At the end of 1939 George Edwards, manager of Weighbridge's Experimental Department, undertook the production of a specially modified Wellington for exploding German magnetic mines. The device was thought up, tested and put into service within three months.
A Wellington had suspended underneath it a 48ft diameter ring containing an aluminium electromagnetic coil for countering Hitler's first secret weapon, the magnetic sea mine that had become a serious menace in coastal and estuary waters.
The DWI project, as it became known, required that every night, photographs of progress made on the experimental aircraft during the day were sent to Winston Churchill then First Lord of the Admiralty.
Christmas 1939 was the time of peak effort, preparing the aircraft for its first flight. The Experimental Department produced an effective countermeasure to the mine without undue risk to the aircraft. The aerodynamics turned out to be perfectly satisfactory.
After the recovery of the first intact magnetic mine the examination of its mechanism and devising of an antidote were undertaken by the RAE and the Electromagnetics Group of the Admiralty Research Laboratory. Two methods were devised by the Admiralty, the ring mounted on an aircraft to detonate the mines and the degaussing system for ships.
The aircraft ring was codenamed Directional Wireless Installation to confuse the enemy. It was known in the workshops as "Down With 'Itler". It was designed and made at Weybridge, and by 20th November 1939, a start had been made on a turntable for making the coil. The coil was made from aluminium strips, 2in wide. This was wound into the coil on the turntable loke a clock spring, along with paper insulation. A balsa-wood fairing, bound with tape, provided a drag reducing casing. The ring mounted on a Wellington was decided as the result of wind tunnel tests. Aerodynamic troubles were anticipated, so arrangements were made for the ring casing to be made in sections for transporting, at first to Chester, for assembly and testing. The first aerodynamics test flght was to be made with the casing empty, and then it was flown to Weybridge for the coil to be installed. It was considered unsafe to make the first flight from Brooklands.
The first Wellington modified was P2516, a Mk IA straight of the Weybridge line. Air Marshal Sir Wilfrid Freeman, Air Member for Development and Production, would only agree to the diversion of one Mk IA from urgently needed production.
The Wellington installation was such that the magnetic field generated in the ring could be focused forward or aft of the midships line by adjusting the angle relative to the aircraft's for-and-aft datum. The detonation of a mine would be caused at, or immediately following, the peak effect of the magnetic field, and the incidental effect, if any, upon the aircraft then depended upon the height and speed. Height had to be low enough to ensure triggering the mine mechanism but not low enough to detonate the mine in the path of the aircraft. Speed had not to be so fast that the magnetic field peaked for too short a period for the mechanism to operate, yet fast enough for the detonation and splash to miss the tail. Regarding the angle of incidence for the ring in flight, the ring would contribute a significant amount of lift. The trim of the aircraft/ring combination required careful attention to ensure level flight and the correct location of the magnetic field. The coil was energised by a Ford V8 engine driving a 35kW (310A) Maudslay generator. Pierson preferred a solution using a Gipsy Six aero-engine driving a 95kW generator. The coil was cooled by air circulated within the casing from an intake in front, with outlets at the sides and rear.
On 13th December, Air Marshals Tedder and Sholto Douglas agreed that three conversions from the Mk IA should be made off the production line, and that subsequent conversions would be installed by Rollason at Croydon, the ring being made there by English Electric.
By 19th December the first coil was finished at Weybridge. The casing had been proof-loaded and sent to Boscombe Down the same day. The aircraft was test flown and taken to Boscombe Down to have the casing fitted. At Farnborough on 20th December it was proposed that Handley Page should make the ring cases for future conversions for fitting by Rollason. Some very early Wellingtons were available, and a trial Gipsy Six/95kW generator combination installed in one of these. A slightly smaller and lighter ring gave a 50% greater magnetic field saving 1,000lb.
The next day, P2516 was flown light to Boscombe and fitted with its ring casing. Tested by Mutt Summers, it ws pronounced satisfactory and he flew it back to Weybridge for the ring to be installed. Despite the high pressure for results no undue risks were to be taken. Summers and Wallis were anxious that the wind at Brooklands should be in the best direction before flying off. On 27th December the Wellington took off and returned to Boscombe for the Ford engine/genitor set to be fitted. The delivery flight was made by Summers flying solo.
