Development and Validation of a Propeller Slipstream Model for Unmanned Aerial Vehicles

Recent interest in high-angle-of-attack flight, aerobatic maneuvering, vertical/short takeoff and landing, etc., of small unmanned aerial vehicles necessitates more detailed modeling of the complex aerodynamics associated with these flight regimes. This includes modeling the effect of the propeller slipstream, also called prop wash, which is the main source of airflow that helps maintain lift and control during near-zero forward-speed flight like that encountered during vertical/short takeoff and landing, as well as during high-angle-of-attack flight/aerobatic maneuvering like hovering. Propeller slipstream models based on conventional theories, such as the momentum theory, have been used extensively in the literature to predict the induced air velocity within the slipstream. However, because these conventional theories consider only acceleration of air within the slipstream and not diffusion, their applicability in regions far downstream of the propeller where diffusion is dominant, is questionable. This paper presents a propeller slipstream model that considers both acceleration and diffusion within the slipstream using simple analytical and semi-empirical equations. The proposed model is shown to be in good agreement with the experimental data for several different propellers and configurations, up to propeller diameters downstream of the propeller plane, with an rms error of in velocity. As well, the proposed model matches the simplicity of conventional models and is therefore suitable for real-time applications.