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Re: Pterosaur arm supination (getting long)
----- Original Message -----
From: "Michael Habib" <firstname.lastname@example.org>
Sent: Tuesday, January 15, 2008 12:51 AM
Subject: Re: Pterosaur arm supination (getting long)
particularly the aerodynamics near the crossover point.
I'm actually surprised to hear that no one has written much on that topic;
it would indeed be excellent (adding to list of things to do someday...)
It's possible that I've missed it. I've seen a number of mentions in
passing in the literature, but nothing in depth.
Ironically enough, it is the latter relationship that occurs to me first,
despite the fact that I actually work largely on bending strength in avian
arm bones! I do wonder, though, if the 12:1 limit may also incorporate
It may well do.
given the variance in bone strength in birds, even within a narrow AR
range. For one thing, any birds with an AR over 12 are going to be
cruising pretty fast;
I know what you are saying, but think this may be a good place to add (not
for your benefit -- you already are aware of the relationship), that the
increase in speed is due to the increase in wing loading, not the high
aspect ratio. The need for speed creates the advantage for high wing
loading. The drag associated with high wing loading creates an advantage
for high aspect ratio when these high speed wings are traveling near the
slow end of their range. Consequently, animals with high aspect ratio are
generally going to be traveling fast even though it is possible to configure
high aspect ratio wings with sufficient surface area for slow flight.
You'll see that latter trend in some aircraft, but compromise tends to limit
the trend toward really large high aspect ratio wings in vertebrate flyers.
BTW, there are other advantages for high wingloading that are not related to
speed, or even flight itself.
no inland species have an aspect ratio that high except for some
small-bodied aerial hawkers.
Once you toss in the oswald efficiency factor, I think you'll find that some
inland species have effective aspect ratios (when utilizing the slots) that
are similar to the effective aspect ratios of the unslotted marine flyers.
As you say, gross aspect ratio does remain higher for the marine flyers.
As such, most high AR birds should be in a speed regime that is beyond the
crossover point related to induced drag.
Only when they are flying at high speed (high for them). When flying slowly
(for them), their lift coefficient will be high enough that induced drag is
a factor. For birds, the advantage to high aspect ratio comes when they are
extracting energy from the atmosphere. When they are traveling rapidly
between lift sources, the advantage to high aspect ratio disappears because
of the reduction in induced drag at low lift coefficients and they retract
their wings to lower the profile drag component instead. This increases the
induced drag due to the lowered aspect ratio, but at the high speed, the
induced drag isn't a factor even with the reduced aspect ratio because of
the inverse relationship between induced drag and lift coefficient.
Do you happen to know where the bone/shaft strength crossover was first
I don't know that it has ever been written up. It's just something that
occured to me one day, and I've not pursued it. I think it is also related
to maximum wingspan in birds, but I've not pursued that either.
In any case, inland soaring forms with slotting do have stronger forelimb
elements, as expected (though, again, I think there are multiple reasons
We agree on a totally disgusting amount of stuff.
I know, it's scary.
More seriously, I fear that folks who don't see our offline discussions will
think we are in total agreement, and I don't want that. I think the
snippets where we disagree potentially illuminate our topic. In my case, I
don't necessarily carry a searchlight, but maybe at least a candle or
, with the lower surface inboard eddy being used to enhance inboard lift
(potentially a beneficial compromise).
That's quite interesting; I'll have to keep it in mind. How much in the
way of air sacs are you included external to the bone spaces?
Quite a bit, but I think it is variable, under control of the animal
(whether passive control or active, I don't know). In any event, the
relatively abrupt transition in airfoil thickness immediately behind the
humerus and proximal r/u creates a high pressure eddy aft of the skeletal
spar that can produce substantial lift without a huge drag increment (there
is a substantial drag increment, but not exhorbitant). As an aside, Bill
Akersten's videos of transient pelican air sac inflation made a real
impression on me.
Overall, your model is presumably a better rapid-glider, and mine is a bit
more generalized, but the differences are slim.
Yeah, the difference between 9.8 and 10.2 sq.M. is within individual
variation for two animals with identical span. The biggest straight flight
difference may be that I presumably load the tail structure relatively more
lightly than you do. We probably load the outer wing similarly. With the
proviso that your loading may be greater because I think you use a heavier
weight for Qn than I do. One of us really ought to take time to do a
detailed mass estimate for Qn based on an allometric blowup of Qsp (I do
realize how speculative that would be, which is why I've not bothered to do
it so far).