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A Technique for Measurement of Static and Dynamic Longitudinal Aerodynamic Derivatives Using the DSTO Water Tunnel.

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Abstract

The DSTO water tunnel's balance and rotary support mechanism provides a measurement capability for longitudinal dynamic derivatives. This report documents the underlying theory and computational implementation of a technique which uses the water tunnel for determination of normal force and pitching moment coefficient derivatives with respect to angle of attack, nondimensional pitch rate and angle of attack rate.

Executive Summary

Determination of an aircraft's dynamic derivatives is an essential prerequisite for accurate modelling of linearised stability and control characteristics. DSTO operates an Eidetics Model 1520 water tunnel, which has been used extensively for flow visualisation tasks involving complete aircraft models. The water tunnel model support mechanism has been modified to allow independent or combined programmed rotational motions in pitch, roll and yaw, and to measure time-varying aircraft loads during motion using either two-component or five-component sting-mounted balances. Because the model support mechanism is restricted to rotational motion, the individual longitudinal dynamic derivatives can not be measured directly. However, combinations of measurements allow all the individual derivatives to be determined. This report details the underlying theory, computational implementation and experimental techniques associated with the measurement of aircraft longitudinal static and dynamic aerodynamic derivatives in the water tunnel.; The individual components of the main longitudinal dynamic derivatives are resolved from measurements using purely rotary oscillations, by combining results measured with different balance sting lengths. Experiments with simulated inputs have confirmed the general validity of the technique, and provided some indication of the data quality from which acceptable results might be obtained.; The difference between tunnel test and flight Reynolds number is large enough to potentially affect the validity of measured dynamic derivatives, depending on the geometry of the aircraft under test. The discrepancy should be specifically considered when planning a dynamic derivative test program. Owing to the small scales, low dynamic pressures and consequent low loads involved in the water tunnel balance system, and the structure of the computational treatment of the experimental results, results of acceptable quality require rigorous control of accuracy and precision at every stage of the experimental and computational process. Measurement and quality control of balance and model geometry are particularly important.; Static and combined dynamic derivatives can be reliably and accurately computed in the presence of very high noise levels on the measured signals, but separated dynamic derivatives require signal-to-noise ratios in the order of 100 dB or better for adequate performance.

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