Classic Aircraft profiled every issue -

Issue 10 - de Havilland Mosquito: Mosquito Prototypes and Testing

In fact the hall is no stranger to fast ladies, either, for in the 17th century KingCharles II’s mistress Nell Gwynne had a cottage nearby. The hall’s very large kitchen became a workshop, and a hangar was constructed abutting the building.

The initial DH.98 design was presented to the Air Ministry in September 1938. For the fighter and fighter-bomber rôles envisaged, there was provision for 20mm cannon to be installed under the nose. Development was hampered by continued Air Ministry interference, trying to impose defensive armament (a mock-up was in fact built featuring two rearward firing machine guns) and provision for torpedoes in the maritime strike role, but de Havilland stood firm in the face of repeated rejections. Aerodynamic purity would not be sacrificed, though even Air Marshal Freeman was not entirely convinced that the type would outperform a Spitfire or German fighters that might be introduced in the by then inevitable war.

Finally, in November 1938, an order for a development prototype was granted, but priority was to be given to the production of Tiger Moth trainers, Rapide light transports (which the RAF called Dominies) and essential variable-pitch airscrews for several other manufacturers. The company was also sub-contracted to produce Airspeed Oxfords. The Air Ministry still interfered occasionally, suggesting that the more powerful Griffon engine might permit installation of a four-gun turret without compromising the top speed. The possibilty was investigated by de Havilland, but as neither Griffons nor turrets were available, only mock-ups were built. The DH.98 was still considered purely a development type.

On December 12, 1939, three months into the war, a prototype was at last ordered, but as Bomber Command chief Sir Edgar Ludlow-Hewitt did not favour an unarmed bomber, it was to be a photo-reconnaissance type under Specification B.1/40, which was duly issued. Then, on the first day of 1940, Air Marshal Freeman ordered a single prototype of what Geoffrey de Havilland called “the fastest bomber in the world”, to have a maximum speed of 397mph (639kmh) at 23,700 feet (7723.7m) and to cruise at 327mph (526kmh) at 26,600 feet (8100m). Range was to be 1,480 (2380m) at 24,900 feet (7600m), and ceiling 32,100 feet (9800m). The order was increased to 50 aircraft, including the prototype, on March 1, 1940.

The Dunkirk emergency put a hold on detail design and construction as Merlin engines were needed for fighters, and none could be spared for prototype construction. Priority was restored only when Lord Beaverbrook, Minister of Aircraft Production, was convinced that enough Merlins would be available. The team worked through the Battle of Britain, losing much time to air raids that required them to retreat to the shelters, but E-0234, the bomber/reconnaissance prototype fitted with Merlin 21s and painted overall in yellow, was rolled out on November 19, 1940, just 10 months and 26 days after its inception. It had been transferred by road to a shed with a 450-yard air strip adjacent to the Hatfield factory. Taxi tests on November 24 were followed next day by the first flight, piloted by Geoffrey de Havilland, Jr, accompanied by John Walker, designer of the engine installation.

The Mosquito performed just as expected, reaching a speed of 220mph (350 kmh). Minor problems were the failure of the main gear doors, tension sprung with bungee cords, to close fully, and of the tailwheel to castor properly. The latter, persisting for some time and eventually leading to a fuselage breakage the following February at the Aeroplane and Armament Experimental Establishment, Boscombe Down, necessitated the hurried replacement by the fuselage of the photo-reconnaissance prototype W4051. It was cured in June-July 1941 by the substitution of a Dowty unit. By early December 1940 the aircraft had received its RAF serial number, W4050, and in January it proved faster than a Spitfire at 6000 feet (1800m).  As the flight envelope was extended, buffeting was encountered due to turbulent airflow from the nacelles striking the tailplane. After various modifications were tried, the buffeting was cured by extending the nacelles aft of the wing trailing edge with fairings which required the flaps to be split.

Despite the accident at Boscombe Down, the test pilots there were very enthusiastic. W4050 had achieved 388mph (624kmh) at 22,000 feet (6700 m), and exceeded its predicted ceiling by about 1,800 feet (550 m). It could climb at up to 2,880 feet (880m) per minute. A demonstration to officials including Lord Beaverbrook and General Henry ‘Hap’ Arnold of the USAAF on April 20, 1941 featured an upward roll on one engine. It made a profound impression on the visitors; General Arnold was keen for the US to buy the type, but was overruled by those who thought, mistakenly, that the Lockheed P-38 would be equally capable in the reconnaissance role. A few days after the Japanese raid on Pearl Harbor in December 1941, the USAAF, by then an Allied service, requested a Mosquito for evaluation.

During W4050’s initial testing, it was given a larger tailplane and more efficient exhaust stubs. When it proved able to carry a bomb load of 4,000lb (1800kg), it was decided to increase the wingspan from 52ft 6 in (16m) to 54ft 2in (16.51m) on subsequent airframes. It briefly acquired a turret, which was quickly abandoned since its drag slowed the aircraft down too much, as anticipated. Much more worthwhile were the two-stage supercharged Merlin 61s with which it first flew on June 20, 1942; they gave W4050 a top speed of 437 mph (703 kmh) at 29,200 feet (8900 m).

On its second flight with the new engines it reached an altitude of 40,000 feet (12200m). Part of this increase in performance was due to multiple ejector exhaust stubs, which W4050 evaluated for night fighter applications; not only did they eliminate the dazzling glow which interfered with the crew’s night vision, but they also added a significant element of what amounted to jet thrust.

