Effects of morphing wings on aerodynamic performance and energy efficiency improvement for ground effect vehicles.

Abstract

In this study, morphing wings in ground effect were investigated with the intent of applying to UAV craft in ground effect to improve the aerodynamic performance using CFD at a Reynolds number of 320,000. First, an aerofoil selection was carried out using RANS with K-Omega SST in two and three dimensions. The NACA6409 was a compromise between high aerodynamic efficiency, high lift, and substantial thickness for structural constraints and space to store morphing systems. Morphing was applied to a two-dimensional aerofoil in ground effect using the FishBAC morphing method and steady-state RANS CFD. Morphing the aerofoil increased the lift due to the reduced distance between the trailing edge and the ground, which enhanced the ground effect. Gains in aerodynamic efficiency were seen for low angles of attack up to 4 degrees with small trailing edge deflections. The same improvements were seen using unsteady dynamic morphing with URANS; this was due to considering UAV actuator speeds for the morphing period which resulted in a quasi-static flow. For the dynamic morphing, it was seen the lift was slightly higher and the drag somewhat lower from the increased detail captured by using URANS. The FishBAC morphing in ground effect was compared to traditional control surfaces in ground effect and morphing in freestream. For the same trailing edge deflection, morphing wings generated more lift and were more aerodynamically efficient due to a continuous surface and smooth changes in geometry. Periodic morphing was carried out in 10% ground clearance in two dimensions, which increased the aerodynamic efficiency by 80.5% and lift by 15.1% whilst reducing the drag by 36.7% for a Strouhal number of 3.58 and a trailing edge deflection of 1%. The increase in aerodynamic efficiency was due to the Von Karman shedding interaction between the shedding vortices and the motion of the ground plane, which caused thrust. Three-dimensional wings with morphing in ground effect were also investigated to see the impact of a finite aspect ratio. An optimisation study was carried out in three dimensions where a tip chord of 20% of the root with a forward wing tip position showed 12.42% higher lift and 35.95% higher aerodynamic efficiency at ℎ/𝑐 = 0.1 compared to a rectangular wing. The low angle of attack at the root with forward wing tip position and small tip chord resulted in the pressure on the lower surface of the wing feeding the wingtip vortex less than the rectangular wing. The small tip chord also increased the aspect ratio of the wing. Camber morphing was applied to a three-dimensional wing where the start and end location of morphing along the span was investigated using steady RANS simulations. Applying the camber morphing along the full span length resulted in smaller trailing edge deflections to achieve the same change in lift compared to applying the morphing for a small proportion along the span. Morphing wingtips using FishBAC morphing in the span direction increased the lift and reduced drag. Starting the morphing earlier in the span direction resulted in a greater proportion of the lower surface being closer to the ground, further enhancing ground effect. For a fixed wing tip clearance of 2%, the root height was varied using FishBAC wing tip morphing to simulate an aircraft varying altitude. It was seen for a ground clearance of ℎ/𝑐 = 0.1 that the drag reduced by 15% and for ℎ/𝑐 = 0.3 the drag reduced by 23%. Finally, span morphing was also investigated to increase the wing aspect ratio, and it was seen the optimised wing efficiency increased from 22.7 for 100% span to 31.43 for 150% span length. Therefore, large spans show increased endurance and range whilst lower spans allow roll of the craft. The study focused on UAV craft where the optimised wing had an endurance of 0.95 hours and a range of 38.65km. The optimised wing endurance was 50.8% and the range was 73.7% higher than the baseline rectangular wing. Increasing the optimised wingspan to 150% increased the endurance to 2.12hours and range to 63.97km. Applying periodic morphing showed the optimised wing had a range of 48.4km and an endurance of 1.59hours which was a gain of 217.5% for the endurance and 252.3% for the range compared to the rectangular non-morphing wing. A span of 150% with periodic morphing showed an increase in range of 460.3% and endurance of 362%.

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