Re: pteros have lift-off

[Michael Habib <mhabib5@jhmi.edu>]

David Peters wrote:

> I see storks and herons taking off from a standing start, perhaps 
> using ground effect before gaining cruise speed. There's no 30º 
> trajectory there.

Storks and herons may use a bit of ground effect in certain conditions, 
but they don't generally require it.  They take off at steep angles 
owing to specifics of limb morphology and very low wing loadings (and 
reasonably short spans).  The difference in launch angle isn't all that 
informative here - it just indicates that a pterosaur probably didn't 
look much like a heron taking off.  They're not built like herons, so 
that's not surprising.  It seems that pterosaurs were built to launch 
fast and powerfully, but not especially steeply (with perhaps some 
exceptions in the smaller body mass ranges).

> 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 mapped sequential photos of a pelican taking off (I can send this 
> via a non DML email). Although the pelican is airborne immediately, 
> the acceleration over a half dozen frames is easy to measure while 
> flying in ground effect. 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? Perhaps a trajectory in 
> big pteros lower than 30º would be equivalent (i. e. ultimately 
> leading to flying at cruise speed)?

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.

> Looks like: 1) crouch simultaneous with wing unfolding and upbeat. 2) 
> hindlimb extension simultaneous with first downbeat. No surprises. 
> Probably common to most birds.

The hindlimb impulse precedes the production of most of the aerodynamic 
force production - at least, that's true for all birds so far tested 
explicitly.

> So, I'm gathering from the video that a large amount of thrust is 
> generated by the wings along with thrust from the hind limbs. Which 
> seemingly relieves the hindlimbs of a large amount of duty -- which 
> was a major concern of yours.

The wings do not produce much launch force - that is the real kicker 
here.  As David M. correctly ascertained, about 90% of the launch force 
(in birds) comes from the hindlimb impulse.  The wings are not 
traveling fast enough to generate much force.  Even hummingbirds, with 
their tiny legs, produce 59% of their launch force from the hindlimbs 
(see work by Tobalske).

The upshot of this is that hindlimb skeletal strength scales very 
rapidly in birds - such that large species have disproportionately 
strong femora.  The humeri scale much more slowly, by comparison.  
Pterosaur scaling patterns are literally reversed.

> 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.

Cheers,

--Mike


Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181
habib@jhmi.edu