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Mechanics and energetics of ground effect in flapping flight Cueva Salcedo, Horacio Jesus de la


Ground effect (GE) is an interaction between a wing and a surface that increases lift and reduces induced drag, stalling, minimum power, and maximum range speeds (Vstall, Vmp,Vmr, respectively). Four bird species utilising GE during flapping flight were studied: Double-Crested Cormorants (Pholacrocoraz auritus) at Mandarte Island B.C., Brown Pelicans (Pelecarius occideniali.s) at Ensenada Méico, Black Skimmers (Rynchops niger) at San Diego California, and Barn Swallows (Hirundo rstica) at Ladner, Williams Lake, and English Buff, B.C. Films were taken at 60—64 Hz in nature and digitised for vertical wing displacement analysis. Flight speeds of swallows and cormorants were measured with a Doppler radar. Best-fit sinusoids of dicular-linear regression (Batschelet 1981), periodic ANOVA (Bliss 1970), and Fourier series (Lighthill 1958, Bloomfield 1976) were used to describe vertical wing movement. Linearised sinusoids were used to show vertical wing displacement and average height of wings above the surface. Methods of evaluating GE for fixed wings were compared to determine simple, realistic calculations for flapping flight. Average interference coefficients were used to evaluate the influence of GE, utilising the theory of Reid (1932). Results were compared to those for oscillating (Katz 1985 ) and banking wings (at an angle to the horizontal) (Binder 1977). The effect of GE on the daily energy balance (DEB) was evaluated in cormorants. DEBs were constructed considering GE, metabolic, reproductive, and fight costs. Power curves were constructed using fixed-wing quasi-steady aerodynamic theory (Pennycuick 1975). Flight speeds Vmp=10.9 ms-1 out of ground effect (OGE), 6 ms-1, in GE (IGE), arid Vmr=16.3 ms-1 OGE, 15.4 ms-1 IGE and flight costs IGE and OGE of cormorants were compared to speeds measured during the 1987 and 1988 reproductive seasons. Cormorants observed at Mandarte Island obtain savings of up to 40% of total power when flying in GE. If cormorants fly with a fixed cargo per trip (0.3 kg) from the feeding site to the nest the predicted number of trips per day is 3—4 as observed by Robertson (1971). Cost of Transport (cost of moving a unit weight a unit distance) was compared for flights IGE and OGE, with and without a load. Foraging radius (Pennycuick, 1979) calculates maximum distance travelled on available energy. Foraging radii ICE and OGE of double-crested cormorants limit potential food sources. Flight IGE is significantly cheaper than OGE for the double-crested cormorant. Flight energy expenditures were compared for the four species. Swallows and skimmers show savings of up to 50% when IGE, but most savings are obtained only at speeds 15 ms-1 to 20% at speeds

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