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Re: Archie a non-flyer? (was:Re: origin of bats/reply 2 to TMK)



On Tuesday, June 24, 2008, at 04:53  PM, Roberto Takata wrote:

Not across the feather, but behind the feather.

You could test it with a paper sheet. Hold it horizontally with your
fingertips at midline. Push the sheet through the air. The front
middle will curl creating a turbulent airflow that cause the drag -
you will feel the air resistance to the paper motion.

The property you are referring to is called flutter, and is especially important in thin foils with substantial aeroelastic properties. The position of the shaft in an avian feather relates to producing the proper contour as well as resisting torsional moments related to the rather anterior center of lift (it's at 25% of chord for an uncambered thin foil). What Jim was saying (quite accurately) is that the air flow over an avian wing isn't laminar as it is, so the shape isn't preventing turbulence. In fact, birds use several tricks to add momentum to the flow, rather than reduce it (especially owls, which have a leading edge structure adapted to produce turbulence in the boundary layer). The trick is the scale of the turbulence in the flow: a turbulent boundary layer will tend to stay attached to the wing surface (which is helpful), while massive flutter causes separation of the flow from the wing and associated large-scale turbulent wakes (and thus a sudden loss of lift and the ground rushing up rather quickly to offer its salutations).


(By the way, many people say that the plane wing cross section shape
create lift, but it is not true, it just reduces drag. What create
lift is the angle of attack, if the shape of the wing was responsible
for the lift, then planes could not fly upside down.)

The camber of the wing cross section increases effective angle of attack, actually. More importantly, the tapered shape forces a stagnation point at the trailing edge, which creates circulation in the air flow. The circulation, superimposed on the transverse flow, produces the lift over the wing (and is balanced by shed vortices behind the wing). So the shape of a wing is critical to lift formation, but camber is not required. Uncambered wings do indeed still work (used in a number of aircraft, including stunt planes), and some aircraft (supersonic ones, to be precise) have wings with an effective reverse camber (bottom side is slightly more convex than upper surface). In the speed regime utilized by flying animals, a cambered foil is generally advantageous (though camber varies substantially across taxa).


Cheers,

--Mike


Michael Habib, M.S. PhD. Candidate Center for Functional Anatomy and Evolution Johns Hopkins School of Medicine 1830 E. Monument Street Baltimore, MD 21205 (443) 280 0181 habib@jhmi.edu