Choongmo Yang

Helicopter Rotor Shape Optimization for the Improvement of Aeroacoustic Performance in Hover

A helicopter rotor is optimally designed for aeroacoustic performance improvement. As shown in previous reports, the blade shapes can be designed to minimize high-speed impulsive noise but tend to have excessively high tapers and be swept back. Since an overly short chord length around the blade-tip region may cause structural problems and safety issues in autorotation, an autorotation index has been introduced to keep the tip region from having excessive taper ratios. In addition, the changes in thickness and camber of the airfoils can also be taken into account to better represent realistic rotor shapes. Aeroacoustic analysis is performed using Kirchhoffs method coupled with computational fluid dynamics analysis, and the optimization is performed using the kriging-model-based genetic algorithm method. Optimization results are presented that show that the designed blades have improved aerodynamic performance and reduced high-speed impulsive noise characteristics. It is found that a more practical blade shape can be obtained by using airfoil transitions and an autorotation index. The results of the analysis of variance and self-organization map indicate that the taper ratios, the swept back, the tip chord length, the protrusion shape, the camber, and the thickness of the root airfoil are the prominent features affecting the aeroacoustic performance of the rotor.

Comparison of Noise Reduction Effect between AFC and Conventional IBC by Moving Overlapped Grid Method

The effect of active flap control (AFC) on the reduction of helicopter blade-vortex interaction (BVI) noise is quantitatively analyzed in comparison with the conventional individual blade control (IBC) using a three-dimensional unsteady Euler code. This code is based on our advanced CFD code for the full configuration of helicopters. The acoustic signal of BVI noise is obtained by combining the CFD code with an acoustic code based on the Ffowcs Williams and Hawkings (FW-H) equation. As a result, it is found that AFC is quite effective for the reduction of BVI noise compared to IBC because AFC yields almost same magnitude of noise reduction as IBC with the small area of movable control surface (4.5% of IBC).