This paper examines the aerodynamic environment of a high speed coaxial compound helicopter in steady level forward flight using CFD-CSD coupling. Numerical simulations have been performed to study trim configurations of an eight-bladed coaxial compound configuration (four blades per rotor) using comprehensive analysis coupled to a vortex wake model. The model is validated using public-domain information and flight test data obtained for the Sikorsky X2 Technology Demonstrator. Rotor-rotor interference follows the expected momentum theory type model, with the lower rotor experiencing larger induced inflow. Thrust sharing between rotors remains nominally constant, with at most ±5% bias over the speed range studied. CFD analysis for the rotors is then introduced at select forward flight speeds to identify key aerodynamic modeling refinements. In particular, impulsive normal force due to rotor blade crossings are not captured accurately by the vortex wake model. The impulsive 8/rev pitching moment is completely missed by the reduced order model. While significant, this aerodynamic pitching moment does not excite a noticeable torsion response from the stiff rotor considered. For future designs with lower stiffness blades, these impulsive airloads may induce additional vibratory hub loads and drive the design of the rotor pitch control system.
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