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Re: Pterosaur arm supination (getting long)
----- Original Message -----
From: "Michael Habib" <email@example.com>
Sent: Sunday, January 13, 2008 4:14 PM
Subject: Re: Pterosaur arm supination (getting long)
Don't worry; I am not quite that easily misled ;)
Never thought you were.... :-)
As far as I can tell, the low AR in vultures and other dedicated inland
soaring forms comes as result of 1) an emphasis on loitering and 2) a need
for steep launch angles.
Plus the ability to adjust effective aspect ratio through the tip slots by
creating 'multiple tips' each with high aspect ratio. August Raspet wrote a
good article about this back in the 50's. I wish he'd pursued it further
before his untimely death.
I'll bet that vultures fuel up very heavily when possible.
I agree. I've seen vultures too gorged to fly till after they sit around
and meditate for a while.
And actually, the aspect ratios in most vultures are not as low as many
people seem to think.
I agree. Particularly when the tip slots are open and increasing the
effective aspect ratio (multiple high aspect ratio tips mitigating the gross
low aspect ratio at the expense of some additional interference drag at the
inboard end of the slots). I don't think anyone has written extensively on
that, but it would make a good topic, particularly the aerodynamics near the
crossover point. BTW, there are two crossover points -- one apparently
related to the relationship between the bending strenth of avian arm bones
and feather shafts (the approximate 12:1 limit), and the other related to
the relationship between lift coefficient and induced drag (the speed at
which individuals will close the tipslots when traveling faster).
I am also familiar with the aspect ratio crossover point; brown pelicans,
specifically, are the highest AR bird with slots and they run an AR of
about 11 to 11.2, as I recall.
Yes. They are why I implied that the crossover point was slightly higher,
roughly about 12.
And, of course, at lower AR not only does the raw drag interplay favor
slot usage, but so does the speed regime in which the animals tend to
travel (since it further favors induced drag mediation). Vultures don't
use slots when gliding rapidly between loitering localities.
Agreed. I tend to think that some of the azhdarchids also used a
combination of loitering (looking for schools of fish) with high speed
travel between lakes and/or rivers and during migration (migration is more
I agree, for the most part.
We agree about so many things that the fun comes when we do have
disagreements, most of which seem to be minor.
My prior comments about reduced aspect ratio were based on the casual
observation that the expansion of MCIV doesn't seem to quite make up for
the distal wing reduction PhIV.
It doesn't, quite. My hunch has been that the shift serves a two-fold
1) Increasing the length of MCIV increases the stride length (leaping
stroke) of the launch, thereby decreasing the required acceleration and thus
the dynamic loads on the animal and the peak power required.
2) The folded wingfinger doesn't extend as far above the back as in other
pterosaurs, thereby providing some increased protection in what may be a
relatively cluttered terrestrial environment.
The two things above imply a need for increased unit spanwise twist in the
outer wing. More about that below.
My thought has been (.......snipped) , but I'm less convinced now that it
was a major factor in the wing shape evolution of azhdarchoids. [I do
suspect that anurognathids may have been shrinking distal wing mass to
reduce inertia (and bump up flapping frequency)].
That said, I do still hypothesize that the change in wing element ratios
within azhdarchoids was related to power output, flapping capacity, and
I've been of the opinion that it is primarily a combination of launch
dynamic and the need to continue to be able to reach the mouth with the
hands as the neck length increased. For my feeding scenario, there is also
a need for the wing to be able to survive transient wingtip water surface
but I think the reduction of inertial effects was a very small
side-advantage, in part because the expansion of MCIV should move more
muscle mass outboard.
Again, me too. We agree on a totally disgusting amount of stuff.
My current suspicion is that the change in wing element lengths in
azhdarchoids was related to an adaptive complex which increased flapping
capacity and launch speed, while also improving ground mobility.
Yes. See above.
As part of this same complex, the dp crest was enlarged, along with the
coracoid flanges (a lot), and the associated musculature.
Yes. I think the enlarged dp cest and coracoid flange were what allowed the
relative robustness and increased weight of the azhdarchoids. As an aside,
the azhdarchoid dp crest is also reshaped in a way that allows it to help
shape the airfoil section of the inner wing, which implies that shaping of
this area was very important in the more heavily loaded pterosaurs.
