[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Senter 2006, Confuciusornis, and humeral mobility
On Mar 23, 2011, at 7:00 PM, Jason Brougham wrote:
> That brings to mind one more big difference between Confuciusornis and any
> modern bird. The primaries are distributed along a second finger which is
> robust and retains mobile joints. Unlike any living bird, therefore, the
> distal primaries can be folded relative to the proximal ones. Wouldn't
> flexion of the wing finger, folding the primaries up before the upstroke
> begins, help reduce negative lift? It does in bats, right? And wouldn't
> such a snap of the wing finger at some point on the power stroke,
> accelerating the distal primaries to very high speeds, significantly
> increase lift?
I'm not sure if there really is that much mobility there, but even assuming
that there is, I don't think there is enough folding there to greatly effect
the fluid force on the upstroke. It would be rather less than the effect of
folding at the wrist or elbow, for example. Those mechanisms might all work to
partially minimize an upstroke during a fast gait, but they would be
insufficient during a slow gait (say, for climb out after launch) because it
won't break circulation - it just shrinks the area a bit. In bats, the folding
is widespread and complex; the flexion at the leading edge is just a small part
of the total deformation. Bats perform some serious origami on their wings
during the upstroke at low speeds.
In order to add much speed to the distal primaries, the snap would need to be
pretty pronounced. Based on the small excursion I am using in the rough
calculations, it's not nearly enough. If you can supply evidence that the
primaries had much greater mobility than 5-8 degrees then please correct me;
that would be interesting (but it needs more like 30 degrees or more, at least,
to make much of a difference). If there were a large excursion in the
primaries, it would help reach full circulation more quickly - maximum lift
would be less affected.
>> 2) there is insufficient excursion on the
>> shortened downstroke to build up to full circulation by the midpoint in
>> the stroke where the primary force production occurs.
> Can you say this with confidence without doing any calculations or
> modeling at all?
No, but I already did the calculations; they're actually pretty easy. Granted,
I'm just doing a rough version. However, the results are not promising for
powered flight with a non-elevated forelimb. The space needed to generate full
circulation without Wagner-effect beating mechanisms is reasonably consistent,
as is the flapping frequency relative to span, wing area, and body mass.
Between those two things, it's not too difficult to demonstrate that the wing
is unlikely to hit full circulation during the stroke. I think people might
give me the evil eye if I start typing out a bunch of algebra at the moment,
but I can punch it out if need be (plain text will make it extra fun).
> I fear we may be in danger of making a priori qualitative
> assumptions when the answer may be quantitative. In other words, an animal
> with limited dorsal mobility of the humerus may have LESS ability to
> generate lift than, say, a pigeon, but it may still have ENOUGH lift for
> powered flight.
It might, because I've only done a very basic run of the calculations, but it's
really quite unlikely at this point. There needs to be some pretty serious
additional factors in order for the lift coefficient to get where it needs to
be for effective powered flight in an animal the size of Confuciusornis.
Basically, it takes some special pleading at this point to find a solution in
which flapping with a wing that does not elevate above the dorsum does better
than an unpowered series. Unless something pretty substantial (more so than
mobile primaries) comes along to change the picture, I would put my chips in
with unpowered right now *if* the shoulder mechanics reconstruction by Senter
is accurate. [Incidentally, let me be the first to applaud your request for
quantitative backing; too few people do that].
> Picture how many different flight strokes could be possible if you add two
> points of wing flexion - at the first and second finger joints. The
> resulting complexity seems daunting to me, at least. I have read that
> bats, which can do this, often fly with different configurations of the
> left and right hand during each wingbeat:
The resulting complexity is indeed daunting if you try to permute all of the
wing positions, but fortunately that isn't required unless you're trying to
reconstruct specific details of wing kinematic. All we need to know is if
there is enough excursion to plausibly provide a near-immediate circulation
maximization. Based on the data available currently, it appears that the
mobility is insufficient to do that (though it could influence control).
Again, bats do much more than simply flex part of the first couple fingers;
they have a huge range of potential motions within the wing and several points
of camber control on each side.
Assistant Professor of Biology
Woodland Road, Pittsburgh PA 15232
Buhl Hall, Room 226A