Berry:
On Sat, 13 Sep 1997, Robert BERRY wrote:
> WM James Davis wrote:
> >
> > To anyone who can help:
> >
> > When I took ornithology class some years ago, it was explained that the
> > lift generated from a bird's wing (secondary feathers) during gliding
> > flight was due to the same forces that lift an aeroplane's wing........
>
>
> It is quite a while since this was posted and unfortunately the thread
> does'nt seem
> to have got off the ground. Flight in birds is more difficult to break down
> than in
No humor please <grin>.
>
> But the short answer is that birds and planes stay up cos their wings deflect
> air
> down.
>
After much thought I have come to the conclusion that such a short answer
does little to explain how the wing of a bird generates lift (your long
answer is more on the money in my opinion). Normally, I wouldn't have
given such a short answer the time of day if it wasn't for the fact that
Gill, in his highly respected ornithology textbook, used the short answer
as his only explanation. This made me wonder if my understanding was out
of date, flawed, etc.
If we can exclude from this discussion the subtantial amount of lift
and propulsion generated by the primary feathers, then I must conclude
that 2/3 of the lift of the inner wing is generated by negative pressure
from above and the rest is generated by positive pressure pushing from
below due to a positive angle-of-attack. Is this what you are refering to
when you say "deflected down"? (I agree that airspeed is also a
relevent factor.)
There is still the intriguing possiblity that the laminar air-flow
hugging the top surface of the wing also generates some downward
thrush at the trailing edge because this airstream is moving faster than
the air surrounding it and because the trailing edge is slant downward. If
it does, however, I fail to understand what it could be pushing against to
cause any lift. Unfortunately, I have not beable to find any literature
that supports this idea.
One of the participants of this listserve suggested that the wing-tip
vortices generated during gliding flight also generates lift, I have also
failed to substantiate this claim -- birds flying in a V formation use the
energy trapped in such a vortex generated by the bird flying ahead of it,
but for a single bird flying alone the wing-tip vortices either
produces drag or degrades the pressure differential between the
top and bottom of the wing that in turn reduces the lift, etc.
I have probably said more than enough, sorry for being so long winded.
Thanks for the input.
Cheers, Jim
|
