In the design of MAVs, several prominent features are identified: (i) low Reynolds numbers (i.e. 10³-10⁵), resulting in degraded aerodynamic performance, nonlinear response to variation of the angles of attack of the wing, and massive flow separation at high angles of attack; (ii) small physical dimensions, leading to much reduced payload capabilities, and some favorable scaling characteristics including structural strength, reduced stall speed, and impact tolerance; and (iii) low flight speed, resulting in an order one effect on the flight environment such as wind gust, and intrinsically unsteady fight characteristics [1,2,3]. For the MAVs generally two types of propulsive system are considered: (i) a fixed wing-based system (that requires additional resources of propulsion); and (ii) a moving wing-based system (that can generate propulsive forces by itself). It is well-known that commercial aeroplanes employ the fixed wing-based system, helicopters the rotating wing-based system, and biological flyers the flapping wing-based system. Although favourable flight performance of moving wing based micro-sized air vehicles is to be expected, the current state of the art and the knowledge of flapping and rotating wings learned from natural flyers and helicopters is still challenging to apply in the vehicle design because of the complicity of the problems. Therefore this paper focuses on a rigid fixed-wing aerodynamics at low Reynolds numbers.

The effects of airfoil shape on low Reynolds number aerodynamics using two airfoils, namely, the SD7003 and the Ishii airfoil, are investigated. The large-eddy simulations are performed with a sixth-order compact finite difference scheme for computing the spatial derivatives of convective and viscous terms, metrics, and Jacobian and tenth-order low pass filter for eliminating spurious components, and second-order backward implicit time integration with inner iterations. Systematic numerical excesses show the feasibility of current simulations to predict flow fields around fixed-wing configurations involving a laminar separation and laminar-to-turbulence transition at low Reynolds number. At the Reynolds number of 2.3x10⁴, two types of thin and asymmetric airfoils are considered. The results show that the upper surface of airfoil shape affects the formation of an laminar separation bubble (LSB) and the transition to turbulence in the three-dimensional flow around the wings and the lower surface of the airfoil shape does not significantly affect the flow structures around the airfoil, however, it can help to increase lift generation if its shape is designed appropriately. Therefore significant influence in the aerodynamic performance is observed.

1

T.J. Mueller, (Editor), "Fixed and Flapping Wing Aerodynamics for Micro Air Vehicle Applications", American Institute of Aeronautics and Astronautics, Inc., Reston, VA, U.S.A., 2001.

2

W. Shyy, Y. Lian, J. Tang, D. Viieru, H. Liu, "Aerodynamics of Low Reynolds Number Flyers", Cambridge University Press, New York, NY, U.S.A., 2008.

3

F.W. Schmitz, "The Aerodynamics of Small Reynolds Number", NASA TM-51, 1980.

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