Re: pterosaur femora sprawl

[jrc <jrccea@bellsouth.net>]

----- 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