Gas Turbine Engines: Operation, Modules and Monitoring
The engine cycle
Understand the Brayton cycle: intake, compression, combustion, exhaust. Know the function of the compressor (axial and centrifugal), the combustion chamber, the turbine, and the exhaust. Understand the meaning of pressure ratio, bypass ratio, and specific fuel consumption. Know the difference between a turbojet, turbofan, turboprop and turboshaft.
Engine systems and monitoring
Understand the fuel system, lubrication system, ignition system, and starting sequence. Know the meaning of EGT, N1, N2, and fuel flow as monitoring parameters, and the significance of a trend shift. Understand the purpose of engine health monitoring and the meaning of a hot start, hung start and wet start.
Traps
- Confusing N1 (fan/low-pressure spool) with N2 (high-pressure spool).
- Assuming a high EGT is always a sensor fault; it may indicate a real engine problem.
- Forgetting that a wet start leaves unburnt fuel and is a fire risk.
Worked example
A turbofan with a bypass ratio of 5 passes 5 kg of air through the bypass duct for every 1 kg through the core. Higher bypass generally improves propulsive efficiency at moderate speeds, which is why modern transport engines use high bypass ratios.
Propellers: Aerodynamics, Control and Maintenance
Propeller aerodynamics
Understand how a propeller generates thrust by accelerating air rearward, and the meaning of blade angle, pitch, and angle of attack. Know the difference between geometric pitch and effective pitch, and the meaning of slip. Understand the fixed-pitch, variable-pitch and constant-speed propeller, and the role of the governor in maintaining RPM.
Control and operation
Know the function of the feathering system, the reverse-pitch system, and the beta range. Understand the meaning of propeller synchronisation and synchrophasing. Know the forces acting on the blade: thrust, centrifugal force, torque and vibration, and the importance of blade tracking and balancing.
Maintenance and inspection
Understand the inspection of blades for nicks, cracks and erosion, and the limits for blending out damage. Know the meaning of a propeller log card and the control of life-limited components. Understand the correct handling of a propeller with a defective governor.
Traps
- Confusing pitch (blade angle) with RPM; the governor controls RPM, not pitch directly.
- Assuming feathering is the same as reverse; they are opposite functions.
- Forgetting that blade damage limits are approved data, not judgement calls.
Rule of thumb
For constant-speed propellers, remember the governor's job: it senses RPM and changes blade angle to hold the selected RPM. If RPM is wrong, suspect the governor or the control linkage before the blades.