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Re: pterosaur femora sprawl




----- Original Message ----- From: "Mike Habib" <habib@jhmi.edu>
To: "Christopher Taylor" <gerarus@westnet.com.au>
Cc: <mhabib5@jhmi.edu>; "'dinosaur mailing list'" <dinosaur@usc.edu>
Sent: Tuesday, June 03, 2008 11:48 AM
Subject: Re: pterosaur femora sprawl



I suspect that your confusion comes from trying to force the poor critter to deploy the wings too early (based simply on the fact that I get that bit of confusion a lot).

I do too.

A quad launching pterosaur would leap first, and then deploy the wings near the max height of the ballistic path.

They would start the deployment immediately upon the hands leaving the ground, but the deployment takes a bit of time (about a quarter second for the bigger pterosaurs). They can't start significantly unfolding the wings till a line drawn from the shoulder to the wingfinger pivot is high enough that the wingtip will clear the ground by the time it is unfolded. Once the outer wings start to unfold, a significant amount of lift is generated, and it starts happening before the animal reaches max height on the ballistic path, and it also increases the leap height over the ballistic height. Max extension does happen at about max height of the ballistic path though (In other words, I'm not disagreeing with Mike -- just pointing out that the wings start doing their thing before they are fully deployed and that that starts happening fairly early. At the time that the hands leave the ground, for a big pterosaur (Quetz, Hatz, etc.), the vertical component of the animal's velocity is about 7 to 8 Meters/second, so that the ballistic path would raise the cg about 2.3 to 3.3 meters during an elapsed time of about 0.68 to 0.82 seconds. This is over twice the duration of a downstroke, so there is plenty of time for the animal to initiate the first downstroke and be on his way before the ballistic path reconnects with the ground (read that as 'no kersplat required'.....)


There is a tendency, I think, to expect that birds flap themselves into the air, and that being bipedal somehow makes ground clearance less difficult. Remember, though, that most living ground-launchers use a leap-first, flap-second kinematic, as it is.

In fact, quad launching *reduces* the problems of clearing the ground,

Indeed it does. Well said, Mike.

because the leap impulse is so much more powerful than a bipedal leap - it is birds that actually should have the most trouble clearing the ground.

Yup.
JimC