Improved wind tunnel testing and data reduction methods using a magnetic suspension system

Abstract

Improved techniques and data reduction methods for aerodynamic force measurements are proposed and tested. The model levitation and force measurements are provided by the magnetic suspension and balance system of Southampton University. Quasi-static and dynamic forces and moments are measured at low tunnel air flow speeds.

A systematic study of the incidence ramp technique established limitations set by the model aerodynamic properties, balance dynamics and data reduction methods. A simple relationship between the maximum ramp rate, the aerodynamic flow response time and the required data smoothing was obtained. This relationship was verified experimentally. The aerodynamic forces on several slender models were measured using slow Incidence ramp rates. The effect of ramp rate on various aerodynamic characteristics was studied using an AGARD-G planform model. The results compare favourably with published work. The drag force is found to exhibit very slow response. It requires correction even at the lowest ramp rate. The results of the ramp testing show finer aerodynamic details than conventional point-by-point testing. The set of data points for a single test display a high degree of self consistency. They demonstrate the potential for a reduced cost per data point from wind-tunnel testing.

The novel method of digital covariance zero crossing for accurate determination of the frequency response of linear dynamic systems is proposed. The method is particularly useful for low frequency sinusoidal signals distorted by heavy superimposed noise. It is shown that the accuracy of calculated joint dynamic properties of two signals depends on their record length, or number of oscillation cycles. A relationship between the signal-to-noise ratios, minimum record length and required accuracy is obtained. A verification of the method is provided by a simple digital simulation.

The method has been applied to or considered for:

(i) the measurement of roll damping derivative at constant incidence,

(ii) the measurement of roll damping derivative with ramped incidence,

(iii) multi-degree of freedom systems for the measurement of combined pitch and heave derivatives.

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