Re: alula

["James R. Cunningham" <jrccea@bellsouth.net>]

Patrick Norton wrote:

> Jim C wrote, in part:
>
> 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.

Remember, those specific numbers were chosen to make the results easy to
visualize, not to represent any specific animal.  As always, I look forward to
your input -- which I always find useful and informative.

> 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

I agree with this statement, but would go on to add that the consequence of the
alula action can be quite large.  But the big increase doesn't appear to result
from the alula acting as a slot.  Rather it appears to be associated with the
action of the longitudinal vortex shed from the leading edge by the alula.  A
different process entirely from the lift increase due to a slot.

> .  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.

This is quite true too.  And for the most part, unrelated to the slot function.
Also, the turbulent energy doesn't necessarily decrease. There can be a
transition from turbulent separated flow to turbulent attached flow due to the
additional energy added to the turbulent boundary layer by the alula.  Don't
forget that shedding a leading edge vortex can increase CLmax as much as 50%
over the maximum achievable by a slotted, flapped wing.  As a visual  example,
compare the deck angle of the Concorde to the deck angle of a Boeing 747 when
they are on final approach.  John McMasters with Boeing pointed this out to me
as a good visual example of the difference between the two forms of lift
enhancement while we were discussing alulas, and I was quite taken with it as an
example.

> 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.

Phylogeny isn't my strength, but speaking as a total layman, Pat sure makes
sense to me. Also speaking as a layman, I think the alula function will develop
in most flying animals that originally had a hand.  Look at the thumb in bats
and digits 1-3 in pterosaurs.  They all desperately need to be able to shed a
vortex where the wing is cranked and there is a discontinuity in the pressure
distribution of the airfoil, and they all have the ability to do so.

However, I'm not an evangelist and this is my last post to the list on this
thread (Pat, I'll be glad to continue it off-list).

JimC