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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 Four Engine Bomber Airplane in Rough Air

A flight investigation was made on a four-engine bomber airplane to
determine effects of wing flexibility on wing strains developed in flight
through clear-air turbulence. The amplification of strain due to flexi-
bility effects was determined by comparing the strains for a unit normal
acceleration that are developed when the airplane experiences a gust with
the strains for a unit normal acceleration that are developed in a pull-up.
At a station near the root the bending strains due to gusts were on $38
average 51 percent greater than the corresponding pull—up strains. e
amplification was found to vary with spanwise location, diminishing
slightly at successive outboard stations except at the most outboard sta-
tion where it increased.

Variations from 180 to 250 mph in the airspeed and from 91,000
to 105,000 pounds in weight had no pronounced effect on the amplification
factor. The amplification was found to be a function of gust-gradient
distance (as measured by the time interval to pass from the lg level—flight
condition to peak acceleration), decreasing as the gradient distance
increased. The shear—strain time histories resembled the bending-strain
histories at outboard stations, and.approximately the same amplification
factors were found for shear strains as for bending strains at these sta-
tions.

Some supplementary calculation studies of the amplification factor
as a function of gradient distance gave results which.roughly substanti~
ate the results found from the flight tests.

A series of flight investigations has been made in rough air with a
two-engine transport airplane and a four—engine bomber airplane to obtain
a practical measure of how wing strains and accelerations induced by
gusts are amplified by transient-response effects asSociated with wing
flexibility. Some results of these investigations have been reported in
references 1, 2, and 5. Reference 1 deals with the acceleration results
for the twin—engine transport and indicates that the peak accelerations
at the fuselage were on the average 20 percent greater than the peak
accelerations at the nodal points of the fundamental mode, the latter
being used as measure of over-all airplane acceleration; reference 2
deals with strain measurements on this transport and indicates that the
peak bending strains were also increased about 20 percent by flexibility
effects.