Recent Improvements in Pressure-Sensitive Paint Measurement Accuracy at Boeing

The accuracy of pressure measurements made with the Boeing Pressure-Sensitive Paint system has been re-assessed. The evaluation was done in light of recent system improvements, which include a new paint formulation and the addition of feedback control for the excitation lamps. A highly rigid test installation that minimized model displacements in the light field was also a factor. The observed accuracies for transonic test conditions range from 0.012 to 0.033 in terms of pressure coefficient. These results represent an improvement of 50 percent. Background and motivation Reference 1 documented the accuracy achieved by the Boeing Pressure-Sensitive Paint (PSP) system late in the year 2000. Uncertainty estimates were obtained from a statistical analysis of over 44,000 comparisons between measurements from PSP and those obtained from conventional pressure transducers. The standard uncertainty of the data from the intensity-based system with in-situ calibration, in a production wind tunnel, ranged from 0.16 to 0.45 psi. In terms of pressure coefficient, this is equivalent to 0.04 to 0.1. In addition, the variable-density capability of the tunnel was utilized to apply several levels of known, constant pressure to the model under quiescent conditions. The uncertainty for these wind-off test points was only 0.14 psi. ∗ Copyright c © 2002 by The Boeing Company. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. † Senior Specialist Engineer, Senior Member AIAA ‡ Senior Engineer § Associate Technical Fellow, Associate Fellow AIAA ¶ Associate Technical Fellow, Senior Member AIAA ‖ Associate Technical Fellow Reference 1 also examined the efficacy of using infrared cameras for temperature correction. It was found that the impact of temperature correction on the uncertainty of wind-on test points ranged from a 30 percent improvement to a 35 percent degradation. In contrast, the improvement was a substantial 60 percent for the wind-off measurements. In light of the significantly reduced uncertainties of the wind-off data, it is obvious that extra sources of error are present in the wind-on data. The effects of these extra error sources are greater than the improvements afforded by temperature correction or in-situ calibration. Two likely culprits are unsteadiness of the excitation light field and motion or deformation of the model in this field. These sources of error arise from the necessity to obtain reference images at quiescent conditions. The reference images are separated in time from the run images, allowing any temporal unsteadiness of the lights to be manifested as error. The reference images are also obtained with the model under a different load than that experienced with the wind on. Not only will the model support system deflect, but the model itself will deform. Both effects mean that for the wind-on image, the model is in a different location in the illumination field than it was in the wind-off reference image. This manifests itself as a change in luminescence ratio that is not due to pressure. This paper discusses the results of a test that addressed these two sources of error. Feedback controllers added to the standard blue lights ensured a steady excitation field. The effects of model motion and deformation were minimized by the use of a plate mounting system on an external balance. Finally, a new paint formulation has been introduced that is less temperature sensitive. Experimental conditions The test was conducted in the Boeing Transonic 1 American Institute of Aeronautics and Astronautics 22nd AIAA Aerodynamic Measurement Technology and Ground Testing Conference 24-26 June 2002, St. Louis, Missouri AIAA 2002-2907 Copyright © 2002 by the author(s). Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 0 2 4 6