[TD-Linux]

The size of most electric motors is proportional to the torque they produce - so if you can spin it faster, you can get more power for the same weight. There are some other factors that make high speed worse - for example, bearing and core losses. The PCB motor is a coreless motor though, so it has no core loss, biasing it even harder towards high speed operation.

Propellers generally get less efficient with higher speeds though, which might be a bigger factor than any of this.

[bboygravity]

> Propellers generally get less efficient with higher speeds though, which might be a bigger factor than any of this.

Unless operated at lower air-pressure (higher altitude) then higher speed is generally more efficient.

Source: Musk interviews about why electronic very high altitude aircraft make sense in a lot of ways (faster, more energy efficient, potentially cheaper, less polluting). But only after battery densities are improved in the future.

[jacquesm]

Musk isn't correct though, or maybe you misunderstood or misquoted him. Higher speed is only more efficient if the craft itself moves at a higher speed.

To explain a bit further: a prop is at its most efficient when it has clean air to work with and that only works if the craft moves forward at least as much as one prop's worth of air in the direction of motion. Less than that and the prop will encounter it's own backwash. This is the reason why variable props exist, to ensure that the prop has enough 'bite' rather than that it just churns the local air. So at low revs you run a higher angle than when you go faster and towards the tips of the prop the angle gets flatter as well.

Which more or less defines the range of a variable prop, once the tips are nearly flat there is nothing more to gain. Another important factor is blade count. A single blade is theoretically most efficient because it can run at the highest RPM before the blade encounters it's own wake again, but there are balancing issues and vibration issues with low (<3) blade count props. And in practice the efficiency gains are offset by complexity, weight (a single blade needs a counterweight) and drag of that weight. Two is common enough though because it is easy to make a sturdy two blader prop. Three is optimal from a longevity and maintenance point of view, and a offers very good efficiency.

And I think you meant 'electric', not 'electronic'.

[pengaru]

> Musk isn't correct though, or maybe you misunderstood or misquoted him. Higher speed is only more efficient if the craft itself moves at a higher speed.

He's got it completely backwards from what Musk said in that interview, see my reply.

[stavros]

Hm, I don't entirely know how this works, but for RC planes we use slower motors and higher prop pitch for efficiency, faster motors and lower prop pitch for acceleration.

[jacquesm]

Yes, that's because you run fixed pitch props.

[pengaru]

FFS dude

If you're going to cite sources like that at least get it remotely close to right.

"For aircraft, or just generally, you want to move a large mass of air slowly. So you can reduce the velocity component of kinetic energy, which goes as the square. You want to move a large amount of mass slowly, not a small amount of mass fast... So, the way you make aircraft engines more efficient is you move a lot of air slowly. Like big fans, basically, big slow fans work great. Small, tiny. fast-moving jets, are very inefficient." - your favorite billionaire, in the exact interview you're inversely misquoting. (JRE #1609 last ~12 minutes is electric planes)

[nullc]

Huh? No way, the energy needed goes up with the cube of the speed, but the mass moved only goes up linearly-- the reason is that you are having to accelerate a small mass a lot rather than a big mass a little. The most efficient is a big slow propeller, but size and mass constraints put limits on how big and slow you can go.

[jacquesm]

A coreless motor has more losses, not less. That's because the field isn't focused as nicely. It's just that the losses aren't in the core but in the air.

Core losses by the way are typically a different kind of loss, they are eddy currents resulting from the fact that the stator laminates are not infinitely thin.