In helicopters, why not have electric motors controlling blade pitch?

Question

From my related question comes this idea of electrically controlled blade pitch in helicopters.

Control linkages in helicopter rotors seem to be pretty complex. This surely incurs a lot of friction, especially if you realize that blades have to "flap" back and forth as they rotate around (in order to compensate for retreating blade stall and/or dissymmetry of lift during forward flight). So the swashplate and linkages must surely undergo a lot of friction.

So why not replace all this with electric motors that control blade pitch? They would be small, inside the rod connecting the blade to the hub. No need for any pitch control to be mechanically connected to any other blade(s), so no friction. A computer would control what the pitch should be given the blade's position during its rotation.

This does not mean the main rotor has to be electrically driven. The turboshaft engines can drive the main rotors mechanically as normal. But there would be an electric generator for the blade pitch motors. Note: I'm pretty sure there's already an electric generator on these engines as the computers and radars and stuff have to get their electric power from somewhere.

Answer 1

How would you communicate with (and power) these motors?

The main rotor is spinning constantly - Wires won't work, they would wrap around the shaft and be shredded. A slip ring and brushes (as used in some electically-actuated propellers) would work, but would also wear away quickly and require frequent maintenance as losing control of a helicopter's main rotor blade pitch is a much more serious situation than losing control of a fixed-wing aircraft's propeller pitch (I believe this captures the consequences pretty well).
Because of the rotor speed balance is critical, so any motors, brushes, etc. would need to be balanced around the rotor hub (either by duplicating the equipment, which also provides redundancy, or by adding dummy weight) - get it wrong and the rotor starts vibrating and may come apart.

The motor would also have to be constantly moving: The swashplate adjusts blade pitch through the full 360 degrees of rotation, smoothly changing the pitch of each rotor blade as they rotate. Duplicating this would require the motor(s) to make constant adjustments as the rotor turns, moving and reversing very rapidly). The level of precision required and the forces involved would likely require high-torque stepper motors and a computerized control system of some kind would be needed to drive them and make the necessary blade pitch adjustments at "rotor speed".

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So far the motorized solution has added at least one motor, a slip ring & brush system to communicate with it, and a fly-by-wire computer to read the flight control positions and appropriately adjust the blade pitch through 360 degrees of rotation (at whatever speed the rotor is operating).
That is already a lot of complexity and a substantially increased chance of failure versus the (relatively) simple mechanical solution of a swash plate, and I'm not even going out of my way to think of failure modes (actual engineers designing these things are far more paranoid, and could probably imagine all sorts of failure scenarios which eventually result in a helicopter plunging to the ground).

Answer 2

Because it would be too complicated (and failure prone) compared to the present system and would offer no great advantages.

First, for all the complexity in the helicopter upper controls, the principle is pretty simple- Align the rotor plane with the (rotating) swash plate, and tilt (or rise) it according to requirements.

Source: helistart.com

This system is used in almost all helicopters, and have operated for millions of flight hours under a variety of conditions and has proven itself over a wide range of helicopter weights.

In order to replace it, the electric motor system should have the following characteristics.

• The electric motor, if used should be highly robust, with extremely low failure rate as it (failure of any one) would jeopardize flight safety. Also, it should be able to deliver significant changes in torque in a rapid manner.
• The power supply to the motor would be critical and the only thing I could think of is the slip-ring system. This would require frequent inspection as power supply (and signal transmission) is absolutely critical for operation. The power supply required would make the electrical system of the helicopter heavier and more complicated (due to redundancy).
• New control algorithms and a flight computer (a fly-by-wire system) has to be developed for this system as it is completely incompatible with the present ones and there is no way to directly transmit the pilot control inputs to the rotor blades as it is done at present. I'm not sure if anyone is going to develop a complicated system (it has to be operational at every point of the rotor rotation as pitch is varying constantly) to replace a system that has been operating well.
• The components should be balanced (i.e. their weights should be balanced) across all the bales as it would lead to vibrations otherwise.

• The rotor would have to be hinged at some point anyway. Then the question becomes how to transmit the rotary motion from the hub side to the blade side. This can be done either by

  • Pitch link, which is practically the same as a mechanical linkage, or
  • Torque transmission through a rotating shaft.

In either case, the system should be able to flex both in up-down direction (due to blade flapping) or in forward-aft direction(due to blade lead-lag movement).

