The Development of Advanced Internal Balances for Cryogenic and Conventional Tunnels

Abstract

The measurement of the aerodynamic forces is the most important task in the wind tunnel at least for aircraft development work. The accuracy and reliability of the balance is the key factor in this test technology. The urgent requirement for more and more accuracy of force testing leads to a demand for more and more balance accuracy. The most urgent demand in this field comes from the Cryogenic Tunnels like the new European Wind Tunnel. Funding from the German Ministry for Research and Technology made possible about 12 years of uninterrupted research in internal wind tunnel balances, especially in cryogenic balances, at the Technical University of Darmstadt in co-operation with Daimler Benz Aerospace Airbus GmbH. This work resulted in an advanced technique of internal balance design, construction,instrumentation and calibration methods. The outcome of this effort is not only a cryogenic balance technology, which allows transport performance measurements in the ETW with a repeatability of less than one drag count but also a considerable improvement of balances for conventional tunnels. For this result all aspects of the balance technology had to be treated. For the balance design a computerised method was developed, which allows an optimisation of the structural design in a short time. Principal aspects of the design were studied with Finite Element analysis for optimised solutions. The technique of the electron beam welded balance was established successfully. This construction method gives considerable advantages with respect to design for optimum structure stiffness and low interference. The difficult problem of strain gaging and wiring for cryogenic environment, which normally includes severe moisture problems, was solved as a result of lengthy investigations. For cryogenic balances a novel arrangement of the axial force measurement system was developed, which minimises the problem of temperature gradient induced error signals. For residual errors of this type numerical correction methods are proven. For balance calibration a new strategy is used. A novel mathematical algorithm extracts a third order measuring matrix (no matrix inversion necessary) from the calibration data set. In a mathematical sense this is the best possible closed solution. In co-operation with Deutsche Airbus and the Carl Schenck Company a fully automatic calibration machine was developed for ETW. A smaller and simplified version of this machine is under construction at the Technical University of Darmstadt. Finite element analysis turned out to be a powerful tool in the development of optimised structures for internal balances. Novel balance structures with minimised linear and non-linear interference and with minimised sensitivity against temperature gradients have been developed and will be demonstrated especially in the paper of Mr. Zhai and Mr Hufnagel.