Swashplate (helicopter)

:"This article is about the swashplate in helicopters, see also the swashplate article which describes all uses of the mechanism."

A swashplate is a device that translates the pilot's (or autopilot's) commands 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.

1 Non-rotating outer ring (blue) 2 Turning inner ring (silver) 3 Ball joint 4 Control (pitch) preventing turning of outer ring 5 Control (roll) 6 Linkages (silver) to the rotor blade # Linkages (black) that make the inner ring turn]

Assembly

The swashplate consists of two main parts: a stationary swash plate and a rotating swash plate. The stationary (outer) swash plate 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) swash plate is mounted to the stationary swash plate 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 swash typically has an anti-rotation slider as well to prevent it from rotating. Both swash plates tilt up and down as one unit. The rotating swash plate is connected to the pitch horns by the pitch links. Alternative mechanics to the stationary (outer) swash plate 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 blade control

Cyclic controls are used to change a helicopter's roll and pitch. Push rods or hydraulic actuators tilt the outer swash in response to the pilot's commands. The swashplate moves in the intuitively expected direction - tilting forwards to tilt the rotor 'disc' forwards, for instance; but 'pitch links' on the blades transmit the pitch information ahead of the blade's actual position, giving the blades time to 'fly up' or 'fly down' to reach the desired position. This creates a difference of lift around the blades, and the helicopter will tilt towards the side with lower lift.

Collective blade control

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.

History

The swashplate was invented by Boris Yuryev [ [http://www.hronos.km.ru/biograf/bio_yu/yurev_bn.html Biography of Boris Nickolaevich Yuryev] (ru)] in 1910, a Russian scientist in the field of aerodynamics. On most modern aircraft the swashplate is above the transmission and the pushrods are visible outside the fuselage, but a few early designs - notably light helicopters built by Enstrom Helicopter - placed it underneath the transmission and enclosed the rotating pushrods inside the mainshaft. This reduces rotor hub drag since there are no exposed linkages.

References