De Havilland Mosquito

The de Havilland DH.98 Mosquito is a British twin-engined, shoulder-winged, multirole combat aircraft, introduced during the Second World War. Unusual in that its frame was constructed mostly of wood, it was nicknamed the "Wooden Wonder", or "Mossie". Lord Beaverbrook, Minister of Aircraft Production, nicknamed it "Freeman's Folly", alluding to Air Chief Marshal Sir Wilfrid Freeman, who defended Geoffrey de Havilland and his design concept against orders to scrap the project. In 1941, it was one of the fastest operational aircraft in the world.Bowman 2005, p. 21.

Originally conceived as an unarmed fast bomber, the Mosquito's use evolved during the war into many roles, including low- to medium-altitude daytime tactical bomber, high-altitude night bomber, pathfinder, day or night fighter, fighter-bomber, intruder, maritime strike, and photo-reconnaissance aircraft. It was also used by the British Overseas Airways Corporation as a fast transport to carry small, high-value cargo to and from neutral countries through enemy-controlled airspace. The crew of two, pilot and navigator, sat side by side. A single passenger could ride in the aircraft's bomb bay when necessary.

The Mosquito FB Mk. VI was often flown in special raids, such as Operation Jericho (an attack on Amiens Prison in early 1944), and precision attacks against military intelligence, security, and police facilities (such as Gestapo headquarters). On 30 January 1943, the 10th anniversary of Hitler being made chancellor and the Nazis gaining power, a morning Mosquito attack knocked out the main Berlin broadcasting station while Hermann Göring was speaking, taking his speech off the air.

The Mosquito flew with the Royal Air Force (RAF) and other air forces in the European, Mediterranean, and Italian theatres. The Mosquito was also operated by the RAF in the Southeast Asian theatre and by the Royal Australian Air Force based in the Halmaheras and Borneo during the Pacific War. During the 1950s, the RAF replaced the Mosquito with the jet-powered English Electric Canberra.

Development

By the early to mid-1930s, de Havilland had built a reputation for innovative high-speed aircraft with the DH.88 Comet racer. Later, the DH.91 Albatross airliner pioneered the composite wood construction used for the Mosquito. The 22-passenger Albatross could cruise at at , faster than the Handley Page H.P.42 and other biplanes it was replacing. The wooden monocoque construction not only saved weight and compensated for the low power of the de Havilland Gipsy Twelve engines used by this aircraft, but also simplified production and reduced construction time.Bowman 2005, p. 9.

Air Ministry bomber requirements and concepts

On 8 September 1936, the British Air Ministry issued Specification P.13/36, which called for a twin-engined, medium bomber capable of carrying a bomb load of for with a maximum speed of at ; a maximum bomb load of that could be carried over shorter ranges was also specified.Sharp and Bowyer 1971, p. 30. Aviation firms entered heavy designs with new high-powered engines and multiple defensive turrets, leading to the production of the Avro Manchester and Handley Page Halifax.Sharp and Bowyer 1971, p. 31.

In May 1937, as a comparison to P.13/36, George Volkert, the chief designer of Handley Page, put forward the concept of a fast, unarmed bomber. In 20 pages, Volkert planned an aerodynamically clean, medium bomber to carry of bombs at a cruising speed of . Support existed in the RAF and Air Ministry; Captain R. N. Liptrot, Research Director Aircraft 3, appraised Volkert's design, calculating that its top speed would exceed that of the new Supermarine Spitfire, but counter-arguments held that although such a design had merit, it would not necessarily be faster than enemy fighters for long. The ministry was also considering using nonstrategic materials for aircraft production, which, in 1938, had led to specification B.9/38 and the Armstrong Whitworth Albemarle medium bomber, largely constructed from spruce and plywood attached to a steel-tube frame. The idea of a small, fast bomber gained support at a much earlier stage than is sometimes acknowledged, though the Air Ministry likely envisaged it using light alloy components.

Inception of the de Havilland fast bomber

Based on his experience with the Albatross, Geoffrey de Havilland believed that a bomber with a good aerodynamic design and smooth, minimal skin area, would exceed the P.13/36 specification.Birtles 2017, ch. 2. Furthermore, adapting the Albatross principles could save time. In April 1938, performance estimates were produced for a twin Rolls-Royce Merlin-powered DH.91, with the Bristol Hercules (radial engine) and Napier Sabre ( H-engine) as alternatives. On 7 July 1938, de Havilland wrote to Air Marshal Wilfrid Freeman, the Air Council's member for Research and Development, discussing the specification and arguing that in war, shortages of aluminium and steel would occur, but supplies of wood-based products were "adequate." Although inferior in tension, the strength-to-weight ratio of wood is equal to or better than light alloys or steel, hence this approach was feasible.

