Comprehensive aeromechanical measurements of a model-scale, coaxial, counter-rotating rotor system
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With the renewed interest in rigid, counter-rotating coaxial rotor designs, and the increased fidelity of fully coupled CFD/CSD simulations, there exists a lack of comprehensive experimental data for a rotor system with which to validate analyses. The goal of this dissertation is to generate a new set of measurements on a model-scale rigid coaxial rotor systems in hover and high speed forward flight. A counter-rotating transmission was built, incorporating 6-component upper and lower rotor load cells for individual hub load measurements. Upper and lower rotor control systems, as well as complementary instrumentation including pushrod load cells, root pitch measurement and blade tip clearance sensors were developed. Two sets of rotor blades were fabricated and characterized using stereoscopic digital image correlation in combination with static and dynamic loads. A novel rotating-frame operational modal analysis successfully identified the first blade flap frequency and aerodynamic damping. Hover testing focused on quantifying the effects of upper and lower coaxial rotor interference when compared to isolated rotors. Statistical analysis of the measured data revealed clear trends with a known confidence level. Due to mutual interference, the upper and lower rotors of the coaxial configuration consumed 18% and 49% more induced power than that of an isolated two-bladed rotor. The coaxial counter-rotating configuration was found to consume 6% less induced power than an isolated, four-bladed single rotor of equal solidity. While torque balanced, the upper rotor was found to produce 54% of the total system thrust regardless of blade loading. Significant four-per-revolution vibratory thrust was observed in the lower rotor, with primary and secondary peaks corresponding to bound vortex and blade thickness interactions respectively. Wind tunnel testing examined the effects of lift offset and rotor phasing at high forward flight speeds. Rotor effective lift-to-drag ratio was found to increase with increasing advance ratio and lift offset, resulting in a 50% peak efficiency gain. The lower coaxial rotor was found to operate at higher lift-to-drag ratio than the upper rotor, due to the reversal of differential upper and lower rotor thrust compared to hover. Lift offset resulted in a decrease in blade tip clearance with a corresponding rise in rotor side force. Vibratory loads increased with advance ratio, with the largest occurring at two and four-per-revolution harmonics. Lift offset decreased vibratory forces while increasing vibratory in-plane moments. The coaxial system experienced reduced vibratory in-plane forces and torque compared to the isolated rotors due to cancellation between upper and lower rotor loads. Adjusting the inter-rotor index angle modified vibratory forces and moments transmitted to the fixed frame, increasing some components while decreasing others.