Why the T-Tail?

"Eric Greenwell" wrote in message
...
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.

Lotsa Libelle booms have been broken, but then the shape may have something
to do with that, or just the sheer numbers of Libelles built skews the
sample.

Several PIK-20 tail booms have unzipped along the lower fuselage seam from
ground loops. No idea how that compares to those that actually broke the
boom. From what I've heard, once repaired, they don't unzip again.

What about those booms that break in two places? Peculiar to make, or type
of accident?

Frank Whiteley

[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

At 08:54 28 October 2004, 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?


Complicated mixing box required for elevator/rudder
control.

Earlier, Peter Wyld wrote:

Complicated mixing box required for
elevator/rudder control.

I don't think that such devices are necessarily complicated. Dick
Schreder designed a variety of mixers for the HP/RS-series sailplanes,
including cats-cradles and arm-on-arm systems, and none of them were
particularly hard to build or maintain. You can view the drawings for
the HP-18 ruddervator mixer he

http://www.soaridaho.com/Schreder/Sc...76_Page_36.jpg

An aft-looking-forward drawing of the HP-18 ruddervator mixer is at
the bottom of that page. Sure, it's more complicated than a direct
connection to an elevator. But compared with some other glider
mechanisms, for instance the flap mixer in the ASW-20 or the gear
retract system on the original Twin Astir, it's relatively simple and
straightforward. Also, it transmits rudder as well as elevator inputs,
so some of its complication results in eliminating a separate rudder
circuit. So, in terms of overall aircraft control system complexity,
it comes out only slightly worse than more conventional
rudder/elevator systems.

Probably the best thing about V-tails is that with them you have one
less tail surface to construct, finish, paint, and mount. You have
fewer hinges, and fewer intersections. You leave more stuff on the
ground, and get more use out of what you do take into the air.

However, as I develop the next-generation HP kit sailplane, I've had
to make the unhappy decision to go with a T-tail. The primary reason
is aesthetics: My surveys suggest that I can substantially widen my
tiny market by offering a low-cost go-like-stink glider that looks
just like all the other go-like-stink gliders. Secondary reasons
include that, in composites, I found it easier to develop mounting
provisions for a conventional T-tail horizontal surface than for an
equivalently-sized set of diagonal surfaces. Other reasons include the
convenience of decoupling the sizing, deflections, and mass-balancing
of separate rudder and elevator surfaces, and the greater ease of
developing and installing control circuits in the aft fuselage.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24
*fuselage shell molds complete, readying wing and tail tooling*

This is the only fatality I am aware of at Avenal, and the
only ruddervator mechanism-related T-tail fatality I have read.

NTSB Identification: LAX92LA393 .
The docket is stored on NTSB microfiche number 48074.
14 CFR Part 91: General Aviation
Accident occurred Friday, September 18, 1992 in AVENAL, CA
Probable Cause Approval Date: 9/14/1993
Aircraft: SPARKS-SCHREDER HP-140V, registration: N704B
Injuries: 1 Fatal.

Prob cause:
" THE LOSS OF PITCH AND YAW CONTROL DUE TO THE SEPARATION OF THE
RUDDERVATOR CONTROL CABLE FROM ITS CLEVIS FORK DUE TO IMPROPER INSTALLATION."

In article ,
Peter Wyld wrote:
At 08:54 28 October 2004, 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?


Complicated mixing box required for elevator/rudder
control.




--

------------+
Mark J. Boyd

Mis-rigging could happen to any type. A school teacher was killed in Wales
one day while I was soaring due to mis-rigging one of the ruddervator
control guides on his v-tailed BG-135, diving in from the top of the winch
launch (his second of the day). It was only the second occasion he'd rigged
the glider IIRC. Could have been a t-tail.

I flew a v-tailed SHK for four years. The rigging of the control guides was
very similar and held the same potential for being mis-rigged as the
aforementioned BG-135. The sad part is that the guide is clearly in view
during control deflection. This is one very specific instance where a PCC
might not have detected the problem, but a visual inspection would. The
fact that he made the first flight successfully attests to this. The actual
disconnect probably happened during ground handling or during he landing
role and wasn't noticed before the second flight.

Frank Whiteley

"Mark James Boyd" wrote in message
news:41818446$1@darkstar...
This is the only fatality I am aware of at Avenal, and the
only ruddervator mechanism-related T-tail fatality I have read.

NTSB Identification: LAX92LA393 .
The docket is stored on NTSB microfiche number 48074.
14 CFR Part 91: General Aviation
Accident occurred Friday, September 18, 1992 in AVENAL, CA
Probable Cause Approval Date: 9/14/1993
Aircraft: SPARKS-SCHREDER HP-140V, registration: N704B
Injuries: 1 Fatal.

Prob cause:
" THE LOSS OF PITCH AND YAW CONTROL DUE TO THE SEPARATION OF THE
RUDDERVATOR CONTROL CABLE FROM ITS CLEVIS FORK DUE TO IMPROPER
INSTALLATION."

In article ,
Peter Wyld wrote:
At 08:54 28 October 2004, 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?


Complicated mixing box required for elevator/rudder
control.




--

------------+
Mark J. Boyd

"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

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."

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.'