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Re: Arms into wings



I'm forwarding this question & answer to the list at Jim's suggestion.   
3 emails are combined into one so my apologies for any confusion.   



Subject:        Re: Arms into wings
   Date:        Mon, 8 Mar 1999 10:14:07 -0500
  From:        jmv <van02@earthlink.net>

"James R. Cunningham" <jrccea@bellsouth.net> on 03/08/99 11:57:08 AM

To:   Janet M. Vandenburgh
cc:
Subject:  Re: Arms into wings

 P.S. Janet, feel free to forward my post to the list if you wish.    I
didn't because you didn't, but it's information they might wish to have.
Jim


"James R. Cunningham" <jrccea@bellsouth.net> on 03/08/99 11:57:08 AM

To:   Janet M. Vandenburgh 
cc:
Subject:  Re: Arms into wings

 jmv <van02@earthlink.net> wrote:

> Hi James !
>      I have Wellenhofer's (sp?) book on Pterosaurs & have looked at the
> (Fantastic!!) detailed pictures of the fossilized wing membranes showing
> the bands of connective tissue laid out roughly parallel to the wing
> finger(at the closest area to the bone, but flaring out as the distance from
> the bone  increases).   That leads me to think I understand what you mean
> by "chord-wise structural support ".
>
>      However I don't have a clue as to what "anisotropic tension in the
> membrane" means.......or "compression battens".    I know what the words
> mean, but I can't 'fit' those definitions to the membranes.        If you
> have the time/inclination to clarify those, I'd be grateful.
>
>        Thanks for your List contributions.  :-)          janet



Hello Janet,
Isotropic tension in a membrane is the same in all directions like the
tension
in the skin of a balloon or beach ball.  Anisotropic tension is
different
in
different directions like the tension in the metal sidewall of an
above-ground
swimming pool.  Look at fingers 3, 4, & 5 in a bat's wing (the ones that
don't
form part of the leading edge of the wing), and look  at a photograph
of  a
hang glider from underneath (there are bunches of these on the web). The
slender internal struts that run from front to rear are battens (as are
the
ribs in an umbrella -- which functions the same way).  They are all
aligned
so
that tension in the membrane puts them into compression at roughly right
angles, moreso on the bottom side of the batten than the top, which
makes
them
bend downwards at the ends.  The fibers in a pterosaur wing don't
function
like
this.  They lie in the lower surface of the wing, are always in tension,
and
are intercalated so that a fiber that starts at the leading edge quits
before
it reaches the trailing edge, and the two lying beside it take up any
loads.
They have different elastic and tensile properties from the membrane,
and
transverse muscles between the fibers allow them to be shifted laterally
and
transversely with respect to one another to help control the shape of
the
lower
wing in both directions (bats have internal muscles in the membrane aft
of
the
elbow that that are not connected to the skeleton and control camber in
a
somewhat analogous way).  In pterosaurs the upper wing appears to take
its
shape aeroelastically to help control flow separation and speculatively,
to
allow the airfoil to be generally thicker than it would be otherwise. 
If
you
think of an umbrella as a circular wing with the center being a shrunken
leading edge, and the ribs (battens)  being bat fingers, you can watch
the
loads on the fingers develop as you slowly open the umbrella.  Pterosaur
wings
don't work this way, but please be aware that there are folks who still
consider the fibers to be speculative and think the wrinkles are caused
by
post-mortem shrinking.

Wellnhofer's book is the best single source of information on
pterosaurs,
but
the illustrations tend to make the wings (and legs) look thinner and
less
muscular than they actually were, and generally don't adequately reflect
the
catenary shape of the wings in the spanwise direction when viewed from
the
front. Note that in the photo of the suspended wing on page 141 of my
copy
of
Welnhofer's book, the tip doesn't turn back up at the joint between
phalanges
IV-2 and IV-3. It just looks that way because Wann hadn't finished
adjusting
the weights on the strings supporting the wing.  Also, in Quetzalcoatlus
(the
only type of pterosaur I've studied in any great detail, and the only
one
for
which I have a wing and shoulder girdle here at the house -- it's the
same
one
that Wann had hanging from strings in the photo on page 141), the wing
doesn't
seem to have hooked into the back leg, but rather at the massive support
structure formed by the leading edge of the sacrum.  Also, please note
that
the
Quetzalcoatlus 'jaws' in the image on page 144 aren't  -- that photo is
of
a
tapejarid from much lower down in the formation, and was not
contemporaneous
with Qsp & Qn.  All of Sibbuck's drawings show the tapejarid beak
grafted
onto
Quetzalcoatlus.  I'd be happy to send you a jpg image of Wann's new
Quetzalcoatlus skull reconstruction if you wish.  Quetzalcoatlus appears
to
have been a fresh-water skimmer that usually flew in ground effect,
though
he
was capable of soaring flight on the same order of efficiency as an
albatross
when needed.  I think they may have been migratory, moving with the
fluctuation
in food supply as they depleted the small lakes they appear to have fed
from.

Hope this helps.  Word pictures can be fuzzy sometimes. 'Specially with
dangling participles like that last one.

Best wishes,
Jim