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

[Michael Habib <mhabib5@jhmi.edu>]

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