Experience of an Automatic Rig for Calibration of Wind Tunnel Balances at FFA

A calibration rig has been designed and built for balances to be used in the new TI500 transonic wind tunnel at FFA. This rig can accomodate sting mounted as well as halfmodel balances with one to six components and typically a normal force of up to 50 kN. An optimized set of calibration loads is generated via menu on a dedicated computer and the loads are applied by means of pneumatic actuators under control of the same computer. This report describes the principles of the rig and some experiences obtained during its development. Operational status of the rig with and without a master balance coupled in series with the object balance is reported as well as various problems encountered in the progress of development to adapt the rig to various balance types and load ranges. Background A new type of automatic rig for calibration of wind tunnel balances was first suggested at FFA in 1982. This rig was a non-repositioning device characterized by attachement of the object balance at its model end to a rigid platform, instrumented and calibrated as a master balance for reference. However, since this was a new concept for calibration of wind tunnel balances, the idea was at f i t cautiously received at FFA. Later on, in 1985. when plans were being developed for our new transonic wind tupnel, T1500, a corresponding new rig was required for high calibration loads and high production capacity. At this time the plans were accepted for such a rig according to the new concept and the rig structure was completed in 1988. This device was designated at FFA as the MK15 wind tunnel balance calibration rig [I]. Description of the MK15 calibration rig. In the following the MK15 calibration rig (Figure 1) is described on the basis of main requirements in the rig specification [2]. Functional principles The non-repositioning principle is used to make the rig a low cost device, adaptable for automation by computer. Necessary corrections of calibration loads may be obtained via deflection measurements and computer software [31. In order to facilitate application of this method, the object balance is installed by its model end directly fastened to the rigid rig frame or to an intermediate very stiff master balance bolted to the rig frame (Figure 2). Use of a master balance requires accurate calibration of this balance preceding the object balance calibration. One reason for choosing the master balance method together with the non-repositioning principle was that better accuracy could be expected since calibration errors due to deflection would be largely reduced. Another advantage would be the possibility of fast object balance calibration since deflection measurements are not needed in this case. Flexibility of application The calibration rig is made to accept a large variety of wind tunnel balances for one to six component loads, half model balances as well as box type and sting mounted balances. All component loads may be applied singly or in any combination. Besides for aerodynamic application this rig may also be used in any other type of multicomponent transducer calibration for e.g. industrial or other fields. Rig configuration The rig frame is made with highest rigidity possible for strength and high natural tkquency. It is cage shaped for closed force loops and has outrigger beams for load application (Figure 1). For best conditions of load symmetry and application flexibility the rig is made with vertical axis. Balances for calibration are attached to the center of the cage floor, for standard sting type balances in FFA coordinates with the x-axis directed downwards (Figure 3).