Re: Diving pterosaurs? (was Re: Lemurs and Feathers) (long)

["James R. Cunningham" <jrccea@bellsouth.net>]

David Marjanovic wrote:

> About 10 days ago, there was a thread on
>
> Klaus Ebel: On the origin of flight in *Archaeopteryx* and in pterosaurs, N.
> Jb. Geol. Paläont. Abh. 202 (3), 269 -- 285 (Dezember 1996)
>
> Well, here's what Ebel writes, and I think there are some reasonable arguments
> in this:
>
> "Rhamphorhynchoids [wrong, long-tailed ones] were not yet adapted to flying
> in the air. The Upper Jurassic *Rhamphorhynchus* has still the long bony
> tail of underwater flyers with a small vertical steering velum which could
> hardly produce an effect when flying in the air [amazingly few people have
> thought about this so far].

I think most everybody that is interested in quantifying animal flight has
thought about it.  Ebel seems to have forgotten that the small vertical tail
vane was located at the aft end of a loong moment arm, which could have made it
capable of producing substantial yaw command authority.  He seems to be
presuming that yaw forces are used to produce turns when flying, which isn't
true.  Turning forces are produced by using the wings to rotate the lift vector
slightly from the vertical toward the center of the turn.  Yaw forces are used
only to coordinate the turn (to minimise skid and slip -- known as 'keeping the
ball centered').

> .............., they did not only reduce their tail but also
> lengthened neck and skull at the same time. As a consequence of these
> skeletal modifications, the centre of gravity experienced a considerable
> shift forward [...].

Not necessarily.  This is too sweeping a statement.  What happened to the mass
of the hindlimbs while the tail was reducing and the neck and skull were
lengthening?  And what happened to the position of the center of lift?

> However, when flying in the air the centre of lift was
> positioned in front of the centre of gravity, leading to a moment that
> hampered the flight capability seriously.

Not necessarily, it just reduces the longitudinal stability, which can be
desirable in a flying animal because it increases maneuverability.  Besides, the
animal can sweep his wings fore and aft to put the center of lift wherever he
wants it to be with respect to the cg.  Sweeping forward is destabilizing, but
there are techniques for compensating for that.

> The animal was flight-mechanically unstable

Perhaps, but if so -- so what?

> and could stay in the air only by permanent flapping.

This doesn't follow from instability.

> Gliding was not feasible because of the labile balance condition.

Makes no sense.  Quetz is unstable, as are most flying animals.  Again, so
what?  Quetz appears to have been an adept glider.  I expect the ramphorynchs
were too.

> Shifting the centre of
> gravity below or slightly in front of the centre of lift to generate a
> condition of indifference or slight stability, in birds also the normal
> situation, became an imperative necessity and decisive improvement for
> flying in the air.

Pterodactyls, bats, and birds are all capable of doing this.  It would appear
that ramphorynchs were probably equally capable of it.  All it takes is sweeping
the wings back, which is stabilizing in and of itself.

>         This shifting of the centre of gravity led to a minimization of
> steering forces,

Moving the cg forward with respect to the center of lift tends to increase the
required steering forces, not minimise them.

> allowed a rapid reaction to disturbances, and thereby guaranteed optimum
> manoeuvrability.

The actual effect is just the opposite.

> Indeed, the shifting of the centre of gravity can be regarded as good evidence
> that
> rhamphorhynchoids were largely underwater flyers."

?????

> Here's how he has formed that concept -- from comparisons with Archie.
>
> rhamphorhynchoids must have been very moderate flyers that were hardly able to
> rise high above
> the water surface. The primitive airfoil section, compared to birds,
> suggests the same conclusion.

What was primitive about their airfoil section?

> "Flying in the air and the occupation of new ecological niches by the
> pterodactyloids became only possible after the completion of this
> modification.

Why?  The following argument seems to ignore aeroelasticity and the effect of
the pressure jump on the shape of the membrane.  I suggest reading Mike
Johnston's Ph.D. dissertation, which discusses this in detail together with a
superb mathematical analysis of the effect of the pressure jump and the
aeroelastic number on shape and geometric stiffness of the membrane.  And of
course, the reciprocal effect of the aeroelastic number and geometric stiffness
on the pressure jump.

> A simple flat integument as an airfoil section would have been
> sufficient only for small forms.

Why would it have been flat?  That seems most unlikely -- that durned pressure
jump again.

> Larger pterodactyloids had to modify also
> the leading edge area and to fit it with a thickened cambered airfoil
> section in order to avoid considerable flight restrictions.

Inserting the skeletal spar and support structures into the membrane would seem
to resolve this issue.

> Although the airfoil section was modified, the extent does not appear
> sufficient to allow
> good performance.

I'd like to see the math that supports that.  It seems to fly in the face of the
work done by Mike Johnston, Richard Smith, and Wei Shyy.

> The reason may be that large types could specialize to
> powerless gliding which was certainly their recipe of success. Soaring [...]
> [is easy enough to tolerate slight misconstructions.]

In quetz, the length of some of the wing bones varies by about 5% from left to
right side.  I expect this is typical of most flying animals.

> Restricted flight ability of large pterodactyloids also follows from the
> 'design' of the wing
> covered by a continuous integument which does not permit high maximum lift
> coefficients and consequently does not allow the low speeds of birds.

The maximum steady state lift coefficient of the quetz airfoil appears to have
been on the loose order of 1.9 to perhaps 2.2.  The highest maximum steady state
lift coefficient that I'm aware of in birds is about 1.63 for the Frigate Bird.
Both animals usually fly at a cl of about 0.9 to 1.0.   Modern birds do better
with maximum unsteady lift coefficient increases because they tend to flap
faster, mostly as a result of having wingspans that are generally shorter than
those of the big pterodactyls.

> Furthermore, the possibility of exercising an effect on the airflow by the
> interplay of feathers was not applicable. Thus, for the minimization of the
> induced drag, induced by the tip vortices, the wing had to be extremely
> tapered.

In both pterosaurs and some birds, the tapered tip is a function of the cranked
wing, since minimum induced drag does not come with elliptical planform or
constant spanloading in cranked wings.  If the wing were not cranked, an
elliptical planform would give minimum induced drag.

> This led to an overproportional wing span, compared to birds, which
> caused a restriction in the ability to take off from the ground, because
> increasing wing span results in a relative decrease of the gap between wing
> tip and the ground [sic]. [...]

This makes some presumptions about launch technique which may not be true.  Due
to probable launch technique, it does not appear to have been a problem for
quetz.