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Lynx Type B Rate Gyro Unit

Technical Information

Catalogue No: C0314
Category: Gyro
Object Type: Sensor/Transducer
Object Name: Lynx Type B Rate Gyro Unit
Part No: 10-008-02EX1
Serial No: 006/70
Manufacturer: Elliott Bros (London) Ltd
Division: Flight Controls [FCD]
Platform(s): Lynx
Year of Manufacture: 1970
Width (mm):
Height (mm):
Depth (mm):
Weight (g):
Location: Rack RAA09 [Main Store]

Rate Gyro
Type No. 10-008-02EX1
Ref. No.
Ser. No. 006/70
Code No. K0656


This is a later prototype version two of which were fitted to the Lynx Helicopter
The Yaw Rate Gyro provides information for the Yaw Autostabiliser and a DC signal proportional to aircraft yaw rate; a Pitch Rate Gyro does similarly.

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

A rate gyro is a type of gyroscope, which rather than indicating direction, indicates the rate of change of angle with time. The gyro has only one gimbal ring, with consequently only two planes of freedom and is used to measure the rate of angular movement. Rate Gyros output a voltage that is proportional to the rate of turn about its sensitive axis. 

The original concept of this Rate Gyro was a design by Siemens dating from the late 1930s used in their K12 Autopilot. The concept was developed further in Germany during WWII where it formed part of the rudder control system on the Henschel Hs 129 and  in the ‘V2’ rocket weapon. After the War the design was brought to RAE Farnborough eventually being offered to industry, By 1953 Elliott Bros had developed the autostabiliser which was according to FLIGHT Magazine of 1953 necessary to correct the ‘snaking’ activity of certain high speed aircraft. This gyro was fitted near to the c of g of the aircraft and operated a remote actuator unit for the rudder servo tab via a control box. The design was further developed into the Type ‘B’ Autopilot.

Each Rate Gyroscope consists of a 1.5" hysteresis gyro motor mounted in a single gimbal to give a precessional torque which is proportional to the rate of instrument rotation about the sensitive axis. A multi-coil assembly is mounted on an arm from the gimbal ring, so positioned that the windings move axially in an annular permanent magnet when the gyro precesses. A second arm carries a fine wire pick-off making contact with a fixed winding which is fed from a 28VDC supply and can be set for correct zeroing of the instrument. The voltage between the centre-tap of the DC supply and the pick-offs fed to one of the coils so that interaction of the magnetic fields opposes the precessional torque; thus the gyro precesses against an "electrical spring”. Other moving coils (the gyro shown has three coils but some models only have two) can be used for transient velocity damping or for enforcing an artificial precession, or can be short-circuited for electromagnetic damping.

The voltage between the pick-off and the supply centre-tap is proportional to the magnitude and direction of the precessional torque, so it is also used as the output signal. If a large capacitor is wired between the pick-off and the spring coil the spring still resists precession, but the gimbal does not return to its "neutral" position when the precessional torque ceases With this treatment, the output signal from the pick-off is proportional to the integrated rate of rotation.

The RAE design was modified by Don Kierley of Elliott Bros but was made in Flight Controls Division not Gyro Division. Variants of the gyro were fitted to Elliotts' Flight Control System for the Jindivik in the late 1940s and subsequently to the Lightning (the Type "B") and the FIAT "Flying Bedstead" and although it was proposed for Concorde it was not actually used.

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