A follow-up letter to Freeman on 27 July said that the P.13/36 specification could not be met by a twin Merlin-powered aircraft and either the top speed or load capacity would be compromised, depending on which was paramount. For example, a larger, slower, turret-armed aircraft would have a range of carrying a 4,000 lb bomb load, with a maximum of at , and a cruising speed of at . De Havilland believed that a compromise, including eliminating surplus equipment, would improve matters. On 4 October 1938, de Havilland projected the performance of another design based on the Albatross, powered by two Merlin Xs, with a three-man crew and six or eight forward-firing guns, plus one or two manually operated guns and a tail turret. Based on a total loaded weight of , it would have a top speed of and cruising speed of at .

Still believing this could be improved, and after examining more concepts based on the Albatross and the new all-metal DH.95 Flamingo, de Havilland settled on designing a new aircraft that would be aerodynamically clean, wooden, and powered by the Merlin, which offered substantial future development. The new design would be faster than foreseeable enemy fighter aircraft, and could dispense with a defensive armament, which would slow it and make interception or losses to antiaircraft guns more likely. Instead, high speed and good manoeuvrability would make evading fighters and ground fire easier. The lack of turrets simplified production, reduced drag, and reduced production time, with a delivery rate far in advance of competing designs. Without armament, the crew could be reduced to a pilot and navigator. Whereas contemporary RAF design philosophy favoured well-armed heavy bombers, this mode of design was more akin to the German philosophy of the ''Schnellbomber''.Batchelor and Lowe 2008, p. 5. At a meeting in early October 1938 with Geoffrey de Havilland and Charles Walker (de Havilland's chief engineer), the Air Ministry showed little interest, and instead asked de Havilland to build wings for other bombers as a subcontractor.Batchelor and Lowe 2008, p. 6.

By September 1939, de Havilland had produced preliminary estimates for single- and twin-engined variations of light-bomber designs using different engines, speculating on the effects of defensive armament on their designs.Buttler 2004, p. 78. One design, completed on 6 September, was for an aircraft powered by a single Napier Sabre, with a wingspan of and capable of carrying a bomb load . On 20 September, in another letter to Wilfrid Freeman, de Havilland wrote "...we believe that we could produce a twin-engine bomber which would have a performance so outstanding that little defensive equipment would be needed." By 4 October, work had progressed to a twin-engined light bomber with a wingspan of and powered by Merlin or Griffon engines, the Merlin favoured because of availability. On 7 October 1939, a month into the war, the nucleus of a design team under Eric Bishop moved to the security and secrecy of Salisbury Hall to work on what was later known as the DH.98.Sharp and Bowyer 1971, p. 34. For more versatility, Bishop made provision for four 20 mm cannon in the forward half of the bomb bay, under the cockpit, firing via blast tubes and troughs under the fuselage.Bowman 2005, p. 11.

The DH.98 was too radical for the ministry, which wanted a heavily armed, multirole aircraft, combining medium bomber, reconnaissance, and general-purpose roles, that was also capable of carrying torpedoes. With the outbreak of war, the ministry became more receptive, but was still sceptical about an unarmed bomber. They thought the Germans would produce fighters that were faster than had been expected. and suggested the incorporation of two forward- and two rear-firing machine guns for defence.Bowman 2005, p. 10. The ministry also opposed a two-man bomber, wanting at least a third crewman to reduce the work of the others on long flights. The Air Council added further requirements such as remotely controlled guns, a top speed of at 15,000 ft on two-thirds engine power, and a range of with a 4,000-lb bomb load. To appease the ministry, de Havilland built mock-ups with a gun turret just aft of the cockpit, but apart from this compromise, de Havilland made no changes.

On 12 November, at a meeting considering fast-bomber ideas put forward by de Havilland, Blackburn, and Bristol, Air Marshal Freeman directed de Havilland to produce a fast aircraft, powered initially by Merlin engines, with options of using progressively more powerful engines, including the Rolls-Royce Griffon and the Napier Sabre. Although estimates were presented for a slightly larger Griffon-powered aircraft, armed with a four-gun tail turret, Freeman got the requirement for defensive weapons dropped, and a draft requirement was raised calling for a high-speed, light-reconnaissance bomber capable of at 18,000 ft.

