This paper presents a comparison between the predicted effect of an increase in backpressure on a turboshaft helicopter engine and the actual results measured in an experimental test program. A generic engine performance program was used to perform a sensitivity study to identify the effect of increases in power turbine exit pressure (backpressure) on other engine performance parameters. The analysis showed that as the backpressure increases the engine increases fuel flow to produce a constant shaft torque (or horsepower), until the maximum power turbine entry temperature is reached. Once this occurs, fuel flow can no longer increase and thus further increases in backpressure cause a decrease in output torque. These predicted results are then compared with the actual effect as measured on a T55-GA-714A engine in a static test facility. The tests involved replacing the standard engine tail pipe with one of three shorter stub ducts which increased the backpressure by employing straight and convergent flow passages instead of the divergent passage on the standard tail pipe. The test-cell data identified that the stub ducts increase specific fuel consumption by between 0.016 and 0.039 lb/hr/hp, while the turbine inlet temperature increased by up to 108 deg F. This temperature increase means that the power output will become turbine temperature limited at a lower ambient temperature than would otherwise occur. Results showed that when temperature limiting exists the power output will be reduced by between 115 and 400 SHP depending on the choice of stub duct.

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