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Comment by dyauspitr | Hacker News Reader

Hunh so for pure lifting our standard helicopter design is apparently the best. How did we stumble on the perfect design half a century ago? Was it sheer luck and constraints or can you actually do the math for this to work it out?

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For a given lift, a larger, lower speed rotor is strictly more efficient than a smaller faster one, all else being equal. So no surprises that when efficiency matters we don’t see quadcopter-type designs. The reason they became ubiquitous is because they are so mechanically simple, all the control can be done with cheap electronics, no need for the great mechanical complexity of a swashplate.

The article makes reference to this being expected from the physics, but I don't think it's particularly intuitive why it should be the case. Unless it's as simple as smaller swept areas needing higher tip speeds for the same lift? That would make the energy loss to drag worse for multirotors.

Higher aspect ratio blades are more efficient. It maximises disc area for a given mass of blade (linear-squared relationship). Blade is held straight due to tension not purely from torsional strength, so it can be made from a material with high tension strength like carbon fibre which is light. Also it positions the fastest moving blade area far out from the fuselage where there is less blown-drag.

The downside is that the tail rotor is purely loss.

Potentially a similar efficiency could be achieved with two large rotors, but then you end up needing to use cyclic roll to counter drag-induced cyclic yaw, or adding extra pitch mechanisms on each motor mount to make differential yaw.

So the wiggle room for efficiency is in the mechanism for counteracting yaw? Either the weight or energy expense of running a tail rotor or quadrotors or variable pitch mounts (or some other clever mechanism)?

Same physics that drives wind turbines to be as large as possible, I believe.

As other commenters say, there's physics reasons one big rotor is most efficient.

The reason the earliest rotary wing aircraft took this configuration is because where was no other choice with the technology of the time. They had to pick the most efficient option for it to be cost effective and have useful capacity.

I am surprised it was not a coaxial design(think the mars copter, but designed for a real atmosphere)

I would think coaxial designs suffer as the lower rotor works in more turbulent non laminar air.

Twice as much rotor weight moving the same cross section of air. What might actually work would be a coaxial where rotor A and rotor B swept different parts of the rotor disc at different rotational speeds, to get blade speeds closer over the whole area, or the twin rotor front and back design, but apparently the structural weight of high power distribution for the dual rotor design is a significant problem, sweeping less disk for the weight than a single rotor system. I think a big part of it is that disk area goes up at the square of rotor length, so having multiple rotors costs you the advantage.

We have also achieved optimal teapot design for perfect flow thousands of years ago.

We have some times very good intuition and ideas.

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