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• August 10, 2016 Create Date
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Metrology Measurements of the DSTO Transonic Wind Tunnel Store Support Arm

The Defence Science and Technology Organisation maintains and operates a number of
experimental aerodynamic test facilities including a transonic wind tunnel. This facility
has three model support systems including a sidewall turntable, a main model
support, and a store model support. The store support system provides six degrees of
freedom, enabling models under test to be translated and rotated in the test section to
predefined positions and orientations. The store support arm uses encoders, linear
displacement transducers, and resolvers to measure the raw motion of its actuation
components. These readings are converted by the data acquisition system into
engineering units through the use of a number of calibration curves. The accuracy of
the translational and rotational readings from the store support system must be
checked periodically to ensure that they are within acceptable uncertainty limits, and
therefore provide quality assurance for test clients.

A series of metrology measurements were conducted on the store model support for
this purpose during February and March 2013. The movements of the store support
arm were primarily measured by a portable coordinate measuring machine, a precision
spirit level, and a mechanical clinometer. These instruments were used to take
translational and rotational readings against a reference metrology arbor installed on
the store support. Readings were compared with those obtained from the store support
system data acquisition system. This considered both absolute readings and relative
readings.

Results were used to update the data acquisition system calibration curves which
convert the raw readings into output engineering units. No significant changes had
occurred, and only minor offset adjustments were applied to the vertical, axial,
upstream yaw, and pitch calibration curves.

The final uncertainties of the store support system exceed those defined in the original
specifications, and this was attributed in part to the uncertainty in the metrology
methodology. The original specifications were based on measurements under
laboratory controlled conditions and the use of highly accurate laser interferometry.
They were not determined with the store support assembled in the test section.

It is recommended that the metrology measurements are repeated within five year
intervals. Improved methods to measure the translational and rotational movements
should also be investigated.