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Re: Patagium attachment Was: Re: Pterosaur.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.