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Re: Tail feathers

tlford@ix.netcom.com wrote:

> 2). Are tail feathers really needed to fly? I know birds do use them to
> break and manevor, but just to fly around, is it really needed?

In general, in non-turbulent conditions, no.  But loss of the tail
affects both modes of longitudinal dynamic stabilty, and spanwise
vorticity shed from the wing can no longer impact the tail, so the
effective lift slope of the wing is modified as well.  The animal has to
increase the amplitude and frequency of its wing sweep adjustments to
compensate for reduced damping, so compensation for turbulence requires
more effort and its overall efficiency falls off. Without the tail, the
frequency of the short-period mode will increase so that the period may
approch the animal's reaction time, thereby allowing less margin for
error. With no tail, yaw stability becomes more marginal in a similar
manner. Without the tail, the animal will be grounded during some
weather conditions when he would otherwise have been able to fly.

>The reason I ask is because of Pterodactyloid pterosaurs don't have tails,
> and I've often wondered how they would manever with out them. Possibly
> very well, if they were like the mocking bird.

I would expect that pterodactyloids used that portion of the
brachiopatagium between the elbow and the femur (in come cases, tibia)
combined with the uropatagium between the tibia and tail as an
approximate equivilent to the bird's tail.  Their flexible membrane wing
also automatically modifies camber with load in a manner which augments
the stability derivitives in the appropriate direction.  Bats do this as
well. Although feather deflection in birds will result in a somewhat
similar trend, it doesn't appear flexible enough to allow as much
augmentation (I have not seen or done the calculations required to
support this last statement - it is only an opinion).
> Tracy