The intrinsic hand muscles associated with digit IV were also expanded,
and this meant the expansion of MCIV, and a reduction of phIV to maintain
Yes. And this also implies a need for increased unit spanwise twist in the
outer wing, which was accomplished by the azhdarchoid lazy Y section in
phalanges IV-2 & IV-3 (there is a hint of it left in the much reduced IV-4,
which has reverted to an almost circular shape -- but, I think if the
azharchoids had survived much longer, IV-4 might have been lost entirely
[unless it was serving as a safety fuse to protect IV-3 from physical
damage] ). As an aside, the strap oval shape of IV-1 substantially
increases its vertical bending deformation and slightly increases its twist
without reducing its ability to resist aftward bending. The azhdarchid
outer wing is trying to tell us something about the compromise between
launch requirements and flight requirements.
AR dropped slightly relative to marine forms, but that may or may not be
Agreed. The difference is minor and the increased energy available on
average from the inland atmosphere might diminish the importance of aspect
ratio compared to other demands on the animal.
The bending resistance in the second third of the wing may also be higher
withe expanded MCIV, but I have yet to confirm this.
Qualitatively it is greater, but I haven't confirmed it either.
The most distal wing, by contrast, seems (to me) adapted to passive
deformation, but only in very limited planes. Not sure what degree of
passive outboard twist there would be, but the lazy-Y cross-section
obviously makes a big difference.
As you know, the lazy Y section substantially increases the spanwise twist
without reducing the vertical bending deformation and while increasing the
resistance to aftward bending (I've mentioned this for benefit of others on
the list who may not be aware of it -- I know that you are aware).
The expanded MC IV also raises the stance, as does the lengthened hind
Yes, and allows the hands (front feet) to move further forward on the
ground, to help support the forward cg of the long, unretractible neck and
Overall, the trend seems to be bigger motors and a higher terrestrial
And increased stride length (leaping stroke amplitude)
This should make launching very powerful, and flapping bursts more
effective. It slightly reduces glide ability because of some inboard drag
effects and a slightly lower AR, but nothing big.
I've done some inboard airfoil sections, and the inboard drag appears to me
to be fairly well mitigated, with the lower surface inboard eddy being used
to enhance inboard lift (potentially a beneficial compromise).
My expectation is that basal azhdarchoids, at least, spent a fair bit of
time foraging on the ground, and launched and flew quite rapidly, covering
very large distances between patchy resources.
I agree with this, particularly the part about patchy resources. I agree
about some terrestrial foraging, but note that it would be impacted by the
very restricted lateral mobility and somewhat limited vertical mobility of
the long neck. Vertical mobility though restricted, was not as restricted
as the lateral, for obvious reasons. I'd prefer that we not talk in much
detail about neck mobility till after Wann publishes.
This may also extend to azhdarchids proper, but it is also possible that
animals like Quetzalcoatlus simply inherited the altered planform and made
it work for things like skimming (though, to be honest, I'm pretty
skeptical of the skimming hypothesis, currently. No offense meant, of
course; I know you have favored it in the past).
And no offense taken. My opinion is based on a combination of three
features that are almost perfectly adapted for intermittant skimming of
schools of small freshwater fish. These being the high wing loading (fast
flight), relatively reduced outer wing length, the long neck and head, and
the hydrodynamic shape of the lower mandible which is remarkably well
adapted for intermittant surface skimming and capture of relatively large
numbers of small fish. I see Quetzalcoatlus as loitering over fresh water
lakes and/or rivers, spotting surface activity, dropping down to make short
skims through that activity, and then climbing to repeat and or loiter
again. Fishing from freshwater lakes implies resource depletion and the
need to travel rapidly to other resources. I also suspect migration, but
don't consider it to be proven by anyone yet. Needless to say, the features
I mention above would as always, be further modulated by other, potentially
conflicting or complementary needs (terrestrial foraging, launch,
anti-choking device, etc.)
BTW, I think you restore Quetz with more wing area than I do, but I restore
with more tail area, so that the total lifting surfaces probably aren't all
that far apart. For Qn, if I remember correctly, my wing area is on the
loose order of 78 or 80 s.f. or thereabouts with a tail area of about 25 or
26 s.f. for a total lifting surface of about 103 to 106 s.f. (roughly about
9.8 sq.M.). Roughly what numbers are you seeing? Off the top of your head
is good enough -- I didn't bother to go look up the ones I just spouted.
All the best,