Lift and power in fruitflies in vertically-ascending flight

Effect of passive wing pitching on flight control in a hovering model insect and flapping-wing micro air vehicle

Passive wing pitching is a hypothesis in insect flight, and it is used widely by most flapping-wing micro air vehicles (FWMAVs). This study analyses the flight control of hovering model fruit fly and FWMAV with passive pitching wings. The longitudinal and lateral control derivatives are obtained by numerical simulation of the fluid dynamic equations coupled with the torsional spring passive pitching system. In contrast to active pitching wings, some of the control derivatives are remarkably changed by passive pitching wings, such asZ_Φ(vertical force produced by unit stroke amplitude),Z_f(vertical force produced by unit flapping frequency), andMψ0(pitching moment produced by unit rest angle). For example, increasing flapping frequency does not lead to an evident increase in lift and may even have a reverse effect. Therefore, the flight control of FWMAV with passive pitching wings should be treated with caution. For wings pitching passively with a torsional spring at the root, the differential change of the angle of attack in the downstroke and upstroke (α_dandα_u) could be achieved by modulation of the rest angle alone; however, the equal change inα_dandα_umay require an otherwise manipulation of the elastic coefficient. Results in this study provide guidelines for the design of FWMAVs in evaluating the effects of different control inputs correctly and formulating a cost-effective control scheme.

Wing kinematic and aerodynamic compensations for unilateral wing damage in a small phorid fly

To investigate the way in which very small insects compensate for unilateral wing damage, we measured the wing kinematics of a very small insect, a phorid fly (Megaselia scalaris), with 16.7% wing area loss in the outer part of the left wing and a normal counterpart, and we computed the aerodynamic forces and power expenditures of the phorid flies. Our major findings are the following. The phorid fly compensates for unilateral wing damage by increasing the stroke amplitude and the deviation angle of the damaged wing (the large deviation angle gives the wing a deep U-shaped wing path), unlike the medium and large insects studied previously, which compensate for the unilateral wing damage mainly by increasing the stroke amplitude of the damaged wing. The increased stroke amplitude and the deep U-shaped wing path give the damaged wing a larger wing velocity during its flapping motion and a rapid downward acceleration in the beginning of the upstroke, which enable the damaged wing to generate the required vertical force for weight support. However, the larger wing velocity of the damaged wing also generates larger horizontal and side forces, increasing the resultant aerodynamic force of the damaged wing. Due to the larger aerodynamic force and the smaller wing area, the wing loading of the damaged wing is 25% larger than that of the wings of the normal phorid fly; this may greatly shorten the life of the damaged wing. Furthermore, because the damaged wing has much larger angular velocity and produces larger aerodynamic moment compared with the intact wing of the damaged phorid fly, the aerodynamic power consumed by the damaged wing is 38% larger than that by the intact wing, i.e., the energy distribution between the damaged and intact wings is highly asymmetrical; this may greatly increase the muscle wastage of the damaged side.