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Re: Origin of flight - long
Norton, Patrick wrote:
> On 4/29 Dinogeorge wrote:
> >The only way to answer this question is to build a model of
> Archaeopteryx and see how it flies. Do some wind-tunnel work. Tails do
> contribute drag, no
> question, but the tail of Archaeopteryx is not like the tail of any
> birds, and it would be premature to accept your statements without some
> supporting evidence.<
> I agree, and I've thought about doing it. And I am aware of at least one
> person who is considering such a project.
Because of the body drag anomaly in wind tunnel tests on dead birds and
models (measured drags in the wind tunnel that differ from the wind
tunnel tests on the same species of living birds by up to 800%), there
might be some difficulty in validating the wind tunnel results. I'm not
suggesting that the wind tunnel tests not be done. They should. But
they should be paralleled with calculations which would allow another
method for comparison.
> > In the fossils, the tail is of a size comparable to the wings, and I
> don't think this is accidental.
Nor do I.
> The tail feathers are apparently
> horizontal and have a positive dihedral angle, for lift, not vertical,
> like a rudder or fin. If Archaeopteryx was more a glider than a flapper,
> its tail would seem to be ideal for maintaining aerial stability.<
> There are two kinds of lift. A typical wing (bird or plane) has its long
> axis perpendicular to the airflow (ie, a long "leading edge"), a cambered
> upper surface and a flat, or relatively flat, lower surface. This allows
> the wing to function as an airfoil. As the airfoil moves through an
> airstream, lift is produced as a result of the Bernoulli effect--the
> creation of a pressure differential (lower on top than on the bottom.)
This may be a bit of an oversimplification. Many airplanes with
cambered airfoils can fly quite handily upside down, with the curved
side on the bottom. They just have to adjust their angle of attack to
compensate. The low pressure side remains on top of course.
> The wings of Archeopteryx were airfoils and produced lift of this type,
> but the tail was not; for at least 3 reasons: its long axis was parallel
> to the airstream; it had no true "leading edge", and it was not cambered.
> The type of lift generated by the tail would have been like a plank--or
> inclined plane--whether it formed a positive dihedral or not.
Planks and cambered airfoils have the same lift slope (approximately
0.116 per degree) and produce the same increment of lift per increment
of angle of attack. The camber just lowers the angle of attack for zero
lift, creates a nose-down pitching moment, and usually lowers the
profile drag coefficient. The induced drag is a function of lift, and
is independent of airfoil camber or lack thereof. Induced drag
predominates at speeds below max L/D; profile drag predominates at
speeds above max L/D. For a gliding wing, total wing drag is at a
minimum when the two are equal at max L/D. Flapping creates additional
lift through unsteady effects. For example, a gliding pideon can achieve
a CL of about 1.5. Flapping at 9.4 Hz, he can reach a CL of about 3.4,
so on launch about 56% of his lift is coming from unsteady effects.
> A positive
> dihedral does not create lift, but it would--as you suggest--provides
> stability while gliding (but I can't see how you determined from a flat
> fossil that the tail formed a dihedral. A tail with a positive dihedral
> would form the letter "V" when viewed in tranverse section. When a
> pidgeon glides, for example, its wings form a positive dihedral.) A flat
> plank will generate a lifting force if it is inclined at a small angle of
> attack, but that also creates a large amount of drag. Additionally,
> although it's true that most airplanes have vertical rudders to correct
> yaw (side to side movement about a vertical axis), the horizontal tails
> of birds do assist the wings in controlling yaw by twisting, therefore
> performing like a rudder. But the primary function of a bird's tail is to
> create drag to reduce airspeed and assist with such things as landing.
At the moment, I haven't looked very hard at Archaeopteryx, so the
following should not be considered as a hypothesis. I'm just throwing
out food for thought. If the animal was producing significant tail
lift, then he was also producing a significant nose-down pitching
moment. There are only two efficient ways to control that. Shift the
center of lift of the wings forward, or shift the cg back, or both. I
would suggest that if the animal's hip structure allowed it, he may have
trailed his legs in flight somewhat like a flamingo, so that they
compensated to some extent for the pitching moment. Also, if (this is a
big if - I haven't checked to see if it is true), I repeat, if his tail
feathers were mounted near the horizontal plane at a significant angle
to his long axis, then he may have been able to use them as a
multi-planar lifting body, seperating them in a manner similar to the
wing tip primaries in buzzards and pelicans, and for the same reason,
increasing Ozwald's efficiency factor and the effective aspect ratio,
thereby reducing the induced drag. Perhaps each feather could even have
been placed in the upwash of the tip vortex from the preceding feather.
If this were true, I would expect the tail to gradually get wider as you
go aft, with a fairly sudden triangular taper near the tip (I don't know
if it does this or not). Ozwald's efficiency factor could be boosted
even more if the legs were tucked in under the centerline of the tail to
create a high-wing fuselage effect. This would let the tail support
itself, the legs, and perhaps the rear body with a slight deficit to
compensate for the pitching moment of the wings, and would leave the
wings supporting only themselves, the forebody, neck, and head, and
would allow them to do their thing without being canted so far forward.
An animal in this configuration could be expected to have a body profile
Cd on the loose order of 0.45, so the wings would need to produce
significant thrust to maintain level flight. However, my initial
unsubstantiated hunch is that they wouldn't require a pectoral/total
mass ratio of more than 5-6% to do so. By comparison, a 0.4Kg pigeon
has a ratio on the order of 17%, sufficient to climb at about 490 fpm.
In summary, I think you all have a neat discussion going on.