Why the T-Tail?

[Warning: somewhat long off-topic ramble ahead]

Earlier, Doug Hoffman wrote:

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

That's my experience as well, but I think that it has more to do with
the lightweight-yet-rugged aluminum semi-monocoque aft fuselages that
Dick Schreder hung on his gliders. I've never personally seen a
Schreder aft fuselage crumpled aft of about the wing root; and yet
I've seen plenty of composite tailbooms broken at or near the fin
root.

One of the substantial issues is how you mass-balance the controls,
and how much. It's easy to look at the centroids of a pair of diagonal
surfaces, and note that it is closer to the fuselage axis than the
centroid of a pair of T-tail surfaces. However, the weight of the
actual tail surfaces often has very little correspondence with the
centroid.

With the Schreder V-tail surfaces in particular, the chunks of
mass-balance lead on the ruddervator end plates move the center of
mass of the combined stabilizer/ruddervator pretty far from the axis
of the fuselage.

With a T-tail, the envelope of the vertical fin gives you some good
opportunities to move the balance masses closer to the axis of the
fuselage. With the rudder, you can concentrate the mass near the lower
hinge. And for the elevator you can either locate the mass balance at
the bellcrank at the fin root, or as in the case of the later LS
gliders just use the elevator push-pull tube itself as the mass
balance.

Of course, the most effective (some might say the only effective) mass
balance is to distribute the counterweight along the hinge line of the
surface. However, the practical experience of the European
manufacturers seems to be that concentrated mass balances can be
adequate if implemented correctly on relatively stiff control
surfaces.

On the other hand, and I think this is what Doug is pointing out, the
thing to watch out for is not necessarily the distance between the
center of mass of the tail surfaces and the fuselage axis. For
groundloop resistance, the distance between the center of mass of the
tail surfaces and the plane of the waterline of the fuselage gets
important. That's the plane (plus and minus a few degrees for dihedral
and wing flex, of course) in which lateral groundloop forces are
applied to the tailwheel. And with a V-tail, the center of mass will
be closer to the waterline plane than to the fuselage axis (by a
factor of .707 for a 90-degree included angle like Dick always used).

As an aside, when Stan Hall located the balance masses at the outboard
ends of the tail surfaces on his pretty little Ibex, he experienced a
flutter mode in which the slender tailboom flexed in torsion. Since he
was using all-moving tail surfaces, he was able to fix the problem by
moving the mass balance weights to the inboard ends of the
stabilizers. His tailboom was more slender than Dick's RS-15 boom, and
much more slender than Dick's semi-monocoque tails, though, so I don't
consider his experience to be particular cause for worry in the HP
world.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24

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,

So why did you say it!

To fill the white space at the bottom of postings, to waste bandwidth
and to confuse readers.

Oh, and it is somewhat on topic because I got it from

http://www.mountainflying.com/mountology.htm

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."

"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."

So why did you say it!



sorry couldn't resist...

Gail

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.
,

At 11:24 02 November 2004, Marian_Aldenhövel wrote:
Hi,

So why did you say it!

To fill the white space at the bottom of postings,
to waste bandwidth
and to confuse readers.

Sorry! That's the wrong 'it.' The 'it' in question
is the hypothetical 'it' referred to in your quote
rather than the quote itself.

I'd guess the answer is because you didn't run the
hypothetical statement past the 'hypothetical' editor
in your mind before you uttered it -- an all too common
failing. grin


Oh, and it is somewhat on topic because I got it from

http://www.mountainflying.com/mountology.htm

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.'

"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