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National Advisory Committee for Aeronautics, Technical Notes - Airfoil Lift with Changing Angle of Attack

Tests have been made in the atmospheric wind tunnel of the
National Advisory Committee for Aeronautics to determine the ef—
fects of pitching oscillations upon the lift of an airfoil.
It has been found that the lift of an airfoil, while pitch?
ing, is usually less than that which would exist at the same
angle of attack in the stationary condition, although exceptions
may occur when the lift is small or if the angle of attack is
being rapidly reduced.
It is also shown that the behavior of a pitching airfoil
may be qualitatively explained on the basis of accepted aerodyh
namic theory.
As the science of aerodynamics is, as yet, in a state of
development, rather than in one of refinement, it is natural that
the steady motion of wings through the air has been studied EXP.
tensively while the essentials of the accelerated motions remain
practically unknown. The necessity for attacking the latter
problem has been felt for some time; the necessity of investi—
gating the forces_which act upon the wings and tail surfaces of
modern airplanes during the rapid maneuvers of which they are ”
capable has focused attention upon this field of aerodynamics.
Other examples of the unsteady motions of airfoils are found in
the autogyro, the helicopter in horizontal or gliding flight, the
feathering blade propeller.and the flutter of airplane wings.
All of the examples just mentioned give rise to the same
questions: (a) Are the instantaneous air forces which act upon
an airfoil while its angle of attack is changing equal to those
experienced by it while in rectilinear motion at the same angles
of attack? (b) If the answer to (a) is negative, what differences
exist? The study of this fundamental problem was suggested by
Dr. Hunk, in wh0se paper (Reference 1) will be found a theoretic—
al treatment (for the wing of infinite aspect ratio) of the ques—
tions stated above.