On 12 December, the Vice-Chief of the Air Staff, Director General of Research and Development, and the Air Officer Commanding-in-Chief (AOC-in-C) of RAF Bomber Command met to finalise the design and decide how to fit it into the RAF's aims. The AOC-in-C would not accept an unarmed bomber, but insisted on its suitability for reconnaissance missions with F8 or F24 cameras.Bowman 2005, p. 12. After company representatives, the ministry, and the RAF's operational commands examined a full-scale mock-up at Hatfield on 29 December 1939, the project received backing.Buttler 2004, p. 80. This was confirmed on 1 January 1940, when Freeman chaired a meeting with Geoffrey de Havilland, John Buchanan (Deputy of Aircraft Production), and John Connolly (Buchanan's chief of staff). De Havilland claimed the DH.98 was the "fastest bomber in the world ... it must be useful". Freeman supported it for RAF service, ordering a single prototype for an unarmed bomber to specification B.1/40/dh, which called for a light bomber/reconnaissance aircraft powered by two Rolls-Royce RM3SM (an early designation for the Merlin 21) with ducted radiators, capable of carrying a bomb load. The aircraft was to have a speed of at and a cruising speed of at with a range of at on full tanks. Maximum service ceiling was to be .

On 1 March 1940, Air Marshal Roderic Hill issued a contract under Specification B.1/40, for 50 bomber-reconnaissance variants of the DH.98; this contract included the prototype, which was given the factory serial ''E-0234''. In May 1940, specification F.21/40 was issued, calling for a long-range fighter armed with four 20 mm cannon and four .303 machine guns in the nose, after which de Havilland was authorised to build a prototype of a fighter version of the DH.98. After debate, that this prototype, given the military serial number ''W4052'', was decided to carry airborne interception (AI) Mk IV equipment as a day and night fighter. By June 1940, the DH.98 had been named "Mosquito". Having the fighter variant kept the Mosquito project alive, as doubts remained within the government and Air Ministry regarding the usefulness of an unarmed bomber, even after the prototype had shown its capabilities.

Project Mosquito

With design of the DH.98 started, mock-ups were built, the most detailed at Salisbury Hall, where ''E-0234'' was later constructed. Initially, the concept was for the crew to be enclosed in the fuselage behind a transparent nose (similar to the Bristol Blenheim or Heinkel He 111H), but this was quickly altered to a more solid nose with a conventional canopy.

Work was cancelled again after the Battle of Dunkirk, when Lord Beaverbrook, as Minister of Aircraft Production, decided no production capacity remained for aircraft like the DH.98, which was not expected to be in service until early 1942. Beaverbrook told Air Vice-Marshal Freeman that work on the project should stop, but he did not issue a specific instruction, and Freeman ignored the request. In June 1940, however, Lord Beaverbrook and the Air Staff ordered that production should concentrate on five existing types, namely the Supermarine Spitfire, Hawker Hurricane fighter, Vickers Wellington, Armstrong-Whitworth Whitley, and Bristol Blenheim bombers. Work on the DH.98 prototype stopped. Apparently, the project shut down when the design team were denied materials for the prototype.Jackson 2003, p. 7.

The Mosquito was only reinstated as a priority in July 1940, after de Havilland's general manager, L.C.L. Murray, promised Lord Beaverbrook 50 Mosquitoes by December 1941. This was only after Beaverbrook was satisfied that Mosquito production would not hinder de Havilland's primary work of producing Tiger Moth and Airspeed Oxford trainers, repairing Hurricanes, and manufacturing Merlin engines under licence. In promising Beaverbrook such a number by the end of 1941, de Havilland was taking a gamble, because they were unlikely to be built in such a limited time. As it transpired, only 20 aircraft were built in 1941, but the other 30 were delivered by mid-March 1942. During the Battle of Britain, interruptions to production due to air raid warnings caused nearly a third of de Havilland's factory time to be lost. Nevertheless, work on the prototype went ahead quickly at Salisbury Hall since ''E-0234'' was completed by November 1940.

In the aftermath of the Battle of Britain, the original order was changed to 20 bomber variants and 30 fighters. Whether the fighter version should have dual or single controls, or should carry a turret, was still uncertain, so three prototypes were built: ''W4052'', ''W4053'', and ''W4073''. The second and third, both turret armed, were later disarmed, to become the prototypes for the T.III trainer. This caused some delays, since half-built wing components had to be strengthened for the required higher combat loading. The nose sections also had to be changed from a design with a clear perspex bomb-aimer's position, to one with a solid nose housing four .303 machine guns and their ammunition.

