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National Advisory Committee for Aeronautics, Technical Notes - An Experimental Investigation of the Normal Acceleration of an Airplane Model in a Gust

In order to provide experimental data as a check of
the theories used in the prediction of applied loads on
airplanes due to atmospheric gusts, an investigation was
made in the N.A.C.A. gust tunnel to determine the influ—
ence of the airplane wing loading, the forward velocity,
the wing plan form, and the fuselage on the reaction 6f the
airPlane to a known gust. Tests were made for four values
of gust velocity and for two gust gradients, namely, the
sharp— —edge gust and a gust rising linearly to full. strength
in a distance of several chord lengths.

The results of the investigation indicate that the
formulas given in a 193? S.A.E. paper by R. V. Rhode en—
titled "Gust Loads on Airplanes” predict the qualitative
effect of gust velocity, forward velocity, wing loading,
wing plan form, and gust gradient on the acceleration in—
crement in a satisfactory manner. The Quantitative agree~ ;
ment is also good for airplanes of normal proportions when _
the fuselage is neglected in the computations; i. e.,_—hen
the wing area intercepted by the fuselage is included as a
part of the wing. Although the agreement between computed
and test results remained good for aspect ratios as‘lbi as
2.0, there is an indication in Technical Note No. 682*that:
in some cases, the influence of the finite span may require
more careful consideration.

The investigation also shows that the value of maxi—
mum lift coefficient for steady flow does not limit the '
acceleration increment in a gust.

In recent years, the importance of the applied wing
loads due to atmospheric gusts has increased. In order to
obtain fundamental data on the magnitude of these loads,
statistical measurements of accelerations and speeds (ref—
erence l) were obtained during flight in rough air. Theo-
retical studies (reference 2) of the response of an air-
plane when a gust is encountered in flight were also pub-
lished.

In regard to the theoretical studies of the reaction
of an airplane to a gust, much progress has been made.
The first analyses of gust loads on airplanes were based
on the simple sharp—edge gust formula, which is derived in
reference-l. Kussner's contribution (reference 2) removed
some of the more severe limitations of the simple formula
hby including the influence of lift lag and the vertical
motion of the airplane. For the practical solution, the
equations of reference 2 were solved, certain-simple gust
shapes being assumed; the results are given in reference 3.
Based on Wagner's classical work (reference 4), a recent
paper by Jones (reference 5) has introduced additional re—
finements in unsteady—lift theory to account for the ef~
feet of finite span on the development of lift. Other
research workers (reference 6) have attempted to refine
the theory still further by removing other restrictions
utilized by desner-in his original work.