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Re: Senter 2006, Confuciusornis, and humeral mobility
On Mar 23, 2011, at 2:31 PM, Jason Brougham wrote:
> And what about non - glenoid means of raising the primary tips above the
> horizontal? Rotation of the humerus around its long axis, which happens in
> living birds, can alone raise the manus above horizontal. Extension of the
> manus could increase this effect, and the effect would increase with longer
> primary lengths, in that the primary tips would be that much farther above
Just getting the primaries above the horizontal is insufficient. Most of the
wing needs to get into a reasonable upstroke to exit circulation effectively
(see below). The problem is not so much that the wing needs to be above the
horizontal, specifically, so much as it needs to have enough room to build and
cut circulation; this happens to require a significant elevation above the
horizontal in birds.
> Your conclusions below seem to suggest that most of the upward and forward
> force produced from a flap comes from the recovery stroke, but surely it
> must really be the power stroke.
The upward and forward force is produced by the power stroke. However, the
recovery stroke will also be aerodynamically active, with opposite sign, unless
the wing can be very heavily rotated (hummingbirds, some bats, many insects),
in which case the sign remains the same (the flow sign, of course, is
reversed), or circulation on the wing can be cut so that it is relatively
inactive in fluid force production (at least rather less so than the
downstroke). In slow flight, non-apodiform birds pull the wing out of
circulation altogether, shedding ring vortices. In faster flight, it is
sufficient to merely reduce the circulation on the upstroke, slowing the animal
slightly, and shedding a continuous vortex stream or ladder. Bats do the same.
Circulation on the wing does not start or stop instantly, it takes time and a
fair number of chord lengths to start and stop full circulation. There are
mechanisms for building circulation very quickly (and most powered flyers today
utilized them, as do most swimmers), but they generally require extremely high
accelerations of the foils over very short distances. Basal birds would be
incapable of using them in cruising flight if the recovery stroke was
kinematically limited (although a transverse snap of the wing during launch
might get circulation started quickly during takeoff - that does not solve the
flapping problem, however).
> Ventral clapping may not produce aerodynamic forces like a neornithine, but
> the forces may well be appropriate for propulsion.
Ventral clapping would pull the animal downward, not upwards. It would,
admittedly, jump-start circulation on the wings just as dorsal clapping does,
but the counter-vortex formed in doing so would generate a low pressure field
ventral to the animal.
> If a starling can take off with a power stroke that starts at 50 degrees
> above horizontal and then continues down a further 90 degrees to the midline
> then, knowing nothing else, it seems that only a minority of lift, something
> like one third, is generated above the horizontal.
It's a bit less than a third for most animals, actually, but that's not the
problem. The problem is that 1) the recovery stroke is limited such that it is
difficult to get the wing out of circulation in time to avoid a negative lift
coefficient and 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. The arc of the wing above the horizontal is not
producing much fluid force, but it is building circulation so that the rest of
the downstroke may do so - it's winding up, as it were.
> On Mar 23, 2011, at 2:19 PM, Habib, Michael wrote:
>> It's an interesting idea, but generally speaking, the half-stroke ventral
>> clapping kinematic you're suggesting would not produce appropriate
>> aerodynamic forces. Without getting into at length, the upshot is that the
>> animal will produce about as much force counter to the direction it needs as
>> it will useful aerodynamic force (i.e. it will go down and back about as
>> much as it goes up and forward). It is an interesting thought, though - it
>> gave me pause for a moment.
>> --Mike H.
>> On Mar 22, 2011, at 10:39 PM, Jason Brougham wrote:
>>> On the subject of humeral mobility and powered flight, is it possible that
>>> the first stages of flapping could involve flapping the wings downwards
>>> In other words, on the lineage leading from a glider to a powered flier,
>>> could the early stages involve gaining lift and extending glides by
>>> flapping the arms from horizontal ventrally down to meet at the midline?
>>> Then, in later stages of refinement, the recovery stroke could be added by
>>> more complex shoulder modifications?
>> Michael Habib
>> Assistant Professor of Biology
>> Chatham University
>> Woodland Road, Pittsburgh PA 15232
>> Buhl Hall, Room 226A
>> (443) 280-0181
> Jason Brougham
> Senior Principal Preparator
> American Museum of Natural History
> (212) 496 3544
Assistant Professor of Biology
Woodland Road, Pittsburgh PA 15232
Buhl Hall, Room 226A