One instance where electric motors are actually used for pitch control is in active vibration control, were individual rotor blades are controlled using piezoelectric actuators for vibration control.

The rotor pitch control you are describing would require control forces which are orders of magnitude above that of Individual Blade Control system used and would require a completely new system. Even in this case (where power requirements are modest), University of Southampton notes,

the dependency of active control on external energy supply can limit its practical applications, particularly in hostile environments, where energy is scarce or unreliable, or where it is impractical to route a power supply.

So, using of electric motors would actually have the effect of increasing cost, weight and complexity of an already working system which is not a good idea in an (critical) aircraft system.

Answer 3

I went on a little search on my university library database, and found a review paper. I'm not sure if you can access it without paying.

Active rotor control for helicopters: individual blade control and swashplateless rotor designs by Ch. Kessler. Link: http://dx.doi.org/10.1007/s13272-011-0001-0

There, I extracted three sources relevant to your question:

1. Kretz, M.: Research in multicyclic and active control of rotary wings. Vertica 1(2), 95–105 (1976)
2. Guinn, K.F.: Individual blade control independent of a swashplate. J. AHS 27(3), 25–31 (1982)
3. Arnold, U.T.P., Fuerst, D., Neuheuser, T., Bartels, R.: Development of an integrated electrical swashplateless primary and individual blade control system. In: 32nd ERF, Maastricht, The Netherlands, September 12–14, 2006

If you're lucky, you can find a way to access these papers - however, my university didn't seem to have subscriptions to the relevant journals (either way, they weren't in the database).

In all these papers, the main motivation was to reduce vibrations caused by aeroelastic effects. This means that the interaction between blade elasticity and airflow causes unwanted vibrations which cannot be solved by a swashplate, since a swashplate can only actuate frequencies at the number of blades times the RPM. Friction from the swashplate configuration is of minor importance. A swashplate is generally just a set of ball bearings, which have the nice property that the forces on them are always at a right angle, i.e. a centripetal force which does not cause energy losses. The only energy loss is due to rolling friction, which is extremely small (I found friction coefficients of 0.005 in a paper on lubrications) for properly designed bearings. All in all, very minor compared to the massive power needed to lift a helicopter up.

Note that electronic actuators are not particularly efficient in all situations. (following part revised to clear up some confusion:) Imagine lifting a heavy box off a high shelf. Even though strictly speaking, you're doing negative work on the box, you still feel tired afterwards because for human muscles and (simple) electric actuators alike, it costs energy to apply a force. In other words, an electric motor also has to provide the negative work on a system, unless there are energy recovery systems integrated. This was actually proposed in one paper to overcome overheating issues. Furthermore, for a constant force (no work), a constant current flow is still needed in an electronic actuator.

Perhaps I best quote Kessler in his conclusion (shortened):

Individual blade control can alleviate a lot of typical helicopter problems:

• reduce the cabin vibration by 80% or even more,

• reduce component loads and power required, [...]

That is the good news. And now the bad: About 58 years of research and development on HHC and IBC have passed by. And no helicopter is equipped with such a system. [...] But even for customers it might be difficult to see an advantage of IBC and a payback. [...] An IBC system would surely raise the purchase price.[...] On the other side, designs get more and more complex, the swashplateless concepts are the far end of this complexity. It should be questioned if this is still reasonable. The advice would be, ‘‘make one step after the other; do not try to do two at the same time.’’

Answer 4

Development is in progress, just a matter of time:

Source: Helicopters are quieter (translation by Google)

Answer 5

Biggest simplification and weight saving would be to totally eliminate the whole mechanical collective & pitch mechanisms and use electrically activated controls to move self energising control flaps on the trailing edge of the blades to drive the required blade pitch alterations in the manner of the Kaman K-Max intermesher - a very successful helio with many years of proven service.

Answer 6

Helicopters certified for IFR must meet the requirements of 14 CFR Part 27 Appendix B, which specifies certain static and dynamic stability that can only be achieved through the use of electrical systems to control the rotor system.

There are a number of different kinds of systems which are used to achieve this certification, and all of them use either electric or electro-hydraulic actuators to achieve stability.

Answer 7

I flew an aircraft which had an electric pitch change without rubbing parts, i.e. it changes by electrical induction. Far too slow, and too complex. Nowhere near fast enough for use in a helicopter.

Sometimes there is a special art and beauty of simple direct or hydraulically boosted linkages. Especially when it all had to be sent via a rotary coupling.