818. Aerodynamics Effects on the Measurement of Mass Properties Using a Trebel Dynamic Balancing Machine

Paper

Abstract

The work described in this paper is concerned with the measurement of mass properties of bodies using a Trebel FVD.500 Dynamic Balancing Machine, and deals specifically with bodies whose external shape is unsymmetrical about the axis of rotation. The rotation of such bodies results in an aerodynamic force which, because the balancing machine assumes that all forces are caused by rotating masses, represents an error in the measured mass properties. It would also constitute an error in dynamic balancing, because weights would be added to the body to compensate for the aerodynamic force as well as the mass forces. The measurements described in this paper were made using a Trebel Dynamic Balancing Machine, and illustrate how this machine can be used to measure aerodynamic force and so correct the machine data to give true mass properties. The relationship between machine readings and the force system all a rigid rotating body are shown to contain a term dependent upon the aerodynamic force and varying inversely as the square of the speed of rotation. This term is of appreciable magnitude at low speeds of rotation, and results in a variation of machine readings with speed of rotation. By carrying out measurements on one particular body over a range of speeds from 50 rpm to 120 rpm, the change of machine reading with speed can be determined, and from this the aerodynamic force can be computed. This process is discussed in the paper. and test results are given and analysed. The main conclusion from this work is that, when aerodynamic effects are appreciable, they can be measured on the Trebel FVD. 500 Dynamic Balancing Machine by carrying out a series of runs at different speeds of rotation in the low speed range. The experience of the authors of this paper is limited to the Trebel vertical two plane balancing machine and they cannot speak of the applicability of the methods to tests using other types of machine. However, it is their view that a basic consideration of the way in which the force system contributes to the machine readings can, as in their particular experience, be of value in appreciating the capabilities of a measuring system.