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National Advisory Committee for Aeronautics, Technical Notes - Some Experiments on Autorotation of an Airfoil

These experiments show that the rate of autorotation of a
monoplane airfoil is reduced by sweepbaek, ceasing entirely
when the sweepbaok is 50°. They confirm previous results on
the increase in rate of rotation with decrease in aspect ratio.
In addition a very serious increase in rate and range of auto—
rotation with yaw is shown.

These experiments were made as portions of two
theses on autorctation, the first (Reference 1) undertaken with
the definite object of studying the effect of sweepback on the
autorotation of an airfoil; the second, (Reference 3) to comp
plete the first and incidentally the effect of yaw was studied.
It is the purpose of this note to give the results of the ex—
perimental work, omitting other than a brief description of
apparatus and method, with little discussion in regard to
causes, reasons, and theory of the phenomena observed. It is
felt that the work itself is of sufficient general interest.

Three airfoils were used, a rectangular 18—
inch by 3-inch clerk Y made of wood, a duralumin 18—inch by
3—inch Clark Y made in halves with sweepbaok adjustable (Fig. 1),
and a wooden airfoil of various spans, 3.46—inch chord of the
section through a Clark Y with 30° sweepback. The arrangement
of the sweepback model is such that the section.along the wing
varies with the angle of sweepbank while that perpendicular to
leading edges is constant.

The rotation apparatus (Fig. 3) is so familiar that only a
brief description is needed. The axis is a steel tube-carried
at either end in ball bearings. The airfoil is held on a rod
passing through the tube._ During the tests the airfoil was al—
ways above the tube. The angle of attack and of yaw may be
' varied. Both are fixed by friction