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PROCEEDINGS OF THE FIFTH INTERNATIONAL CONFERENCE ON ENGINEERING COMPUTATIONAL TECHNOLOGY
Edited by: B.H.V. Topping, G. Montero and R. Montenegro
Computational Aerodynamics and Experimental Investigations of an Inflatable Wing
E. Crosbie1, T. Calder1, C. Chang1, P. Marzocca2, Z. Gürdal3 and J. Hol3
1Imperial College, London, United Kingdom
E. Crosbie, T. Calder, C. Chang, P. Marzocca, Z. Gürdal, J. Hol, "Computational Aerodynamics and Experimental Investigations of an Inflatable Wing", in B.H.V. Topping, G. Montero, R. Montenegro, (Editors), "Proceedings of the Fifth International Conference on Engineering Computational Technology", Civil-Comp Press, Stirlingshire, UK, Paper 175, 2006. doi:10.4203/ccp.84.175
Keywords: inflatable, airfoil, numerical, experimental, unmanned aerial vehicle, performance.
Current military unmanned aerial vehicles (UAVs) are used for strategic and tactical purposes, and there are attempts in progress to satisfy this need at the level of a platoon or an individual soldier. Taking advantage of miniaturisation, small UAVs are being developed to compliment current systems by performing missions that are too dull, dirty, dangerous, or expensive for existing unmanned and manned platforms. Small scale UAVs often require the stowing of their wings and control surfaces into very small volumes to permit gun launch or packaging into aircraft mounted aerial drop assemblies [1,2]. This entails the requirement to reduce or eliminate completely the mechanical complexity of the wing deployment and control surface systems. One technology that has shown promise in achieving this goal is the inflatable wing. There are many advantages present in the inflatable wings. An inflatable UAV would offer significant improvements in portability and weight. They can be packed into volumes tens of times smaller than their deployed volume and without damage to the structural integrity of the wing. Depending on mission requirements, deployment can occur either on the ground or in flight, from an aerial drop assembly, and the wings inflated within a second. The inflatable wing presents a clean wing geometry, it can be deployed without the mechanical complexities involved in conventional wings. As the inherent stiffness of the inflatable wing is considerably lower than its metal-composites counterpart, it is possible to morph or alter the shape of the airfoil to provide aerodynamic control or changes in the flight characteristics of the vehicle at varying velocities. Neal et al. , have analysed methods to radically change the planform of the wing, which has the potential to improve the performance of the vehicle by allowing a single aircraft design to traverse through various flight regimes.
Numerical simulations were used to analyse the inflatable airfoil. The computational model was developed using FLUENTR  and used the SST variant of the turbulence model to solve the RANS governing equations. The computational model was validated against experimental results gathered from wind tunnel testing. A stereolithography model was manufactured with pressure tappings to collect the necessary data. An acceptable agreement in the pressure disributions produced by experiment and CFD was obtained although the computational model did not predict too well the regions of separated flow.
A comparison was made between inflatable profiles with different numbers of cells. The lift and drag forces on the airfoil were recorded for each angle-of-attack. As the cell number increased, there was an increase in the maximum lift coefficient. At moderate to high angles-of-attack the inflatable profiles with 7 or greater cells generated a higher lift coefficient than the smooth NACA profile at the design Reynolds number of 2*105. The stall angle-of-attack increased noticeably for the 10 and 12 cell profiles with the smooth NACA profile lying on par with the lower cell profiles. In the case of the low-cell profiles, the extreme surface gradients close to the trailing edge cause the flow to separate early even though the boundary layer is highly turbulent. For the smooth NACA profile the separation is laminar due to the low Reynolds number and is caused by moderate adverse pressure gradients on the suction surface.
The zero-lift drag coefficient for the profiles decreased steadily as the number of cells was increased. This phenomenon was consistently true as the angle-of-attack was increased. The smooth NACA 0018 airfoil had significantly lower drag at low angles-of-attack however at high angles-of-attack the inflatable airfoil has superior aerodynamic efficiency (L/D) at the design Reynolds number. The growth of the separation impedes the ability of the smooth airfoil to generate lift and for angles-of-attack above 12o the inflatable profile has superior efficiency. At the higher Reynolds number (Re=2*106) the low drag characteristics of the smooth airfoil are preserved to higher angles-of-attack resulting in increased performance. In terms of drag and aerodynamic efficiency, the inflatable profile will only perform better than the smooth section at high angles-of-attack since the local separation and highly turbulent boundary layer will only serve to increase drag when large regions of separated flow are not present.
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