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National Advisory Committee for Aeronautics, Report - Wing Fuselage Interference, Tail Buffeting, and Air Flow about the Tail of a Low Wing Monoplane

This report presents the results of an investigation
of the wing-fuselage interference of a low-wing monoplane
conducted in the N.A.0.A. full-scale wind tunnel on the
“McDonnell ” airplane. The tests included a study of tail
buzfeting and the airflow in the region of the tail. The
airplane was tested with and without the propeller
slipstream, both in the original condition and with several
devices designed to reduce or eliminate tail bufi'eting.
The devices used were wing-fuselage fillets, an N.A.0.A.
cowling, reflered trailing edge of the wing, and stub
auxiliary airfoils.
The use of proper fillets practically eliminated the
wing-fuselage interference and greatly reduced the tail
vibrations due to bufieting. An N .A.0.A. cowling reduced
the bufieting and interference ezfects to unobjectionable
magnitudes at angles of attach: up to within about 3° of
the stall. A large fillet alone gave the greatest reduc-
tion in bufi'eting eject, reducing the tail vibrations to
one seventh their original amplitude, but the combination
of the large fillet and N.A.0.A. cowling gave the best
all-round results. This combination reduced the tail
oscillations due to bufieting to one fourth their original
amplitude, increased the maximum lift 11 percent, de-
creased the minimum drag 9 percent, increased the maxi-
mum lift/drag ratio of the whole airplane 1.9 percent,
and increased the efiectiveness of the elevator about 40
percent at angles of attack in the landing range. The
reflected trailing edge had a minor ejfect and the auxiliary
airfoils in the best position tested were considerably
inferior to the fillets. With the propeller operating, the
interference eflects were practically eliminated, even with
the airplane in the original condition.
The elimination of the wing-fuselage interference
slightly decreased the longitudinal stability of the air-
plane.
Records of the fluctuations in the dynamic pressure of
the air stream at the tail show a prominent wake-fluctua-
tion frequency of the order of magnitude of the natural
frequency of the tail vibrations.