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Re: avian flight




Patrick Norton wrote:

> >Claws have completely inept shapes for this function, I'd say. <
>
> They fit entirely within the size and location parameters of leading edge
> protubernaces long since tested in wind tunnels.

Pat's right.  Typically, vortex generators (vg's) have a height on the order of
1.2 times the boundary layer thickness at the point where they are located.
They affect the flow aft for about 80 vg chord lengths.  They are often paired
(though they don't have to be), and sit at an angle of 15-30 degrees to the
free-stream to assure formation of a longitudinal vortex.  Optimally they have a
parabolic planform, but pretty much any planform will work.  On a Piper
Cherokee, a single pair about 1.25 inch high and 1.5 inch long located just
outboard of the wing root will cause a cruise speed increase of about 4 mph,
plus a decrease in indicated stall speed on the loose order of 4-5 mph.
Location for optimal performance is touchy and installation of too many vg's
will cause the cruise speed to drop.  Vg's work as well as slots and slats and
seem to be better than snags, though they operate by means of a different
mechanism from slats and slots.  Alulas can operate as slots, slats, vg's, or
snags, or can be used to fair the wrist. If I had to choose one or the other,
I'd go with a vg over an alula.

> >And claws of noticeable size coexisted with alulae in early
> ornithothoracines such as *Sinornis*.<
>
> What is a "claw of noticable size"?

I'd say any claw capable of extending 20% of its length into the boundary layer
would be of noticible size.

> >Confuciusornithids (snip) had quite some difficulties in slow flight and in
> manoeuvering because they had no alula.<
>
> I agree with you that that is indeed a testable---which I think would easily
> be proven wrong with the right experimental set up. Alas...

There is a non-destructive way to test this.  Take any modern bird with an
alula, snip the alula feather on one wing only, and let it fly.  The feather
will grow back after the next moult, so no damage will be done to the animal.  I
would hypothesise that  one missing alula doesn't much affect the animal's
flight performance.  Nor should two. I'm looking forward to the results of the
test, should someone decide to perform it.

All the best,

Jim