Collective Bounce Problems on Tiltrotors

The basic mechanism of the collective bounce phenomenon on tiltrotors is discussed. This phenomenon may arise if the pilot’s biomechanics interact with the airframe elastic modes, in particular with the 1st Symmetric Wing Bending (SWB) mode. A simplified aeroelastic tiltrotor model, able to capture the aircraft heave motion and the low–frequency out-of plane wing bending dynamics, is proposed. The model, representative of the Bell XV-15, is validated with data reported in literature. The closed-loop pilot-vehicle system shows that the direct effect of a change in collective input results in a nearly immediate change in thrust, which accelerates the tiltrotor exciting the 1st SWB mode and, in turn, the pilot biomechanics generating an unstable feedback path. Robust stability analyses are performed using the Nyquist criterion for SISO systems, considering the feedback loop between the vertical acceleration at the pilot seat and the collective pitch input. Means of prevention, considering both active and passive devices, are investigated and compared with pros and cons.