Re: Origin of flight

["Norton, Patrick" <Patrick.Norton@state.me.us>]

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   
living
birds, and it would be premature to accept your statements without some   
other
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.

> In the fossils, the tail is of a size comparable to the wings, and I   
don't think this is accidental. 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.)   
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. 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.