# What is the fan pressure ratio in a turbofan?

What is it and how does it affect efficiency and thrust?

• According to thefreedictionary.com, it is The ratio of the fan discharge pressure to the fan inlet pressure. Not sure if this is in the correct context. Jan 26 '21 at 2:06
• The fan is a compressor, the FPR is the compression ratio, usually in cruise conditions. The FPR and the bypass ratio are linked (because the exit pressure of the two flows, bypass and core, must match to ensure proper mixing of the flows at the exhaust). They are selected together, and changing the FPR also changes the characteristics of the core. So it's likely difficult to answer the question. Maybe Optimum Fan Pressure Ratio for Bypass Engines can give you an idea.
– mins
Feb 11 '21 at 18:00
• I’m not convinced mixing of the flows at the exhaust is the main reason for selection of fpr. After reading the paper it mainly seems as a supercharging stage for the compressors. Fighter jet engines have around 30:1 and a fpr of 2 should double that. If you look at a lot of modern commercial turbofans the exhaust is so far from the nacelle both streams should easily be ambient pressure by then. Feb 11 '21 at 18:52