Simulation investigation of operational conditions of rotor for high-speed compound helicopter

The article presents results of simulations concerning possibilities of rotorcraft performance enhancements for compound helicopters with introduced additional wings and propellers. The simple model of helicopter including a point mass of fuselage and a rotor treated as a disk was used for calculations of helicopter flight equilibrium conditions. For the defined flight states, the more detailed model of elastic blade was applied to compute magnitude of rotor loads and level of blade deformations. The model of elastic blade includes out-of-plane bending, in plane bending, and torsion effects due to variable aerodynamic and inertial loads of rotor blades. Equations of motion of rotor blades are solved applying Runge-Kutta method. Taking into account Galerkin method, parameters of blade motion are computed as a combination of assumed torsion and bending Eigen modes of the rotor blade. The six-bladed rotor with stiff connections of blades and hub was applied for comparison of flight envelope for conventional helicopter and versions of compound rotorcraft with additional propellers and with wings and propellers. Simulations indicate that, in the case of compound helicopter configuration, achieving the operational flight conditions at high speed of 400 km/h is possible without generating excessive loads and blade deformations. The results of calculations of rotor loads and generated blade deflections are presented in form of time-run plots and as rotor disk distributions, which depend on radial and azimuthal positions of blade elements. The simulation Keywords: compound helicopter, rotor loads, blade deformation 1.Introduction For classical configuration of helicopter, with main rotor and anti-torque tail rotor, the speed of flight is limited in comparison to fixed wing aircraft. The maximum speed of conventional helicopter is restricted due to high drag associated with compressibility effects for advancing rotor blade and stall phenomenon, which occurs at retreating blade zone. A compound helicopter configuration with added lifting wings and separate source of thrust for propulsion may help to unload main rotor and enhance speed range of rotorcraft. Initial development programs of compound helicopters [6], such as the Bell 533, the Lockheed XH-51A fitted with wings and additional turbojet engines, or the Lockheed AH-56A Cheyenne with wings and pushing propeller and after flight tests were not passed to serial production. Emerging demands for improved performance and progress in composite materials and aerodynamics of rotor blades gave impulse to return to compound helicopter concept. In the last decade, the new experimental compound helicopters with additional propulsion were tested [3]: the American Piasecki X-49 Speedhawk, the Sikorsky X2 and the French Eurocopter X3. It should be mentioned that the Eurocopter X3 achieved speed of 472 km/h setting an unofficial speed record for propeller helicopters. Analytical and experimental researchworks were performed to examine features of compound helicopters, which included effects of varying main rotor tip speed [1], wing-rotor lift share [7, 8] and investigation of dynamic stability characteristics [2]. Basing on collected experiences, the next generation of operational compound helicopter designs are being developed. The American Sikorsky S-97 Raider made the maiden flight in 2015 [4] and the European Airbus Helicopter Racer’s configuration was revealed in 2017 [5]. investigation may help to define demands for rotor of high-speed helicopter.