Why the T-Tail?

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.

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.



--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

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?

Ciao, MM
--
Marian Aldenhövel, Rosenhain 23, 53123 Bonn.
Fon +49 228 624013, Fax +49 228 624031.
http://www.marian-aldenhoevel.de
"I know you believe you understand what you think I said, but I'm
not sure you realize that what you heard is not what I meant."

Marian Aldenhövel wrote:
- With a T-Tail you can build the elevator in one piece so you
can rig and derig more easily (pro).

This is also possible with a conventional tail, see e.g. the Libelle.

Cheers
-Gerhard
--
Gerhard Wesp o o Tel.: +41 (0) 43 5347636
Bachtobelstrasse 56 | http://www.cosy.sbg.ac.at/~gwesp/
CH-8045 Zuerich \_/ See homepage for email address!

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

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 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

In article ,
Marian Aldenhövel 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?

The V-Tail setup is less likely to cause fuse damage in a ground loop.

Regards,

-Doug

Doug Hoffman wrote:
In article ,
Marian Aldenhövel 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?


The V-Tail setup is less likely to cause fuse damage in a ground loop.

Couldn't the T-tail designer just make the fuselage stronger? Or the
V-tail designer make the fuselage lighter to take more advantage of the
lower tail CG, so they both withstand a ground loop just as well?

Maybe JJ or some other glider repairer can tell us how tail booms
commonly fail - torsion or bending, and if there seems to be a
difference in types of failure between the tail types.


--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

The designer could make the fuselage so strong that it would never break, no
matter how fast the ground loop of snap roll. But then the fuselage would be
twice the area, four times the weight, and the glider would never get off the
ground.It is all a trade off in strength to performance.

It is not a given that the tail will break in a ground loop. Most gliders are
designed to survive ground loops with no damange, 'cept maybe a dizzy pilot.

Couldn't the T-tail designer just make the fuselage stronger? Or the
V-tail designer make the fuselage lighter to take more advantage of the
lower tail CG, so they both withstand a ground loop just as well?



Jim Vincent
N483SZ
illspam