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Re: pterosaur take-off analog

Comments inserted.
JimC
----- Original Message ----- From: "Erik Boehm" <erikboehm07@yahoo.com> 
To: <davidrpeters@charter.net>; <habib@jhmi.edu>
Cc: "Mike Habib" <mhabib5@jhmi.edu>; "dinosaur mailing list" <dinosaur@usc.edu> 
Sent: Monday, October 05, 2009 10:27 AM
Subject: Re: pterosaur take-off analog



Only very special gust
scenarios will launch mid-size pterosaurs, and it is almost
impossible to launch a large one by gusts alone


I just want to be clear about terminology:
By a "Gust scenario", do you simply mean "strong winds"
Or do you mean "A rapid change in wind velocity", which is how I interpret "gusts"
Either will lift the animal (or an airplane) off. Neither will keep it there unless sufficient power is added (I'm aware that you know that, but some others may not). 


(easy to
lift it off the substrate very briefly, but that's not
sustainable).
Well, does it need to be sustainable? Say the ptero is sitting on the ground, in a wind 5+mph over its stall speed, it spreads its wings, and lifts up (and backwards), within a second or two, its at its highest point above the ground, and with no further action, it will lose altitude and land back on the ground behind where it lifted off. This assumes level ground, even a slight slope could produce enough ridge lift to keep it in the air. 
All it needs to do is start flapping before it starts to settle back down.
It needs a bit more than that -- it needs time enough for the flapping thrust to accelerate it to flight speed before the deceleration due to drag sets it back down, or alternatively before the lift associated with the gust transient fades away. Since average flapping thrust coefficient is somewhat limited, that places some constraints upon the process.
So then the only question is how much height does it need to get in an effective "flap", and what wind velocity is needed to get the ptero to that height, assuming when it spreads its wings it adopts the optimum AoA?
That's not quite true. It typically won't (read "can't" ) accelerate to flight speed in one flap. Nor will the spanwise AOA distribution be constant. And, the slower the animal is moving with respect to the relative wind, the higher the height of the jump that is required. 


Because mid to
large pterodactyloids could not biped launch effectively
without special conditions, it is very, very unlikely that
they were biped launchers - if they were, we would expect
them to be able to manage it without gusts, even though they
might use them when possible.  And, in fact,
gust-assisted biped launches are still not feasible for most
species.  Gusts do help the quad launch, however.
I think with sufficient wind, biped vs quad becomes a non issue, as the animal won't need to "locomote" in any fashion along the ground, just spread its wings and flap, this of course assumes it can get its wings high enough above the ground that the air isn't slowed too much due to the boundry layer/wind gradient.
No, what you assume that isn't stated is that the average thrust coefficient has to be great enough to accelerate the animal to flight speed before it settles back in. That can take several to many flight strokes.
By the way, what sort of power output do you calculate a large pterosaur needed to sustain flight?
For Q northropi about 1.25 horsepower to sustain level flight. Power required depends upon flight configuration though. The Qn mount that hangs in the lobby of the TMM in Austin would require about 4 hp in that configuration (due to deadline constraints, the ceiling supports were installed before we finished the sculpture). 


A typical hangglider needs about 5 hp to maintain level flight.
However, I would assume the pterosaurs wing flapping is much more efficient than a relatively small diameter propellor (which moves a small volume of air fast, rather than a large volume slow). 
I could easily see a ptero being 2x as efficient as a micro-light prop.


The wing can be about 92-93% efficient.  Most props are closer to 83%.
And even the huge pteros I think were in general lighter than a typical human+glider+ motor combination, and probably had better sink rates. 

That's true.
So my first guess at how much a Quetz would need to maintain level flight in still air: 1.5-2 horse power or 1,200-1,500 watts if you prefer (perhaps as low as only 1 hp, or ~750 watts).
You're a bit low, but in the ball park (assuming a weight on the order of 150 Kg for the pterosaur -- 1.25 hp)
Given that most humans have a peak horsepower output of over 1 hp, and cyclists such as Lance Armstrong can sustain an output over 1 hp for pretty long periods of time (and that the peak horsepower output of a horse is in fact far greater than one horsepower), I think this guess is in the ballpark 

It is.
- as I'd think a large ptero could put out at least 2 hp for at least a minute or two.
Considerably more than that for very short periods while flying anaerobic (say roughly 30 seconds to a minute), but typically somewhat less than that for extended durations. In no-lift conditions, operating as a flap-glider, a 150 Kg Q northropi would have to spend roughly 82% of the time flapping. They would not be able to sustain that for an extended period, but would usually have no trouble sustaining it for long enough to find a source of atmospheric lift.
But this does seem rather high for a creature that only weighed as much as an adult male human,
I presume from that, that you are talking about moderately large pterosaurs rather than the really big ones? 


so I'd also guess they didn't like flapping much,
I'd agree with that. I wouldn't like flapping much either, if I had a choice in the matter.
and like hawks, probably sat on an elevated point when there was insufficient lift to maintain unpowered flight.
Many of these animals didn't hang out in an area where there were any elevated points of note. Nor was there any particular need for elevation. I would say that if a large pterosaur happened to be sitting on a high spot and had the urge, he would probably take off from there (I would). But if he were sitting at the bottom of a slope beside a high spot, he probably wouldn't bother to walk up to the top in order to take off (nor would I, given his launch ability).
As an aside, something that I don't often see mentioned is that for a considerable part of their existance, average temperatures were higher than now, and that higher average temperatures tend to suppress average wind speed. 
JimC