Re: alula

["Patrick Norton" <ptnorton@email.msn.com>]

Jim C wrote, in part:

>a 3.3% increase in lift.  The actual increase would be expected to be less
for several reasons.  An alula can work in this fashion, but not very
effectively, and the slotted scenario doesn't appear to describe the actual
function of the mechanism.<

Interesting calculations. My references describing the alula's function as a
slot are at work, but I'll post them to Jim C next week.

Although I didn't specifically mention lift in my earlier post, it is
certainly reasonable to expect that some increase in lift would result from
the alula acting as a slot. But I would caution against assuming that the
result must be large to be effective.  Birds use their alula only very
briefly and only at the point of stall--when the airstream begins to
separate from the upper (lee) surface of the wing due to the turbulence
created by a large angle of attack. At that critical point, only a small
increase in airstream velocity would be needed to reduce turbulence and
thereby reduce stall speed. I've seen this effect in films of storks landing
on their nests. Just before landing, they "brake" by rotating their wings to
create a high angle of attack. As the bird approaches stall speed, you can
see the feathers on the upper surface of the wing become "ruffled" by
turbulence as the airstream separates from the wing surface. At this point,
the alula is extended you can see the wing feathers return to their normal
position as turbulence decreases.  The whole process takes only a few
seconds and takes place immediately prior to landing.

And Philodor wrote, in part:

>Did the alula develop twice?<

It's certainly possible.  But since it appears to be a very derived flight
feature, I think a simpler explanation is that it arose once (in conjunction
with the carpometacarpus) somewhere within Aves but outside Archaeopteryx.