Prototypes and test flights

On 3 November 1940, the prototype aircraft, painted in "prototype yellow" and still coded ''E-0234'', was dismantled, transported by road to Hatfield and placed in a small, blast-proof assembly building. Two Merlin 21 two-speed, single-stage supercharged engines were installed, driving three-bladed de Havilland Hydromatic constant-speed controllable-pitch propellers. Engine runs were made on 19 November. On 24 November, taxiing trials were carried out by Geoffrey de Havilland Jr., the de Havilland test pilot. On 25 November, the aircraft made its first flight, piloted by de Havilland Jr., accompanied by John E. Walker, the chief engine installation designer.

For this maiden flight, ''E-0234'', weighing , took off from the grass airstrip at the Hatfield site. The takeoff was reported as "straightforward and easy" and the undercarriage was not retracted until a considerable altitude was attained.Thirsk 2006, p. 25. The aircraft reached , with the only problem being the undercarriage doors – which were operated by bungee cords attached to the main undercarriage legs – that remained open by some at that speed. This problem persisted for some time. The left wing of ''E-0234'' also had a tendency to drag to port slightly, so a rigging adjustment, i.e., a slight change in the angle of the wing, was carried out before further flights.Bowman 2005, p. 18.

On 5 December 1940, the prototype, with the military serial number ''W4050'', experienced tail buffeting at speeds between . The pilot noticed this most in the control column, with handling becoming more difficult. During testing on 10 December, wool tufts were attached to suspect areas to investigate the direction of airflow. The conclusion was that the airflow separating from the rear section of the inner engine nacelles was disturbed, leading to a localised stall and the disturbed airflow was striking the tailplane, causing buffeting. To smooth the air flow and deflect it from forcefully striking the tailplane, nonretractable slots fitted to the inner engine nacelles and to the leading edge of the tailplane were tested. These slots and wing-root fairings fitted to the forward fuselage and leading edge of the radiator intakes stopped some of the vibration experienced, but did not cure the tailplane buffeting.

In February 1941, buffeting was eliminated by incorporating triangular fillets on the trailing edge of the wings and lengthening the nacelles, the trailing edge of which curved up to fair into the fillet some behind the wing's trailing edge; this meant the flaps had to be divided into inboard and outboard sections.Bowman 2005, p. 19. With the buffeting problems largely resolved, John Cunningham flew ''W4050'' on 9 February 1941. He was greatly impressed by the "lightness of the controls and generally pleasant handling characteristics". Cunningham concluded that when the type was fitted with AI equipment, it might replace the Bristol Beaufighter night fighter.

During its trials on 16 January 1941, ''W4050'' outpaced a Spitfire at . The original estimates were that as the Mosquito prototype had twice the surface area and over twice the weight of the Spitfire Mk II, but also with twice its power, the Mosquito would end up being faster. Over the next few months, ''W4050'' surpassed this estimate, easily beating the Spitfire Mk II in testing at RAF Boscombe Down in February 1941, reaching a top speed of at altitude, compared to a top speed of at for the Spitfire.

On 19 February, official trials began at the Aeroplane and Armament Experimental Establishment (AAEE) based at Boscombe Down, although the de Havilland representative was surprised by a delay in starting the tests.Sharp and Bowyer 1971, pp. 44–45. On 24 February, as ''W4050'' taxied across the rough airfield, the tailwheel jammed leading to the fuselage fracturing. Repairs were made by early March, using part of the fuselage of the photo-reconnaissance prototype ''W4051''. In spite of this setback, the ''Initial Handling Report 767'' issued by the AAEE stated, "The aeroplane is pleasant to fly ... aileron control light and effective..." The maximum speed reached was at , with an estimated maximum ceiling of and a maximum rate of climb of at .

''W4050'' continued to be used for various test programmes, as the experimental "workhorse" for the Mosquito family. In late October 1941, it returned to the factory to be fitted with Merlin 61s, the first production Merlins fitted with a two-speed, two-stage supercharger. The first flight with the new engines was on 20 June 1942. ''W4050'' recorded a maximum speed of at (fitted with straight-through air intakes with snow guards, engines in full supercharger gear) and at without snow guards.Thirsk 2006, p. 34. In October 1942, in connection with development work on the NF Mk XV, ''W4050'' was fitted with extended wingtips, increasing the span to , first flying in this configuration on 8 December. Fitted with high-altitude-rated, two-stage, two-speed Merlin 77s, it reached in December 1943. Soon after these flights, ''W4050'' was grounded and scheduled to be scrapped, but instead served as an instructional airframe at Hatfield. In September 1958, ''W4050'' was returned to the Salisbury Hall hangar where it was built, restored to its original configuration, and became one of the primary exhibits of the de Havilland Aircraft Heritage Centre.

