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

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.

• What problem are you trying to solve? – Simon Sep 25 '15 at 7:14
• Note that your premise "surely incurs a lot of friction" is wrong. This is just a 'rotational' variety on a camshaft you can find in any car; it is in fact an extremely efficient way to prescribe a certain movement. The reason is that there's a microscopic oil film between all components, which reduces the friction to almost nothing. Besides, don't your actuators need bearings too? – Sanchises Sep 25 '15 at 20:16
• @Simon friction and/or wear and tear of the complex hub linkages that control blade pitch. It may not be a major problem, but if it's reduceable then why not? At least, that was the question. "Why not" seems to be answered by now. – DrZ214 Sep 25 '15 at 20:49
• I have absolutely no idea how much energy is lost due to friction in these parts, but I am equally absolutely convinced it's a rounding error when factored into the energy lost due to drag in the system and friction in the engine and gearboxes. – Simon Sep 25 '15 at 20:59
• @Simon I wouldn't be so sure of that either without a source. Friction/resistance at one end can be felt at the other end of the engine, since they're all moving each other. It's hard to separate friction here from friction there when resistance at one end requires the whole engine to push each gear harder (so each gear gets more friction too) just to overcome the resistance at the end. Again, the blades have to flap 2x per rotation, and the rotors have fairly high RPMs. Doing all that with a swashplate "bumping" control rods up and down seems rather friction-intense to me. – DrZ214 Sep 25 '15 at 21:09

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".

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).

• I would rather have a mechanical link, designed, tested and proven over millions of flight hours, for one thing to push another thing, than a nest of cables, computers and motors. +1 – Simon Sep 25 '15 at 9:04
• In addition to the above mentioned problems, the effect of having to move constantly back and forth at high speed every rotation of the rotor would mean that the complex and more expensive motor would wear out just as fast if not faster than the less expensive mechanical parts. Reversing direction is particularly tough on electric motors because the coil that is active has to remain powered for a longer time creating extra heat on the coil and the associated brush. And the motors would have to reverse themselves twice for every rotation of the motor. – TomMcW Sep 25 '15 at 17:57
• Not sure where the first video "captures the consequences pretty well" was supposed to point... it seems to be a science fiction show not involving a(n) helicopter. – Michael Sep 25 '15 at 19:34
• @Michael Listen to the dialog. ("Oh god, oh god, we're all going to die.") – voretaq7 Sep 25 '15 at 19:44
• How would you communicate with (and power) these motors? That's a great point I never thought of. And hey don't bring a great show like Firefly into this :-) – DrZ214 Sep 27 '15 at 19:57

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.

• While your outline is educational, the statement that new pitch control system would require control forces "orders of magnitude" above the present system...just seems wrong. Whatever huge control forces resist flapping the blades are already overcome by the swashplate and linkages we have today. Electric motors would have to overcome the same resistance, and maybe even a little less if friction is reduced. So unless there's some sort of huge lever arm hidden in the control rods, flapping the blades 2x per rotation requires the same (huge) force whether it's by mechanical or by electrical. – DrZ214 Sep 25 '15 at 21:03
• @DrZ214 I think he meant the forces are orders of magnitude above those of the vibration control system he mentioned, not of the current blade pitch control systems. – reirab Sep 25 '15 at 21:05
• @DrZ214 reirab is correct. I was comparing the required control forces for total control of the h/c with those required in active vibration control – aeroalias Sep 26 '15 at 2:30
• @aeroalias okay, i'll leave these comments here in case anyone else had the same thoughts. – DrZ214 Sep 26 '15 at 2:41

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.’

• I upvoted as soon as I saw all those references, even though I could not access them, because you're the first to cite anything on this thread. Also, you explained the swashplate and it's rolling friction to me, but now I would like to know more about where those aeroelastic vibrations come from. Nevertheless, your analogy about 2 ways to brake a car is wrong. A 1,000 kg car moving at 30 m/s (67 mph) has a kinetic energy of 450 kJ. It will require 450 kJ of energy to stop it. Doesn't matter if it's from a regular car brake turning kinetic energy into vast heat onto the brake pads... – DrZ214 Sep 26 '15 at 2:51
• ...or a regenerative system "absorbing" kinetic energy into an electric generator and into batteries. Both ways need 450 kJ output. One way may be easier and more fail safe and might even have less startup energy, if we can use that term, but in terms of energy, neither one has "energy requirements: almost nothing". Both of them have to do the same output of negative work to stop the car. – DrZ214 Sep 26 '15 at 2:54
• @DrZ214 in a braking car, the majority of its kinetic energy goes into waste heat during tire friction against the road, the brake pads do very limited work in the physical meaning of the word. – Peteris Sep 26 '15 at 8:44
• @Peteris That's the first I heard of that. Do you have a source or further reading link? I'm having a hard time seeing how the brake pads can overheat while the rubber wheels don't seem to wear down all that much. I could be wrong but if what you said is true, it would seem that all the rubber would vaporize before the brake pads start suffering. – DrZ214 Sep 26 '15 at 20:37
• ...plus I have just realized that during auto-rotation, blade flapping is still going on and the hub does not seize up from that---so maybe friction from the swashplate and related linkages is not as bad as it would appear. – DrZ214 Sep 27 '15 at 19:51

Development is in progress, just a matter of time:

• This is a really great find, please do not remove it! – DrZ214 Oct 2 '15 at 21:39
• Okay, sory for the short response. So actually ZF Luftfahrttechnik GmbH from Germany (some of the above cited publications came from ZF, e.g. Kessler and Arnold) is working on the development of swashplateless electrical individual blade control for several years. As far asI know they were the first company working on such actuators. Nevertheless there is similar research going on in USA and China. – Steffen Oct 4 '15 at 18:04

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.

• Hi John, welcome to Aviation.SE. Unfortunately your answer does not seem to directly address the question, but rather it seem to promote discussion. This is not considered well here, as this is not a forum, but a Q&A site. You're welcome to stick around, though, and as soon you will have a little more experience of how things work here (and "reputation") you will be able to join us in chat, where discussion such as this are more than welcome. – Federico Mar 17 '16 at 8:03
• This answer could be improved by explaining the intermesher design in more detail. As it stands, it only adds a term to the discussion without explaining it. As such, it will risk to be removed. – Peter Kämpf Mar 17 '16 at 8:35
• You may add references for people wanting further readings. It helps increasing the quality of each post on this webwite. – Manu H Mar 17 '16 at 9:14
• Although I agree with others that the quality of this post needs improvement, I voted not to delete this one as I think there is some value in this answer. – DeltaLima Mar 17 '16 at 9:30
• @PeterKämpf I noticed 2 terms without explaining: "Kaman K-Max intermesher" and "self energising control flaps on the trailing edge of the blades to drive the required blade pitch". That last one sounds dubious to me. Sounds like a control surface that changes pitch to change the pitch of the blade itself. To me that sounds either impossible or physically very very inefficient. If you already have an electric servo to change pitch of a flap, why not just change the whole blade pitch that way instead of a flap on the blade. – DrZ214 Mar 17 '16 at 12:47

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.