Active/Passive Optimization of Helicopter Rotor Blades for Improved Vibration, Noise, and Performance Characteristics.

Abstract

This dissertation describes an active/passive approach to optimum design of helicopter rotor blades for reduced vibration and noise levels, as well as reduced power consumption. In the active/passive approach, structurally optimized rotor blade designs obtained from surrogate based optimization (SBO) methods were augmented with active control flaps (ACF’s). Multi-objective function optimization techniques were employed to obtain active/passive configurations corresponding to the best trade-offs between vibration, noise, and performance characteristics of the rotor blades in forward flight.

The focus of the initial portion of the work was on the effectiveness of SBO for vibration reduction in forward flight. It was determined that SBO methods could be used to conduct global searches of the design space for reduced vibration designs, even though the surrogates were not accurate everywhere in the design space. Subsequently, it was demonstrated that the Efficient Global Optimization (EGO) algorithm was superior to conventional SBO techniques for vibration reduction at low speed forward flight where blade-vortex interaction (BVI) induces high vibration levels, and at high speeds where dynamic stall is the dominant source of vibration. Since the best design for low speed forward flight differed from the best design for high speed flight, multi-objective function optimization techniques were necessary to find the best trade-off designs for vibration reduction over the entire flight envelope. To this end, the EGO algorithm was extended for surrogate based multi-objective function optimization and the results demonstrate that the modified EGO algorithm located a single trade-off design with vibration characteristics similar to the best designs for both flight conditions.

Finally, ACF’s were used to enhance vibration, noise, and performance characteristics of structurally optimized blades. Using a closed-loop control algorithm and multi-objective function optimization based on EGO, a versatile active/passive design for reduced vibration and noise levels due to BVI was obtained. The design corresponds to $68 - 91%$ vibration reduction and a $2.3 - 2.7$ db decrease in the maximum noise level. In addition, the active/passive approach was used for vibration reduction over the entire flight envelope, while enhancing performance at high speed flight and constraining noise levels at low speed forward flight from increasing.