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National Advisory Committee for Aeronautics, Technical Notes - Flight Investigation of the Effect of Transient Wing Response on Measured Accelerations of a Modern Transport

A flight investigation was undertaken on a transport airplane to
determine the effect of transient wing response in-rough air upon accel-
eration measurements at the center of gravity of the airplane. Flights
were made in clear-air turbulence between altitudes of 3000 and #000 feet
for two speed conditions and different wing weights. Simultaneous accel-
eration measurements were taken at the center of gravity of the airplane
and at several wing stations along the wing span from which the true air-
plane acceleration was determined.

A comparison of the results indicates that the peak recorded center-
of-gravity acceleration increments were, on the average, over 20 percent
higher than the true airplane acceleration increments. There appeared
to be a small change in the acceleration discrepancy with the weight and
speed change involved, but this effect could not be substantiated.

In the flight operation of transport airplanes, atmospheric gusts
constitute a principal source of loads. Knowledge of these loads is
based primarily on V-G type of records and other acceleration measure—
ments taken near the center of gravity of the airplane. A compilation
and analysis of these and related stress measurements taken entirely on
prewar airplanes is presented in reference 1. The interpretation of
flight records has usually proceeded on the basis that the airplane acts
as a rigid body and, for earlier types of airplanes, load estimation by
this means was probably adequate. The different mass distributions,
thinner wings, higher speeds, and various plan forms of more recent air-
planes have caused increased concern with the dynamic effects of wing
flexibility, and the use of peak center—of-gravity acceleration measure—
ments in gust-load studies of these airplanes is open to question. Center
of gravity as used herein refers to a fixed position in the fuselage.

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.