Re: Great in the air, not so good underwater

[don ohmes <d_ohmes@yahoo.com>]

Oddly, I find myself short on time for a Sunday morning. Also, Yahoo mail is 
extremely irritating to use in that it does not differentiate who wrote what 
when in the "reply" mode. So, I will just take a little tiny bit off the top... 
and give a more thorough effort by, say, next weekend.

>"Can you remind me 
again what the markers are to distinguish "primaries" and "secondaries" 
in insect wings?  (or is that another terminology for forewing and 
hindwing?)"

Yes, sorry. Forewing and hindwing. And yes, 1.5-2 atm is a lot of pressure. 
However, the methods of reducing wing area and evaluating performance were 
_very_ crude and low resolution, and wing area reduction was severe, to say the 
least. 

>"...suggest that the rules were different for Cretaceous 
birds (for example, we don't see higher wing loadings in arboreal 
enantiornithine analogs of modern passerines)."

Hmmm. Does your measurement of wingloading in paleo-fliers have an error below 
+/- 10%? What is the of the error in measuring wingloading in extants?

>"...increases in oxygen partial pressure have a 
profound physiological effect for insects.  Vertebrates get a boost as 
well, but to a more limited degree."

The little flies operate real well at O2 levels below standard pressure; I 
believe the effects I observed (if real) are _not_ the results of O2-enhanced 
performance. Sure would be nice if someone with the appropriate tools and an 
open mind would investigate, though.

>"The difference in maximum observed size 
between pterosaurs and birds is well over 2x however, and that seems 
like a larger difference than can be accounted for with a 12% O2 
partial pressure jump."

As far as I am concerned, the difference to be accounted for (empirically, 
please) is the difference between the largest extant fliers and the largest 
extinct fliers, from the perspective of the performance of the extant animals. 
More on that later.

Don

----- Original Message ----
From: Michael Habib <mhabib5@jhmi.edu>
To: dinosaur@usc.edu
Sent: Sunday, December 10, 2006 1:39:35 AM
Subject: Re: Great in the air, not so good underwater

> There
> is an
> There is an alternate hypothesis that accounts for the larger size of 
> flapping fliers in the past, without resorting to educational 
> conjecture about interesting novel aerodynamic systems.

The density hypothesis is indeed interesting (see further notes below). 
  However, the launch differences that Jim and I were discussing are not 
as conjectural as they might seem.  Forelimb-assisted launch is 
reasonably well known in bats at this point, and the different launch 
modes of modern birds have known implications (though some were only 
rigorously tested in the few years).  It is quite apparent now that 
pterosaurs were quadrapedal.  The forelimb-assisted launch is, granted, 
much less certain, but there is significant evidence to suggest that it 
was used (not to mention being quite intuitive).  I'll leave it to Jim 
to reply to that subject more specifically (if he chooses), since he 
has worked on the problem.  I'm putting together a project to more 
quantitatively test the forelimb-assisted launch ability hypothesis for 
pterosaurs.  So more on that in the future.

> Simply put, a denser atmosphere increased the lift available to these 
> organisms. Even the seemingly small density increase (12-15%) 
> concomitant with the increased partial pressure scenarios of O2 at 
> various geologic periods that have been proposed since the early 
> 1990’s may have significantly increased aerodynamic performance in 
> still air; even if O2 related increases in available mass-specific 
> power are ignored. [Names to google include R. Berner, J.B. Graham, R. 
> Dudley, and R. Seymour, among others.]

All good names, and it is indeed a long-running and interesting 
hypothesis.  I am skeptical, however, that such a difference in partial 
pressure of O2 is the best explanation for the large size of 
pterosaurs.  This is the case for several reasons:

1) It is not clear how much of a performance increase a 12-15% 
difference in O2 partial pressure 
ring 
vertebrates (see below).  I think the advantage may have been 
exaggerated to some extent.

2) If the atmosphere did, indeed, make a significant difference, then I 
would expect to see noticeable and measurable differences in planform 
between modern birds and advanced Cretaceous forms.  So far, I have not 
seen anything to suggest that the rules were different for Cretaceous 
birds (for example, we don't see higher wing loadings in arboreal 
enantiornithine analogs of modern passerines).

3) Adaptive differences in launch actually seem more parsimonious, in a 
sense, than evoking atmospheric effects.  It seems odd to me to assume 
that large-bodied pterosaurs, despite being anatomically capable of 
using forelimb-assisted launch, used a less efficient launch system 
instead (less efficient for them, in any case).  By contrast, simply 
invoking the same launch cycle used by some living quadrapedal flyers 
solves the problem in one fell swoop (more or less).


