Closed-Loop Test of a Rotor with Individually Controlled Trailing-Edge Flaps for Vibration Reduction

A control method is proposed to reduce vibrations in helicopters using active trailing-edge flaps on the rotor blades. Each blade is controlled independently, taking into account possible blade and actuator dissimilarities. The rotor is modeled as a linear-time-periodic system, which is identified by linear quadratic estimation (Kalman filter). After an initial open-loop system identification phase using random actuation commands, optimal actuation commands are computed by linear quadratic regulation, and both closed-loop control and system identification are performed simultaneously at each time step. Closed loop tests are conducted using a four-bladed rotor with piezo-bender trailing-edge flaps. The rotor model is fitted on a bearingless model-scale hub and tested on a hover stand. These tests demonstrate the controller's ability to account for blade and actuator dissimilarities and generate different optimal inputs for each blade. The 1/rev normal force could be reduced by more than 90% using small amplitude flap deflections (about +/- 2 deg). During most closed-loop tests, the maximum allowable input to the actuators is reached. It is found that the method used to account for actuator saturation and maintain actuator input within acceptable limits has an important effect on controller performance. The best controller performance is obtained when control inputs are computed by solving a constrained minimization problem.