Re: Great in the air, not so good underwater

[don ohmes <d_ohmes@yahoo.com>]

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. 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.]

I can offer some sketchy, unpublished empirical observations--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. 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).

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. 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. 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.



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.

Don


----- Original Message ----
From: jrc <jrccea@bellsouth.net>
To: mhabib5@jhmi.edu
Cc: dinosaur@usc.edu
Sent: Saturday, December 9, 2006 9:09:55 AM
Subject: Re: Great in the air, not so good underwater


----- Original Message ----- 
From: "Michael Habib" <mhabib5@jhmi.edu>
To: <dinosaur@usc.edu>
Sent: Friday, December 08, 2006 6:50 PM
Subject: Re: Great in the air, not so good underwater


> Just the same, the running launch system has particular constraints and 
> benefits.

I agree.

> Constraint pretty clearly prohibits the forelimb-assisted launch cycle in 
> birds, and thus I doubt they'll ever produce volant members as large as 
> pterosaurs.

Yes.  I suspect that there may be constraints due to feather morphology as 
well, but that's just a hunch.

>  Stabilizing selection is probably hard at work, too, though: despite 
> denying some helpful launch modes, avian bipedality has 
s.  In fact, being bipedal gives birds access to some launch 
> solutions denied to pterosaurs and bats; the ones I have in mind work best 
> at small to medium body sizes.

I agree.
>
> Still, I'll bet nothing beat seeing a Quetzalcoatlus catapault off the 
> ground...makes me sad that they're gone.

Or Hatzegopteryx, or Arambourgiana (sp??).  I tend to think of them as if 
they were still alive, and it jolts sometimes when I'm forced to think about 
their demise.  A cautionary tale for developing methods of meteor control 
during the next few centuries.

All the best,
Jim