Now We Know How Birds Fly
Elementary physical science students know how airplane wings generate lift, but bird flight poses special challenges. The aptly-named swifts, for instance, can practically turn on a dime, dive steeply, and halt in mid-air to catch insects in ways that make a stunt pilot stall. It’s not just flapping, and it’s not just leading-edge feather shape, say some Netherlands scientists publishing in Science1 this week; birds generate leading-edge vortices [LEVs] that provide additional lift and drag for their skillful aerobatics. Instead of using wind tunnels, the researchers figured this out with experiments in water tunnels.
The current understanding of how birds fly must be revised, because birds use their hand-wings in an unconventional way to generate lift and drag. Physical models of a common swift wing in gliding posture with a 60° sweep of the sharp hand-wing leading edge were tested in a water tunnel. Interactions with the flow were measured quantitatively with digital particle image velocimetry at Reynolds numbers realistic for the gliding flight of a swift between 3750 and 37,500. The results show that gliding swifts can generate stable leading-edge vortices at small (5° to 10°) angles of attack. We suggest that the flow around the arm-wings of most birds can remain conventionally attached, whereas the swept-back hand-wings generate lift with leading-edge vortices.
The arm-wings of birds (i.e., the parts near the body) have the conventional airplane-wing shape, but the swept-back hand-wings of swifts and some other birds create the delta-wing jet-fighter look. The leading-edge feathers on these hand-wings are sharp and generate little conical tornados sweeping back from the wing tips that add lift and drag:
LEVs are robust, lift-producing aerodynamic flow systems allowing high angles of attack. At high angles of attack, the drag component of the aerodynamic force is large. We assume that swifts take advantage of the high lift as well as the high drag component of the LEVs to increase their agility in flight. They can, for example, use the high angle-of-attack LEVs to brake in midair without losing height immediately, as they do while catching insects in flight.
The authors do not discuss how this wing system evolved, but in the same issue of Science,2 Müller and Lentink mention that insects have usually been considered the “masters of unconventional lift.” Since birds now are also seen to have “caught onto the same trick,” they suggest that birds, insects and fighter-jet designers have something in common with Darwin: “evolution and aeronautic engineering converged on the same solution—variable wing sweep.”
1Videler, Stamhuis and Povel, “Leading-Edge Vortex Lifts Swifts,” Science, Vol 306, Issue 5703, 1960-1962, 10 December 2004, [DOI: 10.1126/science.1104682].
2Ulrike K. Müller and Lentink, “Enhanced: Turning on a Dime,” Science, Vol 306, Issue 5703, 1899-1900, 10 December 2004, [DOI: 10.1126/science.1107070].
Don’t let that last sentence ruin your day. Watch the Blue Angels, and watch the swifts at dusk sweeping in formation through the air. Enjoy the fruits of intelligent aeronautical engineering design. Maybe the designers of F16 tactical fighter aircraft someday will converge on the solution of building nests under the eaves of a barn, and reproducing exact copies that can grow, sing and catch fast-darting objects in mid-flight.