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Re: Disney's Dinosaur Trailer




Betty Cunningham wrote:

> Question is, would pterosaurs that big have as much mass as a bird that
> big (my understanding is that pterosaurs would be lighter), and if
> depicted accurately would us bird-biased people think it 'felt' right?

Contrary to the literature, pterosaurs aren't that much lighter than birds.  
Their
skeletons have thinner compacta walls along the shaft, but the ends are fairly
similar in construction. And the reduced weight of the bones and lack of 
feathers
may be fairly insignificant compared to the added mass of the larger neck, 
skull,
and legs (at least in Quetzalcoatlus).  The pectorals of a pterosaur may be
slightly smaller than those of a bird of the same total mass, but the 
pterosaur's
inner arm muscles will be more massive than the bird's, because they powered 
their
flight stroke a bit differently.  In any event, for those birds and pterodactyls
that were grossly similar in size and planform, the effective section modulus of
the bone/tendon complex would also have to be fairly similar, because they are
resisting similar loads with similar allowable stresses and strains. Gross 
weight
of the skeleton wouldn't be all that different considering that, though 
pterosaur
skeletons are more lightly built than bird skeletons, the pterodactyls (Q again)
tend to have more total length of bone (longer head, longer neck, longer arm,
longer leg).  The femur and tibia of Q species is pretty durned big and is not a
fragile thing (Betty, I'm sending you a jpeg of a Qsp femur and tibia laid out
next to a scale -- they're big).  In both animals, bone weights are a small
fraction of all-up weight.  Very qualitatively, subtracting one small mass
fraction from another small mass fraction results in a still relatively small 
mass
fraction difference in the bones of the two animals.  Muscle, fat, and other 
soft
tissue densities can be expected to be similar.  Wing membrane thickness' of
pterodactyls seem to be generally underestimated in the literature as a 
carry-over
from a mm vs. cm typo in a paper that dates from the early 70's.

> So next question is: which way do you do it?

I'd make them about as massive as a bird of the same size, but give the 
pterosaur
greater inertia in the wing (more weight outboard of the shoulder).

> Samuel Barnett or Martin Barnett wrote:
> > Dan writes:
> > > what Walking with Dinosaurs could have been if they had had Disney's
> > > budget.  By the way, is THAT pterosaur flying correctly?

Dunno, I didn't see it.  Would like to, though.

> , angle of  attack more believable,

What was the AOA in the trailer?  Pterodactyl wing camber was such that they
developed a lot of lift with a very low angle of attack.  As an example, if they
load the wing with a positive AOA, and then lower the AOA, they can maintain a 
CL
of about 0.7 at a zero AOA.  Since that is roughly the CL at best L/D, having a 0
AOA for that CL tends to minimize the profile drag of the wing at best L/D.  In
cruise, the leading edge can be expected to be slightly lower than the trailing
edge is  at any given section (yes, this is quite possible in a membrane wing.
Once the animal establishes camber under load, he can maintain positive non-zero
camber even with a mild negative AOA.  What he can't do is unload the wing for 
any
significant time.  If he wants to invert camber, he has to do it quickly, and if
he inadvertently unloads the wing, he has to reestablish a load in short order 
or
flutter will set in and make his life miserable.

>  Please > note, I'm NOT saying pterosaurs really flew like this.  If you ask 
> me
> it
> > looks far too much like a frigate bird's flight technique, but hey - if it
> > works, do it!

Frigate birds' wings are among the most lightly loaded of all flying 
vertebrates,
big pterodactyls' among the most heavily loaded.  And they flew in a very
different range of Reynolds numbers.  But there would still have been some 
visible
similarities in flight.  And if you compare a small pterodactyl to a frigate 
bird,
the visual similarities would have been remarkable.  But I'm not saying the two
flew just alike either.  We can be reasonably sure that they didn't.  But in 
this
case, I agree - if it works, do it.

Martin, how would I go about getting a look at a videotape of that trailer?

Best wishes to all,

Jim