Re: Pterosaur arm supination (getting long)

[Michael Habib <mhabib5@jhmi.edu>]

> 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.

It might get close; I'll have to look into it.  The gap is pretty big 
in many cases, though.  Many of the inland species have gross ARs 
around 7.  While this is not a low as some people seem to think it 
would be, it is still a far cry from the 12-16 range seen in marine 
soarers.  Even with the oswald efficiency factor, that is going to be a 
big gap to close.  Some of the inland soarers with slightly higher 
gross AR may indeed have effective ARs close to marine taxa.  Some of 
the coastal eagles, which obviously tow the line between marine and 
inland, are good candidates.  A few of the higher AR vultures may also 
jump the gap a bit.  Again, part of what makes it a bit tricky is that 
many of the inland soaring forms that make the best use of slots are 
also active hunters (raptorial species, in particular), and prey 
pursuit places additional requirements on the flight apparatus 
(planform, muscle power, and bone strength).  This appears to be 
especially true for taxa with extreme prey pursuit behaviors (like 
osprey).

> 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.

True; good point.  I suppose what I had in mind was the fact that for 
many marine species especially, energy extraction itself is often 
improved by moving relatively fast.  This is especially true of 
albatrosses and other procellariiforms.

> 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.

And, in addition, many species (especially Fregata and many inland 
soaring birds) are at quite high altitudes by the time they start to 
move to a new lift source, so they can sacrifice glide angle if they 
want, allowing for a further reduction margin.

> 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).

Interesting though, and quite cool.  It does seem that the sharp 
transition, and the subsequent high pressure eddy formation, would also 
tend to promote flow separation from the wing at high lift 
coefficients, though.  If there was indeed an air sac system under 
control of the animal, with components posterior to the r/u, then the 
effect could obviously be mediated at low speeds/high lift coefficients 
as need be.  Otherwise, I might expect that the contour was kept smooth 
continuously.

Cheers,

--Mike


Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181
habib@jhmi.edu