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In helicopters, the swashplate is used to control the pitch of the blades. Could a set of electric motors, each placed at the root of each blade, be used to control the pitch instead? Has this method ever been attempted? What potential problems might arise?

Advantages I can think of:

  • Fewer parts
  • Ability to create more complex pitch curves
  • Lower complexity, especially for coaxial rotors

Are there any major problems with this approach that I might not be aware of?

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Yes that is possible and has been attempted by the german company Airbus Helicopter Technik GmbH (formerly called ZF Luftfahrttechnik GmbH) in cooperation with Sikorsky. They have published two papers (here and here).

Electric direct actuation of actuators, picture taken from here

The resulting rotor is quite a beast; note that the silver dome in the picture below houses all of the power electronics, which emit quite a bit of heat.

View of the prototype, taken from here

The advantages of installing this system are coupled to the IBC (Individual Blade Control) concept. Hereby, one can actuate each blade individually. This brings a couple of possible benefits

  • Reduction of noise, as one can prepitch the blade before beating through another vortex
  • Reduction of vibrations, by actively countering these (which is why this setup is interesting for Sikorsky)
  • Lower main rotor power demand, as less energy is wasted
  • (However not all of these benefits can be achieved simultaneously)

Obviously the system is quite complex, you need a lot of electronic hardware to manage the powerflow and this hardware has to be mounted on the rotorsystem. Especially interesting is the fact that the actuators are able to regenerate power as the blade is back-driven in some portions of its revolution. Distributing this power to the other actuators is a demanding task from an electronic point of view (something which a swashplate does for free).

View of the electronics under the dome, taken from here

But the gains might outweight this drawback especially for rigid rotor systems such as the Sikorsky X2 rotorsystem, as these rotors are naturally very prone to vibrations.

The authors claim that this system has been successfully tested on a teststand, but not yet in real-life. However, from personal experience I can say that the prototype is quite impressive.

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    $\begingroup$ To be fair, that is one location where cooling your electronics is NOT going to a difficult task! $\endgroup$
    – MikeB
    Commented Jul 31 at 15:12
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    $\begingroup$ @MikeB that's not exactly true. The exterior of the device is going to be in a relatively high airflow environment, but air cooling wont be optimal and that smooth dome is going to see radiation heating from the sun in addition to heat dissipation from the electronics. As this is on the rotor head so attaching cooling fins will be complicated by drag depending on how they are orientated. which might have flight characteristics issues. direct air cooling is difficult because air is an effective insulator. $\endgroup$
    – Rowan Hawkins
    Commented Aug 1 at 14:41
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    $\begingroup$ In the paper, the authors state that at first they considered water-cooling the electronics, but opted to simply space out the electronic boards as much as possible to mitigate the problem. Looks like they were successful. But I agree, that convective cooling of the plate should provide ample cooling power if the helicopter is in free flight. $\endgroup$
    – U_flow
    Commented Aug 1 at 14:53
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This is certainly possible, and as you wrote, would enable more complex control strategies, and with sophisticated programming would make controlling a helicopter much easier.

However, I dare say the system suggested by you is more complex than the current mechanical/hydraulic swashplate arrangement, adding possible points of failure, and to maintain safety, a lot of cost.

The system would need to consist of multiple components per rotor for redundancy, also the electric system of the helicopter would need to be upgraded to a more fail safe configuration. Also the control logic would not be a simple task to program, and again depending on application, double or triple redundancy would be needed for control computing.

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    $\begingroup$ Isn't a swashplate a complex system with lots of single points of failure as well? $\endgroup$
    – Michael
    Commented Aug 1 at 4:39
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    $\begingroup$ @Michael but failures in an electronic system would likely be sudden and catastrophic, whereas in a swashplate they would more typically announce themselves via excessive vibration, sticky controls, loss of hydraulic pressure etc. before actually becoming bad enough to cause a crash. Redundancy can help to some extend in the electronic case, but it incurs a lot of complexity and failure opportunity of its own (you need to ensure the systems don't "fight each other"). $\endgroup$
    – leftaroundabout
    Commented Aug 1 at 8:51
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    $\begingroup$ @Michael It would seem so, but being (mostly) mechanical, the engineering behind the swashplate design is relatively simple and very much proven. This is entirely not the case with electronics, and certainly not with programming $\endgroup$
    – Jpe61
    Commented Aug 1 at 8:58
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The closest we should get to that is Fly-By-Wire helicopters, those have swashplates too but don't directly connect the controls to that.

  1. Motors can and will fail more than mechanical links, and especially in places like the rotor of a helicopter, you really need to make sure everything is working fine and it's ok to have "more" parts.
  2. Cost and complexity will probably also increase, since you have to maintain these motors which I think would cost more than the mechanical links. Complexity is created with all the associated systems that would be required for this. Complexity is reduced in one area but increase in a much worse way in other places.
  3. In case of a failure I think these would be worse. Ensuring power supply to the motors in all scenarios will be difficult.

Really speaking there's not really a need to integrate motors there and swashplates are just fine.

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There's lots of ways this can be done, and I'd highlight https://hackaday.com/2020/06/25/building-and-flying-a-helicopter-with-a-virtual-swashplate/ which simplifies the mechanical side about as far as it could conceivably go.

The big questions are reliability and failure modes: a swashplate allows autorotation in the case of powerplant failure, any alternative which relied on a motor to hold blade pitch at any particular point and didn't automatically revert to the neutral position on failure would be unsafe.

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