Notes on

M Lambert

To describe an individual aircraft's electrical system and its specific components such as the Canberra wouldn’t be difficult but could at best be described as a bit boring. So to inject some interest in the subject I will attempt to go in to a few basic principles that had to be born in mind by the designers after the end of the last war when deciding on what to use and why in their new bomber design. Trying to get in to the mind set of the design office versus the production office with hindsight is an interesting exercise in itself anyway. I will not try to follow all the production changes and differing marks of aircraft as that would take a seriously long time but to give more of a flavour of the aircrafts early electrical system history.

Specifications given to the electrical design team would have included must have power for items of basic equipment, such things as radar, radio, nav aids, air conditioning, cameras etc all of these items have one thing in common they are heavy users of electrical power. Remember, no such things as transistors or processors existed, computers were electrically driven mechanical devices using at best, valve technology.

Turning generators is of course relatively easy to arrange from an ancillary drive gearbox with its input shaft taken from the engine, but design wise the gearbox itself has to be light, reliable and capable of transferring some impressive power at quite extreme temperature ranges. The port and starboard Canberra auxiliary drive gearboxes also provided the drive outputs for the hydraulic pumps.

Generators have to be built to take sustained usage at high altitudes and this was long before the advent of brushless generators that are now commonly available. Brush problems were a common cause of failure on early high altitude systems as a glaze would develop on the brush contact area preventing good surface contact between the brushes and the commutator slowly reducing the generators output. Different carbon composite compounds were gradually developed but it was never a completely solved problem. Brush replacement on an hours used basis was the only real solution. Forced air cooling from the inboard leading edge was employed with the used air outlet on the underside of the wing. It was very important to put in the blanking bungs on an after flight, as they provided excellent nesting areas for any local bird populace to use.

Leaning heavily on wartime experience with multi and twin engine driven systems EE had the additional problem of the Canberra being the first twin engine jet to require a very heavy electrical usage from an already quite power stretched power plant. The Meteor development at the time only required quite a small electrical generating capacity by comparison.

Early Canberra’s used a P3 dual generator system developed by EE giving a total capacity of 400 amps at 28 volts DC. Now that may seem a lot of power but it was soon used up and the generating capacity was reached very quickly so husbanding of the system became a necessity in flight. The lighter you could keep the loads the better the reliability of the system and the failure of one generator would mean an immediate end to a sortie.

The engine coupled gearbox would although geared down, run at a proportion of varying engine speed. This meant that some form of regulation of the output had to be employed to keep the voltage stable at 28 volts and within quite tight parameters of + or – 0.25 of a volt. Today this would be easily achievable with solid state devices but in 1949 the only form of regulating the quite high field current of a high output generator was by using a device called a carbon pile regulator. This relied on the property of carbon allowing more current to pass through it the more you compress it, so by electro magnetically altering the compression at the end of a series of carbon discs you could control the current it passed, to within quite fine limits. Each generator would have its own regulator as there could be a difference in engine speeds of the two systems for all sorts of operational reasons.

To be a little bit more specific, each generator on the P3 system was voltage controlled to 24 volts using two Type 23 regulators and then both Type 23 regulators were controlled by a single Type 32 regulator which was used to bring them both up to 28 volts in unison. This master slave system was designed to keep the load sharing as equal as possible between the two generators.

Later marks of aircraft had the system updated to what was called The Wide Speed Range Generator System. Basically, this was an improved generator with a Type 94 carbon pile regulator per side. The control system was also improved to ensure generators came on line reliably. The voltage regulators were positioned alongside the generators inside the wing leading edge instead of the starborad side hatch in an attempt to minimise the voltage drop in the wiring and to keep then cooler.

So we have established a basic 28 volt bus bar with up to 400 amps available, this 28 volts was available for all the trim actuators, fuel and hydraulic electrically operated valves and lighting etc.

There was however a problem, most radar and navigation (Gee) equipment was designed to run on 115volts at 400 cycles per second. The reasoning, without going in to a great deal of detail, behind this specification was quite simply to keep the weight of the radar internals as light as possible. Voltages of up to 10,000 volts were in regular use and had to be transformed up using steel cored transformers. So the higher the frequency employed meant a reduction in transformer core weight.

This AC had to be generated by a rotary device called an inverter. This was quite simply a 28 volt dc motor coupled to a 115 volt AC generator with the dc motor speed controlled to produce a frequency of 400 c/s all in the one case. Operating at a conversion efficiency of about 20% they consumed a lot of power in their own right before they even started supplying the avionic equipment. Most hard working inverters were designed to be force cooled from the air stream and overheated quite quickly if run fully loaded on the ground for more than a few minutes.

It generally worked out that the aircraft had separate inverters for each piece of equipment and a main and standby for the flight instruments. This wasn’t as bad as it sounds because if a piece of equipment was introduced such as the Tail Warning Radar (Orange Putter) which required 115 volts AC at 1800 c/s it could be catered for by its own inverter tailored for that equipment.

