The aerodynamic and structural study of flapping wing vehicles
This thesis reports on the aerodynamic and structural study carried out on flapping wings and flapping vehicles. Theoretical and experimental investigation of aerodynamic forces acting on flapping wings in simple harmonic oscillations is undertaken in order to help conduct and optimize the aerodynamic and structural design of flapping wing vehicles. The research is focused on the large scale ornithopter design of similar size and configuration to a hang glider. By means of Theodorsen’s theory the aerodynamic forces on a thin aerofoil subject to heaving, pitching, and combined heaving and pitching motions are carefully studied. The analytical method is then employed to calculate the lift acting on the rigid flat plate undergoing small simple harmonic oscillations at different airspeeds and frequencies. The theoretical calculations are compared with experimental results which show reasonably good agreement. However experimental study shows that the wing frame deformation induces extra aerodynamic forces which can change the overall wing performance. Hence an experimental investigation focusing on wing flexibility effect on aerodynamic forces is also carried out. Three wings of similar planform geometry but slightly different degree of flexibility are manufactured for wind tunnel testing. Test results show that the wing deformation not only affects the aerodynamic forces but also the required power for various wing flapping motions. By understanding the aerodynamic performance of flapping wings from both theoretical and experimental studies the preliminary design of large scale ornithopter is carried out based on a hang glider prototype. Theoretical and experimental studies are carried out to validate aerodynamically the loading acting on the wing and finite element analysis is carried out to evaluate the structural strength. In addition, DeLaurier’s method is employed to calculate the aerodynamic forces of flapping wings by taking into account of the wing aspect ratio. The test results show good agreement with the theoretical calculation by DeLaurier’s method. However the FEA results indicate structural failure based on the original calculation by assuming the wing is completely rigid. Modification of aerodynamic modelling is carried out to reassess the structural strength by taking into account of the wing deformation and possible effect due to large angle of attack which shows a much more reasonable stress distribution on the entire wing structure without failure. Furthermore three wing planform and structural modifications are carried out to improve the aerodynamic performance of the flapping wing. Finally the folding wing design case is selected as the optimal design which produces the highest overall positive lift and a variable geometric system is employed to control the folding motion of the wing.
TA Engineering (General). Civil engineering (General)
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