Imagine a dozen 20 pound wild turkeys flying together. They are quite a bit more efficient than a single 240 pound craft/passenger because air displacement grows by the surface area of an object (by the square) while weight grows by the cube. That's why birds get off the ground easily while a human powered airplane needs a hundred foot wingspan.
Mathematically, the efficiency of a helicopter rotor is roughly proportional to its length. Double the diameter and fuel usage falls by roughly half for the same lift. However the lift for an array of rotors grows by the square of length, even though efficiency stays constant. So if you start with a quadcopter running on batteries that’s 10 times as efficient as a full-scale helicopter running on gas, an array of them with the same lift will still be 10 times as efficient. It will also have a smaller cross sectional area at the cost of greater complexity.
So the future of high efficiency lift is almost certainly in using arrays of rotors. The reason that hasn't happened yet is that most aircraft are designed for high speed travel, not hovering. I still want my reactionless drive though!
"So if you start with a quadcopter running on batteries that’s 10 times as efficient as a full-scale helicopter running on gas"
This I have never heard of. In a ratio of energy efficiency against carrying capacity I was under the impression that a normal helicopter massively outperformed a small quadcopter.
My impression with doing lifts with lots of small drones was that you get around this by constantly cycling the swarm.
Also, this still doesn't get around the problem of downwash. Either you shift a massive cross section of air for your force and get very little downwash, but then you end up with something much bigger than a car, or you focus it and get lots. You cannot get around the fact that to suspend a mass you have to move something equivalent downward.
edit - I get what you are saying with the turkeys. However, in a vehicle carrying four people, you have replaced the turkey bodies with a giant blob that has much less lifting potential, especially at low speed, than lots of turkey bodies. You have to dump a lot of useful surface area. Also I note that turkeys do not hover much.
> a normal helicopter massively outperformed a small quadcopter.
not really. The power loading (power divided by surface of the propeller's circle) is the key factor what matters for efficiency (until of course Reynolds number - air viscosity factor - starts to change - happens when we come to Kolibri scale)
In general, helicopter with 3m long blades (6m diameter) on one 500kw turbine or something like 16 of 30kw motors with 1.5m diameter propeller (with the same number of blades as helicopter) each is pretty much equivalent. That is pretty theoretical starting point. After that we can optimize these systems in different ways. 16-copter allows to use somewhat bigger, yet less-bladed, like 2-3 bladed propellers which are more efficient than multi-blades of helicopter (and you can't really have 2 bladed rotor on helicopter). You can use ducts on 1.5m propellers - not a case for helicopter. So all this allows to increase efficiency.
Ya there are definitely many ways of looking at this. I found the link about rotor power halving for each doubling of diameter (where it says: "So in this process the power is halved to (2m)(v/2)2/2 = mv2/(2x2) = P/2."):
Downwash definitely figures into this, as well as using fewer blades.
But, the key point is that we intuitively know that smaller craft require exponentially less power than larger craft. We could build a large craft with a very wide rotor to approach the efficiency of small craft, but I think it will turn out to be easier to use multiple small craft and scale how much weight we lift linearly.
"the key point is that we intuitively know that smaller craft require exponentially less power than larger craft."
This is where your intuition has let you down. Think about a normal helicopter's lifting capability and range as you go between models and full size aircraft. Or why you see bees walking.
Imagine a dozen 20 pound wild turkeys flying together. They are quite a bit more efficient than a single 240 pound craft/passenger because air displacement grows by the surface area of an object (by the square) while weight grows by the cube. That's why birds get off the ground easily while a human powered airplane needs a hundred foot wingspan.
Mathematically, the efficiency of a helicopter rotor is roughly proportional to its length. Double the diameter and fuel usage falls by roughly half for the same lift. However the lift for an array of rotors grows by the square of length, even though efficiency stays constant. So if you start with a quadcopter running on batteries that’s 10 times as efficient as a full-scale helicopter running on gas, an array of them with the same lift will still be 10 times as efficient. It will also have a smaller cross sectional area at the cost of greater complexity.
So the future of high efficiency lift is almost certainly in using arrays of rotors. The reason that hasn't happened yet is that most aircraft are designed for high speed travel, not hovering. I still want my reactionless drive though!