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.

However, the horizontal stab is normally at negative angle of attack,
generating a down force. So vertical stabilizers on the end of the
horizontal stab could reduce these vortices. The tradeoff in extra
section thickness, and interference drag may offset this. For a
T-Tail, it would make the whole Torque/Moment thing much worse as
well.

(JonB) wrote in message om...
Marian Aldenhövel wrote in message ...
Hi,

I have noticed that most if not all modern gliders are built with a
T-Tail (not sure about the term, I am talking about the elevator being
located at the top of the tailfin). While most power-aircraft I know
right up to the airliners have it at the bottom.

What are the aerodynamic or constructive reasons for that?

Ciao, MM

It's nothing to do with aesthetics. It's just a happy coincidence
that aerodynamically efficient structures are beautiful things (and
not just for gliders).


Three reasons that may be significant are that:

1) The stabiliser is likely to be raised above the level of any crop
that the pilot may land in - therefore it will not be removed by
injudicious field-selection.

2) Also, I think I have read that a T-tail configuration produces one
less vortex than a conventional tail arrangement:- a vortex is spawned
from the end of each free tip of a tail surface (stabiliser or rudder)
therefore the top of the fin will not produce a vortex in a T-tail
arrangement (as the stabiliser prevents the fin from having a free
tip in the air stream). A vortex causes drag, therefore a T-tail will
be marginally more aerodynamically efficient.

3) Spin recovery is easier when the stabiliser is not in the
turbulence of a spinning main-plane - as is more likely to be the case
with a T-tail. Therefore a T-tail may be a safer aeroplane for
low-time pilots.



Jon.