Effects of Propwash on Horizontal Tail Aerodynamics of Pusher UASs

In this paper, the propwash phenomenon for pusher configuration unmanned aerial systems (UASs) is studied. The SkyHunter UAS has the propeller placed aft of the fuselage and in front of the horizontal tail with no offset from the zero lift plane. Thus, the propeller slipstream directly flows over the horizontal tail affecting its aerodynamics. To validate the physics based model developed for propwash impacts and to quantify the effects of propwash, the SkyHunter UAS was equipped with an extra pitot tube. The first pitot tube was placed at the nose of the aircraft where the air is undisturbed and it was measuring the aircraft's velocity. The second pitot tube was placed at different locations on the horizontal tail and three flight tests were conducted. Flight test data showed that the current configuration of the SkyHunter: (A) has large variations in the horizontal tail dynamic pressure ratio along the span of the horizontal tail. (B) had the dynamic pressure ratio higher than 1.7 in two of the three investigated locations along the span of the horizontal tail. This suggests that the theoretical estimate of the dynamic pressure ratio (which is 0.93) may be an under estimation. (C) The dynamic pressure ratio varied with time during flight. A change in the horizontal tail dynamic pressure ratio directly leads to a shift in the location of the aerodynamic center of an aircraft as well as changes in the stability and control derivatives of an aircraft. The implication of these changes in small UASs can be significant since they affect the aircraft longitudinal stability, trim elevator, and flight characteristics. In order to mitigate such changes during flight, a redesign was made to the original empennage design where is an offset was added between the zero lift plane and the aerodynamic center of the horizontal tail. This redesign intends to move the horizontal tail above (and away from) the propeller slipstream. Manufacturing of the redesigned horizontal tail was completed recently and it was flight tested. Flight test data from the new design show that the new design successfully mitigates the effects of the propwash.