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Re: Tiktaalik (ptero flight)
Comments inserted below.
Jim
----- Original Message -----
From: "Michael Habib" <mhabib5@jhmi.edu>
To: <dinosaur@usc.edu>
Sent: Thursday, April 06, 2006 6:24 PM
Subject: Re: Tiktaalik (ptero flight)
And, in fact, heavy bracing seems to be related to launch loads in many
birds, as well. Bone stress and strain both seem to be lower in the
forelimb infrastructure of endurance fliers, and higher in those that use
bursting takeoff *. The trend needs further investigation, however, and
is undoubtedly more complex than it currently looks.
I agree wholeheartedly. I do note that in my scenario for quetz launch, the
front feet (hands) are the last thing to leave the ground, and when they do
leave the ground, the beastie is already going faster than steady-state
stall speed. The outer wings seem to me to be unfolded after the arms are
raised high enough that the wings can unfold without the tips whacking the
ground. After that, the rest of the upstroke is pretty much standard. On a
'good' day, no flapping would be required. On a 'bad' day, perhaps 10 to 20
flaps would be required, before an extensive break. I doubt that quetz
could flap for a protracted period because the volume of the trachea seems
to me to be fairly substantial compared to the probable volume of the lungs,
implying that there might be a limit on continuous rates of oxygen exchange.
The exception is wing-propelled aquatics, which have the heaviest bracing
of all, but that's presumably a fluid medium issue (as well as a
thrust-producing upstroke issue). Overall, the launch strategy vs bone
stress pattern is interesting because forelimb (and sternal) robustness in
fossil species is sometimes used as an indication of poor vs advanced
flight ability. In reality, gracile forms may just be endurance fliers,
with poor launch but strong cruising flight; while those with heavy
bracing and/or high-load adapted limb geometry may be adapted to rapid
launch. Rapid launchers can be long or short distance fliers, depending
on other factors (for example, compare a grouse with a pigeon).
Agreed. Particularly re birds. Two thoughts come to mind. Quetz bracing
throughout the skeleton is fairly consistant and isn't all oriented to match
flapping loads or soaring loads. And launch doesn't appear to necessarily
involve flapping anyway.
Of course, the neat thing about Qn and Qsp is that they seem to combine
rapid launch ability with high efficiency burst-flapping flight, which
would seem to make them (and possibly a few other pterosaurs) the only
rapid-launch supersoarers.
You're a good man, Charlie Brown. I agree again. They do have an
extraordinary ability for high efficiency burst-flapping flight (as well as
rapid launch capability), but my take is that quite often they wouldn't have
had to use the burst-flapping mode.
None of the modern avian soaring champs have particular rapid launch
ability (vultures take off from a standstill, but the takeoff velocity
isn't all that high...they just have a low stall speed).
True. And though quetz (and most other pterosaurs) would seem to have been
able to achieve maximum lift coefficients substantially higher than any
birds, quetz stall speed wasn't low -- they were heavily loaded. Some
other, more lightly loaded pterosaurs would have had a relatively low stall
speed.
Cheers,
--Mike Habib