Meditations on a Mosquito – A Lesson in Smart Engineering and Assembly.

Affectionately known as the ‘Mossie’ or ‘Wooden Wonder’ the de Havilland DH.98 multi role two-man aircraft was remarkable for its performance as an unarmed fast bomber, medium-altitude daytime tactical bomber, high-altitude night bomber, pathfinder, day and night fighter, fighter bomber, intruder, maritime strike aircraft and fast photo-reconnaissance aircraft. But equally remarkable were the less known innovative methods by which these birch plywood, spruce and balsa aircraft were engineered, procured and manufactured.

The story of the Mosquito probably began in 1936 when the Air Ministry called for a twin-engine bomber capable of carrying 1400 kg at 275/4000 mph/m which resulted in the Manchester and Halifax aircraft. A year later Handley Page’s chief designer George Volkert put forward the idea of an aerodynamically clean medium bomber carrying 1400kg at 300 mph. In parallel the idea of a small fast bomber was gaining support in the Air Ministry along with the need to consider non-strategic materials. In 1938 the Armstrong Whitworth Albemarle appeared with a plywood and spruce shell attached to a steel-tube frame. So the scene was set for Geoffrey de Havilland to enter the story.

In July 1938 with the shortages of duralumin and steel de Havilland suggested a different approach to high-speed bomber design. A new aircraft should be faster and more manoeuvrable than any foreseeable enemy fighter aircraft and not require heavy defensive armament. A lack of gun turrets would speed up delivery rates and would reduce the crew size to a pilot and navigator. In November that year the Air Ministry directed de Havilland to produce a light reconnaissance bomber capable of 400/5500 mph/m.

After viewing a full-scale mock-up of the light reconnaissance bomber in December the Ministry order the first Mosquito powered by two Merlin 21 engines carrying 450kg at 397/7200 mph/m with a cruising speed of 327 mph and a maximum altitude of 9800m which outperformed the American B17. By further contrast the Mk 1 Spitfire in 1936 had maximum speed of 310 mph and altitude of 9723m. In March 1940 50 Mosquitos were ordered and two months later a long range fighter variant armed with cannon and machine guns was also ordered.

The use of a plywood and balsa sandwich for the Mosquito fuselage by de Havilland was not entirely new, but in war time Britain it took on special significance. Wood was in plentiful supply, and its use meant the design stages could be speeded up, prototypes more quickly produced, an appropriate workforce was available, repair work could be undertaken by carpenters of average skill, and many coachbuilding and furniture workshops could be turned over to sub-contract work on the aircraft. In fact, during the period of its service some four hundred UK sub-contractors were engaged in Mosquito production. And a timber fuselage aircraft was less radar detectable than one made of metal.

The basic idea of the aircraft was to design it as a frameless monocoque shell built in two halves divided vertically along the fuselage centreline and to assembly as far as possible all the internal workings in the separate halves before joining them together. This had the advantage of allowing more people to work more quickly in for example installing the control cables down the port side of the fuselage while at the same time installing the hydraulic plumbing system down the starboard side. The control column was also mounted in the port half-shell with all its’s rudder and elevator cables connected before the joining stage was reached.

mosquito-block

This double half shell construction was undoubtedly the most significant assembly feature of the aircraft. The first stage was to produce port and starboard moulds of the inside face of each fuselage half with the inner Canadian birch plywood layer attached. Onto this inside plywood layer laminated spruce stiffening strips were attached at bulkhead and wing locations to dissipate the concentrated forces occurring at those points. Further longitudinal spruce members were introduced along the half-fuselage edges and at door and other aperture cut out locations. The next stage was the fully glued Ecuadorean balsa-wood infill between the spruce stiffening members which served to stabilise the gap between the inner and out plywood layers. Once sanded down the final three-ply birch outer skin was glued onto the half fuselage and clamped until fully boded. The overall thickness of the birch and balsa sandwich skin was a surprising 11 mm. This sandwich skin was so stiff that no internal reinforcement was necessary from the wing’s rear spar to the tail bearing bulkhead.

Into each half-fuselage large templates would be attached to the inside for accurately locating a complex arrangement of brass ferrules to which all the internal fit out equipment would attach. In the early stages for example the cock-pit, bomb-aiming floors and armour plating would be mounted. And progressively further equipment and services were installed until the two halves were ready to be ‘boxed-up’ using a special ‘v’ joint reinforced with recessed plywood strips running the length of the fuselage. The final fit out stages would be completed and the fuselage would be lowered onto the wing assembly and the tail plane, fin and flap, and undercarriage assemblies would then be completed. The fuselage had a large central section cut-out (braced during construction) that allowed the fuselage to be lowered onto the wing centre-section. After the wing was secured lower panels were replaced and the bomb bay or armament doors fitted. A covering of doped woven cotton Madapolam fabric was stretched tightly over the shell and a coat of silver dope was applied, after which the exterior camouflage was applied.

The fuselage achieved its’ strength by virtue of the tubular form, and by the seven bulkheads running the length of the aircraft and through which the aircraft systems would be threaded. Each bulkhead was constructed in the same fashion as the fuselage itself from a fully bonded plywood and balsa sandwich. The original glue used was Casein-based, but was later replaced by “Aerolite”, a synthetic urea-formaldehyde, which proved to be considerably more durable. Additional strength was achieved by using a laminate sheet on the outside of the plywood in various locations to distribute radial loads. This material was called Micarta and sometimes referred to as Tufnol or as Phenolic sheet. The drawings for the Mosquito engine instrument boards, sub panels and heal boards specify the material to be Fabric Reinforced Synthetic Resin sheet available in varying grades which was the same material specified on Spitfire instrument panel drawings.

The Micarta sheet used in the Mosquito’s construction was bonded to the inner plywood fuselage during manufacture. It was used to reinforce the ferrule holes where the undercarriage structure, engine, wing and tail plane attachment fittings were bolted into position. Fabric Reinforced Synthetic Resin sheet was also used extensively throughout the Mosquito for the flight control cable pulleys, undercarriage leg piston guides, undercarriage door retraction guides and rubbing blocks, undercarriage door bungee cable rollers and cable guides, ammunition box rollers, various removable airframe inspection panels, radio antenna cable supports. Even the aerofoil section trailing aerial mast that protruded below the cockpit was made from Fabric Reinforced Synthetic Resin.

There were many other remarkable aspects of the Mosquito design and assembly process that could be celebrated. The accelerated adhesive setting process for example where a series of thermostats linked to heating elements in different part of the aircraft would ensure a controlled curing process to counter the effects of a draughty factory. There was the rubber block wheel suspension system which avoided the use of complex hydraulics speeding manufacture and maintenance. But the most celebrated aspect of the Mosquito was of course its operational record.

In 1942 while the US War Department considered Hughes’ ‘flying lumber yard’ to counter the Allied shipping losses from U-Boat attacks in the Atlantic, the Mosquito entered into widespread service as a high-speed, high-altitude photo-reconnaissance aircraft. Mosquitos went on to fly high-speed, medium or low-altitude missions against factories, railways and other pinpoint targets in Germany and German-occupied Europe. And from late 1943, Mosquitos were formed into the Light Night Strike Force and were used as pathfinders for RAF Bomber Command’s heavy-bomber raids. As a fighter-bomber in the Second Tactical Air Force, the Mosquito took part in “special raids”, such as the attack on Amiens Prison in early 1944, and in precision attacks against Gestapo forces. The ‘Wooden Wonder’ also played a significant role in RAF Coastal Command strike squadron attacks on Kriegsmarine U-boats and eventually supported the Allied invasion of Normandy.

Tom Frearson