Re: pteros have lift-off

[jrc <jrccea@bellsouth.net>]

----- Original Message ----- 
From: "Michael Habib" <mhabib5@jhmi.edu>
Subject: Re: pteros have lift-off


David Peters wrote:

> Do your figures take into effect 'ground effect' flying?

I don't usually bother, though the calculations could be made more
accurate by doing so.  Ground effect will not allow large pterosaurs to
launch bipedally, though.  The effect is insufficient in magnitude to
solve the problem at hand (that much I have confirmed, at which point I
usually just leave it out).

.......I do take it into account because it helps in low level flight, so 
I've built it into my calculations.  I agree with Mike that it is 
inconsequential during launch.

> Maybe, like pelicans and herons the availability of ground effect means 
> pterosaurs don't have to reach cruise speed within a second after 
> take-off?

......They don't have to reach cruise speed at all, but if they fly below 
minimum power speed, they do have to work harder.  I don't see what an 
arbitrary one scond time frame has to do with it.

> Perhaps a trajectory in big pteros lower than 30º would be equivalent (i. 
> e. ultimately leading to flying at cruise speed)?

.......I think we've all said several times that the launch angle varies. 
Thirty degrees is about the average, not a minimum or a maximum.

Ground effect doesn't help quite that much.  Besides, a biped launch
doesn't get many pterosaurs even close to cruise speed - they need to
get at least close enough for an anaerobic burst window to finish the
job.  And there remains the lingering issue that pterosaurs are so well
built for quad launching and so poorly built, as a general rule, for
biped launching.

........Ground effect becomes important if you plan to stay close to the 
ground for a while.  It isn't terribly important during short duration, high 
power anaerobic burst flight.

> And this is incompatible with small and large pterosaurs because...?

See above - it is incompatible because most of the launch force needs
to come from the initial leap, and the comparatively gracile hindlimbs
in large pterosaurs are insufficient to carry the loads.  Smaller
species are more uncertain - biped launch isn't absolutely forbidden,
but there is no good evidence for it, while the structural scaling
patterns and forelimb morphology of most small taxa continue to
indicate quad launch adaptations.  Lacking any positive evidence for
bipedal launching, the most simple model at present is that even small
species preferred a quad launch.  It appears to be required for big
guys.

........In one of the mid-sized pterosaurs that I'm fond of, the hindlimbs 
are longer and more robust than most other pterosaurs of similar span.  Even 
in that robust pterosaur, the hindlimb power production seems to peak about 
30 to 40 percent of the way through the terrestrial launch, tailing off 
quickly thereafter even though the pes doesn't leave the ground till about 
80 percent of the way through the launch.  The forelimbs are creating power 
from the start and really begin to come on strong about 40  percent of the 
way through the launch, driving hard all the way through liftoff.  Wing lift 
is effectively zero during the terrestrial part of the launch, coming into 
play shortly thereafter.
JimC