The purpose of this investigation was to determine the sensitivity of tiltrotor autorotation characteristics to proprotor design variations. The investigation had two phases: validate CAMRAD II as a tool to simulate low power, descending flight; and evaluate the sensitivity of autorotation characteristics to design variations. Wind tunnel results from four helicopter rotors in axial descent and the XV-15 proprotor in forward flight descent were simulated. The simulation accuracy was evaluated for free wake modeling parameters such as wake age and trailing vortex roll up. The primary tiltrotor autorotation characteristics were identified as: proprotor rotation speed droop, steady rate of descent, and rate of descent reduction during collective flare. The sensitivity of these primary characteristics to design variations (blade twist, taper ratio, number of blades, solidity, and rotor speed) was investigated. Analytical results for the autorotation condition showed that the geometry of the free wake is sensitive to small changes in thrust and trailing vortex rollup. However, proprotor thrust and power in autorotation conditions are not sensitive to free wake age or vortex core radius. It was found that the XV-15 proprotor requires additional inertia to obtain a 1000 fpm descent rate during autorotation flare. Also, increasing the blade solidity and allowing 110% rotor speed were shown to improve autorotation characteristics.
展开▼
