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National Advisory Committee for Aeronautics, Technical Notes - Mass Distribution and Performance of Free Flight Models

This note deals with the mass distribution and perform—
ance of free flight models. An airplane model which is to be
used in free flight tests must be balanced dynamically as well
as statically, e.g., it must not only have a given weight and
the proper center of gravity but also a given ellipsoid of in—.
ertia. Equations which relate the motions of an airplane and
its model are given. Neglecting scale effect, these equations
may be used to predict the performance of an airplane, under the
action of gravity alone, from data obtained in making dropping
tests of a correctly balanced model.
It is shown how a light model, built with little regard to
its mass distribution, may'be loaded with.masses of predeter—
mined weight, shape, position and attitude to give the distri—
bution desired.
It is indicated how this method of distributing the mass
in a body to obtain a desired distribution, together with the
model dropping tests may be used to determine the most desirable
inertia characteristics of an airplane of given shape.
Model dropping tests were recently instituted at the
Langley Memorial Aeronautical Laboratory as a means of studying
the spinning characteristics of airplanes. The model was_dropped
in the spinning attitude from approximately 100 feet height and
motion pictures were taken of its descent.(results soon to be
published).
In free flying models, the mass forces and their moments _
greatly affect the performance, and therefore the quantity and '
distribution of mass cannot be overlooked in the construction
of the model. Since any motion may be considered as made up
of a series of steady helical motions which take place during
infinitesimal time increments, the kinetics of the problem will
be dealt with in two parts. First, the mass distribution re—
quired for the simulation of a steady helical motion of the
airplane will be developed. Second, it will then be shown
that the distribution found for this case is satisfactory for
the simulation of any motion whether steady or not and whatever'
the path.