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Rotary Actuator

Technical Information

Catalogue No: C1163
Category: Flight Control
Object Type: Actuator
Object Name: Rotary Actuator
Part No: AE4742MK1
Serial No: 0444
Manufacturer: Lucas Aerospace/Marconi Avionics
Division: Flight Controls [FCD]
Platform(s): Lynx
Year of Manufacture: circa 1980
Width (mm):
Height (mm):
Depth (mm):
Weight (g):
Location: Rack RAA02 [Main Store]

Lucas Aerospace Ltd.
Bradford, England.
Rotary Actuator
Type AE4742MK1
Serial No 0444
Stroke 60°
Load 44 lbf
A.C. Motors - Volts 115 Phase 1
Amps .15 Cycles 400
D.C. Clutch 25V
Potentiometer Volts +12.0, -12
Customers Code No 60-022-01
N.A.T.O. Code 6TU/6615-99-637-0008
Modifications Incorporated 1, 2 & 3
[white ink markings]
Mod ZY648
E.B.L.(T) Apr. 91.
GAv(R) LCSD 31


This is a Mk1 version of the Rotary Actuator

Linear Actuators: On aircraft linear actuators may be used for flight control surfaces, wing flaps, and spoilers. Hydraulics deliver a great deal of power without taking up too much space or weight, meant that they massively help pilots in undertaking necessary mechanical tasks without using too much energy.

The other advantages of hydraulics are the quick response to the demands that may be placed upon the system. They are also reliable and reasonably easy to maintain. Because they do not use electricity, there is no chance of a shock hazard, and the chances of being a fire hazard are low, which makes them a safer option than other equivalent systems.

Perhaps the main advantage of a hydraulic system is that they can handle a practically unlimited amount of work due to the immense force they can produce. This is all the more important in modern day aircraft.

Hydraulic fluid is pumped from a reservoir, either by an electric, or engine driven pump. It is filtered to keep it clean, and then passes through a selector valve, which relieves extra pressure.

Once it reaches the linear actuator, the fluid power is turned into work by a piston. This power is then used to move an aircraft system or flight control. These actuators can be either single or double acting, depending on the requirements of the system, meaning that the fluid can be applied to one or both sides of the actuator. The selector valve allows for the fluid direction to be controlled, which is necessary for operations such requiring extension and retraction.

The actuating cylinder also contains a reduction gear, which allows it to control the rotating motion to what is needed within the system. Previously, systems would use steel cables connected by pulleys to control motion. The cables that connected the controlling mechanism, such as the pedals, to the controlled surface, such as the rudder, would be subject to expansion due to temperature changes. Hydraulic systems though, are capable of controlling motion without worrying about such concerns, as they do not operate in an environment that is not open to the atmosphere. This means that hydraulic systems not only provide better control for the pilots using them, but also increase response times, making them an imperative part of aerospace engineering.


Rotary Actuators:

Electric rotary actuators:

Stepper motors are a form of electric motor that has the ability to move in discrete steps of a fixed size. This can be used either to produce continuous rotation at a controlled speed or to move by a controlled angular amount. If the stepper is combined with either a position encoder or at least a single datum sensor at the zero position, it is possible to move the motor to any angular position and so to act as a rotary actuator.


A servomotor is a packaged combination of several components: a motor (usually electric, although fluid power motors may also be used), a gear train to reduce the many rotations of the motor to a higher torque rotation, a position encoder that identifies the position of the output shaft and an inbuilt control system. The input control signal to the servo indicates the desired output position. Any difference between the position commanded and the position of the encoder gives rise to an error signal that causes the motor and geartrain to rotate until the encoder reflects a position matching that commanded.

Fluid power rotary actuators:

Hydraulic power may be used to drive an actuator, usually the larger and more powerful types. Fluid power actuators are of two common forms: those where a linear piston and cylinder mechanism is geared to produce rotation (rack and pinion), and those where a rotating asymmetrical vane swings through a cylinder of two different radii. The differential pressure between the two sides of the vane gives rise to an unbalanced force and thus a torque on the output shaft. Vane actuators require a number of sliding seals and the joints between these seals have tended to cause more problems with leakage than for the piston and cylinder type. The three most commonly used types are, vane and helical.

