Gas Turbine Engines, Thrust, Performance and Monitoring
The gas turbine cycle
A turbojet or turbofan operates on the Brayton cycle: intake, compression, combustion, exhaust. Thrust is produced by accelerating a mass of air: F = ṁ × (Vj − V0), plus a pressure term at the nozzle. A turbofan adds a bypass stream, improving propulsive efficiency; the bypass ratio is the mass of bypass air divided by core air. Turboprop and turboshaft engines extract shaft power through a free turbine or reduction gear.
Performance parameters
Engine parameters include N1 (fan or LP spool speed), N2 (HP spool), EGT, fuel flow and oil pressure. Thrust falls with altitude and rises with speed (ram effect) up to a point; high ambient temperature reduces density and thrust. The trap is that N1 is the primary thrust parameter on many turbofans, not N2; EGT is a limit, not a thrust setting.
Starting and monitoring
Start sequence: motoring, ignition, fuel on, light-off, acceleration to idle. A hot start (EGT exceeds limit) or hung start (no acceleration) requires shutting off fuel. Engine monitoring includes vibration, oil pressure and temperature, and trend monitoring for deterioration.
Worked example
If mass flow is 100 kg/s and exhaust velocity 600 m/s with inlet velocity 200 m/s, gross thrust is 100 × (600 − 200) = 40,000 N. Doubling mass flow doubles thrust at the same velocities.
Traps
Confusing EGT with thrust setting, assuming thrust is constant with altitude (it is not), and forgetting that compressor stall and surge are different: surge is a complete breakdown of flow with a bang, stall is local separation. Also, reverse thrust is less effective at low speed and must not be used below the manufacturer's limit.
Rule of thumb
Thrust ≈ mass flow × velocity change; density, temperature and speed all shift the operating point.