A further re-engining with Merlin 77s, flown from October 8, 1942, produced a maximum speed variously quoted as 437 and 439mph (about 705 kmh) in November, the fastest of any Mosquito. The aircraft spent just over three months of 1943 as a test-bed with Rolls-Royce. In 1944, after a long and distinguished flying career, W4050 was grounded and handed over to the Hatfield factory for apprentice training. It appeared in the 1945 film The Mosquito Story before returning to Salisbury Hall, by then the de Havilland Aeronautical school, in 1946, and was struck off charge on June 21, 1947. Incredibly, this historic aircraft was then scheduled for destruction, from which it was rescued by Bill Baird, de Havilland’s assistant public relations manager. He had the aircraft stored at various locations before a place was found at Salisbury Hall, the new owner of which, Walter Goldsmith, had asked for the prototype to be exhibited there, in its birthplace. W4050 went on display on May 15, 1959, and is now the centrepiece of an enthralling museum, the de Havilland Aircraft Heritage Centre. It is currently under restoration by the supporters’ society, with the collaboration and backing of BAe Systems.

W4051 having donated its original fuselage to the first prototype following its accident, W4052 was next to fly, on May 15, 1941. A German spy had been arrested in the area the day before, but he had not learnt of the secret prototype under construction. This was the fighter prototype, designed to specification F.21/40 and armed with four 0.303 machine guns in the solid nose and four 20mm cannons below them, their breeches protruding into the bomb bay. A new crew entrance door in the side replaced that on the underside, and to aid gun sighting, an optically flat bullet-proof windscreen was fitted.  Upgraded Merlin 21s of 1460 hp (1090kw) were installed. In a career lasting until January 28, 1946, when it was unceremoniously scrapped, W4052 evaluated 40 mm cannons, bomb racks, barrage balloon cable cutters, drop tanks and, in 1942, the remarkable Youngman Frill airbrake, which encircled the fuselage behind the wings and lay flat against the skin until it opened up like the frill of a lizard. Intended for rapid deceleration in combat, the brake was not adopted, because lowering the undercarriage proved just as effective. Other items tested included Hamilton airscrews in place of the standard de Havilland Hydromatic propellers, braking propellers and drooping ailerons to act as supplementary flaps.

The delayed W4051, finally given a production standard fuselage, flew on June 10, 1941 equipped for photo-reconnaissance and the first of the type to have the longer wings. A variety of cameras could be carried, and the Mosquito’s adaptable wooden structure made modification to take them a simple matter. Even the camera mounts were made of wood, which was found to transmit less vibration than the steel mounts initially installed. W 4051 was the only one of the three original prototypes to become operational, eventually joining No.1 PRU based at RAF Benson.

After much vacillation, a production order was finally placed on June 21, 1941, for 19 PR aircraft, 176 fighters and 50 unspecified types, which were confirmed as unarmed bombers the following month. Contracts for T.III trainers and F.B.VI fighter-bombers were added in January 1942 in addition to a large order for other variants, bringing the total to 1378 plus 400 to be built in Canada. Eventual production reached 7781 aircraft in 36 marks, with Australia also building Mosquitoes in due course.

Parts for the Mosquito were made in widely dispersed workshops, but particularly concentrated around High Wycombe in Buckinghamshire, where there were several furniture factories and a piano factory. A fuselage could be produced and equipped by six workers in a week. Concrete copies were made of the original mahogany moulds, which were not durable enough for mass production. The only major metal parts were the radiators and their housings, the engines and their mounts, the undercarriage, the rudder and the framework of the fabric-covered elevators, which were eventually also given aluminium skin. Actually, aluminium covered most of the airframe in the form of silver dope, applied over the clear doped Madapolam cotton fabric to fill the remaining weave texture before the top coat of camouflage was added. Spruce formed the wing and tail spars, and was used in combination with various other woods in the remaining structural members. The plywood fuselage skin was made from Ecuadorean balsa wood sandwiched between layers of special thin birch veneer made by Roddis Manufacturing in Marshfield, Wisconsin, America. This and the balsa were of such quality that it was worth running the gauntlet of U-boat packs to import them. Aeromodellers, however, spent the entire war bemoaning the scarcity of balsa wood.

Completed Mosquitos were turned out in Britain by de Havilland at Hatfield, Leavesden and Hawarden, the Standard Motor Company at Coventry, Percival Aircraft at Luton, and Airspee Aircraft at Portsmouth. Canadian-built fuselages were made by General Motors in Oshawa, Ontario, and airframes completed by de Havilland in Toronto. Australian Mosquitoes were built by DH in Sydney. Concern about deterioration in tropical climates eventually led to a substitution of urea-formaldehyde adhesive for the casein glue originally used.

A German contemporary, the Focke-Wulf Ta 154 nightfighter, also made of plywood and resembling the Grumman Tigercat, inevitably came to be called the Moskito, but it enjoyed considerably less success than its British namesake, falling victim to Nazi party infighting and the forced use of inferior ersatz adhesives. It was eventually cancelled after the dismissal of its chief advocate, General Erhard Milch.

In 1943, Reichsmarschall Hermann Goering gave the Mosquito its most famous tribute: “I turn green and yellow with envy. The British, who can afford aluminium better than we can, knock together a beautiful wooden aircraft that every piano factory over there is building, and they give it a speed which they have now increased yet again…”