Wind Tunnel Testing and Aeromechanics Predictions on Slowed Mach-scaled Thrust Compounding Rotorcraft with a Trailing Propeller

A thrust compounded helicopter - a main rotor with a trailing propeller was tested in the Glenn L. Martin Wind Tunnel (GLMWT) to evaluate its performance under different flight conditions. The main rotor rig consists of a hingeless hub with four fully instrumented NACA 0012 blades and a modified Robin fuselage. The propeller rig consists of a rigid Sensenich L26H propeller with four blades. Tunnel tests were carried out for the isolated propeller, and thrust compounded rotor configurations. The isolated propeller tests were conducted with and without the fuselage installed ahead of it to understand the effects of the fuselage on propeller performance. The thrust compound configuration was tested at three different main rotor shaft tilt angles (α_s): -4^◦, 0^◦, and 4^◦, advance ratios (µ) from 0.3 to 0.6, and multiple lift (C_L/σ) and propulsive (C_X/σ) trim targets. Main rotor hub loads, oscillatory blade structural loads, and propeller hub loads were measured for all the tests. The test data was verified with a full vehicle aeromechanical analysis using the University of Maryland Advanced Rotorcraft Code (UMARC). The thrust compound configuration with the main rotor shaft tilt of -4^◦ (rearward tilt) provided the best performance. Thrust compounding with rearward shaft tilt (-4^◦) resulted in a 50% increase in the maximum aircraft lift-to-drag ratio compared to a single rotor helicopter. Half peak-to-peak hub vibratory loads and blade bending loads decreased with thrust compounding. It was observed that for the same lift target (C_L/σ), thrust compounding achieved 20% higher flight speeds than a single rotor.