Introduction of Alternators as the primary power source
E.G. Bowen

Towards the end of 1938 we could foresee that AI and ASV would soon come into service in Fighter and Coastal Command aircraft and the problem of how to supply them with electrical power became an important consideration. The standard RAF generator at that time was a 500 watt, 24 volt DC machine and no other type was contemplated. Most of its output was taken up by the existing aircraft services and there was no power to spare to run the set, however economical. The radar equipment was going to require such a variety of voltages that an alternating supply was essential. A way out of the problem existed on two-engine aircraft where it was customary to fit only one DC generator, leaving a spare drive-shaft on the other engine. Adding a second DC generator and using this to drive an alternator was one possibility, but that would be less than 50 per cent efficient and we needed all of the 500 Watts. So there was no option but to go to an engine driven alternator. The problem was how to get one into service.

An Air Ministry Committee had been sitting on the question of alternating current supplies for years without getting anywhere. They regarded it as an either/or problem – either one used a DC system or an AC system. The forces in favour of one or the other were so evenly balanced that there was a deadlock and no effective action was possible. I discussed the problem with Watson Watt who advocated the direct approach to a manufacturer. I made enquiries which indicted that Metropolitan-Vickers (Metro-Vickers) in Sheffield could probably make the alternator we required. I made an appointment to see the Managing Director, Mr. Fletcher. As time was short, I decided to fly up to Sheffield in one of our Battles towards the end of October 1938. Sergeant Wright was the pilot and we landed at the nearly airfield of Finningley. It happened that we did not have a spare DC generator back at Martlesham, so that after landing I took a spanner, whipped the generator of the Battle and took it hot-foot to the Metro-Vickers works. I laid it on the Managing Directors table and outlined what we wanted – an alternator, not a DC generator, looking exactly like the one on the table, occupying the same space and using the same fixing bolts and spline shaft. We could accept any voltage within reason, but 100 volts was a good figure to shoot for. We knew that with changing engine revs we would have to tolerate something like a two to one variation in frequency, but this was no problem. Also, for economy in transformer iron, a high, rather than a low, frequency was called for, say 500 or 1000 cycles. Needless to say, as engine revs changed the voltage had to remain more or less constant. Lastly, our total output power requirement was certainly not less than 500 watts but anything over that figure would be very welcome. What could be done?

He called in Tustin, his Chief Engineer (who later became professor of Electrical Engineering at Imperial College), and put the problem to him. Tustin went away to do some calculations. After an excellent lunch in the Managing director’s dining room we gathered again and Tustin was very confident about meeting our requirements. He said that the preferred voltage was 80, the frequency range would be 1200 to 2400 cycles and he could be reasonably certain that the machine would deliver 800 watts! The weight would be about the same as the DC machine and he saw no problem in matching the fixing bolts and the spline shaft. Nothing could be better and we settled for that specification on the spot. These were the figures which were used throughout the war and did not change until a long time after the war was ended. Production of the machines presented few problems and Metro-Vickers thought that, if pressed, they could produce the first models in one month. I promised to get the appropriate papers to hem in a matter of days.

I returned to Finningley with the generator under my arm. By this time it was getting late in the day and we did not have time to reinstall the generator in the Battle. We started up on a battery cart and decided we could do the return trip to Martlesham on the aircraft accumulator. The Station Commander was there to see us off and he expressed concern about the failing light. Dusk was already beginning to fall. We said not to worry but asked him telephone martlesham Heath to say that it might be dark when we arrived. There would be no landing lights and no moon, but we would feel our way in. We took off for home, not exactly singing and dancing but reasonably confident we would make it. As we proceeded south, the cloud base got lower and lower and it was this which eventually was our undoing. Navigation was difficult in the poor light and we soon realized we were south of our intended track. We were down to about 800 feet in the Fen country north of Newmarket when quite suddenly we flew through a huge flock of wild geese and hit one of them with a resounding thump. It must have been a large bird because it swung the aircraft appreciably and made a considerable dent in the leading edge about six feet in from the wing tip. It was nearly dark and this shook us up a little. Sergeant Wright decided to make an immediate landing. There was flat, well ploughed land beneath, so he picked a good long paddock and put down. He made an excellent landing and we were very content to have our feet back on the ground. There was a small village nearby and the accepted drill in a case like that was to telephone the nearest airfield for assistance. Sergeant Wright and I pooled our money to pay ofr a telephone call. Our combined financial recourses reached a total of one shilling and fourpence halfpenny. I stayed to guard the aircraft and Sergeant Wright headed for the village clutching the money in his hand.

The village turend out to be Barrow, just south of Newmarket-Bury St Edmunds road, and the nearest RAF station was Mildenhall, where the duty officer reacted promptly and sent a guard and transport. It transpired that we must have flown right over Mildenhall a few minutes before landing and we were extremely unlucky not to have seen it in the failing light. In no time at all, the aircraft was surrounded by sightseers from the village; some kindly souls were good enough to bring piping hot tea and buns and even blankets, which were very welcome in the chill night air. Then came the Air Force and transport back to Mildenhall, where we put up for the night.

The plan was that Sergeant Wright would fly the Battle out in the morning, while I would head for London to arrange for orders to be placed for the alternators. The station adjutant gave me a vouvjer for the train journey and in due course I arrived at King’s Cross. From King’s Cross to the Air Ministry in Kingsway was a journey of several miles and I did not have a penny to my name. It was a very pleasant morning and I walked. I reported to Watson Watt, told him the good news from Metro-Vickers and orders for eighteen engine-driven alternators were placed immediately. I then borrowed £ 5 from the great man and made my way happily back to Bawdsey.

A model of the alternator was running successfully on the bench within a few weeks and the first deliveries were made within a month. They more than met our expectations and we put them into immediate use. We never had to worry about power problems again. The order for the first eighteen alternators was completed before the outbreak of the war and a repeat order for 400 was made in October 1939. Without this machine, airborne radar simply would never have been possible, or at best would have been severely handicapped. The alternator was made in hundreds of thousands during the war – the total British production reaching a figure of 133800 units. The alternator was adopted by all the Allied Air Forces, including the Americans. They finished up using two standards, a 400 cycle alternator to drive US-produced equipment and the British alternator on the many occasions on which they carried British airborne gear.