Half-model-balance for the Cologne-cryogenic-wind-tunnel development and test results

The Cologne Cryogenic Wind Tunnel (KKK) is instrumented with internal balances. There has been high demand from the tunnel customers to make half model tests at high Reynolds numbers. The Mach number range from zero to 0.3 and the relative low costs with regard to the high Reynolds number made the KKK very attractive for half model testing. To realize this capability it was decided to develop a half-model-balance for the KKK. This was a very challenging task, because no previous attempt to solve this problem for a continuous running cryogenic wind tunnel was successful. Fortunately, the tests in the tunnel with the first balance (W519) built were very successful. During the tests a lot of experience was gained and some new problems appeared. The main problem was the unexpected large temperature gradient inside the balance, although there was a very sophisticated heating and insulating device around the balance. A finite element study was made to design a new balance in such a way that the balance is more insensitive to temperature gradients. Unfortunately the balance W519 was destroyed by an accident in the tunnel and we had to build a second one (W523) very quickly. All the things we learned had to be turned into practice very quickly without any tests in the laboratory. Finite element calculations turned out to be a very useful tool to solve this task. We used the external balance of our low speed wind tunnel as a reference balance to give us the "true loads". By this method no realignment of the calibration model was necessary and the calibration process could be done in three weeks. The accuracy of the new procedure, based on recalculation of the calibration loads, turned out to be better than the former one, because the accuracy of the external balance is one magnitude better than the required accuracy of the half model balance and there are no errors of misalignment. The comparison between some measurements, made with the old balance and the new one, showed a very good correspondence. The repeatability of polars with the new balance in the tunnel was about one drag and lift count.