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Re: pterosaur take-off analog




----- Original Message ----- From: "Erik Boehm" <erikboehm07@yahoo.com>

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.

If the wind is highly variable, launching in a gust rather than a lull, is a bad idea.

Agreed. I've done a gust launch at idle power in a Piper Cub. It's not a lot of fun.

They could takeoff in a dead calm, even in today's atmosphere.
JimC

and its the most technically demanding mode of achieving flight,

Almost, but not quite. Launching from water is more difficult than land. They could do the water launch in a calm as well.

whereas launching into a wind is technically much easier.

Agreed.

I just think it could be so easy they could just spread their wings, maybe from a quad position, kind of like when one does a pushup fast enough to clap their hands together.

Takes a big pterosaur half a second to do that first half stroke. Of course by that time using the normal technique, if he wishes, he can be going faster than steady state stall speed even before he raises his wings. Basically, the wind makes it easier, but it wasn't difficult to start with. The biggest pterosaurs have not quite reached their maximum launch capability.

Just a little push with the arms, then quickly spreading them, and maybe they could be airborne into a smooth 20 mph wind, and then flap to slowly start moving forward (relative to the ground) and gain altitude.

For Q northropi, that particular technique would require a steady wind enough faster than 35 mph to give the animal about 32 seconds aloft before it reached 35 mph itself. This is the reason that unlike birds, big pterosaurs are unlikely to just spread their wings and lift off -- they can do that, but can't accelerate fast enough once aloft to keep from settling back in.

And I still think they *could* just lay on their belly, and with sufficient dihedral to get their wings off the ground enough to get airborne. Not to say they would actually do it in practice.

I agree, they can do that. They just can't stay aloft using that technique unless special circumstances are present (an updraft of about 0.6 meters/sec [118 fpm] ). Those are hard to come by when you are inside the shear layer on flat land.
JimC