How is EPR calculated during idle thrust?

EPR, or Engine Pressure Ratio, is calculated during idle thrust using the ratio of the pressure at the engine's exhaust (often referred to as P7) to the pressure at a specified point in the engine's intake (which is P2 during idle conditions). This ratio provides a measure of the engine's efficiency and performance while operating at low thrust settings.

At idle thrust, the engine operates under different conditions compared to full power, so the EPR is a crucial parameter in understanding how well the engine is performing in a lower thrust environment. The P7 to P2 ratio allows engineers to monitor engine performance and detect potential issues without the influences of higher power settings complicating the analysis.

The other options do not accurately define how EPR is calculated:

• Exhaust pressure divided by ambient pressure does not specifically apply to EPR calculations since it does not consider the necessary respective pressures at P7 and P2.

• Static pressure divided by dynamic pressure is more akin to calculating aerodynamic coefficients rather than thrust measurements related to engine performance.

• Engine thrust divided by total engine weight does not relate to EPR; it rather pertains to thrust-to-weight ratio calculations which are useful for understanding engine and aircraft performance dynamics but do not measure pressure ratios.

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