical instabilities of ground and air resonance in addition to whirl flutter. While whirl flutter has been studied by many researchers, air and ground resonance in a tiltrotor has not been explored thoroughly. This dissertation investigates the aeromechanical instabilities of air and ground resonance in a tiltrotor and to assess some passive methods for improving the stability of a soft-inplane tiltrotor.; This research is broken into two parts, addressing the semi-span model and the full-span model. In the first chapters, a semi-span analytical model consisting of a rigid blade rotor coupled to a rigid pylon and an elastic wing is developed and validated. The Boeing Model 222 which was wind-tunnel tested in 1972 is used to validate the current analytical model. This aircraft is used as the baseline for the analytical investigation of ground resonance and air resonance in hover, air resonance in transition, and air resonance and whirl flutter in airplane configuration. Neither ground resonance nor air resonance in hover are predicted for the Model 222 though air resonance in transition and cruise as well as whirl flutter at high speeds are predicted. 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 for air resonance. However, the rotor coupling was quite detrimental to whirl flutter stability and the wing coupling was unable to stabilize all the air resonance regions completely.; The second part of the thesis expands the semi-span model to full-span: fuselage pitch and roll motion, anti-symmetric wing motion, and the second rotor motion are added. The full-span model determined that fuselage motion can couple with rotor low frequency lag to exhibit air resonance. The ground resonance case was stable for the full-span model but showed only a small margin of stability, indicating that a ground resonance instability is possible with a tiltrotor.