Accuracy, Stability, and Confidence: RUAG Relies on HBM Solutions in Wind Tunnels

Experimental tests in a wind tunnel provide the aerodynamics engineer with the necessary data to design and assess the aerodynamic properties of a test object. The tests are performed either with a scaled-down but extremely realistic model of an aircraft, building, and vehicle or with a full-size object (athletes and cars/vehicles).

RUAG Schweiz in Emmen operates several wind tunnels in which measurement data are recorded, processed and evaluated with the highest precision.

Challenge

Despite high investment and operating costs, aircraft and vehicle manufacturers continue to rely heavily on wind tunnel testing. The complexity and requirements of the tests are increasing. High-quality measurement data - both static and increasingly transient - must be delivered reliably and efficiently. Also, the productivity from installation to delivery of the results is essential.

Solution

MGCplus amplifier systems were selected and integrated into a compact and mobile overall system. This results in a fast and flexible connection of measuring chains with standardized connection cables, various measuring sensors, as required for aerodynamic tests, and the configuration via an open interface to the wind tunnel software. Further criteria for the solution are efficient calibration with traceability to national standards and long-term delivery reliability by the manufacturer.

Result

Thanks to the modularity, flexibility, high precision, and excellent long-term stability of the MGCplus/DMP DAQ systems, a wide variety of tests is carried out. Together with the RUAG multi-component balances, a measurement chain is available for determining loads such as lift and drag forces. The chain can be optimally configured to provide the aerodynamics engineer with the necessary data for the improvement/validation of his product.

Wind Tunnel Testing With Realistic Results

The scope of wind tunnel testing performed at RUAG is very large and the requirements with respect to measurements and data acquisition are thus diverse. In the simplest case, the customer is only interested in observing the test object as it is exposed to wind, for instance, an umbrella. In this case, only wind speed needs to be known.

Generally, tests are more complex and the wind tunnel models are equipped with a number of sensors used to obtain quantitative data for loads and pressures acting on the object. The LOSITA wind tunnel model, built and tested in the frame of the European Clean Sky 2 Initiative, is an example of a complex wind tunnel test with high demands on data acquisition. LOSITA has received funding from the European Union's Horizon 2020 research and innovation program under grant agreements CS-GA-2013-01-LOSITA-620108.

The model is equipped with a RUAG six-component balance to obtain the global forces and moments acting on the aircraft. The aircraft is driven by two powerful wing mounted propeller turbines. The propellers will have a significant effect on the airflow around the wing and must thus be incorporated in the wind tunnel model. The forces acting on the propellers are measured with additional 6-component balances installed in the hub of the propellers (Rotating Shaft Balances). From these global force values, the aerodynamicist extracts information about the performance of the aircraft but also its stability and controllability. 

Structural engineers may be more interested to obtain the aerodynamic loads on certain components of the aircraft. Such data is derived from pressure measurements at different locations on the model. For this purpose, the LOSITA model was equipped with more than 700 pressure taps. An alternative or complementary approach is the use of strain gauge component balances designed specifically to measure, for example, the forces on a single control surface.

In addition to these “customer” measurements, several sensors are used to control and monitor the test and the model such as accelerometers, rpm-, position- and temperature sensors. The differing goals of each wind tunnel test campaign and the resulting diversity of deployed sensors put very high demands on the flexibility of the data acquisition system. The instrumentation and specifically the multicomponent balances need to fulfill very stringent customer requirements when it comes to accuracy and repeatability.

The DMP/MGCplus systems have been used in RUAG's wind tunnels and for the calibration of its wind tunnel balances for more than 10 years. The originally specified requirements regarding accuracy, resolution, repeatability and availability are still met or exceeded today. The periodic calibrations also show excellent long-term stability. The measurement results obtained have served and continue to serve our customers as confirmation or as valuable input for the further improvement of their product.

Wind Tunnel Balance

A six-component balance has the task of measuring three orthogonally aligned forces and moments simultaneously with as little mutual interference as possible. In aviation, these are lift/normal force (Z), drag/axial force (X) and side force (Y), and the moments around the roll (L), pitch (M) and yaw (N) axes.

The RUAG balance consists of two halves: the earth-fixed or non-metric and the metric half.

  • Non-Metric Half: It is attached to a solid structure, e.g. the ground or a support, which determines the position of the model in the wind tunnel.
  • Metric Half: It is fixed to the object to be measured, i.e. the car or aircraft model.

Via connecting rods and measuring elements with high-precision strain gauges, mechanical strains are recorded electrically and converted into physical units via a preceding calibration.

RUAG’s 7xx Balance Series

Since the customer requirements on the test object and the forces to be measured are directly dependent on the size of the wind tunnel model, the RUAG balance family offers the possibility of selecting an optimally suited balance for the test in terms of volume and load range.

Precision and Quality Assurance: Measurement and Calibration

Annual calibrations, traceable to a national standard, are required for ISO 9001 quality assurance. This applies to the MGCplus systems used as well as to the RUAG multi-component balances.

A calibration system has been developed for this purpose, with which all MGCplus channels can be checked and calibrated. A DMP40 from HBM is used for the calibration of the balances, which meets the very highest requirements.

About RUAG AG

RUAG AG is a leading supplier, service provider, and integrator of systems and components for civil and military aviation. The company's core competencies include repair and maintenance work, as well as upgrades, development, manufacture, and integration of subsystems for aircraft and helicopters - and this throughout their entire service life. 

Just over 70 years ago, the aerodynamics research department of RUAG's predecessor company, Eidgenössisches Flugzeugwerk F+W, began measuring forces and moments on aircraft. Although the technical possibilities have since been considerably refined, the fundamental questions that engineers ask themselves about understanding physical processes are as relevant today as they were then.

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