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Autostabiliser Computer

Technical Information

Catalogue No: C0547
Category: Flight Control
Object Type: Signal/Data Processor
Object Name: Autostabiliser Computer
Part No: 74D3027-A-1
Serial No: 003/67
Manufacturer: Elliott Bros
Division: Unknown
Platform(s): Harrier
Year of Manufacture: 1967
Dimensions:
Width (mm):
143 
Height (mm):
88 
Depth (mm):
352 
Weight (g):
3,160 
Location: Rack RAA01 [Main Store]
Inscription(s):

Elliott
Autostab Computer
Type No. 74D3027-A-1
Ref No.
Ser No. 003/67

Notes:

This unit was probably one of those used on the Failure Survival Rig in the Hydraulics Lab, for the development of the system, rather than on the actual aircraft.

The given depth excludes the eight 300mm long cables.

In October 1960 Hawkers were ready to fly the first P1127. It was fitted with an Elliotts 20% authority autostabiliser giving artificial damping and stiffness in pitch and roll. The ailerons and tailplane were powered, the rudder was not.

In the spring of 1965 a new contract was obtained, for six development batch aircraft, of a type that was to be known as "Harrier". The Harrier aircraft went into service as an operational type four years later in April 1969. In October 1960 Hawkers were ready to fly the first P1127. It was fitted with an Elliotts 20% authority autostabiliser giving artificial damping and stiffness in pitch and roll. The ailerons and tailplane were powered the rudder was not. In the spring of 1965 a new contract was obtained, for six development batch aircraft, of a type that was to be known as Harrier. The Harrier aircraft went into service as an operational type four years later in April 1969.

No conventional  autopilot was fitted. The type of role for which the aircraft was designed (immediate close support), with its short-duration sorties does not justify such a system, and its absence means that the pilot needs to fly the aircraft all the time. The RAF however, specified a non-duplicated auto-stabilisation system for pitch and roll, not because the aircraft had undesirable handling characteristics but purely as a pilot aid.  Both roll and pitch systems gave short-term damping combined with attitude hold making for easier control in poor weather.

The Harrier auto stabilisation system was designed by Elliotts. Attitude control in the hover was by a jet reaction control system. With the nozzles deflected, high-pressure air was continually bled from the engine to supply roll and pitch nozzles at the nose, tail and wing tips. Pitch control was obtained by increasing the aperture of one of the pitch valves and reducing the size of the other - the total lift force on the aircraft remaining constant. Roll control used differential control of the roll jets. Yaw control came from swinging the pitch jets laterally. The control valves were operated by the stick and rudder pedals in the conventional sense.

Harriers were first delivered to the RAF at Dunsfold in January 1969 when the Harrier Conversion Unit (HCU) was formed.

Autopilot

The basic function of the autopilot is to control the flight of the aircraft and maintain it on a pre-determined path in space without any action being required by the pilot. (Once the pilot has selected the appropriate control mode(s) of the autopilot.) The autopilot can thus relieve the pilot from the fatigue and tedium of having to maintain continuous control of the aircraft’s flight path on a long duration flight. The pilot is thus free to concentrate on other tasks and the management of the mission.

A well-designed autopilot system which is properly integrated with the aircraft flight control system can achieve a faster response and maintain a more precise flight path than the pilot. Even more important, the autopilot response is always consistent whereas a pilot’s response can be affected by fatigue and work load and stress. The autopilot is thus able to provide a very precise control of the aircraft’s flight path for such applications as fully automatic landing in very poor, or even zero visibility conditions. In the case of a military strike aircraft, the autopilot in conjunction with a T/F guidance system can provide an all-weather automatic terrain following capability. This enables the aircraft to fly at high speed (around 600 knots) at very low altitude (200 ft or less) automatically following the terrain profile to stay below the radar horizon of enemy radars. Maximum advantage of terrain screening can be taken to minimise the risk of detection and alerting the enemy’s defences.

