This figure shows events that may occur during a notional flight profile and how they can impact the design of engine clearances. Of particular concern is the pinch point, a minimum clearance condition that can occur during takeoff or reburst. During these events, the rotor/blade assembly expands rapidly because of centrifugal forces on the rotor and heating of the turbine blades. At the same time, the surrounding case/shroud structure expands because of thermal effects, but at a much slower rate. The result is a rapid reduction in clearance. In time, the growth of the casing exceeds that of the rotor assembly, and the clearance increases somewhat. The point in time when the growth rate of the casing first exceeds the growth rate of the rotor is the pinch point. To avoid rubbing at this condition, one must design excess clearance into the turbine. Unfortunately, this additional clearance results in nonoptimal clearances, and increased fuel use at most normal operating conditions, including cruise. In order to realize the full benefit of reduced clearances without damage to the engine, one must control the clearance throughout the flight profile.
Last updated: September 3, 2004 11:01 AM
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