Swashplate (helicopter)

A swashplate is a device that translates input via the helicopter flight controls into motion of the main rotor blades. Because the main rotor blades are spinning, the swashplate is used to transmit three of the pilot's commands from the non-rotating fuselage to the rotating rotor hub and mainblades.

The swashplate consists of two main parts: a stationary swashplate and a rotating swashplate. The stationary (outer) swashplate is mounted on the main rotor mast and is connected to the cyclic and collective controls by a series of pushrods. It is able to tilt in all directions and move vertically. The rotating (inner) swashplate is mounted to the stationary swashplate by means of a bearing and is allowed to rotate with the main rotor mast. An anti-rotation link prevents the inner swash from rotating independently of the blades, which would apply torque to the actuators. The outer swashplate typically has an anti-rotation slider as well to prevent it from rotating. Both swashplates tilt up and down as one unit. The rotating swashplate is connected to the pitch horns by the pitch links. Alternative mechanics to the stationary (outer) swashplate are the hexapod and the universal joint. Swashplates for helicopters having two rotors mounted on the same shaft are much more complex than the single rotor helicopters.

Cyclic controls are used to change a helicopter's roll and pitch. Push rods or hydraulic actuators tilt the outer swashplate in response to the pilot's commands. The swashplate moves in the intuitively expected direction, tilting forwards to respond to a forward input, for instance. However "pitch links" on the blades transmit the pitch information way ahead of the blade's actual position, giving the blades time to "fly up" or "fly down" to reach the desired position, in addition to a 90 degrees advance to account for the gyroscopic precession. That is, to tilt the helicopter forward, the difference of lift around the blades should be maximum along the left-right plane, creating a torque that, due to the gyroscopic effect, will tilt the rotor disc forward and not sideways.

To control the collective pitch of the main rotor blades, the entire swashplate must be moved up or down along its axis without changing the orientation of the cyclic controls. Conventionally, the entire swashplate is moved along the mainshaft by a separate actuator. However, some newer model helicopters remove this mechanically complex separation of functionalities by using three interdependent actuators that can each move the entire swashplate. This is called cyclic/collective pitch mixing.

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Wikipedia