Simple planetary actuators are most commonly used for commercial leading edge slat applications while compound differential planetary actuators offer higher ratios for torque multiplication and are most commonly used for trailing edge flap designs.


Marconi-Elliott Avionics of Rochester designed and produced a modular AFCS, under subcontract to Westland Helicopters, for the utility, Royal Navy and French Navy  versions of the Lynx. A modular, or building-block, system is needed because the aircraft has to operate in several roles.  The auto-stabilisation system is common to all three variants, with a range of auto-pilot facilities being added to meet specific requirements. The basic version, as fitted to the utility Lynx, has holds for airspeed, heading and barometric altitude.  In the Royal Navy aircraft these have been supplemented by a radio altitude hold, and in the French naval variant automatic transition between cruise and hover) and sonar cable hold (operating in both angle and height) are fitted. The heart of the Marconi-Elliott AFCS is the autostabiliser system, which provides attitude stabilisation in pitch and roll. Demands are fed to actuators which in turn drive the main powered flying controls.  In the yaw axis the equipment operates as a rate-damping system to stabilise the helicopter during both manual and automatic flight. All autostabilisation channels are fully duplicated, with monitoring circuits to indicate any discrepancy between the two lanes.  The auto-stabiliser is active throughout the flight envelope, including operation under autopilot control. A unique feature is a simple control loop to provide additional pitch stability by sensing normal acceleration and applying a collective pitch demand to counter the pitch rate divergence and instability induced at high forward speeds with an aft centre of gravity. This system—collective acceleration control (CAC)—has also reduced pitch stick activity, and hence workload, particularly in turbu-lence and during manoeuvring.  The autopilot modes of barometric altitude, radio altitude, radio altitude acquire, heading hold and airspeed hold operate individually or together. For example, barometric altitude, heading and airspeed hold can be used at the same time. Transition and sonar hover require simultaneous control in pitch, roll and collective axes, and the heading-hold mode would normally also be engaged. All autopilot control signals are fed to limited-authority series-mounted actuators and also to full-authority paralleled actuators, which operate to prevent the series units from becoming saturated with control demands during manoeuvres under autopilot control.  All autopilot channels are duplicated in the collective axis, but are simplexed in pitch, roll and yaw.

The AFCS consists of the computer AFCS, computer acceleration control, pilot’s controller, test controller, sonar transition controller, parallel actuators, yaw-rate gyros and stick-position transmitters. Overall system weight varies between 30lb and 50lb, depending on the variant. Mean time between failure (MTBF) is quoted in excess of l.000hr and built-in test equipment facilitates first- and second-line fault diagnosis.

Design work on the Marconi-Elliott AFCS began in late 1969 and the first equipment, comprising the computer acceleration control, was aboard the prototype Lynx on its maiden flight.  Basic development work had been completed by March 1972, when transition-mode equipment was ready for flight-testing.  

The British Army ordered over 100 Westland Aerospatiale Lynx for a variety of roles, from tactical transport to armed escort, antitank warfare , reconnaissance and casualty evacuation. The Lynx AH.1 first entered service with the Army Air Corps (AAC) in 1979. The Mk34 AFCS is fitted to the Army variant.

Four Rotary Actuators form part of the Mk34 AFCS system which provides attitude stabilization in the pitch and roll axes. Stabilization demands are fed to limited authority series actuators which in turn drive the main powered flying controls. In the yaw axis the system operates as a yaw rate damping system to stabilise the aircraft during manual and automatic flight. The Rotary Actuators provide a rotary output of nominally ±60deg for autopilot functions. They are controlled directly from the pilot’s stick switch in pitch and roll for trim adjustments in those axes.

See FLIGHT International, 8 November 1973

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