Physics

Why a Propeller Is Efficient — Until the Tips Go Supersonic

By Lucian — builder & engineer, LK Forge

A propeller turns shaft power into thrust by accelerating a wide column of air by only a small amount, and that single design choice — a large disk over a small velocity increment — is what makes it so much more efficient than a jet at low speed. But the same disk that raises efficiency also has to spin, and its tips move faster than the aircraft itself. We swept the Propeller Lab's actuator-disk model across disk diameter, flight speed, and rpm for a 1.9 m, 2400 rpm default propeller to measure exactly how much efficiency a bigger disk gains, why standing still wastes all of it, and how close those tips already sit to the speed of sound.

 ·  6 min read  ·  measured from the diameter/speed/rpm benchmark

85.4%
efficiency at a 2.8 m disk, up from 76.7% at 1.2 m
1.03
tip Mach at a 2.8 m disk and 2400 rpm — already past the speed of sound
0%
efficiency at zero airspeed, even while producing 4,959 N of static thrust
2.16 m
the largest disk that stays under Mach 0.8 at 2400 rpm

A bigger disk raises efficiency, but raises tip Mach

Efficiency here comes from how gently a disk accelerates the air passing through it: a small disk has to speed a narrow column up a lot to make a given thrust, while a large disk can make the same thrust by speeding a wide column up only a little. That gentler acceleration is what raises efficiency as diameter grows. But tip speed depends only on diameter and rotation rate, not on how gently the disk treats the air — so the same growth that raises efficiency also pushes the spinning tips closer to, and eventually past, the speed of sound.

tip speed = π × (rpm / 60) × diameter

At this lab's 2400 rpm and 62 m/s cruise, moving from a 1.2 m to a 2.8 m disk takes efficiency from 76.7% to 85.4%, while tip Mach climbs from 0.44 to 1.03.

Efficiency versus tip Mach, across disk diameter

Efficiency (blue, left axis) rises steadily while tip Mach (orange, right axis) rises even faster, for the 2400 rpm propeller at 62 m/s cruise, swept from a 1.2 m to a 2.8 m disk. The dashed line marks Mach 0.8, and the dashed vertical marks the 2.16 m diameter where the tips reach it.

0% 20% 40% 60% 80% 100% 00.30.60.91.2 Mach 0.8 limit 2.16 m 1.21.61.92.32.62.8 disk diameter (m) eff. tip Mach

● efficiency (left axis)    ● tip Mach (right axis)

Standing still, none of that thrust does useful work

Efficiency measures how much of the slipstream's kinetic energy ends up moving the aircraft, versus how much is left behind in the wake at a speed the aircraft never reaches. At zero flight speed there is no "behind" to compare against — the aircraft never advances into the energy the disk spent, so efficiency is exactly zero even while the disk produces enormous static thrust. As flight speed rises toward the slipstream's own speed, the gap the disk has to make up shrinks, and efficiency climbs with it, while thrust itself falls off.

Thrust (blue, left axis) and efficiency (orange, right axis) versus flight speed, for the same 1.9 m, 2400 rpm propeller.

0 1000 2000 3000 4000 5000 0%25%50%75%100% 020406290120 flight speed (m/s) N eff.

● thrust (left axis)    ● efficiency (right axis)

The exact numbers

Every row below is a direct sample from the lab's model: a 2400 rpm propeller at a 62 m/s cruise, for the diameter sweep.

Disk diameter Efficiency Tip Mach Thrust
1.2 m 76.7% 0.44 1,856 N
1.6 m 80.7% 0.59 1,953 N
1.9 m 82.5% 0.7 1,997 N
2.3 m 84.1% 0.85 2,036 N
2.6 m 85% 0.96 2,055 N
2.8 m 85.4% 1.03 2,066 N

And the flight-speed sweep, at the default 1.9 m diameter:

Flight speed Thrust Efficiency
0 m/s 4,959 N 0%
20 m/s 3,753 N 50%
40 m/s 2,748 N 73.3%
62 m/s 1,997 N 82.5%
90 m/s 1,436 N 86.2%
120 m/s 1,092 N 87.4%

And the rpm sweep, isolating tip Mach alone at the default 1.9 m diameter:

RPM Tip Mach
1800 0.53
2200 0.64
2600 0.76
3000 0.88

Propeller: 1.9 m default diameter, 2400 rpm default rotation speed, 62 m/s cruise speed. Generated 2026-09-11.

Try it yourself

Open the lab, set your own disk diameter, rpm, cruise speed, and shaft power, and watch thrust, efficiency, and tip Mach move in real time.

Open the Propeller Lab →
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Common questions

Why does a propeller beat a jet at low speed?

A propeller spreads its shaft power across a wide disk, accelerating a large mass of air by only a small velocity increment. In this lab's model that shows up as efficiency of 82.5% at a 62 m/s cruise, climbing further to 87.4% at 120 m/s — high numbers reached at speeds where a small, fast jet of exhaust would still be fighting to catch up with the aircraft. A jet engine gets its thrust from a narrow, fast stream instead, which only closes that speed gap — and only becomes efficient — well up into high subsonic flight. Moving a lot of air slowly, rather than a little air fast, is what gives the propeller its low-speed advantage.

Why does static thrust have zero efficiency?

Standing still, this lab's default propeller still produces 4,959 N of thrust, yet efficiency reads exactly 0%. Efficiency here is the ratio of flight speed to the slipstream speed behind the disk, and with flight speed at zero that ratio is zero no matter how much thrust the disk makes — there is no forward motion for the shaft power to do useful work against, so all of it goes into kinetic energy left behind in the accelerated air. As flight speed rises, that ratio climbs with it, reaching 82.5% at 62 m/s in this sweep.

Why is a bigger disk more efficient?

Thrust is set by how much air a disk accelerates and by how much it speeds that air up, and a bigger disk can produce the same thrust while accelerating a larger mass of air by a smaller increment. That smaller increment is exactly what raises efficiency: sweeping this lab's disk from 1.2 m to 2.8 m at the same 2400 rpm and 62 m/s cruise lifts efficiency from 76.7% to 85.4%, even though thrust itself only rises modestly, from 1,856 N to 2,066 N. A large, slow-turning disk is simply a gentler way to move the same air.

Why does tip Mach limit how big or how fast a propeller can turn?

Tip speed grows with both diameter and rotation speed together, and once the blade tips approach the speed of sound, compressibility losses and noise climb sharply, capping usable diameter and rpm well short of where efficiency alone would want to go. In this lab's rpm sweep, tip Mach runs from 0.53 at 1800 rpm up to 0.88 at 3000 rpm on the default 1.9 m disk — rpm alone can push the tips past Mach 0.8 even without changing diameter. Holding rpm at 2400, the largest disk that stays under Mach 0.8 is 2.16 m, which is why a propeller can only grow so large before tip Mach, not efficiency, sets the limit.