15.1 — Fundamentals

The gas turbine engine is the powerplant of virtually every modern commercial and military aircraft. Understanding its fundamental principles — the conversion of chemical energy in fuel into useful thrust or shaft power — is the cornerstone of all subsequent engine study. This section covers the physics that make gas turbine engines work, and the four main engine configurations you will encounter in aviation maintenance.

Energy, Force and Newton's Laws

Potential Energy

Potential energy is the energy a body possesses because of its position or state. In gas turbine theory, we are mainly concerned with two forms:

• Gravitational potential energy — energy due to height above a reference datum. An aircraft at altitude has potential energy \( E_p = mgh \), where \( m \) is mass (kg), \( g \) is gravitational acceleration (9.81 m/s²), and \( h \) is height (m).
• Chemical potential energy — energy stored in the molecular bonds of aviation fuel. When kerosene (Jet A-1) burns, each kilogram releases approximately 43 MJ of heat energy. This is the primary energy input to every gas turbine engine.

Kinetic Energy

Kinetic energy is the energy a body possesses because of its motion. For a mass \( m \) moving at velocity \( v \):

In a gas turbine engine, kinetic energy appears in the high-velocity exhaust gases leaving the nozzle. A turbojet produces thrust almost entirely by accelerating a relatively small mass of air to a very high velocity. A turbofan, by contrast, accelerates a much larger mass of air to a lower velocity — this is more efficient because kinetic energy increases with the square of velocity, meaning doubling the mass at the same velocity gives twice the momentum (thrust) but only twice the energy, while doubling the velocity at the same mass gives twice the momentum but four times the energy cost.

Newton's Laws of Motion

All three of Newton's Laws are fundamental to understanding gas turbine thrust production:

The Brayton Cycle (Gas Turbine Thermodynamic Cycle)

Every gas turbine engine operates on the Brayton cycle (also called the Joule cycle). It is an open thermodynamic cycle with continuous flow, unlike the Otto cycle (petrol engines) or Diesel cycle which are intermittent. The four stages of the Brayton cycle correspond directly to the four main sections of a gas turbine engine:

Force, Work, Power and Energy

Gas Turbine Engine Types

All gas turbine engines share the same core — compressor, combustor, turbine — but differ in how they convert the energy extracted by the turbine into useful output. The four main types encountered in aviation are:

Turbojet

The simplest gas turbine configuration. All of the air entering the intake passes through the core (compressor → combustor → turbine). The turbine extracts only enough energy to drive the compressor; all remaining energy is converted to a high-velocity exhaust jet that produces thrust. Turbojets are efficient at very high speeds (Mach 2+) but are noisy and fuel-hungry at subsonic speeds. They are rarely used on modern civil aircraft but are still found on some military aircraft and older designs.

Turbofan

The dominant engine type in modern commercial aviation. A large fan at the front of the engine is driven by a turbine (usually the low-pressure turbine via a shaft running through the centre of the engine). The fan accelerates a large mass of air, most of which bypasses the core through an annular duct. The bypass ratio (BPR) is the ratio of bypass air mass flow to core air mass flow:

Modern high-bypass turbofans (e.g., CFM LEAP, Rolls-Royce Trent XWB, GEnx) have BPRs of 9:1 to 12:1, meaning 9–12 kg of air pass through the bypass for every 1 kg through the core. Approximately 75–85% of total thrust comes from the bypass (cold) stream. High BPR engines are quieter and more fuel-efficient than turbojets because they produce thrust by moving a large mass of air at moderate velocity rather than a small mass at high velocity.

Turboprop

Uses the gas turbine core to drive a propeller through a reduction gearbox (RGB). The turbine extracts almost all available energy from the gas flow, leaving very little for exhaust thrust (typically only ~10%). The propeller generates ~90% of the total thrust by accelerating a very large mass of air at low velocity. Turboprops are very efficient at speeds below 400 knots and at lower altitudes, making them ideal for regional airliners (e.g., ATR 72, Bombardier Dash 8) and utility aircraft.

Turboshaft

Functionally similar to a turboprop but the output shaft drives a rotor system (helicopter) or other machinery (ship, generator, pump) rather than a propeller. In a turboshaft, the engine is designed to produce shaft horsepower (SHP) with virtually zero residual jet thrust. Most turboshaft engines use a free turbine (also called a power turbine) — a separate turbine stage that is mechanically independent of the gas generator spool. This allows the power turbine to rotate at a different speed from the gas generator, which is essential for helicopter rotor speed control.