Yesterday I learned that the surface that the helicopter blades create while rotating is very similiar to a gyroscope, and thus gyroscopic precession works there as well. My question is, when the pilot moves the cyclic pitch controls forward, how does that affect the swashplate? Does it rise it in back and lower in front or does it rise on one side and lower on the other?


1 Answer 1


Horizontal displacement: Tilt the rotor

With the rotor blades rotating around a vertical mast, the total lift vector of the rotor is vertical and the helicopter hovers. If a translation is desired, this vector must be tilted so that an horizontal component can appear. This is done by tilting the rotor disc. The rotor disc is the plane of rotation of the blades.

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Rotor tilted forward: The helicopter moves forward.

Rotor tilt: Change blades pitch

The rotor disc is tilted using the lift difference between individual blades. For instance to tilt the rotor forward, each blade creates more lift when it is aft of the mast, less when it is forward of the mast, and the change is progressive between these two positions. The lift difference creates a torque moment on the mast, tilting the mast and the helicopter with it, at least this is our intuition.

As you mentioned, the force doesn't produce its effect instantaneously due to the gyroscopic precession. It is effective only 90° later. So the lift variation must have an advance of phase of 90°. For a forward translation, the lift is maximum when the blade is on the left, and minimum when it is on the right. The momentum is stored into the blade and released 90° later.

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The variation of lift is done 90° in advance.

Lift depends on the angle of attack of the blade. Varying the amount of lift according to the position around the mast leads to varying the AoA around the mast: Increased AoA on the left, decreased on the right.

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Pitch varies according to the radial position of the blade.

Change blades pitch with the swashplate

Changing the pitch of a blade is done using a system of swashplates. A stationary plate is oriented in space by the pilot using the cyclic stick.

Another plate is maintained in contact with the first one, but can turn freely with the rotor. This second plate is forced to rotate in the plane of the stationary plate, thus it oscillates constantly as it rotates. In turn, it creates an oscillation of the vertical rods (horn links). Each horn converts the rod oscillation into a pitch oscillation on the blade. The blade is now pitched according to its instantaneous radial position relative to the mast, and according to the cyclic stick command.

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MBB Bo105, based on photo by Burkhard Domke (source)

Horns are linked to different angular sectors of the swashplate

The vertical rod transmits the stationary swashplate height at its current axial position. By locating the rod 90° ahead of the blade, the link takes care of the rotor precession. The mechanism is so that when the pilot pushes the cyclic forward:

  • The swashplate is tilted forward.
  • When a blade moves left of the mast, its horn link measures the height of the stationary swashplate in its aft sector, where the plate is the highest.
  • The horn is pushed by the link to the maximum, and the blade in turn gets the largest pitch angle.
  • The situation is reversed when the blade arrives on the right. The link is in contact with the forward sector of the swashplate, where the latter is at the lowest position. The blade gets a low pitch angle.

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(Redrawn from source)

Advance for pitch order can be adjusted as required

By adjusting the offset of the horn link which interfaces the horn and the plate, one can adjust to the exact amount of advance required for a given blade.

  • $\begingroup$ The swashplate has to be rigged so that the rotor tries to align itself with it. This is because the fuselage is mostly (or completely in case of teetering rotors) just freely hanging under the rotor, so the rotor must not proceed tilting further before the fuselage has caught up with it. When the swashplate tilts forward to fly forward, the rotor tilting forward reduces the angle between it and the swashplate, so the lift asymmetry decreases until the fuselage catches up. If the swashplate tilted right instead, the rotor would continue to tilt and could hit the mechanical limit and break. $\endgroup$
    – Jan Hudec
    Feb 7, 2022 at 8:16
  • $\begingroup$ Also note that torque transferred to the mast directly by the stiffness of the blade attachments is not subject to the 90° offset, so in many helicopters the offset is not 90°, but only 70°–80°. $\endgroup$
    – Jan Hudec
    Feb 7, 2022 at 8:22

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