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National Advisory Committee for Aeronautics, Technical Notes - A Mechanical Device for Illustrating Airplane Stability

An instrument is described which will illustrate'
completely in a qualitative sense the longitudinal stability
characteristics of an airplane. The instrument is primarily of
use for the lecture room, but it is hoped that ultimately it
will be possible to obtain quantitative results from it.

Introduction. It is a very simple matter to construct a
piece of apparatus which will show dynamic stability or damped
oscillations - for example, a pendulum. A simple device which
can be altered to give any degree of stable or unstable motion
has long been desired, and as far as it is known, the instrument
described below is the only simple method for accomplishing this.

At first it was expected that only the degree of damping of
an oscillation would be illustrated, but as the instrument was
more carefully studied it became evident that nearly every prop—
erty of the flying airplane was represented with astonishing ex—
actness. This is-confirmed by the fact that the equations of
motion work out in almost identical form with those of Bryan and
Bairstow for the airplane.

A description of this instrument is given here because:
first, it shoula be of interest to lecturers on aeronautics;
second, it will allow mathematicians to visualize the actual be—
havior of an airplane without having to make flights; and third,
it holds the possibility of mechanically solving the stability
eQuations.

A complete View of the instrument is shown
in Fig. 1. It consists essentially of a double pendulum,
the lower end of which is a wheel resting on a revolv—
ing drum as shown diagrammatically'in Fig. 3. The drum can be
turned at any speed by an electrio~motor, and the stability and
moment of inertia about the two pendulum axes can be varied at
will.