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In-Flight Validation of Propeller Slipstream Model

This paper presents a semi-empirical propeller slipstream model capable of predicting flow velocities far downstream of the propeller in forward flight conditions. The model is an extension of a model developed for static conditions, to include an incoming freestream flow. The effect of the aircraft’s forward speed on the flow aft of the propeller is demonstrated and compared to that predicted by classical momentum theory. A small UAV platform is then instrumented to measure the slipstream flow at multiple locations behind the propeller. Tests were conducted in a level flight state with a constant propeller speed. In-flight measurements show that the semi-empirical model is significantly more accurate than classical momentum theory near the tail of the platform, with an RMS error of 0.7-2.0m/s. Measurements near the wing of the test platform at an axial location less than 2 propeller diameters downstream showed lower accuracy for both the semi-empirical and momentum theory models, possibly indicating strong unmodeled interference from the wing and ailerons or unknown effects of advancing flow conditions.