A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Fundamental Understanding of Rotor Aeromechanics at High Advance Ratio Through Wind Tunnel Testing(2016) Berry, Benjamin Otto; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The purpose of this research is to further the understanding of rotor aeromechanics at advance ratios (mu) beyond the maximum of 0.5 (ratio of forward airspeed to rotor tip speed) for conventional helicopters. High advance ratio rotors have applications in high speed compound helicopters. In addition to one or more conventional main rotors, these aircraft employ either thrust compounding (propellers), lift compounding (fixed-wings), or both. An articulated 4-bladed model rotor was constructed, instrumented, and tested up to a maximum advance ratio of mu=1.6 in the Glenn L. Martin Wind Tunnel at the University of Maryland. The data set includes steady and unsteady rotor hub forces and moments, blade structural loads, blade flapping angles, swashplate control angles, and unsteady blade pressures. A collective-thrust control reversal---where increasing collective pitch results in lower rotor thrust---was observed and is a unique phenomenon to the high advance ratio flight regime. The thrust reversal is explained in a physical manner as well as through an analytical formulation. The requirements for the occurrence of the thrust reversal are enumerated. The effects of rotor geometry design on the thrust reversal onset are explored through the formulation and compared to the measured data. Reverse-flow dynamic stall was observed to extend the the lifting capability of the edgewise rotor well beyond the expected static stall behavior of the airfoil sections. Through embedded unsteady blade surface pressure transducers, the normal force, pitching moment, and shed dynamic stall vortex time histories at a blade section in strong reverse flow were analyzed. Favorable comparisons with published 2-D pitching airfoil reverse flow dynamic stall data indicate that the 3-D stall environment can likely be predicted using models developed from such 2-D experiments. Vibratory hub loads were observed to increase with advance ratio. Maximum amplitude was observed near mu=1, with a reduction in vibratory loads at higher advance ratios. Blade load 4/rev harmonics dominated due to operation near a 4/rev fanplot crossing of the 2nd flap bending mode natural frequency. Oscillatory loads sharply increase in the presence of retreating blade reverse flow dynamic stall, and are evident in blade torsion, pitch link, and hub load measurements. The blades exhibited torsion moment vibrations at the frequency of the 1st torsion mode in response to the reverse flow pitching moment loading.Item Performance and Loads of Variable Tip Speed Rotorcraft at High Advance Ratios(2015) Bowen-Davies, Graham Michael; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents a lifting-line, comprehensive approach to predicting the performance and loads of high advance ratio rotorcraft. At high advance ratios, the reverse flow region is large and its unique aerodynamics impacts the rotor performance and dynamics more than at conventional airspeeds where they are often ignored. The analysis is refined and augmented with improved modeling of the nearwake in reverse flow, a new aerodynamic model of the fuselage and the root cutout region and corrections to the airfoil properties for highly yawed flow. The analysis is correlated and evaluated against a full-scale UH-60A rotor test to an advance ratio of 1.0 and against an in-house Mach-scaled rotor to an advance ratio of 1.2. High advance ratio performance is predicted satisfactorily for both tests, including predicting the onset of thrust reversal. Despite the high advance ratio, correctly modeling the wake is most important for predicting airloads and the resulting blade bending loads, while yawed flow, nearwake inflow and the fuselage flow disturbances are important for predicting high advance ratio thrust and power. The validated analysis is used to investigate the effect of reverse flow stall, blade twist, root cut-out and shaft angle on high advance ratio performance.