740. Dynamic Balance Error Terms for Rotating Aerospace Vehicles

Paper

Abstract

Rotating aerospace vehicles are usually balanced as a rotor on a dynamic balance machine to determine the position of their principal axes. This paper discusses the error terms in principal axis location that can exist between a rotor on the machine and a free body in space.
The mass properties of a rigid body are defined with respect to a reference system and with respect to a free body. Methods for expressing principal axis location are presented. Several sets of units that can be used are described.
The error terms in using a balance machine to locate the principal axis are discussed. Two types of vertical balance machines are included – two-transducer single-speed and single-transducer two-speed. The machine related errors include machine residual unbalance, induced effects of residual static unbalance, and effects of test article vibration. The effects of non-ideal structural reference systems are discussed. The effects of ambient air are divided into velocity-dependent terms and acceleration-dependent terms. The added mass of air is included in the acceleration-dependent terms. Elastic effects are caused by internal deflections under centrifugal loads. These effects can be important for single-transducer two-speed machines. Other error terms, including configuration changes, are briefly discussed.
The conclusion presents some guides for making error analyses and for selecting the type of balance machine.