EPR info

From:         Willlllbur <ca385e07@nova.umuc.edu>
Organization: University of Maryland University College
Date:         28 Feb 95 22:57:35 
Followups:    1
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I would like to impart some knowledge regarding EPR and the GE TF-39
engines installed of the Lockheed C5-Galaxy.  The TF-39 engine is flat
rated in static take-off thrust for all ambient temperatures up to 89.5
degrees F (standard tropical day) within the limits of maximum turbine
inlet temperature (TIT).  The engine is not run at a fixed maximum speed,
but at a speed to develop the rated thrust.  The variables that increase
the density of the air (decreasing atmospheric temperature - increasing
atmospheric temperature - ram effect inflight) will result in the engine
reaching rated thrust at lower engine speeds.  The flight engineer
calculates the thrust setting relative to ambient conditions and advises
the pilots to set the throttles to provide the desired thrust.  The pilots
use the EPR vertical scale tape instruments to set engine power. EPR
instrument range on the C5 is 1.0 to 7.0 EPR.  Since the EPR
instrumentation is not a direct indicator of fan performance (N1), the fan
speed is used in relation to EPR to determine engine performance.  On the
TF-39 engine, EPR is an indication of mass air energy immediately upstream
of the low pressure turbine which drives the fan.  A pressure ratio 
sensor is connected to the engine inlet total pressure (Pt2) and low 
pressure turbine inlet total pressure (Pt5.4) probes.  EPR is not a direct
indicator of engine performance for the fan or the core engine but a ratio
of fan inlet pressure to high pressure turbine discharge pressure.  Mass 
air flow from the fan and or core engine compressor is directly related to 
rotor speeds in addition to the ambient variables.  Therefore, the 
throttle position to provide a desired EPR is also establishing a mass 
air flow which can be directly related to fan thrust and core engine 
thrust.   

The Lockheed C5B models do not use EPR as on the C5A model.

MSG William R. McGowan
Flight Engineer
709th Airlift Squadron
Dover AFB, DE 19902