Why the T-Tail?

"Robert Ehrlich" wrote in message
...
Udo Rumpf wrote:

Hi,

Thank you all, I have learned a lot. I have also come up
with two more things to consider, both rather minor I suspect:

- The control linkages are propably more complicated
in a T-Tail (con).
- With a T-Tail you can build the elevator in one piece so you
can rig and derig more easily (pro).

Now why are we not seeing more V-Tails? The main pro for T-Tails
seem to be:

- Good ground clearance
- Less drag
- Operates in clean undisturbed air

How does a V-Tail stand up against that?

Ground clearance is not an issue (I speak from experience)
It is questionable as to it having less drag. The theory says yes.
In practise it is not as easy to design a V tail that can match the T
tail.
The lower part of the vertical stab on a T tail is in disturb air as
well.
In case of the V tail you would have two surfaces in the disturb air.
The mixer is a simple and light weight mechanical device. If built and
installed right cross interference is minimal.

Udo

The V-tail is inherently less efficient than tails with separate
horizontal
and vertical surfaces (conventional and T tails) as a pitch stabilizer.
For any speed except maybe just one, the horizontal stabiliser has to
provide
some vertical force in order that the glider remain trimmed. But in order
to
obtain the same vertical force from a V-tail, the normal forces on both
surfaces
need to be higher than the half of the total vertical force, because only
their
vertical component is useful, there are also horizontal components which
cancel
each other. But this increased normal force is lift and so produces an
increased
induced drag.

Not very important if the V is very flat, but then the efficiency in yaw
control,
i.e. as a rudder and fin, is poor and a similar argument may be developed:
now we
are interested in the horizontal component and the vertical (higher)
components are a
nuisance increasing induced drag. However no such component exists in
steady straight
flight, so the inconvenience is less important.

It may happen that in a very well suited situation of steady turn the
above argument
may be reversed in favor of V-tails: other tails need down elevator forces
and outside
turn rudder force, the resulting force being closer to the horizontal
direction than both
the preceding one could be provided with less induced drag by just one of
the ruddervators
if properly oriented. But I think that in performance oriented designs the
priority is
to minimize the drag in straight flight, and anyway this would be in favor
of V-tail just
for (some range around) some very well suited bank angle and speed.

Robert
You are right on all counts.
I think it is still worse due to the fact the elevator and rudder action
has to be combined.
The elevator/rudder chord for the HP V Tail, for example, is 55% chord at
the tip and
45% at the root. The size is dictated due to the combined controls when max
deflexion
is required for both controls, as deflection has to stay around 25 to 30
deg.

To compare the elevator and ruder of a modern T tail which has only 25%
chord and 30%
respectively, which allows for a substantial laminar flow region on both
fixed surfaces.
There is also no question as to the superiority of the T tail regarding the
interference drag.
The T juncture on a T tail is more efficient then the V juncture,
due to the T tail surfaces being aerodynamically off set, also the total
wetted area is less.
Regards
Udo