Ground effect on rotary aircraft has been studied for many decades. Although a large body of research results is now available for conventional helicopters, this topic is just beginning to receive attention in the unmanned aerial vehicle community, particularly for small size UAVs. The objective of this paper is to assess the applicability of a widely-used ground effect model, developed in the middle of last century, for predicting the ground effect on a small rotary UAV. The particular vehicle employed in this work, the Draganflyer X8, is actuated by 8 propellers arranged in 4 coaxial pairs in a quadrotor configuration and is currently employed for the development of autonomous landing capabilities for UAVs. The aforementioned ground effect model gives an explicit relationship between the thrust produced by a single propeller while operating in-ground-effect and the thrust out-of-ground effect, as a function of the normalized propeller height above ground. A series of experiments was conducted with the propellers of the vehicle on a test stand, and with the X8 vehicle in flight, from which we obtained the in-ground and out-of-ground thrusts. Juxtaposition of our results against the theoretical model points to a stronger ground effect on the X8 vehicle than predicted by theory. Discussion of the assumptions underlying the theory and the experimental procedures and their implications on the results obtained is also included in the paper.
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