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National Advisory Committee for Aeronautics, Technical Notes - A Method Utilizing Data on the Spiral, Roll-Subsidence, and Dutch Roll Modes for Determining Lateral Stability Derivatives from Flight Measurements

A method for determining lateral stability derivatives from flight
measurements is obtained by arranging the lateral equations of motion in
such form that information from each of the three modes of lateral motion
may be utilized. This method permits determination of all the important
derivatives without requiring an estimation of any of the derivatives.
The results of an error analysis are given to show the effects of errors
in the measured quantities on the accuracy of each stability derivative
for three representative airplanes.

Calculated and measured.wind-tunnel values of the aerodynamic lateral
stability derivatives are necessary in the design of any modern airplane.
Because of the uncertainty of determination of many of the derivatives
from theory or from.wind-tunnel tests, however, flight measurements of
the derivatives are desirable to check the values assumed in the design
and to provide a basis for further improvement of the aerodynamic
characteristics.

Several methods have been proposed for determining lateral stability
derivatives from flight tests by analyzing transient or frequency-response
data. (For example, see refs. 1 to 5.) These methods allow determination
of all.the important derivatives. One method, known as the vector method
(ref. 5), allows some insight into the effect of errors in the flight
measurements on the accuracy of the derivatives. In this method, which
utilizes data from the Dutch roll mode alone, two of the derivatives must
be estimated or assumed in order to evaluate the others.

The purpose of this paper is to present_a method in which the char-
acteristics of the spiral and roll-subsidence modes as well as those of
the Dutch.roll mode are utilized in an effort to obtain all the important
derivatives. In general, separate flight tests are needed to measure as
accurately as possible the characteristics of the three modes. An error
analysis is made to show the accuracy of flight measurements required to
produce a.desired accuracy of each stability derivative for three repre—
sentative airplanes.