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Re: chriropteran upstrokes and flight take-offs



There are a couple of seeming differences, at least in the pterosaurs I
mess about with.  The heavier pterosaurs tend to go out more
horizontally than bats for a couple of reasons. One, to increase the
horizontal airspeed component -- to reduce the need for lift production
through flapping after liftoff, and to allow an increased thrust
coefficient relative to lift coefficient in order to quickly further
increase the flight speed and consequent lift and to minimise the need
for production of forces through unsteady lift mechanisms (at the slow
flapping speeds achieved by the bigger pterosaurs, unsteady lift
mechanisms aren't as effective as they are in small birds or bats). 
Two, because of the different arm proportions out to their hands, the
timing of force production between the hind and forelimbs is spread out
a bit more, and there is more of a pivoting action with the shoulder
moving  more forward over the hands' ground contact point, effectively
spreading the launch horizontally, extending its duration, and reducing
the peak and average accelerations generated during the process.  In
both quetz sizes when using this technique, the duration of the launch
process is roughly the same as for a typical downstroke during ordinary
flight, but the articulation patterns are different.  Optimal launch
angle appears to be on the loose order of 25-35 degrees from the
horizontal, similar to the launch angle of some frogs and for some of
the same reasons. A prelaunch bobbing action might assist in preloading
some of the tendons, but I haven't really looked at that yet, and it
doesn't appear to be necessary, though the concept does deserve some
additional attention.  These generalizations may not apply to smaller
pterosaurs with different arm and hindlimb proportions.  Like
differences in flight mechanisms between pterosaurs, birds, bats, and
insects; probable launch differences between birds, bats, and pterosaurs
are also illuminating.  Possibly insects too, but I haven't tried to
compare insect launch with pterosaur launch.
Jim

Stephan Pickering wrote:
> I have seen insectivorous bats on the
> ground, foraging for insects...and this is where a
> paper on how the different species of bats initiate
> flight from the ground up, as it were, might, just
> might, offer one insights into flight initation
> mechanisms in pterosaurs.