The basic loop through which the autopilot controls the aircraft’s flight path is by means of an inner and outer loop. The autopilot exercises a guidance function in the outer loop and generates commands to the inner flight control loop. These commands are generally attitude commands which operate the aircraft’s control surfaces through a closed loop control system so that the aircraft rotates about the pitch and roll axes until the measured pitch and bank angles are equal to the commanded angles. The changes in the aircraft’s pitch and bank angles then cause the aircraft flight path to change through the flight path kinematics.

Autopilots in modern complex aircraft are three-axis and generally divide a flight into taxi, take off, climb, cruise (level flight), descent, approach, and landing phases. Some Autopilots can automate all of these flight phases except taxi and take off. An autopilot-controlled landing on a runway and controlling the aircraft on rollout (i.e. keeping it on the centre of the runway) is known as a CAT IIIb landing or Autoland and is available on many major airports' runways today, especially at airports subject to adverse weather phenomena such as fog. Landing, rollout, and taxi control to the aircraft parking position is known as CAT IIIc.
An autopilot is often an integral component of a Flight Management System
An autopilot takes the aircraft's position and attitude from an inertial guidance system and then controls a Flight Control System to guide the aircraft. In such a system, besides classic flight controls, many autopilots incorporate thrust control capabilities that can control throttles to optimize the airspeed, and move fuel to different tanks to balance the aircraft in an optimal attitude in the air.

Autostabiliser

There may be a need for improved damping and stability about all three axes. This can be achieved by an auto-stabilisation system, or, as it is sometimes referred to, a stability augmentation system.

Yaw auto-stabilisation systems are required in most jet aircraft to suppress the lightly damped short period yawing motion and the accompanying oscillatory roll motion due to yaw/roll cross coupling known as Dutch roll motion which can occur over parts of the flight envelope. In the case of military aircraft, the yaw damper system may be essential to give a steady weapon aiming platform as the pilot is generally unable to control the short period yawing motion and can in fact get out of phase and make the situation worse.

A yaw damper system is an essential system in most civil jet aircraft as the undamped short period motion could cause considerable passenger discomfort.

A yaw damper system may be insufficient with some aircraft with large wing sweepback to suppress the effects of the yaw/roll cross coupling and a roll damper (or roll auto-stabilisation) system may also be necessary. The possible low damping of the short period pitch response  can also require the installation of a pitch damper (or pitch auto-stabilisation) system. Hence, three axis auto-stabilisation systems are installed in most high-performance military jet aircraft and very many civil jet aircraft.

Background for Elliott Bros in Autopilots

In the 1950s Smiths Industries  was the traditional supplier of autopilots for transport and bomber aircraft but the RAE [Farnborough | with the UK government wanted an alternative supplier to tackle the new supersonic combat aircraft field. Elliott Brothers (London) Ltd. (at Borehamwood), with its long history of making aircraft instruments, and another company, Louis Newmark Ltd., were asked to go into competition for the honour, by bidding for the two systems required for a 3-axis autostabiliser and the MK13 autopilot. During 1954 Elliott was chosen for both contracts and Newmark was directed into the helicopter controls business. Initial projects involved the development of analogue autopilots for unmanned experimental aircraft and an analogue 3-axis autostabilisation system for a Mach 2 fighter under the official requirement F23/49, which was to become the English Electric Lightning. By 1960 this activity had expanded to cover the autopilots for both the Lightning and Buccaneer transonic strike aircraft as well as complementary development of analogue air data systems for these aircraft.

Most importantly work had begun, also under Ron Howard’s leadership, of a dual channel fail-operative autopilot/automatic landing system for the Vickers VC'10 long-range jet airliner. In 1960 Elliott’s expertise in advanced flight control systems in the UK was recognised by the award of the triplex automatic terrain following autopilot and autostabliser contract for the Mach 2-plus TSR 2 strike aircraft. These two systems, along with the earlier Lightning system, can be considered as the foundation stones of much of what was to follow in Flight Control system developments at Elliott’s.

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