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All of the helicopters by Sikorsky using their X2 technology coaxial rotor system don't seem to have a conventional swashplate and control rod assembly for controlling the pitch of the rotor blades. At least none that is visible from the outside.

All of the X2 technology helicopters are fly by wire, according to Sikorsky so each blade might be fitted with an individual actuator, but I couldn't find any detailed explanation of how the whole rotor setup works.

This theory might also be supported by the sliding contacts between the rotating system of the upper and lower rotor visible in the picture below, but these could also just be there for sensors, anti ice or else.

Does anyone have concrete knowledge on how the Sikorsky X2 rotor system works?

X2 Rotor Hub
Image source: verticalmag.com

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3 Answers 3

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The X2's rotors are of the rigid type (Smithsonian), a 60s invention in which the blades are not hinged.

Apart from that – as far as I searched, even in academic literature – nothing else is confirmed, but two recent patents by Sikorsky for a similar compound helicopter can help fill in the gaps:

  1. Egolf, Thomas Alan, et al. "Sealed hub and shaft fairing for rotary wing aircraft." U.S. Patent Application No. 15/509,422.

  2. Eller, Erez, Steven D. Weiner, and Frederick L. Bourne. "Swashplateless coaxial rotary wing aircraft." U.S. Patent No. 9,248,909. 2 Feb. 2016.

The first patent covers the fly-by-wire description, and in the second, the swashplateless design uses individual actuators (labels 56 below). The system in general – individual blade control (IBC) – was first full-scale tested in the 90s (nasa.gov).

enter image description here

There's also a photo of the X2 without the fairings on the University of Maryland website:

enter image description here
Source: aero.umd.edu

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    $\begingroup$ This is a great answer. As you can see in the bottom of the photo, the system does have a swashplate. You can see the stator and rotor of the swashplate, along with the actuators in red. The rotor is "rigid" in that it has no bearings or hinges, but the blades still twist and bend! This is done by clever use of materials to create a desired stiffness that allows the twisting and flapping to occur, while still providing a decent part life. Imagine taking a plastic ruler and holding it at the edge of a table, and twisting it and bending it. Bearings and hinges wear quickly so are not desired. $\endgroup$
    – Gerry
    Commented Mar 26, 2022 at 6:21
  • $\begingroup$ I wanted to add: you see actuators for the "Individual Blade Control System" (IBCS). It's used to control/tune the flapping of an individual blade, usually to reduce vibration. Again on a rigid rotor, they just twist the blade, they don't rotate it around a bearing! $\endgroup$
    – Gerry
    Commented Mar 26, 2022 at 6:27
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Just as short addendum here: in this particular type of helicopters the rotor is rigid in that the blades are built as stiff as possible and rigidly connected to the hub, apart for the pitch movement. Deformation of the blades (flapping or torsion) is here a no-go simply because the two rotors are so close to each other that any deformation would lead to a fatal strike among the blades of the upper and lower rotor.

So in a rigid desig the blades do not intentionally flap or lead-lag like in a bearingless or in an hingeless design (like the AH145) if not as a side effect since they cannot be built infinitely stiff (and heavy).

That the rotor is rigid (i.e. the blades are stiff) can simply be seen comparing the blades at rest (non rotating) of the X2 with the ones of a "conventional" design: the one of the X2 are almost perfectly parallel to the ground while the others bend downward under their own weight.

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It's simply a set of coaxial conventional variable pitch propellers.

Swashplate and control rod are more useful for flapping the blades, i.e. give the retreating blades more AoA to compensate for it's slower airspeed and equalize its lift to the advancing blades.

For a rigid rotor, the lift is not equalized within the rotor, instead the imbalance is canceled out by the contra-rotation rotor. Since the blades does not need to flap up and down on each rotation, the hub and control mechanism could be a lot simpler than a conventional helicopter with articulated rotor.

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