Correlation of the Trim Limits of Stability Obtained for a PB2Y-3 Flying Boat and a 1/8-Size Powered Dynamic Model Page: 4 of 18
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NACA RM No. L7C01
FULL-SIZE TESTS
Description of Flying Boat
The general arrangement of the four-engine PB2Y-3 flying boat
for which the trim limits of stability were obtained is shown in
figure 1, and pertinent data are listed in table I. The airplane
had been fitted by the manufacturer with a center-line skeg aft
of the second step to improve the directional stability character-
istics, and with a ventilation duct on each side of the keel and
Just aft of the main step to improve take-off and landing stability.
Apparatus and Procedure
An NACA visual trim indicator was used to observe the trim
during the tests; it was located so that the pilot and an observer
could read the indicated trims. An NACA events recorder was used
to record the water speed and to indicate the instant that por-
poising started. The recorded trim from the gyro unit of the
events recorder was not used for the final analysis as it was known
to be affected by acceleration. The accuracy of the readings from
the visual trim indicator was considered to be 0.50.
The trim limits of stability were obtained at nominal gross
loads of 56,000, 61,000, and 66,000 pounds and with the center of
gravity at 28 percent mean aerodynamic chord. The flaps were set
at 00, 200, and 40. The test runs were made at constant speeds,
beginning at about 35 knots and continuing at 5-knot increments
until the maximum .permissible speed. was attained. For each run,
the flying boat was accelerated to speed and the engines were
throttled until the speed was" constant.
To determine the lower trim limits of stability, the elevators
were moved down slowly from a position at which the airplane was
at a stable trim until a lower trim was reached at which porpoising
was encountered; the elevators were then moved up until porpoising
ceased. In attempts to determine the upper trim limits, the ele-
vators were moved up slowly until porpoising was encountered or
the maximum available trim was reached. If porpoising started, the
elevators were lowered until a stable trim was again reached.
Because of the danger involved, the amplitude of the porpoising
was not allowed to build up; consequently, fully developed upper
limit porpoising did not occur, and the upper limit, decreasing
trim, was not determined.3
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Garrison, Charlie C.; Goldenbaum, David M. & Hacskaylo, Andrew. Correlation of the Trim Limits of Stability Obtained for a PB2Y-3 Flying Boat and a 1/8-Size Powered Dynamic Model, report, April 22, 1947; (https://digital.library.unt.edu/ark:/67531/metadc64857/m1/4/: accessed July 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.