Unsteady aerodynamics of flapping flight

Abstract

In the thesis we present a study on the unsteady aerodynamics of insect flight. Motivation for the study is to establish the rational for adopting unsteady aerodynamics principles in the design of Micro Air Vehicles (MAV) and to develop engineering design guidance for the same. Unlike an aircraft with fixed wing or a propeller, birds and insects reciprocate their wings changing the wing direction and the pitch twice in each stroke. Birds and insects generate lift based on unsteady aerodynamic principles. Birds in flight, unlike many insects, not only change their wing direction and pitch but also change the wing area and wing porosity during a stroke. Thus an insect flight is an order less complex for adaptation to a MAV application compared to the bird flight and hence we restrict our discussion here only to insect flight. Study of insect flight is challenging as it involves measurement of small forces cycling at high frequency. Typical forces in the case of a tiny insect like fruit fly will be a fraction of mg cycling at 2()0Hz or more and at the other end of spectrum, for big insects, like butterfly and dragonfly forces will be in the range of 1-2 gm cycling at 5-40Hz. Study of wing kinematics is also challenging as we need to get at least two simultaneous-views of a flying-insect at high frame rate (frame rate has to be at least 10 to 15 times more than the wing beat frequency). From these two views, we have to workout actual orientations of the wing and its variation in time. Keeping these factors in mind, we have developed a two-pronged approach for the study of insect flight dynamics. In one approach, we make a mechanical model, to mimic some salient features of insect flight and carryout flow visualization. In the second approach we are developing tools to study the wing kinematics of an insect while in flight. The thesis has following outline. In the first chapter, we describe some salient features of unsteady aerodynamics principles, low Reynolds number flight, how insect uses unsteady aerodynamics principles for its flight, desirability of adopting insect flight for MAV design. In this chapter we also give scope of the present work In the second chapter we present the design of one-degree freedom model, mimicking insect flight, control system used to drive this model, design of flow visualization setup, test section, flow visualization pictures and discussion based on the flow visualization results. In the third chapter we present some of the preliminary arrangements developed to get wing-kinematics of an insect while in flight. This includes camera system for getting simultaneous front and top view of a flying insect in a wind-tunnel. As the actual flight speed of insect is about I m/s, we have to keep low wind speeds in the wind tunnel and in this chapter we present a procedure for cahbrating anemometer in low wind speed regime. In the fourth chapter we present conclusions and scope for future work.