A sensitive rolling moment balance for use in supersonic blowdown tunnels

Abstract

STRAIN-GAGE balance is a standard device for measurement of aerodynamic loads acting on models in a wind tunnel. The most common type—the six component internal balance—is employed for general, routine testing while special-purpose balances designed to meet exclusive requirements are used for specialized tests. Design of balances meant for use in supersonic blowdown tunnels can often be complicated on account of the need to withstand large transient loads that occur on the models during tunnel starting and stopping and yet have adequate sensitivity. These problems become particularly severe when the loads to be measured are of low magnitude, such as in some special tests. Successful solutions to these problems are often obtained by adoption of some unconventional design concepts and schemes. A recent example of a novel scheme for measurement of low rolling moments in low speed wind tunnels is found in Ref. 1. A special-purpose internal strain-gage balance was developed to measure low rolling moments in a trisonic blowdown tunnel. The capability of the balance to withstand large magnitudes of all the six components of load while retaining a high sensitivity to rolling moment is an important and special feature. The balance was required to measure maximum rolling moments varying between 0.2 and 10 kg-cm with a resolution of 0.002 kg-cm on wing-body models of 36 mm diameter and 380 mm length. The tunnel starting/stopping loads that the balance had to withstand were large owing not only to the relatively large size of models but also to the high pressure ratios required for starting the tunnel (starting pressures at Mach 2.5 and 3 were 5 and 6.5 atm, respectively). The above combined requirements could not be met by the usual balance arrangements employing cruciform or torque tube element and a balance incorporating some unusual design features, described below, was developed. Description The balance is of floating-frame type and the sensing element is essentially a crossed-flexur e pivot formed by four strips connecting the outer sleeve to the inner rod (Fig. 1). The strips are arranged in pairs in two transverse planes and the distance between the strips in each plane is kept as large as possible to resist high pitching and yawing moments. The strips, made of heat treated high strength alloy steel, are 1 mm thick by 9 mm wide and provide adequate sensitivity to low rolling moments. In order to increase the capability of the balance to withstand large rolling moment, an overload safeguard that automatically limits the rolling moment in the strips is provided. The safeguard, Fig. 2, is formed by a stop connected to the outer sleeve and passing through slots in the inner rod. When the balance is subjected to a rolling moment