Turbine Engines
Domain 14.1 of EASA Module 14 examines the gas turbine as a machine: the cycle it runs, how it is built as modules, and how thrust is produced, measured and limited. Questions are usually quantitative or diagnostic - a thrust calculation, a stall/surge scenario, or a question on why bleed valves open at low power.
The cycle and the thrust equation
Turbine engines run a Brayton (constant-pressure) cycle: compression, heat addition at near-constant pressure, expansion, exhaust. Thrust comes from accelerating mass and, at high jet velocity, from a nozzle pressure difference:
F = ṁ(Vj − V0) + (pj − p0)Aj
A high-bypass turbofan moves a large mass slowly, which is why it is quieter and more efficient than a turbojet moving a small mass fast. Bypass ratio (BPR) = bypass mass flow ÷ core mass flow. Specific fuel consumption (SFC) = fuel mass flow ÷ thrust, usually in kg/(N·h).
Compressors, stall and surge
Axial compressors raise pressure a little per stage; the front stages are most vulnerable at low corrected speed because the rear stages choke first. Variable stator vanes (VSVs) and bleed valves therefore schedule at low N2 to keep the operating point off the surge line. Stall is a local breakdown of flow over a blade row; surge is a whole-engine instability in which flow reverses axially, producing a bang, an EGT spike and possible flameout. Centrifugal compressors serve as the final HP stage on some small engines and tolerate inlet distortion better.
Combustion and turbine
Chambers are can, can-annular or annular; annular is shortest and lightest, with the most uniform exit temperature profile. Nozzle guide vanes accelerate and swirl the gas; turbine blades extract work. HP compressor air cools NGVs and blades, usually with film cooling and thermal barrier coatings. Creep and thermal fatigue set blade life, so a turbine gas temperature exceedance is a life-consuming event, not merely a momentary limit.
Worked example
Core mass flow 50 kg/s at 500 m/s; bypass 250 kg/s at 280 m/s; static start so V0 = 0; fuel flow 0.9 kg/s. Thrust = (50 × 500) + (250 × 280) = 25 000 + 70 000 = 95 000 N. BPR = 250/50 = 5. SFC = 0.9 × 3600 ÷ 95 000 = 0.034 kg/(N·h). The fan supplies about 74% of thrust - a typical high-bypass split.
Traps
N1 is the LP spool, N2 the HP spool; do not assume N1 is always the thrust parameter - some engines use EPR. Keep stall (local) and surge (system) apart. Pressure thrust exists only when the nozzle is not fully expanded. Bleed extraction raises EGT and cuts thrust, so never compute performance with bleeds on unless told to.