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Re: Comparing T-rex to pterosaurs was (Re: pterosaur tracks from SVP)



Side loads from wind are not much of a problem if the animal doesn't have the
wing membrane taut at the time, because the slack membrane will simply align
with the wind without producing much force.  And when the wings were folded in
terrestrial position, the membranes were not taut.  Also, the spoon-shaped
joints between PhIV-1&2 and PhIV-2&3 would appear to allow the animal to wrap
the wingtips rather snugly over the back if it so desired (I'm not implying that
it actually did so -- just pointing out that it may be possible).  In Q, the
joint between PhIV-3&4 is plate-shaped rather than spoon-shaped, and it may have
been immobile.

Martin Barnett wrote:

> .  And woe betide it should try moving at
> any speed like this - wing in ground effect would have a field day!

I'd like to make a few additional remarks about wings in ground effect.
Contrary to popular belief, it has been well known since before World War II
that ground effect doesn't necessarily increase lift.  The work demonstrating
this was done by three (retrieved from memory - it may have been two) Japanese
aerodynamicists in the 30's.  They demonstrated that lift in ground effect is a
non-linear function of HAG (height above ground) vs. wing chord (not span)
ratio, and ground effect decreases lift about as much of the time as it
increases lift.  In general, lift cannot be expected to increase as the
HAG/chord ratio is reduced.  Ground Effect does reduce induced drag based on the
ratio of HAG vs. wingspan (not chord), in accordance with the Biot-Savart law,
but induced drag is a function of lift, and if the wings are slack and not
lifting, the induced drag is zero anyway, and ground effect makes no
difference.  This doesn't imply that ground effect wasn't important to some
pterodactyls, some of the time.  The drag reduction it provided was essential to
the skimmer feeding style used by Quetzalcoatlus, and a recent flight
demonstration by a small sailplane (Gary Osoba's Woodstock) with size and
performance similar to Q northropi allowed a simulated skimming run of 600 yards
against a 15 knot headwind, primarily due to the benefit of drag reduction from
ground effect.  The misperception of the effect of 'ground effect' follows from
the fact that a machine (or animal) generally has a given amount of power
available, and when induced drag is reduced through ground effect, then more of
that power is available for increasing the airspeed, which (for a constant angle
of attack or AOA) increases lift.  But the excess lift will raise the device out
of ground effect.  If the machine (or animal) wishes to remain in ground effect,
it can either reduce power for a given AOA (slowing down to the original
airspeed), or it can reduce the AOA, and use the resulting excess power to fly
at a higher airspeed.

Jim

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