Thrust and Turbine Engine Types - ATPL 021 — Airframe, Systems and Powerplant

A gas turbine makes thrust the way a person on a boat makes headway by throwing cargo astern: it accelerates a mass of air backwards, and the reaction pushes the aircraft forwards. Everything in this lesson unpacks that sentence: the formula that counts the push, the five stations that produce it, the three engine types that package it differently, and the efficiencies that decide which type suits which flying.

Thrust from momentum

Thrust is the reaction to accelerating air rearwards: the engine takes in air at roughly the aircraft's speed, adds energy to it, and expels it much faster through the nozzle. The force equals the rate at which momentum is added, which gives the simple thrust formula its two parts: the momentum thrust from the accelerated mass flow, and the pressure thrust that appears whenever the gas leaves the nozzle still above ambient pressure, pushing on the nozzle exit area directly.

Thrust and forward speed

Flying faster feeds the formula from both ends: the difference $V_j - V$ shrinks as flight speed rises, which alone would cut the thrust, but the intake's ram effect packs more air mass through the engine per second, raising $\dot{m}$ . The two effects run against each other closely enough that a jet engine's thrust can be considered approximately constant over the whole subsonic speed range, the working assumption the performance subjects build on.

Five stations and the working cycle

The main components, in gas-path order:

Inlet — delivers the air smoothly, slightly compressed by ram effect in flight.
Compressor — raises the air's pressure many times over, spending the turbine's work to do it.
Combustion chamber — adds fuel and burns it at essentially constant pressure, hugely raising the temperature.
Turbine — extracts enough energy from the hot gas to drive the compressor (and any fan, propeller or accessories) on the shared shaft.
Outlet (nozzle) — converts what pressure remains into velocity, producing the fast jet the thrust formula counts.

Reading the graph is the working cycle in one pass: pressure is bought in the compressor and spent in the turbine and nozzle; temperature is bought in the combustor and spent the same way; and velocity stays modest through the machinery, because the nozzle is where the stored pressure finally becomes jet speed. Note the slight pressure fall across the combustion chamber: burning at constant pressure is the ideal, and a small real loss is the price of pushing gas through it.

One vocabulary note the compressor lessons will lean on: a gas particle inside the engine has three velocities worth naming:

Absolute velocity — the particle's true speed and direction.
Axial velocity — the component along the engine's axis, carrying the flow rearwards.
Circumferential (tangential) velocity — the component around the axis, the swirl the spinning blades add and the stationary vanes remove.

The absolute velocity splits into the axial and circumferential components, and the working-cycle graph plots the axial one.

The family: turbojet, turbofan, turboprop

Turbojet (straight jet) — all air through the core, all thrust from one fast, hot jet. Simple and slim, but noisy and thirsty at airliner speeds.
Turbofan — the turbine also drives a large fan whose air mostly bypasses the core: thrust comes from moving a much bigger mass of air more gently, most of it from the fan's cool bypass stream, the rest from the core's jet.
Turboprop — the turbine's surplus power leaves as shaft power, geared down to a propeller that moves the biggest air mass of all, most gently of all; the exhaust contributes only a small residual jet thrust.

Two pieces of internal architecture recur across the family:

Spool — one compressor-turbine pair on its own shaft. An engine can have one or more: large engines stack two or three, rotating at different speeds, one inside the other.
Free turbine — a separate power turbine on its own shaft, driven by the gas stream but mechanically independent of the core, so the propeller's speed is not chained to the compressor's; many turboprops (and helicopter engines) use one.

Bypass ratio

The bypass ratio measures how a turbofan divides its air: the mass flow through the bypass duct (the cold stream) divided by the mass flow through the core (the hot stream). High-bypass engines, ratios of 5:1 and far beyond, are the quiet, economical airliner standard.

Equivalent horsepower

A turboprop's output is awkward to state in one number, because it delivers both shaft power to the propeller and a little jet thrust from its exhaust. Equivalent horsepower (EHP) is the bookkeeping answer: the shaft horsepower plus the horsepower equivalent of the jet thrust (thrust horsepower), so the whole output can be quoted as one figure.

The efficiencies

Propulsive power — the useful part of the output: thrust times flight speed, the rate at which the engine actually does work on the aircraft.
Propulsive efficiency — propulsive power divided by the power given to the jet. It is high when the jet leaves only a little faster than the aircraft flies, because energy left as high leftover jet speed is energy wasted stirring the sky: moving a large mass gently beats moving a small mass violently.
Thermal efficiency — how much of the fuel's heat becomes gas kinetic energy at all.
Total efficiency — the product of the two: fuel energy to useful propulsion in one number.

Thermal efficiency rises with compressor pressure ratio: the harder the air is squeezed before burning, the larger the share of the added heat the cycle can convert to work, which is why every generation of engines compresses harder than the last.

The curves are the family's division of labour in one picture: propellers for slow aircraft, big fans for airliners, the pure jet only where speed is everything. Each type is most efficient where its exhaust speed best matches its flight speed.

Specific fuel consumption

Turbine SFC follows the piston definition's spirit with different units, and the exam wants the distinction: a turbojet's or turbofan's SFC is fuel burned per unit of thrust per hour, a turboprop's is fuel burned per unit of power per hour, because thrust is what the jet sells and shaft power is what the turboprop sells. Lower is better in both currencies, and the high-bypass revolution was, above all, an SFC revolution.

What the exam asks

The thrust and bypass calculations are worked-example marks; the station-by-station gas behaviour and the efficiency-versus-speed reasoning carry the understanding marks. The overview, following the learning objectives:

Describe how thrust is produced and calculate it with the simple formula, including pressure thrust

State that thrust is approximately constant over the subsonic speed range

List the main components and describe pressure, temperature and axial velocity through the working cycle

Distinguish absolute, circumferential (tangential) and axial velocity

Describe the types, how turbojets and turbofans make thrust and turboprops make power, spools, and the free turbine

Define bypass ratio and calculate it

Describe equivalent horsepower

Define propulsive power and the propulsive, thermal and total efficiencies, and the pressure-ratio effect

Explain how propulsive efficiency varies with forward speed for the three types

Define SFC for turbojets and turboprops