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National Advisory Committee for Aeronautics, Report - A Wind Tunnel Test Technique for Measuring the Dynamic Rotary Stability Derivatives at Subsonic & Supersonic Speeds

A method is described for measuring the dynamic stability
derivatives of a model airplane in a wind tunnel. The char—
acteristic features of this system are thatsingle-degree-ofjfieedom
oscillations were used to obtain combinatimzs of rolling, yaw-
ing and pitching motions; that the oscillations were excited
and controlled by velocity feedback which permitted operation
under conditions unfavorable for more conventional types of
oscillatory testing; and that data processing was greatly simpli-
fied by using analog computer elements in the strain-gage
circuitry.

The system described is primarily for measurement of the
damping derivatives 01,, (damping in roll), 0mq+ 0% (damping
in pitch), 0,,— 0.}, (damping in yaw), and the cross derivatives
Cir-01;, (rolling moment due to yawing) and 0,, (yawing
moment due to rolling). The method of testing also permits
measurement under oscillatory conditions of the static derivar
tives 0,5 (rolling moment due to sakleslip), 0“, (yawing moment
due to sideslip), and 0.,“ (pitching moment due to angle of
attack). All these derivatives are of particular importance in
estimating the short-period oscillatory motions of a rigid
airplane.

A small number of experimental data are included to illustrate
the general scope of results obtainable with this system.

INTRODUCTION

One important problem in the dynamic motions of air-
planes is the nature and the stability of the oscillatory modes.
In measuring the dynamic stability derivatives which apply
to these motions there are certain advantages in employing
oscillation methods in a. wind tunnel, and the development
of such methods has always been attractive to investigators.
Most of the early measurements of damping in pitch were
made from oscillation tests of a model in a wind tunnel.
Damping in roll and damping in yaw have also been measured
in this way but, in general, experimental difficulties have
prevented the wide application of this method to the lateral
motions. This is particularly true in the case of the cross
derivatives, yawing moment due to rolling, and rolling
moment due to yawing, although in one recently developed
method (ref. 1) the yawing moment due to rolling has been
successfully measured using a two—degreeof—freedom
oscillatory technique.