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Re: Senter 2006, Confuciusornis, and humeral mobility

On Mar 22, 2011, at 10:47 PM, Jason Brougham wrote:

> Do you know of any gliders that do not have a caudal source of drag? I
> mean a tail, abdomen, tentacles or other structure that is positioned
> posteriorly from the main glide surface(s)? The only taxa I can think of
> are gliding frogs, but perhaps their pes webbing is larger and provides
> more drag than that of the manus. If a glider (and now I mean man - made
> gliders also) was not stabilized in this way couldn't they end up spinning
> like a frisbee?

Most habitual gliders do have tails, true, but several have quite low-drag 
tails (Draco, for example, can shift its tail to generate center of mass 
changes, but its drag profile is pretty small).  Note that many small 
rainforest arthropods glide and maneuver with no dedicated airfoils at all, 
however, and many of them lack a real "tail" structure (plus you can remove the 
abdomen and they continue to glide just fine - see work by Robert Dudley).

Lack of a tail will not make a glider spin.  It could make the glider more 
unstable in pitch (and yaw, to some extent), requiring active control.  As it 
is, manmade gliders are not using their tails in the same way as animal 
gliders.  Sailplanes and the like have a tail structure that does, in fact, add 
substantial stability.  In gliding animals it is a control surface (in most 
cases by producing both aerodynamic forces and by shifting the center of mass 
relative to the center of lift).  Flying animals (powered and unpowered) do not 
fly like a static synthetic glider.  This is a major concern I had with the KU 
study, as well - it is questionable whether launching a static, fixed-position 
gliding model is sufficiently informative for the task at hand.

> Half of Confuciusornis specimens do not have tail feathers. Has anyone
> ever tried one in a wind tunnel to see if it is stable or if it pitches
> wildly and spins around the long axis of its wingspan?

Not that I am aware of, though you wouldn't actually need to do a wind tunnel 
run to calculate if the animal is unstable in pitch.  If you can build a model 
for a wind tunnel, then you have already either estimated or assumed body mass, 
body shape, wing shape, wing position, and wing compliance.  At that point, you 
can just calculate the pitching moment at any given position by hand, and it's 
not terribly difficult to figure out if it's stable in pitch.  An afternoon's 
work, give or take, once you have all of the above variables (that being the 
tricky bit, of course).  Getting drag profiles is much more complicated, and 
that's really where tossing a model in a wind tunnel is helpful (such as the 
Microraptor Xu et al. run).


--Mike H.

Michael Habib
Assistant Professor of Biology
Chatham University
Woodland Road, Pittsburgh PA  15232
Buhl Hall, Room 226A
(443) 280-0181