Gas Turbine Performance Aspects: Flat Rating, Flex Temperature, and EPR
Thrust and Pressure
- Air Acceleration: As the engine accelerates the air, dynamic pressure increases and static pressure decreases (Bernoulli's principle).
- Specific Thrust: Defined as Net Thrust per unit of Air Mass Flow.
- If intake airflow increases (due to higher speed/density) while net thrust remains constant, specific thrust decreases.
Flat-Rated Engines
Turbine engines are often "flat-rated" to provide consistent performance across a range of temperatures and to protect the engine components.
- Flat-Rated Range (Cold/Standard OAT):
- Occurs when Outside Air Temperature (OAT) is below the "flat-rated temperature" (typically around ISA +15°C).
- Thrust is limited by internal pressure limits (Maximum Mass Flow / Structural limit).
- The engine produces a constant maximum thrust regardless of the OAT dropping further.
- Temperature Limited Range (Hot OAT):
- Occurs when OAT is above the flat-rated temperature.
- Thrust is limited by EGT/TGT (Exhaust/Turbine Gas Temperature) to prevent melting or creep of turbine blades.
- As OAT increases, maximum available thrust decreases.
Reduced Thrust Take-off (Flex / Assumed Temp)
Used to extend engine life by using less than maximum thrust when takeoff performance allows (light weight, long runway).
- Principle: The FADEC/Computer is "tricked" into thinking the outside air is hotter than it actually is.
- Method:
- The pilot enters an Assumed Temperature (or Flex Temp) into the FMS.
- This assumed temperature is higher than the actual OAT.
- Since the engine is "temperature limited" at high OATs, the computer calculates a lower N1/EPR limit.
- Result: The engine produces less thrust, running cooler (lower EGT), reducing wear.
Thrust Indication: EPR
- EPR (Engine Pressure Ratio): A primary thrust parameter on many jet engines (others use N1).
- Definition: The ratio of Low-Pressure Turbine Exhaust Pressure ($P_{exit}$) to Compressor Inlet Pressure ($P_{inlet}$).
- $EPR = \frac{\text{Total Pressure at Turbine Exit}}{\text{Total Pressure at Compressor Inlet}}$
- Sensors: Can be electromechanical bellows or digital transducers.