Why the T-Tail?

Doug Haluza wrote:
A wake vortex is generated at each end of a wing generating lift in a
free flow. So the vortex is generated at both ends of the fin, not
just the "tip". Therefore, the T-tail does not produce one less
vortex. Also, since the vertical stab is usually at zero angle of
attack (except when maneuvering or flying with one wing low) there is
no vortex most of the time, anyway. So this is not a factor at all.

The Fundamentals of Sailplane design (pages 147-148) has a discussion of
the empennage types. A selective summary of this is..

* the conventional tail (fuselage mounted) isn't used because of poor
ground clearance
* the cruciform tail (ASW-17, Liable) improves the clearance but creates
increased interference drag due to the four corners created at the
intersection of the horizontal and vertical stabilizers
* the Vee tail is the most difficult to achieve the desired spin and
other control responses and tends to have higher induced drag
* the T-tail avoids all the above, with the high placed weight of the
horizontal surface being the main challenge

Not addressed in the FOSD book, but as Bob K mentions (and Waibel and
others), other factors affect the designers choices, such as aesthetics
and manufacturing costs.



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Eric Greenwell
Washington State
USA