Re: Pterosaur arm supination (getting long)

[jrc <jrccea@bellsouth.net>]

----- Original Message ----- 
From: "Michael Habib" <mhabib5@jhmi.edu>
To: <dinosaur@usc.edu>
Sent: Tuesday, January 15, 2008 10:47 AM
Subject: Re: Pterosaur arm supination (getting long)


> It might get close; I'll have to look into it.  The gap is pretty big in 
> many cases, though.

Yes, but consider that the aspect ratio of the individual tip slot feathers 
can be quite high, and their induced drag is low.   Consequently the gross 
induced drag can be low (accent on the 'can' as opposed to 'will' ).  August 
Raspet commented on this in a paper back in the 50's if I remember 
correctly.

> It does seem that the sharp transition, and the subsequent high pressure 
> eddy formation, would also tend to promote flow separation from the wing 
> at high lift coefficients, though.

Undersurface, yes.  At all lift coefficients above the bistable aeroelastic 
flutter limit.  It is a case where flow seperation works to the advantage of 
the animal.  Turbulent attached flow ain't always the ideal condition, and 
turbulent seperated flow isn't always bad (on top of the wing, it is bad :-(

> If there was indeed an air sac system under control of the animal, with 
> components posterior to the r/u, then the effect could obviously be 
> mediated at low speeds/high lift coefficients as need be.

Low speed/high lift coefficients are when you want it to occur.  It's a lift 
enhancer.

>  Otherwise, I might expect that the contour was kept smooth continuously.

Impossible in Azhdarchids behind the humerus.  The rate of curvature can be 
smooth and probably is, but it is very high rate (sharp curves).  There will 
be a seperation, and in this instance, it is a good thing.

Another aside.  I've noticed differential preservation immediately behind 
the elbow in some pterosaurs, leading me to speculate that some of them may 
have had 'nacelles' faring the volume immediately behind the thick, 
protrusive elbow -- either fat pockets, or air sacs.  I consider that to be 
highly speculative though.
JimC