15.1 Gas turbine fundamentals — energy, Newton, Brayton cycle and engine types

Scope and exam

Module 15 is taken by A1, A3, B1.1 and B1.3 (EASA: 60 questions for A, 92 for B1). 15.1 is level 1 (A) / 2 (B1): potential and kinetic energy, Newton's laws, the Brayton cycle; relationships between force, work, power, energy, velocity and acceleration; construction and operation of turbojet, turbofan, turboshaft, turboprop and geared turbofan engines.

Basic mechanics refresher

Quantity	Formula	SI unit
Force	F = m × a	newton (N)
Work	W = F × d	joule (J)
Power	P = W ÷ t = F × v	watt (W)
Kinetic energy	KE = ½ m v²	J
Potential energy	PE = m g h	J
Momentum	p = m v	kg·m/s

Newton's laws

1. A body stays at rest or in uniform motion unless acted on by a force.
2. Force = rate of change of momentum (F = m a). For an engine: thrust = mass flow × change in velocity.
3. Every action has an equal and opposite reaction — the engine accelerates air rearwards, the air pushes the engine forwards.

Key fact: A jet engine produces thrust by accelerating a mass of air. Thrust can be increased by increasing mass flow or increasing the velocity change; accelerating a large mass a little (turbofan) is more efficient than a small mass a lot (turbojet).

Worked example (basic thrust): Mass flow 50 kg/s, inlet velocity 0 (static), jet velocity 600 m/s. F = ṁ (V_j − V_a) = 50 × (600 − 0) = 30,000 N (30 kN).

The FAA handbook gives the same relationship in imperial units: F = Ms (V2 − V1) ÷ g, with Ms in lb/s and g = 32.2 ft/s². Example from the handbook: 100 lb/s accelerated from 600 to 800 ft/s gives 100 × 200 ÷ 32.2 ≈ 621 lb of thrust.

The Brayton (constant-pressure) cycle

Stage	Where	What happens
Intake	Inlet duct	Air enters at about ambient pressure
Compression	Compressor	Pressure and temperature rise, volume falls (work done on the air)
Combustion (heat addition)	Combustion chamber	Fuel burnt at (approximately) constant pressure; temperature and volume increase greatly
Expansion	Turbine and exhaust nozzle	Pressure and temperature fall, velocity rises; turbine extracts work to drive the compressor; the remaining energy produces thrust or shaft power

Compare with the piston engine Otto cycle, where combustion is at (approximately) constant volume and the four events happen in sequence in the same cylinder. In a gas turbine all four events happen continuously and simultaneously in different parts of the engine.

Gas-flow changes through the engine:

• Pressure rises through the compressor, falls slightly in the combustor, falls through turbine and nozzle.
• Temperature rises in the compressor, rises sharply in the combustor, falls through turbine and nozzle.
• Velocity is kept fairly constant through the compressor, is reduced in the diffuser before the combustor, then increases strongly in turbine NGVs and the nozzle.

Divergent ducts (subsonic) slow the air and raise its pressure (diffuser, inlet); convergent ducts speed it up and lower pressure (nozzle, NGVs) — Bernoulli's principle.

Engine types

Turbojet

Single gas stream; all thrust from the hot jet. High specific thrust, high fuel consumption and noise; efficient only at high speed.

Turbofan

A fan (driven by the LP turbine) accelerates a large mass of air, most of which bypasses the core.

• Bypass ratio (BPR) = mass of air through the bypass duct ÷ mass of air through the core. Low-bypass (military, around 1:1 or less) to high-bypass (5:1 to over 10:1 on modern airliners).
• High-bypass engines get most of their thrust (often around 75–80 %) from the fan.
• Separate (short cowl) or mixed exhaust.

Turboprop

Gas generator drives a propeller through a reduction gearbox. Most of the energy goes to the shaft (the FAA quotes about 80–85 %), with a small residual jet thrust. Can be fixed-shaft (propeller geared to the compressor shaft) or free turbine (separate power turbine).

Turboshaft

Power turbine drives an output shaft — helicopters and APUs. Almost all energy extracted as shaft power; exhaust gives negligible thrust.

Geared turbofan (GTF)

A planetary reduction gearbox between the fan and the LP spool lets the fan turn slowly (good fan efficiency, low noise) while the LP compressor and turbine run fast (fewer stages, higher efficiency). Enables very high bypass ratios.

Spool arrangements

Arrangement	Shafts	Notes
Single spool	One (N)	Simple; compressor stall margin limited at off-design
Twin spool	N1 (LP: fan/LP compressor + LP turbine), N2 (HP compressor + HP turbine)	Shafts concentric; spools run at their own best speeds
Triple spool	N1 (fan), N2 (IP), N3 (HP)	Short, rigid engine, fewer variable vanes

The HP spool drives the accessory gearbox (through a radial drive shaft) and is turned by the starter.

Station numbering (typical)

0 ambient → 1 inlet → 2 compressor (fan) inlet → 3 compressor exit → 4 combustor exit / turbine inlet → 5 turbine exit → 7/9 nozzle. Parameters are named by station, e.g. Pt2, T4 (turbine entry temperature), EGT measured around station 5.

Exam traps

• Gas turbine combustion is at constant pressure (Brayton); piston at constant volume (Otto).
• Divergent subsonic duct = velocity down, pressure up.
• Bypass ratio is a mass flow ratio.
• The starter and accessory gearbox are on the HP spool.