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National Advisory Committee for Aeronautics, Technical Notes - Notes on the Stalling of Vertical Tail Surfaces

A discussion is given of the important aspects of the
stalling of vertical tail surfaces. The type of instabil-
ity encountered is described and the possibilities of in—
advertent occurrence are noted. The influence of direc~
tional stability on the behavior of an airplane when the
tail stall takes place is discussed. In this connection,
flight tests of a twin—engine airplane in which the ver—
tical fin area was increased are cited. The reasons for
inadequate directional stability in certain modern de~
signs are accounted for and the properties and applica—
tion of dorsal fins are discussed. In addition, the chief
factors regulating the requirements for conventional fin
area are given, in which connection a simplified criterion
for directional stability is presented.

It may be concluded that the stalling of vertical
tail surfaces is not in itself a dangerous condition.
Provided sufficient directional stability exists at large
angles of sideslip. the tail stall may occur with modern
airplanes, as with those of the past, without the knowl—
edge of or concern to the pilot.

A deficiency in vertical fin area has been a rela*
tively common occurrence in airplanes during the past few
years, and in many cases it has been necessary to increase
the vertical tail area of the'original design after pre—
liminary flight tests. The difficulties experienced have
been manifested in various ways. In some cases it has-
been an annoying directional oscillation, a conventional
form of directional instability. Another source of annoy—
ance has been the development of large negative-pitching
moments in sideslip.

This condition has been particularly objectionable
when experienced on airplanes of low longitudinal
stability, since it requires that the elevator
movement must be carefully coordinated with the rudder
movement to prevent diving upon entry into a sideslip and
stalling on recovery from a sideslip. Another difficulty
experienced is that in some cases the unstable moments
have exceeded the maximum capacity of the-fin, so that
when sideslips have been produced intentionally or in—
advertently due to moments produced by the rudder, asym~
metric power or gusts, stalling of the vertical fin has
been produced and a reversal of rudder force experienced.
This latter condition is the one given chief consideration
in the present paper. Tho‘chief basis for this discussion
is data accumulated in the flight research laboratory of
the National Advisory Committee for Aeronautics, in an ex—
tensive flight investigation of the flying qualities of
various airplanes.. This investigation has included tests
of airplanes of varied size from small, light, two—place
airplanes to the largest multiengine bombers.