CALIBRATION UNCERTAINTY OF AN EXTERNAL SIX-COMPONENT WIND TUNNEL BALANCE

Over the last four years, a methodology has been developed for the assessment of the calibration uncertainty of the external balance at the Subsonic Wind Tunnel No. 2 (TA-2), Centro Tecnico Aeroespacial, (CTA), Brazil. Previous estimations of uncertainty did not follow metrology standardization and were simply based on standard deviations of the wind tunnel balance calibration and assumed contributions from tunnel correction procedures. At TA-2, the method for the estimation of combined standard uncertainty in measurements of aerodynamic loads has evolved and nowadays, it includes the employment of the general law of propagation of uncertainty to the curve resulting from the multivariate regression, correction for recognized systematic errors, and the uncertainties components due to the load cell calibrations and readings. Also, the number of degrees of freedom is calculated in order to provide the expanded uncertainty of the measurement. Despite the improvements, some contributions to the uncertainty have been overlooked, such as the calibration uncertainty in the weights applied to the loading system, nonlinearity of the functional relationship, the standard deviations of the measured dependent variable and the computation of the covariances between the fitted parameters. A more recent assessment of the friction forces and corresponding uncertainties has been carried out in order to better evaluate the contributions. A qualitative and quantitative comparison is presented, related to the evolution of the methodology. *