Assessing Long Term Wind Tunnel Balance Performance and Uncertainty from Multiple Calibrations - Revisited

Abstract

NIST defines the intent of an instrument calibration as follows: “to eliminate or reduce bias in an instrument’s readings over a wide range for all continuous values”. Performance of wind tunnel balance calibrations provide traceability to a known standard and remove any large known interaction biases from the instrument. One contributor to the calibration uncertainty is the calibration procedure or process which propagates into the balance uncertainty as a bias (systematic error). One of the basic assumptions of all calibrations is that the reference standard transfers flawlessly thereby yielding a perfectly repeatable calibration every time a calibration or check standard is performed. Mistakenly, calibration coefficient changes are attributed solely to changes in the balance’s mechanical and/or electrical systems. Evaluating math model (calibration curve) adequacy requires replicate measurements. Replication allows one to accurately compute a model-independent estimate of the process standard deviation. The standard practice of performing a single calibration does not expose the uncertainties in the calibration process nor does it offer the ability to track the long-term performance of a balance. Representing calibration data with an appropriate least-squares curve fit will result in a calibration with favorable uncertainties even in the presence of large process bias. This paper will revisit a summary of methods currently in use at the Boeing Aerodynamics Noise and Propulsion Laboratory in Seattle to assess strain-gage balance calibration uncertainties and long-term health using the results of multiple calibrations over the last decade. Nomenclature AIAA = American Institute of Aeronautics and Astronautics ANP = Aerodynamics, Noise, and Propulsion BTWT = Boeing Transonic Wind Tunnel OFAT = One Factor at a Time NIST = National Institute of Standards and Technology (an agency of the U.S. Department of Commerce) 1 Lead Engineer, Aero/Noise/Propulsion Laboratory, P.O. Box 3707/1W-77 51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 07 10 January 2013, Grapevine (Dallas/Ft. Worth Region), Texas AIAA 2013-0546 Copyright © 2013 by The Boeing Company. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. American Institute of Aeronautics and Astronautics Copyright © 2012 Boeing. All rights reserved 2