[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]


Tim Williams wrote:

> A continuous lift surface allows the torso+wings act as a single
> functional unit.  This expanded surface (torso+wings) is able to push
> against a greater volume of air during the descent.

Then, I take it you are looking at vortex lift with a low aspect ratio lifting
body as opposed to lift mechanisms with a relatively high aspect ratio wing?

> As for patagial gliders (flying squirrels, flying possums, colugo) they have a
> gliding surface (patagium) strung between the fore- and hindlimbs and
> contiguous with the body wall on each side of the body.  Hence, the patagia
> meet the torso to form a single effective gliding surface.

Low aspect ratio vortex lift.  Somewhat similar to landing mode in a Concorde,
but not to bird flight.

> I also have doubts; I suspect tertials might have a lesser chance of being
> preserved than the secondaries and primaries.

Yeah, me too -- but I don't feel very strongly about it.

> You're going to have to explain "CL max".  ;-)

The maximum achievable lift coefficient for a specific wing in steady-state
conditions.  For a pigeon it is about 1.54, for a frigate bird, about 1.63 (this
is the highest steady-state CLmax that I'm aware of in birds).  For an unflapped
Selig S1223 airfoil section, it is about 2.2.

> Oh, I see what you're driving at.  But are these aims necessarily mutually
> exclusive - lift and maneuverability.

No, they're not.

>  If the proavian is flapping its proto-wings, either for thrust (such as
> Burgers and Chiappe's
> running-takeoff model)

I find this unlikely, for the reason I mentioned earlier.  Besides, these
beasties appear to possibly have had sufficient leg strength to achieve stall
speed by leaping without having to run.  More like a quail launch.

> or steering (a la Garner et al.'s descending "pouncing proavis") would it also
> be a good idea to enhance your effective lift surface as well?.

Sure, though I don't think it would happen because it was a 'good idea'.  I'd
expect it to happen through random selection.  I also expect it would have been
just as useful in escaping predators as in attacking prey.

> Hence, if the feathers are developed *equally* down the length of the
> forelimb, from wing-tip to armpit, the outer feathers can generate thrust
> and the inner feathers can generate lift concurrently.

Sure -- and the inner feathers can generate thrust and the outer feathers can
generate lift concurrently.  These two functions are not entirely separate.

> Increasing stability might be an advantage at an early stage of proavian
> evolution, when the biped just wants to alight feet-first (since the
> forelimbs can't be used for support, like a cat).

This seems to imply that the animal wants to be in flight before the wings are
developed, which I tend to think unlikely.  I'd prefer a scenario where the
avian wings initially develop for some other purpose and gradually become useful
for flight.  You're probably aware that I'm more interested in pterosaur flight
than in bird flight.  Though I prefer this scenario for birds, I do not for
pterosaurs.  Also, using the low aspect ratio tail to develop sufficient lift
(upload) to offset the weight of the tail plus most of the weight of the
hindlimbs will provide yaw stability while also providing an effective tail
download that will enhance longitudinal stability  (because of the usual deficit
--as you no doubt know, aircraft generally fly with a tail download to offset
the nose-down pitching moment of the wing -- animals don't always do that).  I'd
expect the animal to be moving in the direction of decreased stability almost
from day one.