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The paper describes an Autoniatic Calibration Machine to calibrate internal windtunnel balances. The autoiiiatisation is mainly achieved by using a specially designed force iiieasuring device, similar to an external windtunnel balance. The loading of t h e exaiiiinee is performed by nieans of six force generators, according to the s i s aerodynaiiiic components of t h e balance.Described a r e t h e relevant aspects of accuracy, repeatability and signal resolution, the niechanical design of the calibration rig, the controlling and safeguarding of the applied forces. and soiiie general aspects for the positioning and alignment of the internal balance to b e calibrated. INlRODUCTTONThe most iniportant measuring tool in a wind tunnel is the force measuring balance. Decades of developnient resulted in high precision balances for conventional low speed and high speed tunnels. However additional iniprovenient of accuracies is still needed and internal balances for cryogenic tunnels a r e still f a r away from the required accuracy. An important part of this progress may b e achieved by an advanced calibration procedure and by application of advanced calibration algorithms. These algorithms as well as the use of strain gage balances in cryogenic tunnels ask for a much larger calibration data b a s e than conventional methods. Since for internal balances the calibration procedure with standard hand operated equipment needs a large aniount of manpower, for econoiiiic calibration of internal balances an autoniatic calibration rig is essential. In the c a s e of the cryogenic balance autoiiiation is even more important. Apart from the automatic operation, the rig has to fulfil sonie special requirements resulting froiii the increased accuracy demands. GENERAL REQUIREMENTSThe following list of requirements is divided into two sections. Section I summarizes the absolutely necessary requirements, any solution which does not cover the items listed was definitely excluded. Section I1 covers "second priority" items, Section I 1. Accuracy in the order of 0.10 o/oo. 2.Repeatability at least twice as good as accuracy. Resolution (smallest increnient of load detectable) atleast five times b e t t e r than accuracy. 4.Application of a l l calibration loads without any inodificatioii of original niounting and alignment of balance.5. Estreiiiely precise controlling and nieasurenient of balance alignment as priiiie condition to reach required accuracy and repeat ability. Section I16. Considerably improved operation speed coiiipared with conventional methods. e. g. one coniplete force calibration (all s i s cotnponents) in one day, resulting in about 20 sec. time for any single loading step. 7.F r e e space around the balance to allow the installation of a climatic chaiiiber.8. Mininiuiii required nuniber of openings through the climatic chaniber in order to reduce temperature eschaage and risk of force bypassing caused by icing problems.9. Safeguarding of balance in c a s e of overloading by faulty operation. 470