Emulation of Whirl Flutter on a Stable Helicopter Using Trailing Edge Flaps

Rotor-structure-coupling is a major concern within the development of helicopters as it can lead to unstable oscillations in the coupled rotor-fuselage system. Apart from the well-understood phenomena air- and ground-resonance, a type of rotor-structure-coupling similar to whirl flutter came into focus recently. High frequency whirl flutter of this kind is favored by high unsteady aerodynamic loads and the low stiffness in light-weight upper deck structures. This fact increases the likelihood of occurrence of high frequency whirl flutter for modern helicopters as the demand for performance enhancement and light-weight structures constantly grows. The emulation of this phenomenon using active control on a full scale helicopter offers great opportunities for the studying the whirl flutter phenomenon itself as well as countermeasures for stabilization. To this end the emulation of whirl flutter on a combined rotor-fuselage model of a non-serial BK117 demonstrator is investigated. State feedback control is used to destabilize a suitable eigenmode of the combined rotor-fuselage system. In order to ensure the whirl flutter characteristics of the destabilized eigenmode, an eigenstructure assignment method after Moore is used for the controller design. The success of the emulation is validated by modal analysis as well as simulation of the closed loop system. Finally, as a first step of evaluating the applicability of this control concept for a real helicopter, the influence of control command limitation on closed loop operation is investigated.