[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]


Thought I'd re-open a discussion on biomechanics for a moment, break the
pace on hadrosaurs and ovis for a little and talk about something that
Colin wanted my response to. Sorry for the delay, I had a bit more to
think about on this, and perhaps I felt what i had replied to before,
general biomechanically speaking, was sufficient at the time. For a
belated replay on concepts of flying snakes and how an animal without any
wings can "fly," I will re-introduce what I had written and what Colin had
written in reply (in this case, all text surrounded by double corner
brackets <<>> or mine, and single brackets <> are Colin's, and my current
reply will be without brackets):

I wrote:

<<The arms provide the power, feathers the lift,>>

and Colin McHenry (cmchenry@westserv.net.au) wrote:

<Any structure can provide lift under certain circumstances. I can make a
brick fly if I propel it fast enough at the right angle of attack.  A
facetious point, but worth remembering.>

  Lift in any tossed object is one thing, throwing something, including
oneself, with effects designed to catch air rather than those that do so
accidentally, such as a brick, are another.

<<In birds, flight feathers can generate lift at relative air-speeds (i.e.
the speed of the airstream over the surface of the feather - not the total
airspeed of the animal) that are low enough so as to be within the range
that a muscle powered vertebrate can induce by flapping its wings.  Even
flight feathers are not, on their own, a magic ingredient for producing
lift - they do so within a certain range of relative air-speeds and angles
of attack.  All feathers - and any other integumental structures - produce

It might sound like an obvious and silly point to make (and Jim, kick me
I've got it wrong), but it is worth remembering because if you forget it
then it can lead to some faulty logic, as in...>

<<but feathers will not make the animal fly on their own, contra proposals
about *Longisquama*'s supposed aeronautical ability.>>

<Well, we've established that feathers on their own don't make any animal
fly, so to use this logic to make statements about _Longisquama_'s
flight performance seems a little disingenuous.  In fact, the presence of
feather-like structures on _Longisquama_ might suggest that it is worth
doing some basic analysis to see if _Longisquama_ might have been able to
fly on its own.  I guarantee that I could make it fly (by shooting it out
a cannon, for example - it worked for Rocky in Chicken Run, after all),
a better question would be whether it could 'fly' - either by powered
flight, or by gliding/parachuting from an elevated surface - in a way that
might have been realistic biologically (not to mention non-fatal).   It
seems to me, just from the appearance of the beast, that some sort of
might have been within the boundaries of biological achievability for

  I have proposed, to my credit perhaps, that models be tested on the
aerodynamics of a structure and system as is found in *Longisquama*'s
holotype. The problem here is a radiating, non-overlapping set of vanes
with little assymmetry and distal inflection that correspond to no known
flight structure. This does not mean they couldn't fly, but that
conventional theory doesn't hold for them, and any true testing should be
mechanical and practical in nature. However, I was trying to say that the
basic design is flawed in that more than their structure, they are not
anatomically sound, and the idea of them as flattened, venate
"parafeathers" requires data we do not have. It is also impossible to
assess their cross-sectional anatomy without casting and modelling the
reverse side of the cast as a duplicate of the other, where study by Reisz
and Sues opine that these were bubble-like, bulbous and inflated objects,
rather than flat vanes. The structures do not essentially parallel design
in feathers in that they are unlike any aerodynamic structure entirely.

<<Studies in flying snakes show that the skin flabs allone (integument) do
virtually nothing but impose drag on the snake,>>

<???!!!  But surely imposing drag - and lots of it - is exactly what a
parachuting animal needs its integument to do?  And this is a parachuting
animal, right?  I mean, just because it's called a flying snake, surely
no-one was seriously suggesting that it moved through the air by
powered flight?>

  It's not drag that makes something fly, in the sense of a bird anyway,
but the powering of the structure that produces drag to increase the
resistence by forcing it down and backward, thereby impelling the
structure, increase presure, and exceed thje pull of gravity and create
positive upward/forward movement, rather than reducing rate of falling.
All a flying snake and the flying frogs do is slow their descent. Snakes
also "throw" themselves brick-like from trees so that they may land on a
much lower, farther target, another tree say. Feathers, and skin flaps,
and whatever *Longisquama* has if they are not in fact leaves (I kinda
doubt that, though), do nothing to flight, the arm anatomy and mechanic do
the flapping, and that's how they fly. This is essentially all I was
pointing out. 

<It's not gliding in the sense of a glider 'plane, or a soaring bird, but
surely most animals that get called 'gliders' - flying frogs, flying
squirrels, sugargliders, for example - are really parachutists of some
description (with varying ability to control their descent)?  The only
'glider' that might be closer to a glider 'plane that I can think of
offhand is _Draco_.>

  *Draco* is not capable, to my perusal of the literature, of being able
to produce positive lift above the angle and moment of descent, which
human gliders occassionally do because they are large and their flight
territory is surrounded by thermals. Describing these animals, the
parachuting structure is one large, continuous membrane, and flying frogs
have, relative their mass and size, four medium-sized membranes.
*Longisquama* has several different sized, largest in the middle or
rear-trunk, slats that radiate rather than forming a single membrane; in
thisd they resemble the birds primaries, yet these act differently
aerodynamically from a membrane since they are designed to allow air to
flow through them. Such a structure in a parachuter is counter-productive
to drag, or *Longisquama* was one hell of a poor parachuter, and
exaptation developing longer, borader structures would be ridiculous for a
poor glider/parachuter/incipient flyer. Mammalian leapers like sifakas
have mats of brachial hair that clump and form essentially a membraneous
structure, rather than the millions of slats they would be if they were
stiff and radially arranged and non-clumping.

<Now, I can't resist observing at this point that feathers are excellent
structures for producing drag, and lots of it :-)...>

  Yes, but feathers do more than generate drag, they have an assymetry in
the outer tail and wings that are designed to produce pressure variants
above and below, and work in a flapping motion by twisting during the flap
so as to produce minimal drag on the upstroke, maximum drag and maximum
pressure on the downstroke. I beleive only soaring birds use their wings
as more of a gliding mechanism due to absence of a regular flapping
because they move from thermal to thermal, as does a human glider.


Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

Do you Yahoo!?
HotJobs - Search new jobs daily now