''W4051'', which was designed from the outset to be the prototype for the photo-reconnaissance versions of the Mosquito, was slated to make its first flight in early 1941. However, the fuselage fracture in ''W4050'' meant that ''W4051's'' fuselage was used as a replacement; ''W4051'' was then rebuilt using a production standard fuselage and first flew on 10 June 1941. This prototype continued to use the short engine nacelles, single-piece trailing-edge flaps, and the "No. 1" tailplane used by ''W4050'', but had production-standard wings and became the only Mosquito prototype to fly operationally.Thirsk 2006, p. 269.

Construction of the fighter prototype, ''W4052'', was also carried out at the secret Salisbury Hall facility. It was powered by Merlin 21s, and had an altered canopy structure with a flat, bullet-proof windscreen; the solid nose had mounted four .303 British Browning machine guns and their ammunition boxes, accessible by a large, sideways hinged panel. Four 20-mm Hispano Mk II cannon were housed in a compartment under the cockpit floor with the breeches projecting into the bomb bay and the automatic bomb bay doors were replaced by manually operated bay doors, which incorporated cartridge ejector chutes.Thirsk 2006, pp. 125, 134.

As a day and night fighter, prototype ''W4052'' was equipped with AI Mk IV equipment, complete with an "arrowhead" transmission aerial mounted between the central Brownings and receiving aerials through the outer wing tips, and it was painted in black RDM2a "Special Night" finish. It was also the first prototype constructed with the extended engine nacelles. ''W4052'' was later tested with other modifications, including bomb racks, drop tanks, barrage balloon cable cutters in the leading edge of the wings, Hamilton airscrews and braking propellers, and drooping aileron systems that enabled steep approaches and a larger rudder tab. The prototype continued to serve as a test machine until it was scrapped on 28 January 1946. ''4055'' flew the first operational Mosquito flight on 17 September 1941.

During flight testing, the Mosquito prototypes were modified to test a number of configurations. ''W4050'' was fitted with a turret behind the cockpit for drag tests, after which the idea was abandoned in July 1941. ''W4052'' had the first version of the Youngman Frill airbrake fitted to the fighter prototype. The frill was mounted around the fuselage behind the wing and was opened by bellows and venturi effect to provide rapid deceleration during interceptions and was tested between January and August 1942, but was also abandoned when lowering the undercarriage was found to have the same effect with less buffeting.

Production plans and American interest

The Air Ministry authorised mass production plans on 21 June 1941, by which time the Mosquito had become one of the world's fastest operational aircraft. It ordered 19 photo-reconnaissance (PR) models and 176 fighters. A further 50 were unspecified; in July 1941, these were confirmed to be unarmed fast bombers. By the end of January 1942, contracts had been awarded for 1,378 Mosquitoes of all variants, including 20 T.III trainers and 334 FB.VI bombers. Another 400 were to be built by de Havilland Canada.Bowman 2005, p. 24.

On 20 April 1941, ''W4050'' was demonstrated to Lord Beaverbrook, the Minister of Aircraft Production. The Mosquito made a series of flights, including one rolling climb on one engine. Also present were US General Henry H. Arnold and his aide Major Elwood Quesada, who wrote "I ... recall the first time I saw the Mosquito as being impressed by its performance, which we were aware of. We were impressed by the appearance of the airplane that looks fast usually is fast, and the Mosquito was, by the standards of the time, an extremely well-streamlined airplane, and it was highly regarded, highly respected."

The trials set up future production plans between Britain, Australia, and Canada. Six days later, Arnold returned to America with a full set of manufacturer's drawings. As a result of his report, five companies (Beech, Curtiss-Wright, Fairchild, Fleetwings, and Hughes) were asked to evaluate the de Havilland data. The report by Beech Aircraft summed up the general view: "It appears as though this airplane has sacrificed serviceability, structural strength, ease of construction and flying characteristics in an attempt to use construction material which is not suitable for the manufacture of efficient airplanes." The Americans did not pursue the proposal for licensed production, the consensus arguing that the Lockheed P-38 Lightning could fulfill the same duties. However, Arnold urged the United States Army Air Forces (USAAF) to evaluate the design even if they would not adopt it. On 12 December 1941, after the attack on Pearl Harbor, the USAAF requested one airframe for this purpose.Bowman 2005, p. 20.

