Hover Test of a Mach-scale Rotor-Model with Active Blade Tips

The smart active blade tip (SABT) rotor is a rotor with an on-blade vibration reduction system, incorporating blade tips that are independently pitched with respect to the main blade. The blade tips are used as on-blade control surfaces for vibration reduction. The experimental testing focussed on a small scale rotor on a bearingless Bell-412 hub. The fabricated Mach-scale active-tip rotor has a diameter of 1.524 m, a blade chord of 76.2 mm and 10% span active tips. The blade tips are driven by a novel piezo-induced bending-torsion coupled actuator beam, located spanwise in the hollow mid-cell of the main rotor blade. In hover at 2000 rpm (tip Mach 0.47), at 2 deg collective, and for an actuation of 125 Vrms, the measured blade tip deflection at the first four rotor harmonics is between ±1.7 and ±2.8 deg, increasing to ±5.3 deg at 5/rev with resonant amplification. The corresponding oscillatory amplitude of the rotor thrust coefficient is between 0.7•10−3 and 1.3•10−3 at the first four rotor harmonics, increasing to 2.1•10−3 at 5/rev. The active blade tip rotor was also used as a testbed for the evaluation of an adaptive neural-network based control algorithm. Effective background vibration reduction of an intentional 1/rev hover imbalance was demonstrated. The control algorithm also showed the capability to generate desired multi-frequency control loads on the hub, based on artificial signal injection into the vibration measurement. The research program demonstrates the technical feasibility of the active blade tip concept for vibration reduction and warrants further investigation in terms of closed loop forward flight tests in the windtunnel and full scale design studies.