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Wow, there were a bunch of very interesting designs, Probably telling the winner was a very tradition design, but I love seeing wild stuff being tried.

The kite copter was my favorite, but the large rotor powered by small tip rotors was a close second.

> a conventional single-main-rotor helicopter that weighed 13.3 kg (29.3 pounds) and hauled 51 kg (112.4 pounds)

I think this was the sweet spot for now and we'll need to wait for anything that maintains that ratio at double the carrying capacity.

Slop city, it felt like I was having an LLM chat.

Your assumption is correct, they were prohibited:

>3.7 Heavier than air: The UAS must be a heavier-than-air aircraft. The use of any lighter-than-air gases to provide buoyant lift is strictly prohibited. Teams may use chemically inert gases for subsystem actuation, safety inerting, and component rigidity, provided that there is no buoyant advantage and the gas is contained in components operating at pressures indicative of mechanical function.

https://www.darpa.mil/research/challenges/lift/rules

They probably feel like the athlete who missed breaking a record, like the shipping company which just failed to load the goods in 4 trucks and now needs 5, like the painter who just needed that extra 50 cm on his ladder: "darn, so close".

They've probably got companies lined up to give them jobs that will pay multiples of those. The runner up is even more interesting, with a slightly larger reason to feel like they should have won it. They did ~3nm (out of 4 iirc) of 4.04:1 ratio.

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.

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.

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

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

We have some times very good intuition and ideas.

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.