Design and manufacture

Overview

While timber construction was considered outmoded by some, de Havilland claimed that their successes with techniques used for the DH 91 Albatross could lead to a fast, light bomber using monocoque-sandwich shell construction. Arguments in favour of this included speed of prototyping, rapid development, minimisation of jig-building time, and employment of a separate category of workforce. The ply-balsa-ply monocoque fuselage and one-piece wings with doped fabric covering would give excellent aerodynamic performance and low weight, combined with strength and stiffness. At the same time, the design team had to fight conservative Air Ministry views on defensive armament. Guns and gun turrets, favoured by the ministry, would impair the aircraft's aerodynamic properties and reduce speed and manoeuvrability, in the opinion of the designers. Whilst submitting these arguments, Geoffrey de Havilland funded his private venture until a very late stage. The project was a success beyond all expectations. The initial bomber and photo-reconnaissance versions were extremely fast, whilst the armament of subsequent variants might be regarded as primarily offensive.

The most-produced variant, designated the FB Mk. VI (Fighter-bomber Mark 6), was powered by two Merlin Mk 23 or Mk 25 engines driving three-bladed de Havilland hydromatic propellers. The typical fixed armament for an FB Mk. VI was four Browning .303 machine guns and four 20-mm Hispano cannons, while the offensive load consisted of up to of bombs, or eight RP-3 unguided rockets.

Performance

The design was noted for light and effective control surfaces that provided good manoeuvrability, but required that the rudder not be used aggressively at high speeds. Poor aileron control at low speeds when landing and taking off was also a problem for inexperienced crews. For flying at low speeds, the flaps had to be set at 15°, speed reduced to , and rpm set to 2,650. The speed could be reduced to an acceptable for low-speed flying. For cruising, the optimum speed for obtaining maximum range was at weight.

The Mosquito had a high stalling speed of with undercarriage and flaps raised. When both were lowered, the stalling speed decreased from . Stall speed at normal approach angle and conditions was . Warning of the stall was given by buffeting and would occur before stall was reached. The conditions and impact of the stall were not severe. The wing did not drop unless the control column was pulled back. The nose drooped gently and recovery was easy.Air Ministry 1945, p. 29. (FB 6 notes).

Early on in the Mosquito's operational life, the intake shrouds that were to cool the exhausts on production aircraft overheated. Flame dampers prevented exhaust glow on night operations, but they had an effect on performance. Multiple ejector and open-ended exhaust stubs helped solve the problem and were used in the PR.VIII, B.IX, and B.XVI variants. This increased speed performance in the B.IX alone by .

Fuselage

The oval-section fuselage was a frameless monocoque shell built in two vertically separate halves formed over a mahogany or concrete mould. Pressure was applied with band clamps. Some of the 1/2—3/4" shell sandwich skins comprised 3/32" birch three-ply outers, with 7/16" cores of Ecuadorean balsa. In many generally smaller but vital areas, such as around apertures and attachment zones, stronger timbers, including aircraft-quality spruce, replaced the balsa core. The main areas of the sandwich skin were only thick. Together with various forms of wood reinforcement, often of laminated construction, the sandwich skin gave great stiffness and torsional resistance. The separate fuselage halves speeded construction, permitting access by personnel working in parallel with others, as the work progressed.Bowman 2005, p. 15.

Work on the separate half-fuselages included installation of control mechanisms and cabling. Screwed inserts into the inner skins that would be under stress in service were reinforced using round shear plates made from a fabric-Bakelite composite.

Transverse bulkheads were also compositely built-up with several species of timber, plywood, and balsa. Seven vertically halved bulkheads were installed within each moulded fuselage shell before the main "boxing up" operation. Bulkhead number seven was especially strongly built, since it carried the fitments and transmitted the aerodynamic loadings for the tailplane and rudder. The fuselage had a large ventral section cut-out, strongly reinforced, that allowed the fuselage to be lowered onto the wing centre-section at a later stage of assembly.Ricker, 1944 p. 6.

For early production aircraft, the structural assembly adhesive was casein-based. At a later stage, this was replaced by " Aerolite", a synthetic urea-formaldehyde type, which was more durable. To provide for the edge joints for the fuselage halves, zones near the outer edges of the shells had their balsa sandwich cores replaced by much stronger inner laminations of birch plywood. For the bonding together of the two halves ("boxing up"), a longitudinal cut was machined into these edges. The profile of this cut was a form of V-groove. Part of the edge bonding process also included adding further longitudinal plywood lap strips on the outside of the shells. The half bulkheads of each shell were bonded to their corresponding pair in a similar way. Two laminated wooden clamps were used in the after portion of the fuselage to provide supports during this complex gluing work. The resulting large structural components had to be kept completely still and held in the correct environment until the glue cured.Jackson 2003, p. 8.

