Hello Julian
The feather lifting represents the suction, due to vortical flow posterior
to the separation of flow on the upper surface. It CAN result in breaking up
the reverse flow, if kept up long enough ( probably not as it occurs very
briefly)
Whenever this occurs, it effectively changes the profile, making it thicker.
Thicker profiles have ( again , in steady flow) the characteristic of
smoother stall, i.e. the lift does not drop as much when drag starts going
up. The little feathers, in mid wing also start oscillating, which will
again promote turbulence and reduce separation.
Unfortunately, these generic statements are proven , as I said before, for
steady flow. It would be instructive to see PIV data close to the feathers.

Danny Weihs

----- Original Message -----
From: Julian Vincent <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, March 24, 2003 2:46 PM
Subject: Stall of birdwing when landing


> We have a project looking at compliant airofoils on a compliant mounting
> (stiff fuselage/tail assembly).  This has raised the following questions:
>
> When a bird comes in to land, just before it finally drops on to its
> perch it more or less stalls.  At this point the feathers on the
> trailing edge lift.  What does this represent in terms of flow around
> the wing section?
> Are the feathers giving a conformable upper surface (presumably yes)?
> Does this retain laminar flow over the upper surface despite the
> apparent stall condition (presumably also yes, but what's the detail)?
> What's happening to air flow beneath the lifted feathers?  Is it
> turbulent; does the gap between the lifted feathers and the rest of the
> wing get occupied by air leaking around the trailing edge from the lower
> surface, or what?
>
> Anyone got any references??
> --
> Julian FV Vincent
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>
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