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EstimatingtheAirspeedVelocityofanUnladenSwallow.pdf

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EstimatingtheAirspeedVelocityofanUnladenSwallow

Estimating the Airspeed Velocity of an Unladen Swallow Hashing out the classic question with Strouhal numbers and simplified flight waveforms. After spending some time last month trying to develop alternate graphic presentations for kinematic ratios in winged flight, I decided to try to answer one of the timeless questions of science: just what is the airspeed velocity of an unladen swallow? What do you mean, an African or European Swallow? To begin with, I needed basic kinematic data on African and European swallow species. Although 47 of the 74 worldwide swallow species are found in Africa,1 only two species are named after the continent: the West African Swallow (Hirundo domicella) and the South African Swallow (Hirundo spilodera), also known as the South African Cave Swallow. Since the range of the South African Swallow extends only as far north as Zaire,2 I felt fairly confident that this was the non-migratory African species referred to in previous discussions of the comparative and cooperative weight-bearing capabilities of African and European swallows.3 Kinematic data for both African species was difficult to find, but the Barn or South African Swallow (Hirundo spilodera) European Swallow (Hirundo rustica) European Swallow (Hirundo rustica) has been studied intensively, and kinematic data for that species was readily available. It’s a simple question of weight ratios A 54-year survey of 26,285 European Swallows captured and released by the Avian Demography Unit of the University of Capetown finds that the average adult European swallow has a wing length of 12.2 cm and a body mass of 20.3 grams.4 Because wing beat frequency and wing amplitude both scale with body mass,5 and flight kinematic data is available for at least 22 other bird species,6 it should be possible to estimate the frequency (f ) and amplitude (A) of the European Swallow by a comparison with similar species. With those two numbers, it will be possible to estimate airspeed (U). In order to ma