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Ok, so I seem to have caused confusion, I'm good at that.  
Let me attempt to explain my point (I did have a point, 
really I did...)

On Thu, 02 Aug 2001 19:01:07 -0700 "James R. Cunningham" 
<jrccea@bellsouth.net> wrote:

> >  If the only gliding surface we are considering is distal wing elements,
> > I don't see how it would make any difference.
> Why not?

I was implying that this would not be enough surface to 
slow the animal's rate of fall.  Not a very broad gliding 
surface is presented by the distal feathers only.  
Therefore, when the animal leaps, it does not make much 
difference in terms of how hard it hits at the end, or in 
staying aloft.  It just makes a cool whooshing noise.
> >  If anything, the arms would be forced back towards the shoulders,
> Why?

The gliding surface is not anchored to anything other than 
the forearm.  Therefore, when the animal leaps, pressure is 
exterted only on the arm, which would be forced backwards, 
causing it to spin.  The animal spins because nothing on 
the rear of the animal is acting as a gliding surface.  
That is, resistance is far greater towards the shoulder 
than around the hips and pelvic area, so the torso becomes 
a pivot point.

> > Modern gliders all use broad surfaces across all four limbs, so that the 
> > braking
> > force
> > is applied across most of the animal and a broad membrane.
> Are you referring to parachuting rather than gliding?

All modern "gliding" animals actually use parachuting, 
good call on making the distinction. I should have made it 

> > Absolutely.  Gliding is not unpowered flight, it is slowed
> > falling.
> Er uh, I wouldn't tell a sailplane pilot that.

Neither would I, she (or he) would not be pleased.  But 
gliding marsupials and rodents don't glide like a 
sailplane, they parachute (very well, but it is still not a 
sailplane model).

> >  Modern gliders slow their rate of descent such
> > that they have time to steer,
> Modern gliders often deliberately increase their rate of descent, since it can
> increase their airspeed by the same amount.  This is why racing sailplanes 
> carry
> water ballast to increase their weight and wingloading.

Yes, again, true gliders do.  But modern "gliding" animals 
are parachuters, as mentioned.

> >  Gliding possums, squirrels, and others all show
> > this same technique.  All of these gliders have very low
> > terminal velocities in free-fall, thanks to the gliding
> > surface.
> Good gliding surfaces don't inherently produce low terminal velocity.

True.  But good parachuting surfaces have to.  At least, 
they have to be able to, or else it gets really messy when 
the landing part of the trip occurs.  (I have this really 
sick image of exploding possums in my head now...)

> > That is, while it going forward more than down, the animal must land lower 
> > than
> > it
> > takes off.
> That depends entirely upon local atmospheric lift conditions.  It is quite
> possible for a good glider to land higher than the takeoff, depending upon how
> much energy is extracted from the atmosphere during the flight.  On some days 
> even
> a crappy glider can accomplish it.

Agreed, but only the manmade variety.  A possum glider or 
flying squirrel would be hard pressed to use atmospheric 
lift.  The don't get high enough, and (more importantly), 
are not in the air long enough. They are also too heavy.

However, it occurs to me now that these animals might have 
a much greater mass, relative to volume, than a sailplane 
(as they are mammals, not avian species). They might also 
be lighter per volume than an avian or near avian animal, 
so your model may work better than I expect.  (This 
relative mass difference could be checked rather easily.  I 
may be able to verify it directly, actually, if you wish to 
find out).

> I'd phrase this as 'determined by launching force, atmospheric lift 
> conditions,
> and available vertical drop distance, with the distance being very dependent 
> upon
> how fast the animal glides, with speeds diverging in either direction from the
> optimum being counterproductive'. This latter is because profile and 
> interference
> drag increase with increasing flight speed while induced drag decreases with
> increasing flight speed.

True enough, good call.

Michael Habib
Student, Biology Department
University of Virginia