With the clutch engaged the anchor point (plate 1) is held by the irreversible wormscrew reducer. When the pilot moves the control, the output shaft, shear pin and plate 2 move against the force law spring: this gives the break-out force and the force–displacement gradient. Beyond the detent the force law switches send the override information to the APMs. The RVDTs report the control position. Release the control and the spring brings it back to the anchor point.
The Trim Release switch on the cyclic stick or collective lever (or the permanent trim release button on the AFCAU) declutches the trim actuator: the anchorage is removed and the pilot can change the anchor point without any artificial feel. The damping device limits the displacement speed. When the button is released the clutch engages again and the flight controls anchor on the new position; the AFCSs synchronise their memories.
The DC motor, fed with 28 V pulses from an APM, drives the wormscrew reducer, the clutch, the damper and the reduction gear: the anchor point moves at 3°/s. Through the spring the control follows – this is the beep trim commanded by the pilot, or the auto-trim function that keeps the series actuators around their centre position. If the pilot holds the control, the spring deflects, the force law switches give the override information and the APM stops driving.
If the mechanism jams, the pilot keeps full authority: when the force on the control exceeds the shear pin limit the pin shears and the output shaft is freed from the plates. At all times the pilot has control priority, with total control capability.
Yaw: the directional trim actuator is of the friction type – a hysteresis clutch with a rotor and a stator equipped with permanent magnets north and south and the ring gear replaces the mechanical force feel. Otherwise its mode of operation is the same as the pitch, roll and collective actuators.
1 RVDTs – position of the output lever
2 Double microswitch – force law switches
3 DC (stepping) motor
4 1st stage reduction – wormscrew
5 Magnetic clutch
6 Damper
7 2nd stage reduction gear
8 Plate 1 (anchor side)
9 Force law spring
10 Plate 2 (control side)
11 Shear pin
12 Output shaft