Development of device for optical measurement of spin rate of wind tunnel models

In wind tunnel tests of free-spinning missile models it is of interest to measure the rotation rate of the models. As it is essential to provide free rotation of such models with minimum friction in the bearings, the instrumentation for measuring the rotation rate must not introduce any additional braking torque so that contactless measurement is preferred. Also, instrumentation must fit in the constrained space in the model, most of which is usually occupied by a wind tunnel balance. A simple device for optical measurement of the rolling rate of the models for the T-35 wind tunnel was developed in the Military Technical Institute in Belgrade. The device comprises two infra-red optical proximity switches and a black/white striped mask mounted on a convenient coaxial cylindrical surface inside the model. The outputs from the optical switches are fed through a conditioner into the wind tunnel data acquisition system. Rotation rate is determined from the measurement of the frequency of pulses generated by the switches when the striped mask is rotating in their field of view. Furthermore, as the optical mask is produced with two ribbons of stripes phase-shifted by 1/4 of the period, so that the outputs of the optical switches are in quadrature, the direction of rotation can be detected.