title: Hummingbird wing efficacy depends on aspect ratio and compares with helicopter rotors.
abstract: Hummingbirds are the only birds that can sustain hovering. This unique flight behaviour comes, however, at high energetic cost. Based on helicopter and aeroplane design theory, we expect that hummingbird wing aspect ratio (AR), which ranges from about 3.0 to 4.5, determines aerodynamic efficacy. Previous quasi-steady experiments with a wing spinner set-up provide no support for this prediction. To test this more carefully, we compare the quasi-steady hover performance of 26 wings, from 12 hummingbird taxa. We spun the wings at angular velocities and angles of attack that are representative for every species and measured lift and torque more precisely. The power (aerodynamic torque × angular velocity) required to lift weight depends on aerodynamic efficacy, which is measured by the power factor. Our comparative analysis shows that AR has a modest influence on lift and drag forces, as reported earlier, but interspecific differences in power factor are large. During the downstroke, the power required to hover decreases for larger AR wings at the angles of attack at which hummingbirds flap their wings (p < 0.05). Quantitative flow visualization demonstrates that variation in hover power among hummingbird wings is driven by similar stable leading edge vortices that delay stall during the down- and upstroke. A side-by-side aerodynamic performance comparison of hummingbird wings and an advanced micro helicopter rotor shows that they are remarkably similar.
authors: Mechanical Engineering, Stanford University, 416 Escondido Mall, Stanford, CA 94305, USA Experimental Zoology Group, Wageningen University, PO Box 338, 6700 AH Wageningen, The Netherlands.
authors: Jan W
authors: JW
authors: Kruyt
authors: Experimental Zoology Group, Wageningen University, PO Box 338, 6700 AH Wageningen, The Netherlands.
authors: Elsa M
authors: EM
authors: Quicazán-Rubio
authors: Physics Department, Eindhoven University of Technology, PO Box 516, 5600 MB Eindhoven, The Netherlands.
authors: GertJan F
authors: GF
authors: van Heijst
authors: Department of Zoology, University of British Columbia, 4200-6270 University Boulevard, Vancouver, British Columbia, Canada V6T1Z4.
authors: Douglas L
authors: DL
authors: Altshuler
authors: Mechanical Engineering, Stanford University, 416 Escondido Mall, Stanford, CA 94305, USA dlentink@stanford.edu.
authors: David
authors: D
authors: Lentink
mesh_terms: D000401:Aircraft / Q000295:instrumentation; D000818:Animals; D001696:Biomechanical Phenomena; D001717:Birds / Q000502:physiology*; D005426:Flight, Animal / Q000502:physiology*; D014921:Wings, Animal / Q000502:physiology*
keywords: aspect ratio; hovering; hummingbird; performance; quasi-steady; wing