This paper presents an investigation of the effects of rotor-speed-ratio (RSR) and inlet crosswind distortion on the off-design performance of a contra-rotating propelling unit. A dedicated wind tunnel has been built in order to study the off-design performance in distorted and undistorted conditions. A miniature 8-hole pressure rake has been calibrated to measure the total and static pressure field across the blade span, upstream and downstream of the propellers. A data analysis algorithm has been developed for the rapid exportation of desensitized and reliable mass-averaged characteristics, while wall blockage effects are accounted for incorporating a correction factor. Investigation of rotor-speed-ratio effects at undistorted inlet conditions show that, efficiency is the highest when the front rotor operates at rotational speed equal to 90% of the aft rotor’s speed.

For the optimum RSR, the effects of crosswind inlet distortion are investigated. Crosswind mass-flow is generated by a 16-fan array, accommodated in a secondary square wind-tunnel, with its center axis perpendicular to propeller main flow direction. In order to achieve realistic crosswind flow characteristics, the pressure side of the secondary wind tunnel is being used, after proper flow conditioning. Total and static pressure distribution highlights the location of a primary and a secondary stall cell, both upstream and downstream of the propellers. The results indicate a spanwise shift of the inlet pressure profile towards the direction of crosswind. A rotation of the wake is also observed due to propellers’ remaining swirl. Finally, the effect of distortion on the overall performance is being investigated, by the exportation of averaged total pressure rise for the various examined inlet distortion cases. Mass-flow is slightly increased; however, total pressure rise is dramatically reduced due to crosswind flow effects. Especially for the case of landing under extremely strong crosswind, the ratio between total pressure rise and inlet dynamic head is 4.2 times lower, compared to undistorted inlet operating conditions.

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