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Re: Pterosaur arm supination (getting long)

Comments inserted,
JimC

----- Original Message ----- From: "Michael Habib" <mhabib5@jhmi.edu>
To: <dinosaur@usc.edu>
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 speculative).

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 purpose.
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)]. 

Me too.

That said, I do still hypothesize that the change in wing element ratios within azhdarchoids was related to power output, flapping capacity, and launch dynamic,

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 impacts.

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 aspect ratio.

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 significant.

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 limbs.

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 big head.

Overall, the trend seems to be bigger motors and a higher terrestrial stance. 

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,
JimC