Lightning aircraft and components

By the early 1950’s all automatic flight control systems were designed to reduce to the absolute minimum the number of moving parts. Suitable transistors were not readily available until the mid-1950s and valves were not suitable for the military jet environment. So for a short period the Magnetic Amplifier came into prominence. These had been used during WWII largely by German designers of automatic flight controls. Post war higher permeability magnetic materials and new germanium and silicon diodes became available and gave a significant improvement in the reliability of the ‘Mag Amp’. The Elliott Mk 13 and subsequent automatic flight control systems installed in the English Electric Lightning are representative of such technology. These have magnetic "operational amplifiers" in which the majority of gearing adjustments in the computers are effected in the amplifier feedback loops. These also employed the newly available silicon diodes and ultimately the Lightning system was designed so as to be able to withstand a temperature environment limited only by the dissipation capability of the silicon junctions.

The Autostabiliser/Autopilot for the Lightning provided largely automatic control in any of a variety of flight modes. The system drove fast response electro-hydraulic actuators to give three-axis stabilisation. The details of the system were classified; but the ‘FLIGHT’ report of July 1961 gave some idea of the capability:

"A feature of the control panel of one of them was the use of mechanical interlocks between mode selectors to 'save panel space. A climb setting covers optimum climb performance under autopilot control and the inclusion of "track" and "glide" switches indicates automatic or near-automatic landing.

"Automatic throttle control is included. The main hand control is designed for use with the right hand outside the field of vision. Airpass has a controller for the left hand. It was stated that the autopilot hand controller could govern either extent or rate of turn, according to the mode in use. The barometric height lock is monitored, especially at low altitudes, by a radio altimeter to avoid excessive pitch demands.

"Another logical assumption is that the Elliott autopilot is also linked to the radar fire-control, to fly the Lightning on the correct trajectory to effect the most economical interception. All signals from the system are passed as demands to the autostabiliser actuators inserted in each control circuit. All four autostabiliser actuators (there are two in the aileron circuit, one in each wing) are Hobson electrically signalled, rotary hydraulic motors, with a linear output connected to the appropriate control system in such a manner that it moves the surface but not the cockpit control.

"Artificial feel is provided about all three axes. In the aileron circuit a simple torsion bar is inserted between the control column and the (normally fixed) aileron-trim drive, to provide feel directly proportional to stick deflection. In the rudder and tailplane systems, any control movement is resisted by a separate feel unit, in which deflection from neutral pushes a piston against hydraulic pressure governed by the feel simulator according to q (dynamic head) pressure, which varies with airspeed and altitude. These hydraulic feel units may be cancelled by a cockpit switch, and are automatically disengaged by a landing-gear DOWN selection. Further centring forces are provided by coil springs in each feel unit, which remain operative in the event of loss of hydraulic pressure or pitot/static differential, and a non-linear spring unit in the rudder circuit applies additional centring force and feel to the pedals.

"Trimming is effected from cockpit switches, the rudder having a double switch on the port console and the other surfaces a four-way thumb switch on the control column. Each switch controls an electric actuator with a linear output which displaces the complete control run; the aileron trimmer is attached to the control-column torsion bar and the other units are linked to the autostabiliser/feel assemblies in the rear fuselage.”

This was one of the early products from the Aviation Division established at Borehamwood in 1953/54 and eventually transferred to Rochester.

FARL demonstrated their Fly-by-Light system (in which fibre-optic cables are used instead of wires to pass signals between aircraft electronics boxes) at the 1980 SBAC show held at Farnborough. Following this they were approached by Airship Industries who were attempting to enter the military market, for which an airship is ideally suited for early warning, coastal surveillance and other long duration roles. The original system involved cables and pulleys and gave a heavy pilot workload as a result. Having realised that some form of electronic flight control system was required they ran into another problem that the airship was largely fabricated of non-conducting material and that the electrical cables would give dangerous lightning conductors.

FARL proposed the novel fibre-optic system which also included a highly innovative electrical actuator. The GEC Avionics design team was led by Consultants Staff Ellis and Peter Keay with Paul Buckingham, Trevor Hall, Dave Larner and Phil Lamb of FARL assisting. In January 1983 Marconi Avionics (FARL) signed a contract with Airship Industries Limited to design and build a prototype optically signalled FCS for the SKS600 airship. Delivery of the system was made in September 1983. The maiden flight was made on October 23 1988 fly-by-light by Airship Industries' chief test pilot Dave Burns at the company's US facility in Weeksville, North Carolina.

The major features of the FCS are:
● Fibre optic data transmission between the Flight Control Computer (FCC) and Actuator Drive Unit (ADU) to provide electrical isolation. Single fibres are to be used with data transmitted using Manchester encoding with MIL-STD-1553 format. Two independent supplies are provided.
● A duplicate system with pilot changeover in the event of a failure.
● An electrical actuator incorporating two dc servo motors with integral gear box The Actuator Drive Unit contains all the actuator electronics and fibre optic interface.
● A microprocessor-based Flight Control Computer.

The fibre optic cable used was a single optical fibre produced by GEC Optical Fibres Ltd and packaged by AEI Cables Ltd. Twelve fibre optic links are used within the Airship Flight Control System (FCS). These links handle all the data transmission between the Flight Control Computer (FCC) and Actuator Drive Units (ADU); the FCC being mounted in the gondola and the ADU are mounted within the surfaces on the rear of the Airship.

Eight of the twelve fibre optic links within the Airship FCS carry all surface position information from the FCC to the ADU and are therefore a vital part of the FCS. The remaining four fibre optic links transmit the status information from each ADU to the FCC for diagnosis.

The Air Data Transducer contains an aneroid capsule, or diaphragms, which expands and contracts with the pressure input from the Pitot tube. The case around the diaphragm is airtight and is vented to the static port. The difference between the pitot pressure and the static pressure is called dynamic pressure. The greater the dynamic pressure, the higher the airspeed.
The units have a circular alloy base 6.7cm diam. with four fixing feet protruding. Unit surmounted by Perspex/plastic cylindrical lid. A solenoid within a circular armature and pitot & static barometric connections are visible. The solenoid coil's motion is sensed by an electromagnetic rotary sensor (around the 6-toothed disc).
The current in the solenoid produces a counter-force against that produced by the pitot-static air pressure difference across the diaphragm attached to the coil. External electronics continuously controls the coil current to keep the rotary sensor signal in the middle of its range (probably its null point). With this balance achieved, the coil current is exactly balancing the air-pressure difference and therefore proportional to it. Suitably manipulating the coil current value yields airspeed.

These little units were used in the Autostabiliser on such aircraft as the Lightning and later the VC10.