Design and analysis of rotor systems with multiple trailing edge flaps and resonant actuators

by Kim, Jun-Sik.

The purpose of this thesis is to develop piezoelectric resonant actuation systems and new active control methods utilizing the multiple trailing-edge flaps’ configuration for rotorcraft vibration suppression and blade loads control. An aeroelastic model is developed for a composite rotor blade with multiple trailing-edge flaps. The rotor blade airloads are calculated using quasi-steady blade element aerodynamics with a free wake model for rotor inflow. A compressible unsteady aerodynamics model is employed to accurately predict the incremental trailing edge flap airloads. Both the finite wing effect and actuator saturation for trailing-edge flaps are also included in an aeroelastic analysis. For a composite articulated rotor, a new active blade loads control method is developed and tested numerically. The concept involves straightening the blade by introducing dual trailing edge flaps. The objective function, which includes vibratory hub loads, bending moment harmonics and active flap control inputs, is minimized by an integrated optimal control/optimization process. A numerical simulation is performed for the steady-state forward flight of an advance ratio of 0.35. It is demonstrated that through straightening the rotor blade, which mimics the behavior of a rigid blade, both the bending moments and vibratory hub loads can be significantly reduced by 32% and 57%, respectively. An active vibration control method is developed and analyzed for a hingeless rotor. The concept involves deflecting each individual trailing-edge flap using a compact resonant actuation system. Each resonant actuation system could yield high authority, while operating at a single frequency. Parametric studies are conducted to explore the finite wing effect of trailing-edge flaps and actuator saturation. A numerical simulation has been performed for the steady-state forward flight (µ = 0.15 ? 0.35). It is demonstrated that multiple trailing-edge flap configuration with the resonant actuation system can reduce the required trailing-edge flap hinge moments by 37% to 61% in each individual actuator compared to single-flap iii configuration for high speed flight conditions. A novel resonant actuation concept is developed to efficiently realize the helicopter vibration and blade loads control. The resonant actuation system (RAS) is achieved through both mechanical and electrical tailoring. With mechanical tuning, the resonant frequencies of the actuation system (includes the piezoelectric actuator and the related mechanical and electrical elements for actuation) can be adjusted to the required actuation frequencies. This obviously will increase the authority of the actuation system. To further enhance controllability and robustness, the actuation resonant peak can be significantly broadened and flattened with electrical tailoring through the aid of an electric network of inductance, resistance, and negative capacitance. A piezoelectric resonant actuation system model is derived for active flap rotors. The optimal values of the electrical components are explicitly determined. An equivalent electric circuit model emulating the physical actuation system is derived and experimentally tested to investigate the initial feasibility of the piezoelectric resonant actuation system. It is demonstrated that the proposed resonant actuation system can indeed achieve both high active authority and robustness. It is shown that the actuator authority is significantly increased from 1.25 to 4.5 degrees as compared to the static value, with wide operating bandwidth of 8 Hz. In addition to this, the RAS is compared to an equivalent mechanical system to provide better physical understanding. Design guidelines of the RAS are derived in dimensionless forms. Feed-forward controllers are developed to realize the electric network dynamics and to adapt the phase variation. The control strategy is then implemented via a digital signal processor (DSP) system. Performance of the resonant actuation system is analyzed and verified experimentally on a full-scale piezoelectric tube actuator for helicopter rotor control. Promising results are illustrated that the actuator stroke is increased 2 to 3.5 times compared to its static value with bandwidth of 5 to 10 Hz. iv