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When reading about the lifting capacity of swashplates (as found in helicopters) vs fixed blade systems (as found in e.g. DJI drones), I read a lot of comments about the fact that the bigger rotors of a helicopter are more efficient. But how does that link to the system (fixed vs swashplate) and how much efficiency are we talking about? Now, I know that the question is way too simple for the complexity of the problem, also after reading these questions 1, 2, 3

The factors that influence this problem is also part of the 'quest' and I hope to reach a ballpark figure. It also links to the complexity of the use of a swashplate or even a virtual one. A very complex system can be more efficient in the long run, but is it worth it. That is the underlying question.

If possible could you explain formula's? I had math and physics back in the day until the age of 18 (VWO for the dutchies), so it is rather rusty.

edit: the core of the question is the system efficiency vs 1 or multirotor

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    $\begingroup$ This ain't that easy to answer, as aerodynamics do not scale in a simple manner. Drones and helos are soooo different in size, and there is huge variation in their niches... Way outside my comfort zone ;) $\endgroup$
    – Jpe61
    Feb 3, 2021 at 11:18
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    $\begingroup$ Not to mention that drones and (non-drone) rotorcraft come in quite a range of sizes (likely overlapping - if not now, then certainly quite soon). $\endgroup$
    – Toby Speight
    Feb 3, 2021 at 13:27
  • $\begingroup$ A helicopter is generally massively more efficient than a multirotor, in part because of the rotor diameter and in part because of the fuselage's wetted area. BTW, drones are anything which fly autonomously, so there are drone helicopters as well. $\endgroup$
    – Kenn Sebesta
    Feb 3, 2021 at 16:48
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    $\begingroup$ Drone is a misleading word here. You could turn a large helicopter into a drone, just by adding some actuators & electronics so that it could be controlled by a remote operator instead of a pilot. E.g. popsci.com/technology/article/2013-04/… $\endgroup$
    – jamesqf
    Feb 3, 2021 at 17:21
  • $\begingroup$ The (first) edit was an improvement (removing the word "drone") -- but arguably it invalidated an existing answer-- it seems the most correct course of action would be to roll back and allow original asker to post a new question if he/she wishes-- but maybe the answerer doesn't mind in this case-- ?? We can still upvote the answer after all even if it no longer fits the present version of the question; it should be pretty obvious to everyone that it suited the question as it stood at the time it was posted-- $\endgroup$
    – quiet flyer
    Feb 3, 2021 at 20:06

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The question does not really make sense: a helicopter is a kind of aircraft, a drone is a mode of how a vehicle is controlled.

I can rip the seats and controls out of a helicopter and put some clever software, some cameras, and some transmitters inside, and boom: I have a drone that can carry exactly as much as it could before – probably even more because humans and safety systems are heavy, and we got rid of both of those.

In fact, that's exactly what some companies are doing. The Northrop-Grumman MQ-8C Fire Scout is literally just a Bell 407 with some added electronics and software. And while this particular drone is designed for combat, there are others which are designed for cargo delivery. For example, two uncrewed Kaman K-MAX were deployed to Afghanistan already in 2011.

Here is a video about the Office of Naval Research's Autonomous Aerial Cargo/Utility System AACUS program. The goal of this program is to have a software/hardware package that can be installed in any helicopter to make it fully autonomous. There is no reason you couldn't install it in a Mil Mi-26, for example.

So, in short: by turning an existing helicopter into a drone, a drone can by definition lift just as much as a helicopter. By designing a helicopter specifically as a drone, you could potentially lift even more, because you can get rid of all the weight for the cockpit, seats, life support, safety systems, controls, etc. You could also remove a lot of redundancy, if you consider occasional crashes acceptable when no humans are on board.

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Swashplate compared to fixed-blade system

Makes no difference. The swashplate is there for controlability, not efficiency.

Of course single-rotor fixed-blade system can't be used, because it wouldn't be controllable, and with four or more rotors nobody uses swashplate, because it is controllable without it, but that's orthogonal to efficiency.

Single-rotor compared to multi-rotor

The efficiency depends on area for weight and RPM. Increasing the area reduces the induced power (energy carried away by the slipstream), but it also increases the parasite power (energy needed to overcome parasite drag of the larger, faster moving blades), so there is some optimal size for given lift.

Now the thing is that as weight increases, the optimal area increases slightly slower, and the best possible efficiency decreases. From this it follows that four rotors of quarter the lift each are more efficient than one big rotor.

This is however offset by the weight of the structure and transmission needed to use four rotors, and by larger turbine engines being more efficient. That, plus all the safety benefits of a swashplate system is why single-rotor helicopters are more efficient in practice.

For small sizes the optimization criteria. Due to the square-cube law small scale UAVs have plenty of power for their need, so the efficiency is not that important, the materials are plenty strong for the need, and the simplicity of fixed blades driven by four separate electric engines is the main reason to choose multi-rotor setup.

Other reasons for swashplate

  • Swashplate systems are capable of efficient auto-rotation, so even in case of an engine failure, a helicopter can still glide to a safe(ish) landing. This requires reducing the blade pitch, so fixed-blade systems can't do it.
  • Swashplate can compensate for the different amount of lift on the advancing and retreating side by adjusting the blade pitch, so it copes better with high flight speeds.
  • Larger rotors have significantly larger moment of inertia (moment of inertia grows with fifth power of diameter!), so their RPM can't change quickly enough to give practical control response and variable blade pitch is needed to control their lift instead.
  • Multi-rotor with separate engine for each rotor have a problem dealing with engine failures. If an engine fails, they have to reduce power on the opposite side by the same margin and still not only have enough power to stay aloft, but also enough spinning rotors to stay controllable. That's why the Volocopter has so many rotors (rather than just four).
  • Electric propulsion does not provide the power-to-weight ratio needed for large helicopters, but separate electric motors is what makes multi-rotors simple.
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  • $\begingroup$ If I understand it correctly you write that controllability is orthogonal to efficiency. I do not really understand 'orthogonal' in the sentence, would you mind to elaborate? $\endgroup$
    – Arie
    Feb 5, 2021 at 18:54
  • $\begingroup$ ‘orthogonal’ in this sense means independent. You pick some parameters for sake of one and it does not have much effect of what parameters you need to pick for the other. $\endgroup$
    – Jan Hudec
    Feb 5, 2021 at 20:47

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