Development of a Piezoelectric Servo-Flap Actuator for Helicopter Rotor Control

Abstract

An actuator using a piezoelectric bender to de ect a trailing edge servo- ap for use on a helicopter rotor blade was designed, built, and tested. This actuator is an improvement over one developed previously at MIT. The design utilizes a new exure mechanism to connect the piezoelectric bender to the control surface. The e ciency of the bender was improved by tapering its thickness properties with length. Also, implementation of a nonlinear circuit allowing the application of a greater range of actuator voltages increased the resultant strain levels. Experiments were carried out on the bench top to determine the frequency response of the actuator, as well as hinge moment and displacement capabilities. Flap de ections of 11.5 deg were demonstrated while operating under no load conditions at 10 Hz. Excessive creep at low frequencies precluded the measurement of achievable hinge moments, but extrapolation from de ection and voltage characteristics indicate that if properly scaled, the present actuator will produce ap de ections greater than 5 deg at the 90% span location on an operational helicopter. In addition, the rst mode of the actuator was at seven times the rotational frequency (7/rev) of the target model scale rotor. Proper inertial scaling of this actuator could raise this modal frequency to 10/rev on an operational helicopter, which is adequate for most rotor control purposes. A linear state space model of the actuator was derived. Comparisons of this model with the experimental data highlighted a number of mild nonlinearities in the actuator's response. However, the agreement seen between the experiment and analysis indicate that the model is a valid tool for predicting actuator response.