M.Sc Thesis
M.Sc Student Keren Yuval Tests on a Hovering Flapping Wing Department of Aerospace Engineering Professor Haim Abramovich Professor Rimon Arieli

Abstract

For several decades, men have been designing and flying micro air vehicles. Due to their small dimensions and low forward speed, MAVs fly at low Reynolds numbers. Most studies up to date, designing insect inspired flapping wings, used an elastic membrane stretched within a rigid frame. In the present study we had designed a hybrid wing structure; the wing’s geometry resembled a rotor blade, and the wing’s flexibility resembled an insect’s flapping wing. The wing was designed to be flexible in twist, and thus to maintain the aeroelastic advantages of a flexible wing, while rigid in the spanwise direction. The use of “thick” airfoil made it possible to achieve higher strength to weight ratio by increasing the wing’s moment of inertia.

To achieve an optimal design, we had developed a simplified quasi-steady inviscid mathematical model that describes the aerodynamic and inertial behavior of the flapping wing. We built a flapping mechanism that imitate the insects’ flapping pattern, and conducted a set of experiments for various parameters. We then corrected our mathematical model according to the tests results, and thus compensating for the viscid increase of drag and decrease of lift, that was neglected in the calculations. We had used this model to calculate the propelling efficiency of the hovering wing at different design parameters. We finally validated our model on a second set of experiments based on a smaller wing flapping at a higher frequency.

The propelling efficiency of the designed wing was less than the efficiency of a rotating wing of the same size, due to the high inertial moment required to accelerate and decelerate the wing during each stroke. However, as the wing span decreases, the efficiency increases exponentially. Unlike stationary wings, our model shows that smaller Aspect Ratio increases the propelling efficiency of the flapping wing. We had found that the flapping frequency has no effect on the propelling efficiency. On the other hand, an enhancement of the flapping stroke amplitude improves the propelling efficiency, as the mean lift increases for the same inertial moment. This enhancement is practically difficult to achieve using the four-bar linkage mechanism usually used for flapping wings today, and may be a goal for future development.

A direction for future research would be to manufacture a shorter wing with low Aspect Ratio and to measure its performances. A different flapping mechanism should be designed, which will enable larger stroke amplitude, and will enhance the lift using the clap-and-fling mechanism.