Aerodynamic power and mechanical efficiency of bat airframes using a quasi-steady model

Abstract

Previous bat aerodynamic power models are refined by (1) varying the value of wing profile drag with lift coefficient, which varies with both flight speed and Reynolds number, (2) allowing for the aerodynamic cleanliness of head, body, ears and tail in calculating parasitic drag values at various speeds and according to airframe type, (3) incorporating models of wingbeat amplitude and frequency in the power calculations, and, (4) upgrading the allometric, phylogenetically corrected relationship between basal metabolic rate and body mass using data from 98 bat species. The fidelity of the aerodynamic power model is assessed using published wind tunnel data on a bat in steady glide. By comparing empirical published metabolic power (Pmet) values with values derived using the new aerodynamic model, we update estimates of inflight musculoskeletal mechanical efficiency (g) for the airframes of eight bat species at steady level flight speeds. Furthermore, we calculate the increase in g at high speeds. The bats assessed range in body mass from 0.01 to 1 kg, and the comparison covers the speed range normally used by free-flying bats during their excursions. At their best endurance flight speeds (Vend), g — 1.52 Ln (mbat) +11.44 (%). At speeds > Vend, η — η@Vend+ 1.3 (V— Vend) (%). These equations yield accurate Pmet estimates for flight speeds within the range used for the steady level flight.