How a Plane Works · Chapter 02
A wing only works while air streams past it, so something must haul the plane forward against drag. Every aircraft engine ever built answers the same way: grab something, throw it backwards, and lean on the reaction. The whole family tree — propeller, jet, rocket — comes down to what you throw and how fast.
· ONE IDEA
Thrust is mass thrown per second, times how much faster it leaves than it arrived. That one sentence buys you the whole chapter. A propeller takes an enormous bite of air and nudges it gently backwards. A jet takes a smaller bite and hurls it. A rocket carries its own material to throw — a few tonnes of propellant, ejected at kilometres per second — and so owes the atmosphere nothing at all.
READInside a turbofanThe annotated cutaway — a huge gentle throw and a small violent one, in one machine.TAKEAWAYThrust = mass per second × extra speed. Every aircraft engine is a different split of that product.
· THE PROPELLER
Accelerating air wastes energy in proportion to how violently you treat it — the swirl and rush left behind the aircraft is energy you paid for and abandoned. So the gentlest throw wins on fuel, and at low speed nothing is gentler than a propeller: spinning wings (chapter 1 again, rotated) that move a house-sized column of air a little faster. Its weakness is speed itself: as the aircraft approaches the speed of its own thrown air the push fades, and somewhere past 600–700 km/h the bargain sours — a propeller can be forced faster, but only at deafening, fuel-hungry expense.
DEMOThe turbopropA jet engine spinning a propeller — best of both, up to a point.TAKEAWAYThe gentlest throw wastes the least energy — the propeller is unbeatable on fuel, until speed catches it.
· THE JET
The jet runs the piston engine’s squeeze–burn–expand story continuously: a compressor packs air tight, fuel burns in it without pause, and the hot rush spins the turbine that drives the compressor before blasting out of the back. No reciprocating parts, colossal power per kilogram, and no speed limit anywhere near the propeller’s. The modern airliner engine — the turbofan — is the elegant compromise: a small, ferocious jet core spinning a huge, polite fan, so most of the air gets the propeller treatment while the core supplies the muscle.
TAKEAWAYSqueeze, burn, blow — continuously. The turbofan wraps a propeller-like fan around a jet core.
· THE ROCKET
Wings need air; jets need air; the rocket needs nothing but its own tanks. Carrying both fuel and the oxygen to burn it, a rocket works in the stratosphere, in space, anywhere — and delivers thrust out of all proportion to its size. The price is written in its own mathematics: every second of burning spends propellant the rocket had to lift in the first place, a compound-interest problem that makes orbit one of the hardest bargains in engineering.
DEMOThe rocket labBurn a real stage against the rocket equation.TAKEAWAYThe rocket owes the atmosphere nothing — and pays for it by lifting every gram of its own throw.
FOR THE SELF-CHECK