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• December 14, 2016 Create Date
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National Advisory Committee for Aeronautics, Technical Notes - A Comparison of Theoretical and Experimental Wing Bending Moments During Seaplane Landings

A smooth—water—landing investigation was conducted with a small
seaplane to obtain experimental wing—bending—moment time histories
together with time histories of the various parameters necessary for the
prediction of wing bending moments during hydrodynamic impact. The
experimental_results were compared with calculated results which include
inertiaeload effects and the effects of air—load variation during
impact. The responses of the fundamental mode were calculated with the
use of the measured hydrodynamic forcing functions. From these responses,
the wing bending moments due to the hydrodynamic load were calculated
according to the procedure given in R. & M. No. 2221. This comparison
of the time histories of the experimental and calculated wing bending
moments showed good agreement both in phase relationship of the'oscil—
lations and in numerical values.

The effects of structural flexibility on the wing bending moment
were large, the dynamic component of the total moment being as much as
97 percent of the static component. Changes in the wing bending moment
due to the variation in air load during impact were of about the same
magnitude as the static water—load component.

Recent trends in the design of aircraft have led to an important
increase of the stresses produced in wings by landing impacts. Two
significant factors contributing to these increased stresses during
landing are an increased proportion of the seaplane weight in the wings
and an increased structural flexibility, since, in most cases, these
factors have caused the ratios of the times to peak of the applied
landing loads to the quarter period of the fundamental mode to approach
a critical value.

Several simplified methods have been developed for determining the
inertia loads in wing structures during landing impacts, and studies
have been made of the landingehnpact inertia loads in simplified
structures with the use of the principles of these methods. (See refer—
ence l and bibliography.) Although experimental investigations have
been made to determine the magnitudes of inertia loads in actual
airplane structures (bibliography), little correlation of theory and
experiment has been made concerning the nature and magnitude of inertia
loads in airplane wings during actual landing impacts.