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Re: pterosaurs, bats, flying theropods

Original Message by James R. Cunningham
Tuesday, 24 December 2002 01:37

> David Marjanovic wrote:

[Aerodynamics questions at the bottom.]

> > And, apparently, dissimilar in another crucial aspect: between the
> > actinofibrils, there was no space for skin muscles, was there?
> [...] Also, it might be possible for
> 'skin' muscles to overlay the actinofibrils.

Which may or may not be elastic enough to let the wing change shape. This 
could be the most difficult part to find out. -- Looks like the skin overlay 
the actinofibrils so closely that any skin muscles in the brachiopatagium 
should be visible. But of course I won't fix myself on that.

> > Looks like
> > that, having stiff wings with a fixed shape and lacking an alula,
> > pterosaurs were limited to flying the ways swallows, swifts,
> > confuciusornithids and albatrosses do/did it.

(Pretty much what I read out of, or into, the few papers I've read on that 
topic. Including Rayner's chapter in the Ostrom Symposium volume.)

> What on earth would make one think that pterosaurs had stiff wings with
> a fixed shape?

Depends on how stiff the actinofibrils were. As they were only 0.05 mm thick 
and not exactly bone, I probably exaggerated, even though Wellnhofer (p. 
152... of the German version at least) shows 5 of them per mm, and the bulges 
they make into the skin touch. -- Of course I don't mean they were unable to 
slide past each other or spread a little; this would have made any wing 
folding impossible. I mean practically fixed wing breadth and camber, unlike 
the situation in bats and more like in birds. I should have written that.

> If they did, why would they have needed all the
> structural specializations for controlling the aeroelastic number?

Got me as expected -- I don't know what the aeroelastic number is.

> > HP Jim Cunningham wrote:
> > > For low frequency flapping, the
> > > membranes will likely win hands down, at least in pterosaurs.
> >
> > Why in pterosaurs?
> Because during low frequency flapping, lift from unsteady flow effects
> do not predominate, and the maximum steady-state lift coefficient for
> pterosaurs is about 30% greater for pterosaurs than it is for birds and
> 40-50% greater than that for bats.  Pterosaurs appear to have a
> steady-state CLmax of about 2.1-2.2.  Birds typically range from roughly
> about 1.54-1.65.  Frigate birds have the highest avian steady-state
> CLmax that I'm aware of, at about 1.63-1.65.  Bats range roughly from
> 1.3-1.5.

So this depends on the wing shape rather than on the presence or absence of 
feathers, right?

In the following I will represent what I think I understand of aerodynamics 
as facts, to save space and to make it easier for others to point out just 
where my mistakes lie.

> > Apparently that's why they don't need an alula.
> Doesn't the thumb in some bats act as a functional equivalent to an
> alula, just as fingers 1-3 of some pterosaurs do?

Can it, thin as it is? An alula is broad enough to bend the airstream from 
below rearwards over the wings. (I don't know of studies that point either 
way, of course.) And then you say "some" bats...
        3 pterosaur fingers together look more like an alula. Not a very big 
but then albatrosses, swifts and hummingbirds don't have big ones either. 
Such birds simply avoid slow speeds (respectively move their wings so fast 
that relative airspeeds are always high).
        Anyway, I forgot the propatagium which has a similar effect to an 
directing, when it is spread downwards which increases camber a lot, air that 
hits the front part of the wing from below at low speeds backwards instead of 
forwards (forwards would be stalling). Likewise, assuming the hindlimbs were 
involved in the wing at least as much as in the famous *Pterodactylus* 
specimen, they were certainly able to deform the wing. Not to mention a 
(cr)uropatagium in the first place which was able to add a lot of camber.

Merry Xmas again, who knows which of my 2 mails today reaches you first :-)