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National Advisory Committee for Aeronautics, Technical Notes - Effects of Airplane Flexibility on Wing Bending Strains in Rough Air

Some results on the effects of wing flexibility on wing bending
strains as determined from flight tests of a Boeing 3-29 and a Boeing
B-hTA airplane in.rough air are presented, and the experimental results
for the 3-29 airplane are compared with results from an analytical study.
The results are presented as frequency-response functions of the bending
strains at various spanwise wing stations to gust disturbances. For the
3-29 airplane, the effects of the first and second symmetrical bending
modes yield moderate strain amplifications in rough air all along the
airplane span. Calculations involving one or preferably two structural
modes appear to yield reliable estimates of the flexibility effects on
the strains. For the B-hTA airplane, the dynamic amplifications appear
to be quite large, particularly in the midspan region, but these ampli—
fications are partially balanced by large and favorable static aeroelas-
tic effects associated with this swept—wing airplane. In addition, some
indirect results from the B—LTA investigation suggest that spanwise
variations in turbulence have a significant effect on the responses of
airplanes with large spans.

Previous flight investigations (refs. 1 to 5) have indicated that
wing flexibility could cause substantial amplification in the wing strains
in rough air for such straight—wing airplanes as the Douglas DC-5,
Martin 2-0—2, and the Boeing 13—29. Also, by application of power—
spectral methods of analysis, good correlation has been obtained between
the measured root bending strains and calculated results for these three
airplanes (ref. 5). As a continuation of the work in this area, the
3-29 investigation has been extended to cover the bending-strain ampli-
fications at several other wing stations. In addition, a flight inves-
tigation involving the swept-wing Boeing B-flTA airplane has been under-
taken in order to assess the significance of the wing sweep on the
elastic response in rough air.