Multi-Plane High Speed Balancing Techniques and the Use of a High Specific Stiffness Ti-Borsic Material for Vibration Control.

This report documents results of multi-plane high-speed balancing demonstration of a flexible rotor and a preliminary design analysis for a high specific stiffness composite material shaft. Both studies had as their objective the management of small turbofan engine low pressure shaft bending critical speeds. The prototype flexible rotor was successfully balanced through 3 critical speeds reaching a maximum of 28,000 rpm, which was 74 of the maximum intended rotor speed of 38,000 rpm. Balancing for operation above the 4th critical speed, which was predicted to occur at 33,000 rpm, was prevented due to a sub-synchronous rotor instability. Causes of the instability were attributed to the configuration of the squeeze film bearing damper and the engine rotor support structure as opposed to any limitation of the balancing techniques employed. The preliminary composite shaft design was completed assuming that a Ti-Borsic metal matrix composite with 60 fiber volume and 40 metal matrix would be used. This stiff shaft was designed as a direct substitute for the multi-plane high speed balancing demonstrator rotor. Analytical studies indicate that the composite rotor will have a 24 third critical speed margin when operating at 38,000 rpm.