AIAA - American Institute of Aeronautics and Astronautics
Scope
"Executive summary The purposes of this document are (1) to provide an overview of the calibration of subsonic and transonic wind tunnels and (2) to provide a basis for commonality within the wind tunnel community in the area of wind tunnel calibration. Various standards have been developed over the years in the aeronautics industry@ many of which pertain to ground testing. These standards have made it easier to share information and hardware@ and have allowed the community to improve its overall effectiveness. This document@ however@ is not intended to set standards for wind tunnel calibration. Details of calibrating a wind tunnel vary from facility to facility according to the type of tests conducted@ the operating envelope of the tunnel@ and the physical constraints of the facility@ which makes having a rigid definition of calibration procedures inappropriate. Instead@ this document provides a set of recommended practices that the reader can use to develop a complete tunnel calibration program. This document focuses on general calibration practices and principles that should be incorporated into the calibration of any tunnel. It provides recommendations on calibration hardware and instrumentation based on the current knowledge of the authors@ along with a list of references that readers can use to develop detailed calibration schemes for their tunnels. Detailed examples have been included to provide insight into the current calibration activities at existing wind tunnel facilities. In addition@ this document brings to light topics such as statistical quality control that have only recently been applied to tunnel calibration. This guide strongly recommends that wind tunnel calibrations become a regular part of the operational cycle for any wind tunnel. A planned schedule of calibration tests should be created and executed to ensure that the wind tunnel is operating properly. Beyond this baseline requirement@ planned calibration tests offer the advantage of constructing an extensive database describing the tunnel operation@ which can be very useful in determining the cause of changes in a flow field. In short@ wind tunnel calibrations should be thought of as a regularly scheduled maintenance activity or diagnostic test. Instead of a series of disconnected tests@ the calibration activities should take the form of an ongoing test program. Properly planned@ recurring@ and well-documented calibrations of a wind tunnel provide several benefits to the tunnel operator and end user. ? They ensure that the wind tunnel is operating as expected and are useful in identifying problems in the wind tunnel circuit. ? They provide potential customers with a documented assessment of the tunnel calibration and are essential in determining overall data quality. ? They provide data essential for interpretation and correction of test data. ? They provide archival documentation of tunnel operating conditions@ so that modifications to the wind tunnel may be assessed for their impact on the operating conditions. ? They aid in establishing statistical process control on wind tunnel test data by providing a database of wind tunnel parameter variability. ? They aid in identifying data anomalies that are attributable to the wind tunnel itself@ not to the variability in the calibration process. ? They may indicate@ by comparison with previous calibrations@ that portions of the wind tunnel circuit or instrumentation are in need of repair or recalibration. ? They facilitate tunnel-to-tunnel data comparisons. Since calibration is considered a critical item in the health monitoring of a wind tunnel@ this guide recommends that experienced personnel be assigned to the execution of calibration tasks. In instances where the calibration tests are perceived as routine@ they are commonly used as training exercises for less experienced personnel in the operation of the wind tunnel. Although training is important@ it is also important that an experienced person lead the calibration activities. To maximize the value of a regular program of wind tunnel calibration@ it is also important to maintain some level of personnel continuity over the course of the calibration program. A core group of personnel should be assigned to develop@ execute@ and report on the calibration test program with other personnel included as needed. Forming a team@ as opposed to a single lead engineer@ to be responsible for all aspects of the tunnel calibration program is the most desirable arrangement since the corporate knowledge can be passed between the team members. This cross training of personnel allows for continuity of calibration execution and task knowledge in the inevitable occurrence of personnel turnover. Although this document is intended more for the practitioners who will conduct the tunnel calibration activities@ it contains important points that managers in charge of wind tunnel operations should consider. The first of these points@ which has been largely ignored over the years@ is the strategic importance of a properly calibrated wind tunnel. A properly calibrated wind tunnel is required for timely@ effective product development. Despite this fundamental point@ wind tunnel managers often consider calibrations an option instead of a necessity. There have been instances of major wind tunnels operating for years between significant calibration activities. In the current environment of increased data accuracy requirements and reduced time available for tunnel tests@ it is imperative that accurate and complete wind tunnel calibrations are established@ maintained@ and placed under process control. To not do so is to place at risk substantial investments in wind tunnel testing@ which is unacceptable given the alternative of simply maintaining the wind tunnel calibration. In looking at the business case for regular wind tunnel calibration@ the question should not be ""Can we afford to?"" but rather ""Can we afford not to?"" Regardless of which way the question is posed@ the answer requires a valuation in terms that are understood economically by those in control of the investment. We believe that the operator of the wind tunnel should have a vested interest in taking the lead to establish such a valuation. Some of the major items discussed in this Recommended Practice follow: ? The importance of detailed pretest planning and the use of uncertainty analysis in the planning phase ? The selection of calibration hardware and instrumentation that address the particular facility's operating range and size@ calibration history@ and type of tests conducted ? The introduction of statistical process control as a means of tracking the wind tunnel calibration history and ensuring that the facility is operating within acceptable margins ? The frequency of calibration tests@ both full or check calibrations@ and how the timing of the calibration supports the process control philosophy ? The need for a consistent calibration team composition and the identification of a lead or project engineer ? The importance of documentation and the types of documentation required@ ranging from engineering logs to formal test reports. The bottom line is that during a tunnel calibration@ everything must be documented. It is then imperative that the results of the calibration and any related implications to the tunnel be quickly communicated to the facility staff and to the test customers ? A nomenclature list is included in Annex A to aid readers. "