RT Journal Article
SR Electronic
T1 Aerodynamics of gliding flight in common swifts
JF The Journal of Experimental Biology
JO J. Exp. Biol.
FD The Company of Biologists Ltd
SP 382
OP 393
DO 10.1242/jeb.050609
VO 214
IS 3
A1 Henningsson, P.
A1 Hedenström, A.
YR 2011
UL http://jeb.biologists.org/content/214/3/382.abstract
AB Gliding 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. Ameasured wake area behind bodyARwing aspect ratiobwwingspanbttail spanbt,waketail wake spanbw,maxmaximum wingspanbw,obsobserved wingspanbw,wakewing wake spancmean wing chordCDdrag coefficientCD,parparasite drag coefficientCD,proprofile drag coefficientCLlift coefficientDdragDindinduced dragDparparasite dragDproprofile dragggravitational accelerationkinduced drag factorLliftmass flow ratemmass of the birdReReynolds numberSbbody frontal areaSwwing areaUairspeedU∝freestream flowUbairspeed behind bodyUbgairspeed of best glideUmsairspeed of minimum sinkUssink speedUtruetrue airspeedWweight of the birdΓcirculationΓwaverage circulation of the wingtip vortexΓtaverage circulation of the tail vortexθglide angle relative to horizontalνkinematic viscosityρair density