How the jet made propellers obsolete

In 1944 the fastest piston fighters in the world met an aircraft 150 km/h quicker than anything they could build, powered by an engine with no pistons at all. The propeller had not been out-engineered — it had hit a wall written into the physics of air, and the jet simply did not share it.

The propeller's wall

A propeller is a set of wings moving in a circle, and like any wing it works well only below the speed of sound. Here is the trap: the blade's tipspeed is the aircraft's speed plus the spinning speed, so the tips go sonic long before the aircraft does. Past that, shockwaves camp on the blades: drag soars, thrust collapses, and adding engine power just makes louder noise. Around 700 km/h the best piston fighters of the war were shovelling in hundreds of extra horsepower for a handful of extra km/h. The propeller was finished as a fast machine — no metallurgy could negotiate with Mach 1.

Strokes become flow

The jet's answer was to abandon the whole rhythm of the piston engine. Instead of suck-squeeze-bang-blow taking turns in a cylinder, the turbojet does all four continuously, each in its own dedicated station: a compressor squeezes an unbroken river of air, fuel burns in it steadily, a turbine skims off just enough power to pay the compressor, and everything left blasts out of the nozzle as thrust. No reciprocating parts, no valve events, no propeller — the working parts only ever rotate, which is why a jet could spin at speeds and temperatures no piston assembly could survive.

Why jets love speed

The deeper reason the jet won is that speed helps it. As the aircraft flies faster, onrushing air rams into the intake and arrives pre-compressed — free work the piston engine could never collect. And a jet's thrust holds up at speeds where a propeller's pull is dissolving in shockwaves, so its power— thrust times speed — keeps climbing exactly where the propeller's gives out. The two technologies do not merely differ; their curves cross. Below about 600 km/h the propeller is honestly better. Above it, the jet owns the sky, and always will.

Twenty years to extinction

What followed was one of the fastest extinctions in engineering. The Me 262 flew operationally in 1944; the Comet carried airline passengers by jet in 1952; the 707 made the piston airliner obsolete by 1958. Twenty years after the first jet squadron, no new long-haul aircraft above the weather was designed with a propeller. The jet then spent the next half-century learning economy — wrapping itself in an enormous fan until most of its thrust came from slow, quiet air — but that is evolution. The revolution was 1944: the day speed stopped being the propeller's business.