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Re: pterosaur take-off analog




----- Original Message ----- From: "Erik Boehm" <erikboehm07@yahoo.com>
To: "jrc" <jrccea@bellsouth.net>
Cc: <dinosaur@usc.edu>
Sent: Monday, October 05, 2009 4:39 PM
Subject: Re: pterosaur take-off analog


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?

Yes, the latter is what I'm saying.

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.

Not quite. After popping it up in altitude, what remains of the excess speed with respect to the wind will create excess drag that will decelerate it unless it descends to make up the energy loss or flaps enough to make up for the excess drag due to the 10 mph of excess wind speed. It's not a question of airspeed, its a question of drag. The animal doesn't have power enough to cope with a large drag excess while maintaining level flight with no atmospheric lift available and will slow down until it reaches its normal cruise speed. It can't maintain power enough to fly at or near stall speed for very long (though it can fly at less than steady state stall speed for short periods, using unsteady effects).

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.

Agreed.

It would be flying 10 mph faster than it needs 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.

I have no problem with it moving backwards relative to the ground. I do have some issues with it coping with the excess drag in the amount of time available to it. That would be very pterosaur specific though.

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.

Only for a limited time. These animals were not full-time flappers. Couldn't process enough oxygen for that.
JimC