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National Advisory Committee for Aeronautics, Technical Notes - A Full Scale Investigation of the Effect of Several Factors on the Shimmy of Castering Wheels

A full-scale investigation has been conducted to de-
termine the effect of various factors on the shimmy of
castering wheels. The factors considered were the geo-
metric arrangement. the tire types, the variations of
load, the spindle moment of inertia. and the tire infla—
tion. _A comparison of the results of the present inves-
made. The constants needed in the calculations to deter-
mine the damping required for a castering wheel were meas—
urod.

The results indicate that solid friction appears to
be impracticable as the sole damping agent for castering
nose wheels on large airplanes. Also it was concluded
that the existing theorv is adequate for calcu.lating the
damping required to prevent shimmy The caster angle and
the spindle moment of inertia were found to influence the
solid friction required to prevent shimmy. The effect of
variations in the type and the pressure of the tire was _
insignificant.

The National Advisory Committee for Aeronautics has
conducted a full— scale investigation of the shimmy (spindle
oscillations due to dync.mic instability) of castering
wheels. In addition to the determination of the effect
of the physical properties of a castering wheel on the
tendency to shimmv‘ data Were obtained by which the appli-
cability of a previous theoretical investigation (refer—
ence 1) could be examined.

The investigation included the following factors that
must be considered in the design of a castering-wheel sys—
ten: the geometry of the caster system (caster angle,
fork offset, and caster length): the load; the spindle
moment of inertia, the wheel moment of inertia; and the
tire inflation. These variables-were examined on the
basis of their effect on the tendency to shimmy while
the test wheel was towed over a uniform bump. This test—
ing does not contribute data amenable to an analysis
leading to quantitative design information that would be
applicable to all sizes and types of castering-wheel sys-
tems; it is believed, however. that a qualitative analysis
is valid and is useful for systems where solid friction
is used.