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AIAA R-093 Document Information:
Title
Calibration of Subsonic and Transonic Wind Tunnels
American Institute of Aeronautics and Astronautics
Publication Date:
Jan 1, 2003
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.
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