Bridging the Gap between Pressure-Sensitive Paint and Balance Measurements

Abstract

We consider the question of how to reconcile integrated forces and moments from a Pressure-Sensitive Paint (PSP) measurement with measured forces and moments from a balance. We show that it is possible to compute the smallest change in pressure distribution that would be required to reconcile any differences between the two sets of data. We refer to this as the gap distribution and show that it can be expressed in terms of a set of basis functions that are determined by the geometry of the test article. The use of these gap basis functions allows sensitivity factors to be defined by which individual forces and moments can be expressed in terms of a common unit of measure, namely the magnitude of the implied gap distribution. We apply this gap analysis to data from a wind-tunnel test of the NASA Orion Command Module, for which both PSP data and balance data are available. While it is clear that there are errors in the PSP measurements, the analysis suggests that there are also problems with the balance data, especially for the normal component of force. This finding is consistent with earlier doubts that had been cast on the accuracy of the balance data. The analysis procedure involving a pressure gap distribution should be applicable to the comparison of balance data to integrated forces and moments from Computational Fluid Dynamics calculations and other techniques.