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National Advisory Committee for Aeronautics, Technical Notes - Effects of Airplane Flexibility on Wing Strains in Rough Air at 35,000 Feet as Determined by a Flight Investigation of a Large Swept Wing Airplane

A flight investigation has been made on a large sweptbackswing
bomber airplane in rough air at an altitude of 55,000 feet in order to
determine the effects of wing flexibility on wing bending and shear
strains and to compare the results with results previously obtained at
low altitude (5,000 feet) and reported in NACA Technical Note 1:107. The
effects of wing flexibility on the wing strains were, on the average,
about 20 percent larger at the higher altitude. Representative values
of the amplification factors varied from about 1.5 at the root stations
to about 2.5 at the midspan stations.

Flight investigations of the effects of airplane flexibility on
the wing strains that develop during flight through rough air have shown
that substantial amplifications of the strains may occur. (See, for
example, refs: 1 to 5.) Analytical methods for calculating the struc-
tural response of unswept—wing airplanes to atmospheric turbulence
involving simple wing-bending modes have been developed and are reported
in references 6 to 8, and results of these calculations show good cor-
relation with the results of flight-test studies for the unswept-wing
airplanes considered. For swept-wing airplanes, however, the responses
in rough air are likely to be more complicated since the structural
response of a swept—wing airplane may be expected to involve significant
effects of torsion on the airplane aerodynamics, on the stability of the
airplane, and on the structural strains. Flight tests were, therefore,
undertaken in order to obtain information on the magnitude and character
of the effects of flexibility and aeroelasticity on the strains in rough
air for the case of a flexible sweptback—wing airplane and to provide
experimental data for comparison with analytical results.

An analysis of the flight-test measurements made at an altitude of
5,000 feet and a Mach number of approximately 0.65 is presented in refer—
ence 5. The results of the analysis of reference 5 indicate that both
dynamic and static aeroelastic effects have a large influence on the
wing bending and shear strains. The bending-strain amplification fac-
tors reflecting the dynamic effects alone were found to vary from
approximately 1.25 at the root to 2.7 at the 0.60-semispan station.