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.
