OPR is an abbreviation used in aerospace engineering
for 'Overall Pressure Ratio' - a measurement of the 'efficiency' of jet (turbojet
) engines. The number is produced quite simply; it is the ratio
of the air pressure
immediately behind the engine to that immediately ahead of it. Thus it measures how well the engine produces thrust
- the higher the OPR, the higher the efficiency of the engine. High-bypass turbofan
s increase their OPR not by hotter burning but by mixing colder
(and thus higher-density
) bypass air with the engine core
'hot' flow; this denser air is pushed to higher velocity
by the hotter air. As the Exhaust Gas Temperature
lowers, it indicates a rising efficiency as energy
is expended more as thrust than heat.
Early jet fighters had OPRs in the very low teens, typically; the modern fighter is much more efficient. The Eurofighter 2000 has an OPR of around 23; the F-22 around 22-25. For comparison, the F-4 Phantom II had an OPR of approximately 16. This increase in engine efficiency has brought with it lower fuel consumption as well as more power. This doesn't always translate to higher top 'sprint' speeds; the turbofan (through its larger cross-section) suffers more from blade drag at higher speeds; however, it does make takeoffs shorter, faster and safer.
Fighter aircraft are said to have an OPR, whereas air transport engines themselves usually are measured separately; this is due to the fact that fighters typically have their engines embedded within the airframe. The geometry and mechanical design of the air intakes, airflow inside the fuselage, and the exhaust all affect the OPR. Since transport engines are typically mounted outboard of the airplane in standardized nacelles, their OPR is distinct from the aircraft type.