The installation was completed on 28th December but bad weather precluded full-load trials the next day. On the 30th P2516 was flown by Summers. The Ford engine was not adequately cooled. R.C. Bob Handasyde, then a flight test observer and soon to become a Vickers test pilot described in an interview what it was like to work in the stiflingly hot atmosphere of the Wellington's fuselage during initial tests.
"I was in charge of works at the back, which consisted of the Ford V8 engine driving the dynamo to provide the current. We experimented with the first mine ever discovered by the naval boys off the Medway. It was sticking up in the mud when the tide went out, and they defused it using rubber spanners or something. They took the magnetic part out and sent it down to Boscombe. There they stuck it out in the middle of the aerodrome. The DWI pilot was Bruin Purvis, and we flew for about two hours, flying directly over the thing at 10ft, 20ft 30ft, etc, then so may yards either side of it. Every time we flew past it the magnetic thing clicked. The experts at Boscombe were able to get a beautiful polar diagram until we were to far out for it to dip. At thet particular time at Boscombe, January 1940, it was the coldest they had had for may years, something like -20C. Inside the Wimpy the temperature was about 120F I had to work stripped to the waist, sweating hard; it was most uncomfortable putting on an Irvin jacket afterwards. When they took the first one down to Manston for proper trials, they actually blew up a mine on the first sortie. They kept very low and it was pretty exciting, sitting down at the back of the Wimpy seeing the water suddenly heave up."
Maurice Hare was one of the Weybridge test pilots at the time of the DWI development, before testing Wellingtons at the new Chester factory.
"It was very normal to fly. I felt rather apprehensive at the sight of that awful great thing surrounding the cockpit. It was enormous. Otherwise, apart from being limited in speed, it handled perfectly normally, but it was a weird feeling with this thing (it seemed) wingtip to wingtip and well out beyond the nose. I well recall the rush to get it ready, a really intensive effort to get it out in time over Christmas 1939. I also remember wrecking my very valuable wristwatch by being too near the thing when it was switched on in the works. The magnetism completely wrecked the watch."
Trevor Westbrook found that cooling could be improved by using air ducts. On 30th December the RAE advised Vickers that Tedder had agreed that four Mk IAs were to be converted at Weybridge. Obsolete Mk Is were to be used for the conversions, by Rollason, using locally made English Electric coils. On 8th January 1940, Boscombe informed the Vickers board "Summers says it works".
Design work for subsequent conversions, with smaller rings, was Vickers reasonability. Farnborough undertook the major modifications to the Welligton Is before flying them to Croydon for assembly of the ring and powerplant. Vickers kept its own assembly rig until four Mk IAs had been completed. These were to be known as DWI Mk Is (Type 418) and the higher-powered, smaller ring installations DWI Mk II (Type 419).
By 15th January Weybridges second aircraft was ready to fly from Boscombe Down to Manston for trials in earnest over the Thames Estuary. A week later No 3 had passed flight test and the coil was being tested before fitting to the casing., it being delivered to Boscombe on 25th January and completed two days later. The forth followed soon after. The 22nd of February was a big day for all those that had been involved in this hectic programme. Three DWI Wellingtons were flying that day, and two mines were exploded quite safely. Early in March the DWI had become routine for Vickers. On 2nd March Summers flight tested the first of the Wellington Mk Is modified by Rollason.
Ten more were to be modified to DWI Mk II standard, one of which was flown to the Middle East, where several others were similarly converted. There they cleared magnetic mines from the Suez Canal and North African harbours. They flew satisfactory and the Weybridge Experimental Department had the doubtful satisfaction of seeing photographs not only of their own "baby" at work, but also of a German version doing similar work clearing Allied mines. With the availability of suitable degaussing coils for ships, the DWI was superseded in British coastal waters, but continued its work in the Middle East for some time.
This work gave rise to another development which was to revolutionise submarine warfare. It was called DWI Mk III, but this title was a disguise. It was for avery different purpose: the Leigh light.