For finishing, a covering of doped madapollam (a fine, plain-woven cotton) fabric was stretched tightly over the shell and several coats of red, followed by silver dope, were added, followed by the final camouflage paint.

Wing

The all-wood wing pairs comprised a single structural unit throughout the wingspan, with no central longitudinal joint. Instead, the spars ran from wingtip to wingtip. There was a single continuous main spar and another continuous rear spar. Because of the combination of dihedral with the forward sweep of the trailing edges of the wings, this rear spar was one of the most complex units to laminate and to finish machining after the bonding and curing. It had to produce the correct 3D tilt in each of two planes. Also, it was designed and made to taper from the wing roots towards the wingtips. Both principal spars were of ply box construction, using in general 0.25-in plywood webs with laminated spruce flanges, plus a number of additional reinforcements and special details.

Spruce and plywood ribs were connected with gusset joints. Some heavy-duty ribs contained pieces of ash and walnut, as well as the special five ply that included veneers laid up at 45°. The upper skin construction was in two layers of 0.25-in five-ply birch, separated by Douglas fir stringers running in the span-wise direction. The wings were covered with madapollam fabric and doped in a similar manner to the fuselage. The wing was installed into the roots by means of four large attachment points.Batchelor and Lowe 2008, p. 7. The engine radiators were fitted in the inner wing, just outboard of the fuselage on either side. These gave less drag. The radiators themselves were split into three sections: an oil cooler section outboard, the middle section forming the coolant radiator and the inboard section serving the cabin heater.Jackson 2003, p. 87.

The wing contained metal-framed and -skinned ailerons, but the flaps were made of wood and were hydraulically controlled. The nacelles were mostly wood, although for strength, the engine mounts were all metal, as were the undercarriage parts.Jackson 2003, p. 9. Engine mounts of welded steel tube were added, along with simple landing gear oleos filled with rubber blocks. Wood was used to carry only in-plane loads, with metal fittings used for all triaxially loaded components such as landing gear, engine mounts, control-surface mounting brackets, and the wing-to-fuselage junction. The outer leading wing edge had to be brought further forward to accommodate this design. The main tail unit was all wood built. The control surfaces, the rudder, and elevator were aluminium-framed and fabric-covered. The total weight of metal castings and forgings used in the aircraft was only ."History of the de Havilland Mosquito."
Archived fro

''RAAF Museum Point Cook'', 2007. Retrieved: 13 August 2009.

In November 1944, several crashes occurred in the Far East. At first, these were thought to be a result of wing-structure failures. The casein glue, it was said, cracked when exposed to extreme heat and/or monsoon conditions. This caused the upper surfaces to "lift" from the main spar. An investigating team led by Major Hereward de Havilland travelled to India and produced a report in early December 1944 stating, "the accidents were not caused by the deterioration of the glue, but by shrinkage of the airframe during the wet monsoon season". However, a later inquiry by Cabot & Myers firmly attributed the accidents to faulty manufacture and this was confirmed by a further investigation team by the Ministry of Aircraft Production at Defford, which found faults in six Mosquito marks (all built at de Havilland's Hatfield and Leavesden plants). The defects were similar, and none of the aircraft had been exposed to monsoon conditions or termite attack.

The investigators concluded that construction defects occurred at the two plants. They found that the "...standard of glueing ... left much to be desired."Bowman 2005, pp. 15–16. Records at the time showed that accidents caused by "loss of control" were three times more frequent on Mosquitoes than on any other type of aircraft. The Air Ministry forestalled any loss of confidence in the Mosquito by holding to Major de Havilland's initial investigation in India that the accidents were caused "largely by climate" To solve the problem of seepage into the interior, a strip of plywood was set along the span of the wing to seal the entire length of the skin joint.

Systems

The fuel systems gave the Mosquito good range and endurance, using up to nine fuel tanks. Two outer wing tanks each contained of fuel.Air Ministry 1945, p. 11. (FB 6 notes). These were complemented by two inner wing fuel tanks, each containing , located between the wing root and engine nacelle. In the central fuselage were twin fuel tanks mounted between bulkhead number two and three aft of the cockpit.Bowman 2005, p. 22. In the FB.VI, these tanks contained each, while in the B.IV and other unarmed Mosquitoes each of the two centre tanks contained .Air Ministry 1943, p. 6. (B IV notes).Air Ministry 1944, p. 6. (PR VIII, PR IX, B IX, PR XVI, B XVI notes). Both the inner wing, and fuselage tanks are listed as the "main tanks" and the total internal fuel load of was initially deemed appropriate for the type. In addition, the FB Mk. VI could have larger fuselage tanks, increasing the capacity to . Drop tanks of or could be mounted under each wing, increasing the total fuel load to .

