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National Advisory Committee for Aeronautics, Technical Notes - Flight Investigation of the Effect of Transient Wing Response on Wing Strains of a Twin Engine Transport Airplane in Rough Air

A flight investigation was made on a twin-engine transport airplane
to determine the strains associated with the dynamic behavior of the
wing during flights through rough air. Flights were made in clear-air
turbulence for different wing fuel loads and for two speed conditions
at altitudes between 3,000 and h,000.feet. Slow pull-ups were made in
smooth air to obtain data for a reference quasi-static condition.
Simultaneous measurements were made of nodal-point acceleration and of
wing strains at a number of spanwise stations.

The bending strains per unit normal acceleration in gusts were
approximately 20 percent higher than those in slow pull—ups for all
measuring positions and flight conditions of the tests. The dynamic
component of the wing bending strains appeared to be due primarily to
excitation of the fUndamental wing bending mode. The data on spar
web strain exhibited a rather large amount of scatter, and the esti—
mated web strain amplifications for gusts relative to pull—up condi—
tions varied considerably with measuring station.

Although the treatment of gust loads on airplanes has usually
been made on the basis that the airplane is rigid, the dynamic response
of airplane wings due to gusts has been of concern since early gust-
load studies (see reference 1). Higher speeds, thinner wings, larger
proportions of the total mass carried in the wings, and so forth have
_ led to increased concern as to the effects of wing flexibility for
postwar airplanes in flights through gusts. Although a number of
investigations of the structural dynamic respOnse have been made.

(see list of references in reference 2), almost all the investigations
to date have been either in regard to the development of analytical
methods for calculating structural responses of airplanes in gusts or
in making trend studies. In order to evaluate these various methods
of predicting structural dynamic response and to establish the magnitude
of structural dynamic-response effects appropriate to present-day
airplanes in actual operations, rough-air flight data are needed.