Sean A. Commo

Wind-Tunnel Balance Characterization for Hypersonic Research Applications

Wind-tunnel research was recently conducted at the NASA Langley Research Center s 31-Inch Mach 10 Hypersonic Facility in support of the Mars Science Laboratory s aerodynamic program. Researchers were interested in understanding the interaction between the freestream flow and the reaction control system onboard the entry vehicle. A five-component balance, designed for hypersonic testing with pressurized flow-through capability, was used. In addition to the aerodynamic forces, the balance was exposed to both thermal gradients and varying internal cavity pressures. Historically, the effect of these environmental conditions on the response of the balance have not been fully characterized due to the limitations in the calibration facilities. Through statistical design of experiments, thermal and pressure effects were strategically and efficiently integrated into the calibration of the balance. As a result of this new approach, researchers were able to use the balance continuously throughout the wide range of temperatures and pressures and obtain real-time results. Although this work focused on a specific application, the methodology shown can be applied more generally to any force measurement system calibration.

Statistical Engineering Perspective on Planetary Entry, Descent, and Landing Research

ABSTRACT Statistical engineering emphasizes developing and leveraging statistical methods and tools to help identify and solve large, complex problems. Within NASA, these large, complex problems are known as the agencys “Grand Challenges.” Research in planetary entry, descent, and landing technologies is one of these challenges and is an expensive, resource-intensive endeavor that benefits from the rigorous approach of statistical engineering. This article highlights the contributions of statistical engineering to the Mars Science Laboratory mission and, more generally, planetary entry, descent, and landing research. For example, a new approach utilizing response surface methods was developed for characterizing a complex measurement system. In addition, we reflect on areas where early implementation of a statistical engineering approach can increase the overall impact of the research objectives.

Thermal and Pressure Characterization of a Wind Tunnel Force Balance Using the Single Vector System

Wind tunnel research at NASA Langley Research Center s 31-inch Mach 10 hypersonic facility utilized a 5-component force balance, which provided a pressurized flow-thru capability to the test article. The goal of the research was to determine the interaction effects between the free-stream flow and the exit flow from the reaction control system on the Mars Science Laboratory aeroshell during planetary entry. In the wind tunnel, the balance was exposed to aerodynamic forces and moments, steady-state and transient thermal gradients, and various internal balance cavity pressures. Historically, these effects on force measurement accuracy have not been fully characterized due to limitations in the calibration apparatus. A statistically designed experiment was developed to adequately characterize the behavior of the balance over the expected wind tunnel operating ranges (forces/moments, temperatures, and pressures). The experimental design was based on a Taylor-series expansion in the seven factors for the mathematical models. Model inversion was required to calculate the aerodynamic forces and moments as a function of the strain-gage readings. Details regarding transducer on-board compensation techniques, experimental design development, mathematical modeling, and wind tunnel data reduction are included in this paper.

Development of the In Situ Load System for Internal Wind-Tunnel Balances

Results from the Facility Analysis Verification and Operational Reliability project revealed a critical gap in capability in ground-based aeronautics-research applications. Without a standardized process for check loading the wind-tunnel balance or the model system, the quality of the aerodynamic force data collected varied significantly between facilities. The In Situ Load System was developed to provide a standard for facilities in the check-loading process. The system includes both the hardware and a statistically rigorous process that facilitates the ability for the user to make defendable decisions on the performance of the system. The compactness and simplicity of the system reduce customer costs unrelated to achieving the research objectives, while simultaneously improving the knowledge about the accuracy of the test data collected. While the focus is on the check-load process, the hardware and methods are also applicable to the in situ calibration of a balance or wind-tunnel model system.

Experimental Design Considerations for Calibration of Semispan Force Measurement Systems

Experimental design considerations for the development of calibration load schedules are discussed for the characterization of traditional five-component semispan balances used in aerodynamic ground testing applications. Detail is given on traditional semispan balance design, use of these types of balances, and a survey of some of the calibration systems currently used to calibrate these measurement systems. Techniques are presented to develop experimental calibration designs used to calibrate these instruments, with consideration given to accounting for physical limitations existing within these calibration systems. The techniques provided rely on traditional statistical engineering approaches, leveraging off of statistics-based experimental design techniques and analysis metrics used to assess the characteristics of the designs. Methods used for optimal design techniques are presented, with a case study given that details the comparison of these statistics-based metrics for traditional and optimized cal...

Prediction Interval Development for Wind-Tunnel Balance Check-Loading

The current approach used to apply uncertainty intervals to balance estimated loads is based on the root mean square error from calibration. Using the root mean square error, a constant interval is applied around the estimated load and it is expected that a predetermined percentage of the check-loads applied fall within this constant uncertainty interval. However, this approach ignores additional sources of uncertainty and assumes constant uncertainty regardless of the load combination and magnitude applied to the balance. Rigorous prediction interval theory permits varying interval widths but fails to account for the additional error sources that are unrelated to the mathematical modeling. An engineered solution is proposed that combines prediction interval theory and the need to account for the additional sources of uncertainty from calibration and check loading. Results from a case study using the in-situ load system show improved probabilistic behavior in terms of uncertainty interval capture percenta...