Mean aerodynamic chord --  chord of an assumed rectangular airfoil representing the mean chord of an actual airfoil.   Metal fatigue--  weakening of metal due to microscopic changes in molecular structures caused by vibration or exposure.   Micron --  one millionth of a meter.   N1 system --  gas producer.   N2 system --  power turbine and shaft.   Nacelle --  an enclosed shelter (cowling) on an aircraft for a power plant.   Nozzle --  channel through which gas is conveyed to the rotor vanes of a turbine. Its purpose is to convert pressure into velocity. 
 

 
 
飞机发动机术语英文解释（O-P）
 

Orifice --  opening having a closed perimeter through which a fluid may discharge. It may be open to the atmosphere, or it may be partially or completely submerged in the discharged fluid.   Otto cycle --  a constant volume cycle, with four distinct operations performed intermittently. Reciprocating engines operate on this cycle.   Outside air temperature --  commonly abbreviated as OAT, the temperature of the air outside the engine.   Overspeed --  RPM in excess of design; 100 percent RPM.   Overspeed governor, N2 --  gearbox mounted on engine inlet housing and driven from the power shaft. 
 

 Overspeed governor, fuel control --  part of the torquemeter system, an individual pumping unit which, with the tachometer drive assembly, sets the torquemeter oil pressure.   Overtemperature --  temperature in excess of maximum allowable design temperature at the turbine exit.   PD 680 --  cleaning solvent for exterior of engine and its attached components.   Planform --  form or shape of an object, as of an airfoil, as seen in plan view or from above.   Power --  the rate of doing work; work per unit of time.   Power-to-weight ratio --  relationship between power and weight. Turbine engines produce more power for weight than reciprocating engines.   Power turbine (N2) --  turbine that is free and independent of the gas producer system. It develops rotational shaft power.   Pressure oil system --  method of supplying oil under pressure to engine parts.   Pressure pumps --  devices to put oil into the system.   Pressurizing and drain dump valve --  device to prevent flow of fuel to the nozzle until enough pressure is built up in the fuel control. One also drains the fuel manifold at engine shutdown and traps fuel in the upper portion of the system to keep the fuel control primed for faster starts.   Primary air --  air that mixes with fuel in the combustion chamber to form a combustible mixture. The ratio is 15 parts of air to 1 part of fuel.