The Significance of Ground Effect to the Aerodynamic Cost of Flight and Energetics of the Black Skimmer (Rhyncops Nigra)

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Abstract
The kinematics and aerodynamics of flapping and gliding flight by the black skimmer were investigated to evaluate the significance of ground effect to the foraging and daily energy budget of skimmers. Ground effect is an increase in lift and decrease in drag of an aerofoil when close to the ground. The duration of upstroke and downstroke, the wing movements and the pronation/supination of the wings during flapping flight of the skimmer are similar to other birds. Wing-beat frequency was 3.1 s-1 and flight velocity was 9.1 m s-1. The wing stroke was markedly asymmetric, with the majority of the stroke occurring above the plane of the body. During skimming, wing beats are intermittent and of low amplitude; flight velocity is 10.3 m s-1. Induced power, parasite power and profile power of skimmers were calculated after Tucker (1973) in the absence of ground effect, and the glide angle and sink velocity were calculated for gliding skimmers. Ground effect was shown to markedly reduce induced power requirements, and hence total power requirement, of flapping flight, and reduce the glide angle and sink velocity during gliding. The hydrodynamic drag of the lower mandible was estimated to be 10-4 N, which is insignificant compared to the total aerodynamic drag (0.4 N). Ground effect was shown to markedly increase foraging efficiency and facilitate the attainment of a positive daily energy balance. The significance of ground effect to other flying vertebrates was discussed. Note: Department of Zoology, University of Cape Town, Cape Town, Republic of south Africa.