Stabilization of helicopter blades with severed pitch links using trailing-edge flaps

The feasibility of using trailing-edge flaps to reconfigure a helicopter rotor blade following a failure of the pitch link is addressed, which makes the blade free floating in pitch and otherwise uncontrollable. A coupled rotor-fuselage model is developed that allows for rotor anisotropy. A new, optimization-based, trim procedure is developed to determine the dynamics of the failed blade and the flap inputs required for reconfiguration. The trailing-edge flap can correct the otherwise catastrophic consequences of a pitch link failure. The residual 1 and 2/rev (revolution) components of the hub loads are reasonably small with a one-harmonic flap input and essentially disappear if a second harmonic is added to the flap input. The required flap deflections are high but not unreasonable. The flap acts by generating a rigid-body pitching motion of the free-floating blade that matches the angles that otherwise would have been generated by the swashplate. The steady-state flapping motion of the reconfigured blade is very nearly identical to those of the undamaged blades. The solutions are very sensitive to phase errors in the first harmonic of the flap inputs. The sensitivity is lower for the constant and the second harmonic inputs. The result suggests that if a helicopter rotor is equipped with trailing-edge flaps for other purposes such as vibration or noise reduction, these flaps could be used as emergency control surfaces.