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Re: pterosaur take-off analog
----- Original Message -----
From: "Erik Boehm" <email@example.com>
To: <firstname.lastname@example.org>; <email@example.com>
Cc: "Mike Habib" <firstname.lastname@example.org>; "dinosaur mailing list"
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
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).
lift it off the substrate very briefly, but that's not
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.
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
- 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
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