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National Advisory Committee for Aeronautics, Technical Notes - The Effect of Longitudinal Moment of Inertia Upon Dynamic Stability

Free flight tests have been carried out by the National
Advisory Committee for Aeronautics to show whether the longitudi-
nal oscillations of a standard SoE.5A airplane are noticeably
affected if its longitudinal moment of inertia is increased.
These oscillations were taken by means of a self—recording instrur
ment, the airplane having first its ordinary moment of inertia
and then one increased by 14%. The period of oscillation was
slightly longer after the increase of the moment of inertia, but
the damping was not affected.
It has been shown by a mathematical study of the motion of
an airplane undergoing an oscillation,* that an increase in the
moment of inertia of the airplane tends to make the oscillation
unstable. As it is sometimes desirable from the point of View
of the designer to increase the moment of inertia of an airplane.
it was thought that some actual tests to determine its_effect on
the damping would be of interest. As far as it is known no
other tests of this rature have ever been made.
The airplane used in this test was an S.E.5A (Fig. 1),
selected because of its static longitudinal stability and be-
cause of the comparative ease with which the longitudinal mom—
ent of inertia could be increased. In the two runs made the
conditions were exactly the same excepting for the distribution
of teight. In each case the airplane nae allewed to oscillate
with free ccnttolsl-while the oscillation was recorded by means
of a N.A50,A. recording air-speed meter.