Re: Patagium attachment Was: Re: Pterosaur.net

["jrc" <jrccea@bellsouth.net>]

This might be a good place to mention that although pterosaurs could and 
almost certainly did use wing retraction to reduce profile drag, it doesn't 
work quite the same way as in birds and bats.  In pterosaurs, there is an 
additonal requirement to reconfigure the inner wing appropriately to control 
the nonlinear interaction between the inner wing tension (both spanwise and 
chordwise) and the outer wing tension.  For example, if the inner wing 
membrane is allowed to slacken during the retraction, then the outer wing 
will slacken to a lesser degree thereby increasing the outer wing pressure 
jump which increases the outer wing camber, requiring that the aoa of the 
outer wing be reduced, which moves the line of maximum membrane camber 
aftward.  If left uncompensated, this can reduce the flutter margin and will 
not necessarily reduce profile drag.  Increasing the inner wing tension of 
the retracted wings during a dive by use of hindlimb positioning has 
implications for uropatagium tension, flutter, and drag that put constraints 
on the articular positioning of the hindlimbs so that neither the wing nor 
the uropatagium will flutter.  There are several ways the animal can 
accomplish this, and several other ways that will keep one of the surfaces 
from fluttering while triggering flutter in the other.  I've not looked at 
Mark's drawing to see if his positioning appears to comply with  the 
constraints -- I'm simply using this as an opportunity to point out that the 
relationships are quite nonlinear.  For example, folding at the 
metacarpophalangeal joint will require simultaneous anterior deflection of 
the pteroid in order to constrain the reduction in tension of the outer wing 
and its associated increase in pressure jump and camberline reshaping (which 
can be done).  These things ain't airplanes, birds, or bats and one should 
not emulate the others too closely when working on pterosaur flight 
mechanics.
JimC

----- Original Message ----- 
From: "David Peters" <davidpeters@att.net>
To: "Mark Witton" <Mark.Witton@port.ac.uk>
Cc: <dinosaur@usc.edu>; "Mike Hanson" <mhanson54@comcast.net>
Sent: Monday, January 18, 2010 9:57 PM
Subject: Re: Patagium attachment Was: Re: Pterosaur.net


> > As for the
> > posture of the legs in the Darwinopterus image, the animal is meant to
> > be diving, swooping its wings back and narrowing the uropatagia to
> > lessen the wing area and minimise profile drag: why not use the legs as
> > extra control devices to alter the shape of the wings?
>
> Sorry, I didn't understand the dive concept. For me a diving pterosaur, 
> like a diving bird and bat, would have folded its wings posteriorly at the 
> metacarpophalangeal joint. I thought Darwinopterus was raising the 
> wingtips in a V shape. If it IS diving, it is overextending the elbow and 
> the fully deployed and limp brachiopatagia are going to flutter like a 
> flag on a speeding car IMHO.