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!

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.

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.

http://www.pitt.edu/~jdnorton/Goodies/TE-antiTE/

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.

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.

not really. Octocopters are much easy for a scheme with tilting of a couple of rotors and thus having efficient plane style horizontal flight than tilting the whole machine like helicopter. As result - higher speed, longer range.

> and everyone having flying cars based on pushing air will not be able to use them from the street due to the problem of blowing pedestrians over.

You statement is true for helicopters. The octocopters are much efficient with pushing of the air. Google e-volo. https://www.youtube.com/watch?v=CzP0Zqxam7E (or the old work - Piasecki VZ-8 Airgeep - much less of airwash effect)

As for the prop wash of an octocopter, try standing right next to one that's taking off caring just 5 kilos of payload (batteries and DSLR). I have, and trust me, it gets really, really, windy. And that's with a 5 kilo payload, with a copter capable of generating maximum of 12 kilos of trust. No scale that up to 1000 kilos.

[1] http://personal.osi.hu/fuzesisz/strc_eng/

with correct design for the frame and ducts, the same airmass down flow would generate 20-30% more of static trust than the same airmass flow in case of helicopter. And that airflow will not have that blade end vortex which causes helicopter crashes and is just straight efficiency loss until that.

I did spend my time with trust calculators and some other systems :)

One 300x8 inch prop will generate roughly 1 ton of thrust while spinning at 330RPM and consuming 12kW of energy. So, let's say we go with your prediction, and assign a 30% air flow efficiency to the one prop system. So in reality, according to you, it is generating 700 kilos of thrust. 700/4 is 87.5 kilos. So now we need to figure out how big of a prop we need to generate 1/8 of the thrust of the 300 inch prop, or roughly 88kg. I am going to use 60 inch props, because that will allow us to space 8 props in a circle roughly the same size as the original 300 inch prop.

Ok, so a 88x8 prop, spinning at 2350 RPM will generate roughly 88 kg of thrust wile consuming 6.7 kW of energy! That's 6.7 kW x 8 props, or a total of 54 kW, or almost 4.5 times less energy efficient than the one prop system, and that's taking into account your proposed 30% efficiency gain from multi-bladed systems and ducts.

I build these things all the time, there is a reason why the industry still uses helicopters with a single prop. It IS more efficient.

And another thing. 8 60in props aren't equivalent to 1 300 in wrt power loading. You need 16 of 60in to get equivalent of 1 240in prop.

Anyway, you've just calculated an octocopter with 700kg+ thrust on 60kw power. Is it bad? My neighbor's 400kg single-seater has 100kw gas turbine and it isn't 4x excessive power.

Based on what, please provide at least some reference to an equation or calculator.

>And another thing. 8 60in props aren't equivalent to 1 300 in wrt power loading. You need 16 of 60in to get equivalent of 1 240in prop. >Anyway, you've just calculated an octocopter with 700kg+ thrust on 60kw power. Is it bad? My neighbor's 400kg single-seater has 100kw gas turbine and it isn't 4x excessive power.

Can you give me an example than using some kind of calculations from a reliable source, not the "my neighbor" anecdotes. For instance, what makes you think that equivalent for a 240in prop would be 16 x 60 inch props? I understand that 16 60 inch props would have roughly the same area, but you do realize that 16 props arranges in such a way as not to create interference with each other would take up significantly more area? This is the main problem of a multirotor, you wither have to use smaller, less efficient props, or you have to make it much larger to accommodate the props.

In any case, when you are at your computer, if you feel like it, please do calculate, with references, a single prop machine lifting 1 ton vs an octo lifting the same 1 ton. I understand that a machine that lifts 1 ton would need to provide more than a ton of thrust, of course, but let's make it simple. Just a simple calculation. One prop producing 1 ton of thrust vs 8 props which can be arranged in such a way at to take up no more then 150% as much area as the original 1 prop, producing the same 1 ton of thrust.

If you really can show me that 8 props can be as efficient as 1 prop, please, by all means, prove me, and every one else who designs flying machines, wrong. But please do so with calculations I can verify.

That's because a 1 ton helicopter with 1 ton of thrust would be useless.

taking just blades into account - yes, though not that "massively". But the moment we consider the whole system - attach the blades onto a shaft, connect that shaft to reductor, to gas turbine, etc... ie. if that system is helicopter than bigger blade is still better, but if we change the whole system, ie. octocopter built using electric motors - the equation starts to change and we have good efficiency with octocopter and, which is more important, - much better control (eight electric motors with some of them allowed to be tilted gives precise control compare to very sluggish system of one big rotor driven by one big gas turbine), which addresses your point about " hovering around a minimum of a ton" - impossible task to do safely for helicopter in urban environment and pretty easy to do with ducted octocopter (you can't really have ducted helicopter precisely because of the size of the blades)

As far as i see, the lack of good control (not efficiency, range, payload, etc..) and direct danger from unshrouded blades is that doesn't allow to fly helicopters in any (even close to)urban environment, and especially for regular people.