Distributed propellers slipstream effects on wing at low Reynolds number

Based on the research about the distributed electric propulsion (DEP) technology applied on high-altitude long-endurance (HALE) solar-powered unmanned aerial vehicles (UAVs), the aerodynamic characteristics of the FX 63-137 wing under the distributed propellers slipstream effects in a tractor configuration at low Reynolds numbers are numerically studied. The numerical simulations are achieved by quasi-steadily solving the Reynolds-Averaged Navior-Stokes (RANS) equations based on the multiple reference frames (MRF) method, the kT-kL-ω transition model, and the hybrid grids. Firstly, the numerical results of the FX 63-137 wing and a practical propeller X1 are compared with the experimental data to validate the accuracy and flexibility of the method. Secondly, the aerodynamic properties of the distributed propellers/wing integration are compared among different rotation rates of propellers. Lastly, the detailed flow structures formed on the wing surfaces are sketched and analyzed. The results show that (a) significant lift benefits can be achieved for the reason that both the speed and the dynamic pressure of the incoming flow are enhanced by the propellers slipstream; (b) the turbulence added to the free stream by means of the distributed propellers slipstream prevent the formation of laminar separation bubble (LSB), but apparent horizontal vortexes can be observed at the slipstream boundaries at the same time; (c) the lift augmentation on the down-wash side of the wing is slightly stronger than that on the up-wash side at low Reynolds numbers, which results from the mechanisms that the LSB formed on the windward side of the wing is correspondingly found to be slightly shorter than that on the leeward side.