Aerodynamics of gliding flight in common swifts

Abstract

SUMMARYGliding flight performance and wake topology of a common swift (Apus apus L.) were examined in a wind tunnel at speeds between 7 and 11ms -1 . The tunnel was tilted to simulate descending flight at different sink speeds. The swift varied its wingspan, wing area and tail span over the speed range. Wingspan decreased linearly with speed, whereas tail span decreased in a nonlinear manner. For each airspeed, the minimum glide angle was found. The corresponding sink speeds showed a curvilinear relationship with airspeed, with a minimum sink speed at 8.1ms-1 and a speed of best glide at 9.4ms -1. Lift-to-drag ratio was calculated for each airspeed and tilt angle combinations and the maximum for each speed showed a curvilinear relationship with airspeed, with a maximum of 12.5 at an airspeed of 9.5ms -1. Wake was sampled in the transverse plane using stereo digital particle image velocimetry (DPIV). The main structures of the wake were a pair of trailing wingtip vortices and a pair of trailing tail vortices. Circulation of these was measured and a model was constructed that showed good weight support. Parasite drag was estimated from the wake defect measured in the wake behind the body. Parasite drag coefficient ranged from 0.30 to 0.22 over the range of airspeeds. Induced drag was calculated and used to estimate profile drag coefficient, which was found to be in the same range as that previously measured on a Harris' hawk.

Aerodynamic performance of the feathered dinosaur Microraptor and the evolution of feathered flight

“…Unsteady trajectory performance, rather than aerodynamic efficiency, seems to have been the main selective pressure optimised by Microraptor's flight style. Although comparative data from living birds 26 show that (if our fossilbased calculations are correct) the wing loading (ca. 50 N m À 2 ) of Microraptor is almost exactly at the mean for birds of the same weight, its L/D was much lower (many birds have L/D in the range 10-12 (ref.…”

“…50 N m À 2 ) of Microraptor is almost exactly at the mean for birds of the same weight, its L/D was much lower (many birds have L/D in the range 10-12 (ref. 26), whereas we calculate this for Microraptor at 4.7 (:1)). This must be, in part, because of the fact that (as we show) Microraptor's planform had an intrinsically less efficient configuration compared with living birds and also because it had to operate in a low-L/D region of its flight envelope in order to be aerodynamically stable.…”

Span efficiency in hawkmoths

“…If we compare these results from the moths with those of the desert locust-which was analysed using the same method [9]-all but D. elpenor have lower values of e i than the locusts. Furthermore, the value of e i ¼ 0.83 (k ¼ 1.2), which has been used as rule of thumb before in various models of animal flight [5,[26][27][28][29][30], is likely to be a large overestimate of the aerodynamic performance of insects.…”

Wake structure and kinematics in two insectivorous bats

“…[16 -18]). Similarly, some aspects of wake architecture may be characteristic of insects, birds or bats [7], but there is variation within each group that appears to be associated with kinematics, wing morphology or both (insects: hawkmoths [19] versus locusts [19,20]; birds: blackcaps [21] versus swifts [22]; bats: Pallas' long-tonged bats [23] versus Brazilian free-tailed bats [24]). …”