An improved method of calibrating a wind-tunnel force balance involves the use of a unique load application system integrated with formal experimental design methodology. The Single-Vector Force Balance Calibration System (SVS) overcomes the productivity and accuracy limitations of prior calibration methods.
A force balance is a complex structural spring element instrumented with strain gauges for measuring three orthogonal components of aerodynamic force (normal, axial, and side force) and three orthogonal components of aerodynamic torque (rolling, pitching, and yawing moments). Force balances remain as the state-of-the-art instrument that provide these measurements on a scale model of an aircraft during wind tunnel testing. Ideally, each electrical channel of the balance would respond only to its respective component of load, and it would have no response to other components of load. This is not entirely possible even though balance designs are optimized to minimize these undesirable interaction effects. Ultimately, a calibration experiment is performed to obtain the necessary data to generate a mathematical model and determine the force measurement accuracy.
In order to set the independent variables of applied load for the calibration experiment, a high-precision mechanical system is required. Manual dead-weight systems have been in use at Langley Research Center (LaRC) since the 1940s. These simple methodologies produce high confidence results, but the process is mechanically complex and labor-intensive, requiring three to four weeks to complete. Over the past decade, automated balance calibration systems have been developed. In general, these systems were designed to automate the tedious manual calibration process resulting in an even more complex system which deteriorates load application quality.
The current calibration approach relies on a one-factor-at-a-time (OFAT) methodology, where each independent variable is incremented individually throughout its full-scale range, while all other variables are held at a constant magnitude. This OFAT approach has been widely accepted because of its inherent simplicity and intuitive appeal to the balance engineer. LaRC has been conducting research in a "modern design of experiments" (MDOE) approach to force balance calibration. Formal experimental design techniques provide an integrated view to the entire calibration process covering all three major aspects of an experiment; the design of the experiment, the execution of the experiment, and the statistical analyses of the data.
In order to overcome the weaknesses in the available mechanical systems and to apply formal experimental techniques, a new mechanical system was required. The SVS enables the complete calibration of a six-component force balance with a series of single force vectors.
This new system improves on the "trusted" aspects of current manual calibration systems. The SVS enables the efficient execution of a formal experimental design, is relatively inexpensive to manufacture, requires minimal time to operate, and provides a high level of accuracy.
The system allows for single vector calibration, meaning that single, calibrated dead-weight loads are applied in the gravitational direction generating six component combinations of load relative to the coordinate system of the balance. By utilizing this single force vector, load application inaccuracies caused by the conventional requirement to generate multiple force vectors are fundamentally reduced. The system features significantly fewer components than the LaRC manual system and therefore fewer sources of systematic error. The primary components include a non-metric positioning system, a multiple-degree-of-freedom load-positioning system, a three-axis orthogonal accelerometer system, and calibrated weights (see figure).