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 nearly 4:1 ratio makes a 25kg drone very interesting from a personal mobility perspective. Something you could fly a few miles with and then easily roll around an office. Sadly scaling in aerodynamics is notoriously non-linear.
>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.
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.
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?
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.
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.
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.
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.
The kite copter was my favorite, but the large rotor powered by small tip rotors was a close second.
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.
>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
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.
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.
We have some times very good intuition and ideas.