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