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Re: uprightedness (was Re: SICB Report, part 1 (long)]



Betty,

I'd suggest that losing the tail is a prerequisite for uprightness.  I
couldn't resist that comment.  Actually, I think the two went hand in hand.

It doesn't appear to me that the constraints of fluid mechanics result in
aerodynamic considerations forcing a shortening of the tail. Pheasants do
pretty well with a long tail.  Archies' tail doesn't appear to me to have
been a handicap in flight -- more of a benefit, I'd say.  Speaking from
memory (I'm too lazy to drop offline and look up my calculations), Archie
flew with a tail upload, with the tail CL at about half the wing CL
(supporting most of the weight of the legs and producing about twice the
drag of the wings at L/Dmax).  When gliding, his L/Dmax (flattest glide
ratio) was about 4:1 (compared to a pigeon's 6:1).  If he had a pectoral
mass fraction on the order of 10% (I don't know what it really was - haven't
looked it up or calculated it - do any of you folks have an estimate?), then
his rate of climb would have been about 290 fpm in today's atmosphere
(compared with a pigeon's ROC of 490 fpm with a 17% pectoral mass
fraction).  I don't see the lack of a supracoracoideus as a disadvantage
since Archie probably didn't take off flat-footed from a standing start like
a pigeon does, and neither a cursorial launch nor an arboreal launch would
require the supracoracoideus.  In essence, very preliminary calculations
make me suspect that Archie was a fairly effective flyer, probably a better
fit for his specific flight niche than a pigeon would have been.  On the
other hand, I also suspect pigeons fit the pigeon niche better than Archie
would have.  In short, Archie's flight capabilities don't appear to be all
that primitive from a strictly aerodynamic/fluid mechanics perspective.
Incidentally (and I haven't checked this out), if Archie were capable of
spreading his tail feathers laterally, producing slots  like pelicans and
buzzards when they spread their wingtip feathers, then the induced drag of
archie's tail would have fallen to about half that of the wings, and his
glide performance would have approached that of the pigeon. But with the
tail feathers spread and separated laterally (if that were possible), the CL
max of his tail would would have fallen about 30%, so that it would no
longer have been on the order of 50% greater than the steady state CL max of
his wings.  Steady state, a pigeon's wings can achieve a CL max on the order
of 1.54.  I'd expect Archie's wings to have a steady state gliding CL max at
least 1.3, and probably closer to 1.5.  His tail would have been able to
achieve about 50% more than that when in the low-aspect ratio
configuration.  When flapping, a pigeon can use transient unsteady effects
to achieve a CLmax of about 3.4.  Preliminary to calculations, I'd speculate
Archie would be able to get up to roughly two and a half or three before the
lack of a supracoracoideus would limit the unsteady lift contribution of his
wings.

Cheers,

Jim Cunningham

Betty Cunningham wrote:

> Birds fly with their body parallel to the ground.
> As James will point out, it's a damned efficient shape for flight.
> So the mechanics of flight do NOT call for uprightedness.
> However most birds are upright.
> The ratites seem to be less upright than many of the other bird lines
> (which is why they make such good models for theropods) but most ratites
> don't fly and may not have in their lineage for a very very long time.
>
> I suggest that uprightedness was a prequisite for losing the tail.
> You can fly (abeit awkwardly) with a tail for drag but the length can
> get in the way when you stand upright.
>
> How upright was Archie?  Caudipteryx?  Seinoeropteryx?  Those danged
> duckbird things of the Eocene?
> How much tail did each one have?
>
> -Betty
>
> > Nicholas J Pharris wrote:
> > My suspicion would be that at the same time as aerodynamic
> > considerations forced the shortening of the tail in the line leading
> > to modern birds, balance was maintained through the locking of the
> > femora in a forward position (effectively sacrificing a leg segment in
> > return for an aerodynamically suitable tail), along with  some
> > reorganization of the posture, as Betty suggested.>