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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This paper presents a methodology for preliminary sizing of unconventional rotorcraft using a physics-based approach to estimate the weight of primary load-carrying members and rotor efficiencies. The methodology is demonstrated for a quadrotor biplane tailsitter, a tilt-body configuration that can operate in both helicopter and airplane mode. A beam lattice framework for the airframe structure is iteratively adjusted in the sizing loop to accommodate the limit loads. A similar semianalytical approach is followed to size and estimate weight of the rotor blades. Using this analysis, a consistent combination of vehicle macrodimensions (rotor radius, wing span) and tip speed as well as detailed design parameters (spar height, skin thickness, and cross-section weight) are obtained simultaneously. To compare the effectiveness of various power plants within a weight class, the sizing methodology was modified to identify the payload for three different vehicle takeoff weights: 20, 50, and 1000 lb. To enable operation within constrained urban canyons, the effect of restricting maximum vehicle dimensions to 10 ftfor the 1000-lb designs is also examined. An electric transmission model is used in these designs owing to its relative insensitivity of transmission efficiency to the operating RPM. A variable-pitch and variable-RPM rotor design allows for control redundancy within each rotor.