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https://en.wikipedia.org/wiki/Stewart_platform

Also, are there any helicopters in service that use this mechanism? I was thinking along the lines of using it instead of the part of the swashplate assembly that is used to change the collective pitch.

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closed as unclear what you're asking by Sanchises, Ralph J, TomMcW, Pilothead, Gerry Jun 8 '18 at 2:40

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ Swash plates just control individual blade pitch as the blades go around. Are you thinking along the lines of a Stewart Platform to move the entire rotor disc as a unit like on a gyroplane? $\endgroup$ – John K Jun 7 '18 at 13:26
  • $\begingroup$ Please edit your question to explain whether you are referring to controlling a swash plate with a hexapod-like construction or tilting the entire rotor head - for now, I voted to close as "Unclear what you're asking". On an additional note, a hexapod controls six degrees of freedom, while you only need three degrees of freedom: pitch+roll (cyclic) and axial (collective). $\endgroup$ – Sanchises Jun 7 '18 at 13:36
  • $\begingroup$ Very related $\endgroup$ – TomMcW Jun 7 '18 at 18:58
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    $\begingroup$ Possible duplicate of Why does a helicopter need cyclic control rather than just tilting the whole main rotor? $\endgroup$ – TomMcW Jun 7 '18 at 19:57
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To replace an effective technology, propose a better technology for that particular application.

The Stewart platform (as shown) would add both weight and complexity, and therefore cost and potential performance limitations, to a function already accomplished with less weight and less complexity. As @Therac pointed out, tilting the rotor is not necessarily impossible, but this mechanism would not replace the swashplate.

Direct motors or servo flaps can do that, at the cost of requiring constantly correcting computer control.

This means that you have to build yet another subsystem to do that: adds weight, adds cost, adds complexity. Granted, complexity itself is not an "all stop" design consideration (or helicopters would not be flying today :) )

Let's consider the next issue: consequences of failure. If you put this system into place, what happens when it fails/breaks? What does the rotor system do, or not do, as a consequence of that? How are the loads being transmitted through the airframe/flight control system in this case?

The Stewart platform is fit for purpose for a variety of applications (one being motion in a flight simulator) but when it comes to helicopter design, you have to earn your way on to the aircraft in terms of the weight required. Until you can explain how this system is better than the system that has been shown to work, what incentive is there to apply that? Given its proposed relationship to the flight controls, which are critical to both function and flight safety, there isn't an incentive readily apparent.

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The purpose of the swashplate isn't to tilt the rotor head. It's to pitch each blade as it passes through various sectors of the disc.

In a Stewart platform, both plates rotate at the same time. In a helicopter swashplate, the control plate stays stationary. Rotor blades pass over the stationary plate and sort of follow its profile in pitch.

Tilting the rotor is not necessarily impossible, but it would not replace the swashplate. Direct motors or servo flaps can do that, at the cost of requiring constant correct computer control.

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    $\begingroup$ Also I think tilting what essentially is a huge gyro won't be nice for the helicopter itself -- but I'm not sure. $\endgroup$ – ymb1 Jun 7 '18 at 16:32
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This Wikipedia on Swashplate (aeronautics) indicates

Alternative mechanics to the stationary (outer) swashplate are the hexapod and the universal joint.

So it would seem possible. Very difficult to find anything useful searching for helicopter & hexapod tho, nothing appropriate is coming up, everything seems to be drones, or using the Gough-Stewart platform as the bed for things like testing.

Physik Instrumente (PI) has broad line of Stewart Platform motion controllers, my impression is that these look a lot more complex than a swashplate.

There's this paper on Development of swashplateless helicopter blade pitch control system using the limited angle direct-drive motor (LADDM) which is much more interesting reading.

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  • $\begingroup$ I have read both the wikipedia article and the LADDM paper. I wanted to know if it would be feasible to use this mechanism in a helicopter to tilt the rotor head and if not why? $\endgroup$ – user31222 Jun 7 '18 at 15:03
  • $\begingroup$ @user31222, theoretically everything is feasible, but why? Just think: Stewart platform is a 6-DoF machine, while the swashplate is 3-DoF (plus each needs to pass continuous rotation). It's an order of magnitude more complexity, and what for? $\endgroup$ – Zeus Jun 8 '18 at 6:28
  • $\begingroup$ Enstrom helicopters use the Universal (Hookes) Joint design. Control feedback not as well mitigated with this design. $\endgroup$ – Walker Jun 11 '18 at 17:33

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