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The Response of Turbine Engine Rotors to Interference Rubs,
ARMY RESEARCH AND TECHNOLOGY LABS CLEVELAND OH PROPULSION LAB
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In a typical aircraft gas turbine there are many instances in which rotor rubs occur. Two of the most common are blade tip and seal rubs, which are caused by thermal mismatch, rotor imbalance, high g maneuver loads, aerodynamic forces, etc. Current interest in fuel efficiency is a consideration which drives the engine design toward closer operating clearances. Thus increasing the probability of rotor rubs. The interaction of a rotor with its case, rotor rubs, has been studied. A steady state interaction between a rotor with a rigid case neglecting friction at the interface and a steady state interaction between a linear flexible rotor and case including friction at the interface were studied. The critical transient situation in which the rotor bounces off the case was not considered. It is known that rotor rubs can have an important effect on the rotor dynamics. When a rotor rubs on the case, a frictional force is generated which can drive a rotor to whirl in a direction opposite to the direction of rotation, backward whirl. This frictional force is relatively constant up to the backward whirl speed at which the rotor rolls around the case. Since this rolling contact speed is proportional to the rotational speed of the rotor times the ratio of the diameter to the rotor clearance, the whirl speed can be hundreds of times the rotational speed of the rotor and thus be potentially very dangerous. Author
APPROVED FOR PUBLIC RELEASE