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Magnetic support and balance system
In these systems, the model is supported by magnetic forces without any solid support and its aerodynamic interference; the aerodynamic forces are measured through the electrical currents required by the magnetic fields to keep the model in equilibrium with the desired attitude. The first system was built at ONERA in 1957. Since then, many plants have been built in Europe, the United States and Japan.
7.3.5.1Principle of operation
A magnetic support and balance system is composed of five elements:
1. a magnetic core within the model;
2. a set of electromagnets placed outside the test chamber to keep the model in the desired position;
3. an optical system for detecting the position of the model;
4. a system for monitoring the intensity of the electrical current flowing in the electromagnets;
5. an overall management system.
In the plant built at the NASA Langley Research Center (LARC) in 1964, we see in Figure 7.27 a simple example showing the principle of operation of the system. The balance has a single component because the test model is a sphere, the stream is directed downward and hence the direction of the aerodynamic force (drag) coincides with that of gravity.
The system for detecting the position of the model consists of a light beam incident on a photocell. The magnetic field, provided by a magnet coaxial to the wind tunnel, supports the weight of the sphere which intercepts the light beam; the downward displacement of the model due to the aerodynamic force makes the illumination of the photocell vary which results in sending, through its control system, more current in the electromagnet to restore the original position of the model. From the increase in electrical current the intensity of aerodynamic drag can be inferred.
