Sailplane & Gliding 1966 - Lakes Gliding Club

V-TAILS
By JOHN GIBSON
HE recent inconclusive correspon­
in SAILPLANE & GLIDING abollt
Tdence
V-tails, and Slingsby's Newsletter blaming
Dick Johnson's Austria accident on
its V-tail, have goad~d me into an
attempt to extract some facts about this
configuration. There are V-tailed gliders
with perfectly normal spin-recovery,
while fatal spins from an ample height
for recovery are not unknown with
ordinary-tailed g.)jders. Any glider will
spin if its c. of g. is too faI aft. One
cannot possibly prove that. beeause a
glider with a V-tail could not be got
out of a spin on one occasion, V-tails
are "dangerous" or tend to result in poor
spin-recovery. Since one V-tail is almost
exactly like another, it is hard to visualise
a reason for unsatisfactory characteristics
which do not apply to all V­
tailed gliders.
Critics should read NACA RepOrt 823.
which studied the V-tail very thoroughly.
In particular, they would note that the
V-tail has a much higher stalling angle
of attack or sideslip than an ordinary
tail, and that a fighter (King Cobra, I
think) had a slightly improved spinrecovery
when fitted with a V-tail of the
same volume coefficients as the original
ordinary tail. No evidence or hypothesis
was found to suggest that a V-tail could
in any way degrade normal flying Qualities.
Beneficial effects on eontrol forces
and a 15 peI cent reduction of tail drag
due to reduced interference .are among
incidental features discussed.
Fin volume coefficients for the Olympia
2, Dart, 463, Foka, Phoebus, Edelweiss
and Fauvette, for example, are all
about 0.026, and for the Blanik and
Skylark 4, about 0.031. None of these
has a bad spinning reputation that I
know of. British sailplanes speak for
them!\elves, and I am assured by M.
Witt, Breguet's chief test pilot, that the
Fauvette has perfectly normal spinrecovery.
The Fauvette's designer went
on to design the Choucas and Edelweiss
with V-tails, so he at least has
faith in them!
On the other hand, the Austria's fin
volume coefficient is only about 0.018,
The author is an aircraft design engineer working on aerodynamics
witll responsibilities for design of flying control
systems. He has been gliding since 1954.
as is the SHK's, with a correspOndingly
small tail volume coefficient of about
0.33 (all the others range from about
0.45 to 0.72). One might suspect from
this that the Austria would have less
margin against spinning troubles than
other gliders, but this has nothing to do
with the tail configuration.
The classical causes of failure to have
satisfactory spin-recovery are one or
more of:
(I) large distribution of mass along
the Y-axis (the wing);
(2) inefficient body section producing
Iow body damping;
(3) deficiency in side area producing
low body damping;
(4) shielding of the rudder by the tailplane.
The Dart and Skylark, for example,
have efficient body sections (elliptical)
and plenty of side area, but blot their
copybooks a little by blanketing al least
50 per cent of their fin and rudder area
in spins with the tailplane wake. The
Austria has an inefficient body section
(circular) and not much side area, but
does not blanket its fin. One could surmise
that a Slingsby tail would worsen
the Austria's spin and a V-tail improve
the Dart's.
Calculation of the non-dimensional
anti-spin body damping shows the Austria
to possess about two-thirds of the
Dart 15 value and about the same as the
Dart 17. The Austria has about four
times the unshieIded rudder anti-spin
volume of the Dart, giving a total predicted
anti-spin moment greater than
either Dart version. Complete prediction
requires inertia data I do not have, but
a high ratio of pitching to roIling inertia
is favourable. The Dart appears better
than the Austria in this respect.
Such considerations tend to suggest
reasons why the Austria might have
poorer spin recovery than many other
gliders (if it really does), but the V-tail
is not one of them. Will anyone, therefore,
who can produce a rational case
against the V-tail please oblige us with
the data to prove it?
357