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National Advisory Committee for Aeronautics, Technical Notes - A Study of Autogiro Rotor Blade Oscillations in the Plane of the Rotor Disk

An analysis of the factors governing the oscillation
of an autogiro rotor blade in the plane of the rotor disk
showed that the contribution of the air forces to the re-
sultant motion was small and that the oscillation is es—
sentially a direct effect of the rotor—blade flapping moe
tion. A comparison of calculated oscillations with those
measured in flight on three different rotors disclosed
that the calculations gave satisfactory agreement with ex-
periment. The calculated air forces on the rotor blade
appear to be larger than the experimental'onee. but this
discrepancy can be attributed to the deficiencies in the
strip analysis.

An autogiro rotor blade is connected to the rotor hub
in such a way as to permit two articulations, one about
the horizontal hinge and one about the vertical hinse.

The horizontal hinge axis is perpendicular to the blade
span axis and to the rotor axis and permits the blade to
oscillate freely in a plane containing the blade span_axis
and the rotor axis. The vertical hinge axis is parallel
to and offset from the rotor axis and permits the blade to
oscillate in the plane of rotation. This articulation is
required because the forces acting on the_blade in the
rotor disk are of an unsteady nature and experience has
shown that heavy stresses and uncomfortable vibrations
arise if this articulation is not used. In addition, mo-
tion about this hinge is damped by friction so that tran—
sient vibrations will quickly subside.

A knowledge of the motion about the vertical hinge is
of value for several reasons. The nature of the oscilla-
tion Must be known in order to determine intelligently the
amount of damping required by the blade. In addition, a
verifiied analysis of this oscillation suggests the possi—
bility of subjecting all rotor characteristics to some
form of mathematigaligpaly_is an__lessens the dependence
of the designer—upon empirical rules. This paper presents
an analysis of this phase of the blade motion and shows a
comparison between calculated and measured results which
supports the validity of the analysis.