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National Advisory Committee for Aeronautics, Technical Notes - Flight Investigation of Transient Wing Response on a Four Engine Bomber Airplane in Rough Air with Respect to Center of Gravity Accelerations

In continuation of flight studies of transient wing response initi-
ated on a two—engine transport airplane, a flight investigation was
undertaken on a four-engine bomber airplane to determine the effect of
transient wing response in rough air upon acceleration measurements at
the center of gravity of the airplane. Flights were made in clear-air
turbulence for two speed and two weight conditions. Simultaneous accel-
eration measurements were taken at the center of gravity and at several
stations along the wing span from which the true airplane acceleration
was determined.

An analysis of the results indicated that the recorded center—of;
gravity acceleration increments were, on the average, equal to the true
airplane acceleration increment amplified by a factor of approximately
1.28 and further increased by approximately 0.01g. Within the accuracy
of the results, there were no important changes in this relationship
that could be attributed to variations in speed and weight.

In the flight operation of transport and bomber airplanes, atmos—
pheric gusts constitute a principal source of loads. Knowledge of these
loads is based.primarily on V—G type of records and other acceleration
measurements taken in the fuselage near the center of gravity of the
airplane and interpreted on the basis that the airplane acts as a rigid
body. For some of the newer and larger airplanes, interpretation of
acceleration measurements on a rigid body basis may lead to errors
attributable to the effect of transient wing response in gusts. The
results of a flight investigation made on a two—engine transport air;
plane in rough air (ref. 1) have shown that, for the case investigated,
the use of fuselage accelerations in evaluating gust measurements can
lead to errors, averaging about 20 percent.

Two principal effects of transient wing response on center-of—
gravity acceleration measurements exist. One is a vibratory effect due
to the excitation by the gusts of the natural_modes of vibration of the
airplane which causes the acceleration measured at the center of gravity
to differ from the true airplane acceleration. The other is an aero-
dynamic effect due to the fact that the transient response causes a
change in the total aerodynamic load that acts on the airplane. When
vibratory response of the wing is important, therefore, center-of—gravity
acceleration measurements may not be adequate for gust studies.