Wing-Pilot Vertical Bounce in Tiltrotors

摘要

The basic mechanism of the vertical bounce in tiltrotors in hovering flight is discussed. This rotorcraft-pilot coupling phenomenon arises when the pilot's biomechanics interact with the airframe elastic modes, in particular with the first symmetric wing bending mode. For this reason, it can be referred to as wing-pilot vertical bounce. This work proposes a simple mathematical model to predict the phenomenon. The XV-15 tiltrotor is used as benchmark. The closed-loop pilot-vehicle system shows that the direct effect of a change in collective input, through a vertical power lever, results in a nearly immediate change in thrust, which accelerates the aircraft, exciting the symmetric wing bending mode and, in turn, the pilot biomechanics, leading to a feedback path that could easily become unstable. Robust stability analyses are performed to take into account the large variability of some influential parameters. The tiltrotor shows a significant proneness to this rotorcraft-pilot coupling problem, which must be considered from the earliest phases of the design, especially when fly-by-wire architectures are considered. Means of prevention, considering both active and passive devices, are investigated and compared.