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Re: Slack membranes and Wing In Ground Effect (was Re: Comparing T-rex to pterosaurs)




Martin Barnett wrote:

> It looks to me as though a slack membrane would reduce the animal's control
> over the crosswinds, not limit their effect, because you essentially have an
> M-shape (hip-armpit;armpit-elbow;elbow-wrist;wrist-fingertip) with a loose
> skin attached to it all the way round, hanging in between.

You left out the wingfinger pivot, but yeah, they do fold a lot.  I'm not going
to argue too hard, because my perception is that neither you nor I think many,
if any, pterosaurs were bipedal, and both of us think that large bipedal
pterosaurs would have been unstable.  However, the folded wings would not have
been much of a stability problem for four-legged pterosaurs (which is how I
generally think of them).  But I'm not prepared to say that no pterosaurs were
bipedal.

> Still inherantly unstable for biped locomotion.

see above.

>  The more I think about
> biped locomotion the more I think it seems fraught with difficulties.  I'm
> not denying Pterosaurs could do it - just that it seems more trouble than
> it's worth.

Me too.

>   During flight I'm sure WIGE would have been very useful to
> pterosaurs because it's all designed to make a large flier more energy
> efficient than a slightly higher altitude, smaller one.  see
> http://www.amsc.belvoir.army.mil/ground_effect.htm for some interesting
> points about WIGE (sorry, no palaeobits).

Unfortunately, the Belvoir website's description of ground effect is somewhat
flawed.  For example, it says that during ground effect a cushion of compressed
air forms.  In the range of velocities incurred, air is essentially
incompressible, and significant compression doesn't occur in the condition
described, so I would suggest that the author of the Belvoir paper go back and
take another, closer, look at his sources.

>  What I meant by WIGE having a
> field day is that it would be acting on the wings only and would pitch the
> body out of the way - ie forward and down where the head wanted to take it
> in the first place.

Sam, I don't follow this.  Ground effect acts to reduce the losses to the vortex
shed at the wingtips, by reducing spanwise circulation, so it affects the body
and wings similarly.  For a given airspeed, the amount is quantified by the
aspect ratio and lift being produced.  Why should ground effect pitch the body
forward and down? -- I've never noticed that when making a low level pass in an
airplane.  What you do notice if you don't reduce power is that, since you are
going faster, you have to lower the AOA to limit your lift to the sum of the
weight of the aircraft plus the tail download, so you deliberately lower the
nose.  It has no tendency to want to do so of its own initiative.

Jim