Gorgeous new research highlights some of the differences between fixed-wing flight and birds. Researchers trained a barn owl, tawny owl, and goshawk to glide through a cloud of helium-filled bubbles illuminated by a light sheet. By tracking bubbles’ movement after the birds’ passage, researchers could reconstruct the wake of these flyers.
As you can see in the animations above and the video below, the birds shed distinctive wingtip vortices similar to those seen behind aircraft. But if you look closely, you’ll see a second set of vortices, shed from the birds’ tails. This is decidedly different from aircraft, which actually generate negative lift with their tails in order to stabilize themselves.
Instead, gliding birds generate extra lift with their maneuverable tails, using them more like a pilot uses wing flaps during approach and landing. Unlike airplanes, though, birds rely on this mechanism for more than avoiding stall. It seems their tails actually help reduce their overall drag! (Image and research credit: J. Usherwood et al.; video credit: Nature News; submitted by Jorn C. and Kam-Yung Soh)
Bilbo
Very interesting. Light “small” planes are not a curiosity. Airbus is planing to replace satelites with flying drones, much cheaper to build and mantain, and more eficient for telecomunication networks.
Any small discovery can be a huge step ahead.
Nicole Sharp
No, they’re not a curiosity. I have a whole tag’s worth of posts dedicated to them: https://fyfluiddynamics.com/tagged/micro-air-vehicles/
Jason Hackl
Airplane horizontal stabilizers generated negative lift because doing so stabilizes the airplane when 1.) the center of gravity is ahead of the center of pressure and 2.) when the wing itself has a negative pitching moment. Does the article address any further tricks the owl uses for pitch stability with a (destabilizing) lifting tail?
Nicole Sharp
Here’s what the authors have to say about the function of the tails of these birds:
“We conclude, therefore, that the tail does not contribute to passive pitch stability with a longitudinal dihedral mechanism but, in addition to its role in moment generation when manoeuvring (e.g. Gillies et al., 2011), acts as an aerodynamic wing ‘flap’, expanding the aerodynamic planform area. However, whereas aircraft flaps are required for stall avoidance and increase drag, bird tails produce aerodynamic lift even when not near a stall limit, and act to reduce overall drag at low Reynolds numbers.”
Basically they conclude that birds use their tails very differently than we do with airplanes. Some of that has to do with a bird’s ability to change geometry quickly and effectively (compared to an airplane) and some of it is driven by the fact that birds benefit more from reducing viscous drag since they fly at much lower Reynolds numbers than conventional aircraft.