The pilot controlled flight instrument inverter Type 100A was of course the one that you never wanted to fail so it wasn’t by chance that the designers placed it in the pressurised cockpit in front of the rudder pedals. This kept the brush problems down and the temperature fairly constant for this inverter as it had to run faultlessly for the whole time. Quite noisy though and could be heard through the cloth helmet and bone dome. Oil pressure gauges were also fed power via the same source. This inverter was indeed a very reliable piece of equipment and became the standard instrument inverter for most 1950's aircraft both military and civilian. Later marks of Canberra were updated to carry a transistorised inverter for special requirements.

Now back to the basic design concept we can add in the valved radios to the power requirement loop, these operated from the 28 volt bus bar and had internal motor generators to provide the valves high tension requirements. This made them heavy constant current consumers that had to be supplied even if you lost a generator. Remember that losing an engine would automatically shut down the generator on that side as well, but even though you would have a lot on your mind if that happened, you or the Navigator had to remember to close down the radar invertors pretty quickly otherwise the other generator would fail soon after.

Batteries in the early Canberras were of a wartime design which could be used for inverted flight, the type was shared with many other aircraft of the day. Totalling four in the Canberra they were lead acid 12 volt 25 ampere hour. Wired in a series parallel arrangement to give 24 volts at 50 A/H. Enough for about 20 minutes of standby power if you shut everything off except the one radio you needed and two fuel pumps. Two 12 volt 4 amp/hour lead acid batteries fitted in the cockpit were provided as a standby supply which supplied the Turn and Slip indicator and could be utilised to blow the canopy and hatch detonator bolts if required. Last but not least a small 1.2 volt Nickel Cadmium battery was fitted just in front of the rudder pedals to supply a last ditch lighting circuit for the pilots stand-by compass and main panel. These batteries were there in case you had a total power failure at night, giving the pilot some 20 mins of turn and slip operation plus some panel lighting. The navigator would use the torch they always carried to try and assist with headings for the the E2A emergency compass. Not an enviable scenario this, compounded by the engines not being pressure fed from the pumps in the fuel tanks would mean their performance would have also have been reduced.

Later on in the Canberra’s life a great weight saving type K alkaline battery was introduced, also reducing considerably the risk of acid corrosion on the airframe.

Wiring in the early Canberra’s was based on the a type of wartime cable called Pren, a treated rubber covered multi-stranded cable coming in varying thicknesses depending on the current it had to carry. The outer sheathing of rubber was susceptible to oil impregnation and sun hardening.

But it was state of the art at the time and when protected it survives well to this day in many working airframes and cockpits.

Weight saving design led to the use of aluminium plug shells with aluminium sockets on most of the main wiring loom interconnecting plugs and sockets. This gave the electricians a hard time until the advent of a suitable silicone lubricant in the late 1950s. They were fine when left undisturbed but once removed for whatever reason the ability to re-mate a 26 pin plug successfully couldn’t be guaranteed. The infamous tailplane actuator was a 28 volt split field actuator designed for intermittent use that was directly coupled to the all-flying tail to provide in flight trimming of the tailplane. Initially controlled by a single pole switch on the right hand horn on the pilot's spectacle this was modified after accidents involving tailplane runaways to incorporate an additional isolation switch to control an isolation contactor. This extra safety feature was to prevent a tail plane run away should the direction switch fail in a closed condition. In addition to this the tailplane travel extremes were reduced to prevent complete loss of elevator control above certain Mach numbers should the tail plane actuator fail at one of its limits.

Fire warning around the engines was provided by Graviner detectors that relied on a metal barrel expanding under intense heat. This contained contacts that made and put on a warning light in the pilot's view. Action had to be taken by the pilot to discharge the extinguishers appropriately.

Rubber covered crash contact strips were located under the front and rear outside fuselage of the Photo Reconnaissance aircraft to provide immediate discharge of the explosion suppression system in the belly tank should a wheels up situation develop. This explosion suppression was achieved by having vertical sealed columns of a very rich fuel being explosively released inside the tank to upset the fuel to air ratio.

Crash switches located in the side hatches would if tripped during a crash, isolate the generators and discharge the fire extinguishers automatically.

That I think covers the basic electrical design. What has to be born in mind is that the Canberra had a very long service life with hundreds of different requirements having to be addressed. Surprisingly with time I imagine the actual power required from the generator system actually reduced. This would reflect the move away from rotary inverters valved radios and the introduction of solid state equipment. This in turn led to the main electrical generation system remaining almost the same as it left the manufacturer. I know from inspecting “The Blue One” that it is still fitted with its original P3 system, giving it a life of some 56 years. This was a system that during my apprenticeship in 1957 they were dropping from the syllabus as it was out of date. Without going in to a lot of detail on individual circuitry I hope I have given a flavour of the early Canberra electrical systems.

M. Lambert, 2008