> I can offer some sketchy, unpublished empirical observations--
>
> 1). I found that if the wings of carpenter bees (X. virginica) are
> trimmed such that the lengths of the primaries and the secondaries are 
> equal to 90% of the original lengths
> of the secondaries, bees cannot even (< 2 seconds)
> achieve lift-off.

It's been some time since I looked at insect wings.  Can you remind me 
again what the markers are to distinguish "primaries" and "secondaries" 
in insect wings?  (or is that another terminology for forewing and 
hindwing?)

> In air of
> ~1.5-2 atmospheres pressure, some even (incredibly!) manage controlled,
> sustained (>10 seconds) hovering and lift-off is effectively 100%.
> (I use “hovering” here in the strict, still-air, flight-kinematics
> sense, which has no relation to the stationary soaring observed in
> larger birds.) I offer this to show that hyper-dense air can, as 
> intuition would indicate, significantly reduce constraints relative to 
> wing-loading (lift) and wing shape (control).

Very cool experiment.  I do note, however, that 1.5-2 atmospheres is a 
lot of pressure (relatively speaking).

>
> 2). When testing the vertical ascent capabilities of various 
> drosophilids,
> I found that on average, 90% of a given sample of wild-type D.
> melanogaster would starve if required to fly (at sea level pressure)
> up a vertical tube (~6.5 cm inside diameter, ~1.8m in height,
> and fluon-coated to prevent “cheating”) to obtain food. Pressurizing 
> the
> tube to only 1.2 atmospheres reduced the starvation rate to ~10%. My
> (tentative) conclusion was that, due to unsteady-state effects, the
> response of aerodynamic performance to flight medium density was
> non-linear, at least in small fliers.

Seems like a decent (if somewhat speculative) conclusion to me.  I 
suspect, however, that the major difference you saw is likely to be 
limited to small-bodied flyers.  In particular, I agree that the 
relationship is non-linear, and I suspect the curve plateaus at higher 
Re.  Just like you pointed out, the unsteady-state dynamics are 
probably playing a huge role in your result.

> As small fliers are more
> vulnerable to viscous effects than large fliers, it is my opinion that
> birds may receive even larger relative benefits from small density
> increases than insects.

I'm not sure about this.  Granted, since birds fly in a more 
inertial-dominated flow regime, it seems like density changes could 
matter quite a bit.  However, I suspect that the small insects are more 
sensitive to Re changes.  For one thing, I would think that they should 
respond more strongly to alterations of drag regime than a bird.  Small 
insects get more useful momentum flux out of drag, for one thing 
(though Drosophilia flies around a Re of 100, where vortex generation 
is still the primary source of momentum flux and L/D ratios are still 
well above 1)  The other issue, of course, is that tracheal diffusion 
rates are a rate-limiting step for insects.  As R. Dudley emphasizes in 
his text on insect flight, increases in oxygen partial pressure have a 
profoun
logical effect for insects.  Vertebrates get a boost as 
well, but to a more limited degree.

> Testing the load-carrying capacity of
> pigeon-sized birds at increased pressures might clarify the
> relationship between size, flight medium density and performance in 
> flapping fliers,
> just as variable-density wind tunnels were once used to manipulate the 
> Re
> number when designing airplane wings. Astonishingly (to me), this has 
> apparently never been done, although it appears straightforward.

That does seem like a good idea.

> On the peer-reviewed level--
>
> R. Dudley, P. Chai, and others performed experiments within the last 
> decade with hummingbirds and reduced flight-medium density which in my 
> opinion can only be classified as elegant. Of particular interest is 
> confirmation of the intuitive perception that wing-stroke amplitude 
> and frequency increase as flight-medium density decreases. The 
> corollary is that increased atmospheric density reduces wing-stroke 
> amplitude/frequency, with obvious implications for take-off scenarios 
> in large animals with long wings.

Yes, they're quite nice studies.  One advantage of the particular 
approach they took (using a Helium mixture) is that viscosity was kept 
relatively constant at varying air densities.  The question, however, 
is how much of a change in stroke amplitude and/or frequency larger 
flyers demonstrate when density increases.  I suspect that while 
atmospheric changes would have had an effect on ancient flyers, most of 
those changes would be compensated for with relatively minor changes in 
planform and/or kinematics.  The difference in maximum observed size 
between pterosaurs and birds is well over 2x however, and that seems 
like a larger difference than can be accounted for with a 12% O2 
partial pressure jump.

In any case, very interesting stuff (thanks for sharing the information 
on your insect flight experiments!), and there is obviously plenty of 
work still to be done.  I suspect that large pterosaurs would have been 
perfectly viable in today's conditions, but we'll see what further data 
shows.

Cheers,

--Mike H.