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Re: pterosaur take-off analog



--- On Mon, 10/5/09, jrc <jrccea@bellsouth.net> wrote:
> > But if the wind is blowing 25 mph steady state, and
> the ptero has a 15 mph stall speed, it doesn't need to
> accelerate once up at all, merely flap enough keep itself
> from decelerating below 15 mph airspeed.
> 
> For talking purposes let us assume that the wind is blowing
> steady state at 10 mph more than the steady state stall
> speed of the animal (so that, for talking purposes, we don't
> have to worry about the mass of the specific animal). 
> Now, as in your description above, that means that the
> animal has to accelerate 10 mph before it settles back to
> the ground. 10 mph is 14.67 ft/sec.  Average net thrust
> coefficient for many of the mid to large pterosaurs (to a
> good first approximation) is about 0.05.  Now, v=at so
> t=v/a or 14.67/(0.05*32.16) = 9.12 seconds.  That means
> it takes the pterosaur about 9 seconds to reduce its
> deceleration enough so that it won't settle back to the
> ground.  I'll let you calculate how high the pterosaur
> would have to be to avoid settling back in in the first 9
> seconds, but I think it is apparent that it's not an issue
> of "merely flapping".  There's a lot of settling going
> on there.  Much easier to just do the quad launch and
> not have to worry about it.


Ummmmm, I am not following you.
If the wind is blowing a steady 10 mph faster than the ptero stall speed, why 
would the ptero then have to accelerate another 10 mph? At this point the ptero 
would now be 20 mph higher than stall speed...

Or are you saying the ptero has 9 seconds before it decelerates to below stall 
speed, ie when it has decelerated 10 mph, it will have settled back on the 
ground?

If you stick a pterosaur in air above its stall speed, wings spread, it is 
already flying, and all it needs to do to keep flying is what it normally does 
when flying.

25 mph wind, zero ground speed, 25 mph airspeed
 and 
0 mph wind, 25 mph ground speed, 25 mph airspeed 
are the same as far as anything that flies is concerned.

It would be flyin
ds to, all it needs to do is flap enough to keep flying.

It may move backwards relative to the ground, but it should have no problem 
staying up.

At any rate, when it reaches just above stall speed, the deceleration it needs 
to overcome to maintain its airspeed, is the same as in normal flight, and 
steady state flapping was obviously enough to overcome this.