WIND TUNNEL INSTRUMENTATION TECHNIQUE BALANCE DESIGN PROBLEMS

The most important piece of instrumentation in a wind tunnel is the balance, which at present time normally is designed as an'internal strain gage balance. Since airplane performance can be evaluated only from precise force measurements in the wind tunnel and since performance is the ultimate goal of airplane development, the accuracy of the balance is a key problem in wind tunnel work. The author describes a successful technology for balance design and fabrication. In order to avoid hysteresis and internal friction, a balance should be designed and fabricated from one single piece of metal. This leads to difficult fabrication methods which are avoided by the MBB technology to assemble the balance from pieces by an electron beam welding process. By a special heat treatment the original material behaviour is restored. Sophisticated gage application methods and calibration methods contribute to an excellent balance accuracy and reliability. Especially the problem of temperature effects and temperature gradient compensation is discussed in the paper; an outlook on balance technology for cryogenic wind tunnels is given.