Optimal Design of Rotor Blade for a Winged Compound Helicopter at High Advance Ratio

Abstract

This paper proposes an optimal main rotor blade geometry for a high-speed compound helicopter. Recently, compound helicopter or tilt rotor has been developed to achieve high-speed flight while preserving VTOL capabilities. In the compound helicopter, asymmetric flow field on the main rotor increases as the flight speed increases. In other words, reverse flow region on the retreating side spreads to an extent as high as from 50% to more than 80%. In such a flow field, different blade geometries are deemed to be suited than conventional main rotor blades. Optimization methods are applied in this study to propose a new blade geometry. In particular, design parameters of blade geometry are limited to twist angle, chord length, span position of root portion, tip sweepback angle, and tip dihedral angle, while an elliptic airfoil is assigned to the blade root portion and a SC1095 airfoil for the blade main portion. As a result, a new blade shape which is remarkably different from the conventional helicopter is obtained. The optimal blade's root portion position is approximately forty percent of the whole blade length. And a blade with a sweep-forward angle shows higher effective lift-drag ratio than that with sweepback angles. The comparison with UH-60A on rotor performance in hover and high-speed forward flights are also conducted. The optimal blade shows only half of effective drag coefficient in high-speed flight compared to UH-60A, while maintaining nearly same hovering performance.