Six-component rotating strain-gauge balance for coaxial rotors testing

DOI: 10.34759/vst-2020-1-53-64

Аuthors

Manvelyan V. S.

Central Aerohydrodynamic Institute named after N.E. Zhukovsky (TsAGI), 1, Zhukovsky str., Zhukovsky, Moscow Region, 140180, Russia

e-mail: vagan.manvelyan@tsagi.ru

Abstract

Aerodynamic strain-gauge balance is employed to study the total loads on an object streamlined by the airflow in aerodynamic experiment. As a rule, the total loads are being represented by six components, namely by three forces along the orthogonal axes and three moments around the vectors of these forces. The strain-gauge balance is a special measuring device, which operation principle is based on the strain-gauge effect. Rotating strain-gauge balance is employed to measure loads affecting rotating object.

Coaxial rotor is a system with two airscrews rotating in opposite directions. To analyze the processes while coaxial rotor operation and of airscrews interaction, it is necessary to measure loads on each airscrew, i.e. both on the one rotating clockwise and the other rotating counter-clockwise. To solve the set task two rotating strain-gauge balances were developed in Central Aero-hydrodynamic Institute named after professor Zhukovsky (TsAGI) – one for each airscrew.

All over the world, companies such as RUAG (Switzerland), NLR (Netherlands), ONERA (France), etc. are engaged in rotating strain-gauge balance development. The most common design of rotating strain-gauge balance is a monoblock of a cylindrical shape. The external rigid rim is fixed to the internal cylindrical support by the beams used to be measure the loads. The external rigid rim is coupled with the internal hub by the beams, serving to loads measuring. The external rim is coupled with the screws hub, and internal hub is coupled with the shaft of the installation, which rotates the screws. Thus, the beams, on which the strain-gauge resistors, forming the measuring bridge, are glued, are deformed, and measuring strain-gauge resistor bridges convert the beams deformation into electric signal.

One of the most significant aspects of the design is the number and shape of the beams and the scheme of strain gauge gluing. The most widespread structure includes trapezoidal shape beams at the front view, and eight beams, connecting the rim and the hub, namely, a two beams in each of four packs. The main disadvantage of such structure is low value of the signaling stress under the strain-gauge resistor, pasted for lateral force measuring, and high mutual effect of the components, which leads inevitably to higher error value (more than 1,5% of measuring range).

To avoid the above-mentioned issues, the new structure of the strain-gauge balance was developed in TsAGI. The design is similar to the one described above, but it is based on the unique shape and increased number of beams from eight to twelve, i.e. three beams in each of four packs. Computations confirmed that the signaling stress under the strain- gauge resistors pasted for lateral force measuring increased, while mutual effect of the components decreased. Alongside with other solutions, increasing the number of beams and their unique shape ensures lower value of the expected error (less than 1% of measuring range). The expected error will be confirmed by future studies on the results of static and dynamic calibration.

Keywords:

rotating strain-gauge balance, rotor testing, coaxial rotors, experimental basics of flying vehicle design

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