Compound Helicopter: Insight and Optimization

Abstract

The paper presents an insight into the complex task of design and optimizing a compound helicopter configuration. It introduces the “drag versus power chart” (DP chart) as a tool for separating the rotors, thrusters, wings, and fuselage contributions and understanding their optimal combination in a generic compound configuration. The analysis shows the dependency of the optimal configuration on the efficiencies of the rotors, the thrusters, and the wings, and a way to carefully examine the effect of many design parameters. As such, the analysis may be applied to various configurations including single and coaxial rotor systems. Among other conclusions, it is shown when and why a thruster is absolutely essential for high speed and clarifies the role of the wing in such cases. The paper also supplies a unique optimization process, which is based on a comprehensive and detailed nonlinear free-wake analysis of a compound configuration that includes a thruster and fixed wings. The optimization process is twofold: First, for a global search, a variety of randomly selected configurations are analyzed to determine an initial hover-forward flight Pareto frontier. Then, various types of local analyses are carried out to improve the above frontier. Such successive frontier refinements lead to an improved, detailed, and continuous frontier that may be exploited for a variety of missions. The configurations on the resulting Pareto frontier show design trade-offs between configurations that are more efficient in hover and those that are more efficient in high-speed forward flight.