Det här kan vara en bra fungerande definition
The Flat Rating Concept
The rating of a jet engine is the thrust performance that is
guaranteed by the manufacturer for a new engine under specific
operating conditions such as, take off, maximum continuous, climb,
cruise.
Some non Prat and Whitney (P&W) engines are rated to a constant
compressor speed (RPM). P&W rates there engines to a constant exhaust
gas temperature (EGT). This is referred to as the flat rating concept.
To get your head around this concept you must understand the following
principle.
The temperature and density of the ambient air vary inversely.
Lower temperature = > Higher Density
Higher Temperature = >Lower Density
The amount of airflow (lbs/sec) through the engine is a function of
compressor speed and air density. It is greater when the compressor
speed and density are high.
The compressor speed is a function of the energy available to the
compressors turbine. That energy comes from the combustion or air and
fuel, so the turbine turns faster when the fuel flow rate is greater.
The compressor speed is also a function of the airflow through the
compressor. Higher rates of air flow reduce the speed of the
compressor.
The compressors rotational speed and the amount of airflow through the
compressor are independent, they affect each other though.
The turbine inlet temperature is proportional to the energy available
to turn the turbine. The exhaust temperature is proportional to the
turbine inlet temperature. So a higher EGT corresponds to a larger
amount of energy to the turbine so it can turn the compressor faster.
When EGT is held constant, or lowered the result is a prolonged hot
section life and at the same time provides the thrust to meet the
certification requirements.
Eller en bit mer kortfattad från wiki
When an engine is Flat rated it means that an engine of high
Horsepower rating is constrained to a lower horsepower rating. The
engine output in this case will always remain the same, but when
atmospheric conditions such as high temperatures and high altitude
(Hot and High) reduce the power output of the engine it has more
headroom before it falls below the limited maximum output.