The design of the Mark VI allowed for a provisional long-range fuel tank to increase range for action over enemy territory, for the installation of bomb release equipment specific to depth charges for strikes against enemy shipping, or for the simultaneous use of rocket projectiles along with a drop tank under each wing supplementing the main fuel cells. The FB.VI had a wingspan of , a length (over guns) of . It had a maximum speed of at . Maximum take-off weight was and the range of the aircraft was with a service ceiling of .

To reduce fuel vaporisation at the high altitudes of photographic reconnaissance variants, the central and inner wing tanks were pressurised. The pressure venting cock located behind the pilot's seat controlled the pressure valve. As the altitude increased, the valve increased the volume applied by a pump. This system was extended to include field modifications of the fuel tank system.

The engine oil tanks were in the engine nacelles. Each nacelle contained a oil tank, including a air space. The oil tanks themselves had no separate coolant controlling systems. The coolant header tank was in the forward nacelle, behind the propeller. The remaining coolant systems were controlled by the coolant radiators shutters in the forward inner wing compartment, between the nacelle and the fuselage and behind the main engine cooling radiators, which were fitted in the leading edge. Electric-pneumatic operated radiator shutters directed and controlled airflow through the ducts and into the coolant valves, to predetermined temperatures.Air Ministry 1945, p. 14. (FB 6 notes).

Electrical power came from a 24 volt DC generator on the starboard (No. 2) engine and an alternator on the port engine, which also supplied AC power for radios. The radiator shutters, supercharger gear change, gun camera, bomb bay, bomb/rocket release and all the other crew controlled instruments were powered by a 24 V battery. The radio communication devices included VHF and HF communications, GEE navigation, and IFF and G.P. devices. The electric generators also powered the fire extinguishers. Located on the starboard side of the cockpit, the switches would operate automatically in the event of a crash. In flight, a warning light would flash to indicate a fire, should the pilot not already be aware of it. In later models, to save liquids and engine clean up time in case of belly landing, the fire extinguisher was changed to semi-automatic triggers.Air Ministry 1945, p. 44. (FB 6 notes).

The main landing gear, housed in the nacelles behind the engines, were raised and lowered hydraulically. The main landing gear shock absorbers were de Havilland manufactured and used a system of rubber in compression, rather than hydraulic oleos, with twin pneumatic brakes for each wheel.Bishop 1995, Appendix 1 The Dunlop-Marstrand anti-shimmy tailwheel was also retractable.

Operational history

The de Havilland Mosquito operated in many roles, performing medium bomber, reconnaissance, tactical strike, anti-submarine warfare, shipping attacks and night fighter duties, until the end of the war. In July 1941, the first production Mosquito ''W4051'' (a production fuselage combined with some prototype flying surfaces – see Prototypes and test flights) was sent to No. 1 Photographic Reconnaissance Unit (PRU), at RAF Benson. The secret reconnaissance flights of this aircraft were the first operational missions of the Mosquito. In 1944, the journal ''Flight'' gave 19 September 1941 as date of the first PR mission, at an altitude "of some 20,000 ft".

On 15 November 1941, 105 Squadron, RAF, took delivery at RAF Swanton Morley, Norfolk, of the first operational Mosquito Mk. B.IV bomber, serial no. ''W4064''. Throughout 1942, 105 Squadron, based next at RAF Horsham St. Faith, then from 29 September, RAF Marham, undertook daylight low-level and shallow dive attacks.Wooldridge 1993, Frontispiece. Apart from the Oslo and Berlin raids, the strikes were mainly on industrial and infrastructure targets in occupied Netherlands and Norway, France and northern and western Germany. The crews faced deadly flak and fighters, particularly Focke-Wulf Fw 190s, which they called ''snappers''. Germany still controlled continental airspace and the Fw 190s were often already airborne and at an advantageous altitude. Collisions within the formations also caused casualties. It was the Mosquito's excellent handling capabilities, rather than pure speed, that facilitated successful evasions.

The Mosquito was first announced publicly on 26 September 1942 after the Oslo Mosquito raid of 25 September. It was featured in ''The Times'' on 28 September and the next day the newspaper published two captioned photographs illustrating the bomb strikes and damage. On 6 December 1942, Mosquitoes from Nos. 105 and 139 Squadrons made up part of the bomber force used in Operation Oyster, the large No. 2 Group raid against the Philips works at Eindhoven

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