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



On Tue, Jun 24, 2008 at 9:03 PM, Michael Habib <mhabib5@jhmi.edu> wrote:
>  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.

It is *reducing* the turbulence *behind*.

> 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).

In most cases not directed to behind (except, for exemple, humming
birds that could fly backwards).

>> (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.

Not the cross section shape - ok, if you have a cross section shape
that is circular it will not work. But as far as you note that the
curvature could be greater in the bottom face of the wing (the
negative or reverse camber), we could dismiss the widespread talk that
the air flowing faster over upper face of the wing than over the
bottom face creates a differential pressure (by Bernouilli effect)
that creates lift.

[]s,

Roberto Takata