The Brayton cycle, engine types, thrust production, thermal and propulsive efficiency, ram effect and flat rating.
Notes
Fundamentals & performance — summary notes
Free
Main ideas
A gas turbine works on the Brayton (constant-pressure) cycle in four continuous stages — suck, squeeze, bang, blow: the compressor raises pressure, the combustor adds heat at constant pressure, and the turbine and nozzle extract energy and accelerate the gas rearward.
Thrust is Newton's third law in action: F = ṁ·(V_jet − V_inlet). The engine produces forward thrust by accelerating a mass of air rearward; a high-bypass turbofan moves a large mass slowly (efficient, quiet), a turbojet a small mass fast.
Engine types trade the same core differently: turbojet (all thrust from the jet), turbofan (most thrust from the bypass fan), turboprop (most power to a propeller), turboshaft (all power to a shaft, e.g. helicopters/APUs).
Efficiency has two parts: thermal efficiency (how well heat becomes mechanical energy — favoured by high pressure ratio and turbine entry temperature) and propulsive efficiency (how well that energy becomes thrust — favoured by a jet velocity close to the aircraft speed, i.e. high bypass).
Thrust falls with altitude (lower air density/mass flow) and with rising ambient temperature, and rises with forward speed through ram effect. 'Flat rating' holds a guaranteed thrust up to a corner-point temperature by setting the engine to a limit.
⚠ Exam trap: specific fuel consumption (SFC) is fuel burned per unit thrust per hour — LOWER is better; and a high-bypass fan gives high propulsive efficiency because it accelerates a large mass of air only slightly, not because the jet is fast.
Key formulas
Thrust
F = ṁ·(V_jet − V_inlet)
Brayton cycle
compress → heat (const P) → expand
Solved examples
Why does a modern high-bypass turbofan burn less fuel and make less noise than an early turbojet of the same thrust?
It generates thrust by accelerating a very large mass of bypass air by a small amount, rather than a small mass to a very high jet velocity. A jet velocity closer to the aircraft's speed means higher propulsive efficiency (less energy wasted as leftover kinetic energy in the jet) and a lower, quieter exhaust velocity.