Re: pteros have lift-off

[jrc <jrccea@bellsouth.net>]

BIPEDAL LAUNCH



Bipedal launch would be facilitated either by bipedal running, or bipedal 
leaping.  There are two scenarios for launch by bipedal running, and one for 
bipedal leaping.  Reconstructions of pterosaurs in the bipedal posture 
usually have the angle of the spine oriented roughly about 20 to 30 degrees 
from the vertical.  The consensus among pterosaur workers is that the 
trailing edge of the wing membrane is bound to the hindlimb.  This presents 
constraints on the wing's launch angle of attack that do NOT apply to birds 
since the pterosaurs' inboard chord must now remain approximately parallel 
to the spine. Let's discuss those pterosaurian constraints.  A few quick 
thoughts follow.



1)      Running bipedal launch



a)      In typical bipedal posture with spine about 20 to 30 degrees from 
vertical - the angle of attack of the wing is closer to vertical than 
horizontal, approximately 42 to to 52 degrees greater than the stall angle 
of attack.  Therefore the underside of the wing is facing forward and 
creating substantial flatplate drag.  This  is equivalent to trying to run 
with a drag chute strapped to your waist.  Any effort to provide lift by 
flapping creates a "lift" vector that is oriented mostly to the rear, 
slowing the animal even further.  Very little of the lift vector (about 
34% ) is oriented in a direction that helps to lift the animal. Most (about 
94%) is oriented aftward and acts to slow the animal's forward speed.  This 
means that the animal will have to run very fast and then suddenly pitch 
forward in order to lift off.

b)      An alternative is to rotate the torso, neck, and head forward from 
the hips into  a near horizontal orientation that directs most of the lift 
vector upward.  The downside is that the cg is now well forward of the feet. 
This is equivalent to a human holding a 5 meter piece of pvc pipe by one end 
and extending it out horizontally in front while also bending the torso 
horizontally forward from the hips.  It is very difficult (or impossible) 
for a human to either stand or run in that position, and a pterosaur would 
incur similar difficulty for the same  reasons.



2  Leaping bipedal launch



a)  Bipedal leaping implies creating a force vector that is aligned fairly 
closely with the spine (approximately 20 to 30 degrees from the vertical for 
the bipedal posture).  Therefore, the launch angle is also at about 20 to 30 
degrees from the vertical. Consequently, most of the velocity created by the 
leap is vertical (about 87 to 94%) and a smaller fraction (about 34 to 50% 
is oriented parallel to the ground in a direction that will facilitate 
flight.  In the horizontal direction, the same wing orientation, drag, and 
lift issues apply as are incurred in the running '1 a' scenario.



Any proponent of bipedal launch must have addressed these issues 
numerically.  Does anyone wish to comment?

JimC