Strain Gauge Balance Developments

The New No. 8 Performance Balance

The performance testing of civil transport aircraft models in the 2.74m x 2.44m transonic wind tunnel at ARA currently achieves its high degree of drag accuracy and repeatability by a combination of several factors. Key to this is a quickly performed Mach number traverse, the so-called CD0 run, where data is recorded at a nominally constant model incidence setting across the Mach number range. The purpose of these runs are to minimise the effects of balance temperature drifts seen in the longer runs that are required to execute normal polars and to establish an accurate baseline drag measurement to which data from the normal polars are corrected. Whilst a very reliable and highly developed procedure for achieving accurate drag measurement, these CD0 runs can take up to 20% of the tunnel testing programme.

A balance research and development programme has been on-going at ARA for some years now. One aspect of this programme has been the evolution of new designs for a 57.15mm diameter performance balance. Recently, ARA has been successful in the production of a design that is effectively totally insensitive to temperature drift throughout the range of temperatures generated during wind tunnel testing. Central to this achievement has been:

• Use of finite element structural and transient thermal analyses.
  
• The development of a FE thermal transient analysis technique to characterise the drift performance of a balance design.
  
• The validation of axial force drift predictions against wind tunnel test data.
  
• The development of a new balance design methodology.
  
• Developments in data processing techniques.

The balance performance has been fully validated in the TWT using the ARA Reference Model, a high fidelity generic civil transport model regularly used to quality assure the facility and associated data measuring and acquisition  systems.