Abstract
The flight performance of animals depends greatly on the efficacy with which they generate aerodynamic forces. Accordingly, maximum range, load-lifting capacity and peak accelerations during manoeuvres are all constrained by the efficiency of momentum transfer to the wake. Here, we use high-speed particle image velocimetry (1 kHz) to record flow velocities in the near wake of desert locusts (Schistocerca gregaria, Forskål). We use the measured flow fields to calculate time-varying span efficiency throughout the wing stroke cycle. The locusts are found to operate at a maximum span efficiency of 79 per cent, typically at a plateau of about 60 per cent for the majority of the downstroke, but at lower values during the upstroke. Moreover, the calculated span efficiencies are highest when the largest lift forces are being generated (90% of the total lift is generated during the plateau of span efficiency) suggesting that the combination of wing kinematics and morphology in locust flight perform most efficiently when doing the most work.
Highlights
Any flying device, whether it is an insect, a bird, a bat or a man-made aircraft, generates lift with a certain efficiency
particle image velocimetry (PIV) has been used in several studies of aerodynamics in animal flight over the last decade, but recently techniques have been improved by developments in high-speed cameras, laser equipment and software
A batch of 25 adult desert locusts were obtained from a breeder (Livefoods Direct, UK) and five of these were chosen for experiments based on their flight ability, wing condition and overall vigour
Summary
Whether it is an insect, a bird, a bat or a man-made aircraft, generates lift with a certain efficiency. There are two ways of determining the effect of an uneven downwash distribution on span efficiency, either from actuator disc theory [2,3] or lifting line theory [4] The former leads naturally to the induced power factor, kind, and the latter to the inviscid span efficiency, ei, but one is the reciprocal of the other (for a thorough description of these, see [5], and for how they are related, see [6]). [6,7,8,9]) These models have assumed an even induced flow and elliptical loading distribution and, latterly, to account for the fact that the wings of the flying animal were performing less well than an ideal wing, the induced power factor, kind, has been included to adjust the estimate of the animals’ performance. Time-resolved wake measurements are possible, so-called because the sampling frequency is several times greater than the wingbeat
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