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National Advisory Committee for Aeronautics, Technical Notes - Wind Tunnel Investigation of Fuselage Stability in Yaw with Various Arrangements of Fins

The greater portion of the fixed vertical tail sur-
faces of aircraft is required to counteract the directional
instability of conventional fuselage shapes.5 Methbdfi‘have ”‘
therefore oeen suggested of reducing the maximum value of
. the un‘stable fuselage moment to permit a reduction of the H
vertical tail. One method which has been emploved on com—“_
mercial aircraft, is the addition of a narrow strip of fin_r _;
2 NASA Technical Note No. 785

area, referred to as a "dorsal" fin. along the top center
line of the fuselage ahead of the usual vertical tail sur—
face. Another method is the shaping of the rear of the
fuselage into a wedge, effectively adding fin area at the
top and the bottom. A third method recently susqested is
the addition of a sharp~edge protuberance along the verti~
cal center line of the forward portion of the fuselate.

It was thought that such a protuberance, by disturbing the
flow over the down—wind side of the yawed fuselage, might
decrease the magnitude of the negative pressure in that
region forward of the center df gravity and thereby reduce
the unstable moment. “

These methods are primarily intended to reduce the
maximum value of the fuselage.yawing moment, which—occurs’
at moderately large angles of yaw where vertical tail sur~
faces of conventional aspect ratios are normally stalled.
Any reduction of slope of the yawing—roment curve in the-
vicinity of zero yaw is incidental. None of the methods
is expected appreciably to increase the drag of the fuse-
legs for the unyawed condition of the airplane. "

In the reported investigation two fueelase shapes_
were tested in combination with fin area_at various loca—
tions on the fuselage to prove'the effectiveness of each.”
of the three.methods.

The two fuselage shapes used in this investigation _“
are shown in figures 1 and 2. one of theée'fuseiages is a
body of revolution that was previously used for the wing-
fuselage investigation reported in reference 1. The other
shape was obtained by fairing the rearward portion of the
fuselage with modeling clay as shown in figures 2,‘3. and 4.
The fuselages will hereinafter be referred to as "fuselage