Authorititvely described in 1943 as the fastest operational aircraft in the world, the de Havilland Mosquito was the first modern first-line machine of all-wood construction to go into service. It was a twin-engined mid-wing cantilever monoplane built in two basic versions - bomber and fighter. Of these two there were many sub-variants, including day and night bomber and intruder types.
Structurally, the most outstanding feature of the aircraft was the fuselage built on the balsa-plywood sandwich principle introduced by the de Havilland Company prior to the war and first used in the four-engine Albatross civil transport. The power plant consisted of two Rolls-Royce Merlin engines.
Armament of the Mosquito fighter was a combination of four 20mm Hispano cannon and four Browning .303 machine guns, although as with everything about the Mosquito, a number of variations were possible, including the replacement of the 20mm cannon for a single 57mm gun with just two Brownings for sighting purposes!
Mk XVI Mosquito converted to carry H2X radar for the US 8th Air Force with additional slipper fuel tanks under the wings.
The first stage of fuselage half-shell assembly saw the bulkheads and other members of the internal structure located in slots in the mould.
Stage two saw the inner skin and between skin structure put into place.
Stage three of Mosquito fuselage construction was the fitment of balsa-wood fillers that formed the central portion of the sandwich.
After the outer shell was fitted into place, this was then covered with flexible steel bands under tension in order to provide the required pressure for bonding all the glued elements together.
Equipment was installed inside the two half-fuselage sections prior to boxing up. In this case it is a bomber variant being built, but the process was the same for both fighters and bombers. The trunnion mounting for the wing attachment pick-ups is seen in the centre of the two halves.
The rear floor was fitted, rudder and elevator-operating linkage mounted in the cockpit floor and a start was made with the electrical wiring, and with the plumbing of the oxygen and hydraulic systems.
This view, from the tail end of the fuselage shows the boxing up fixture that brings the two half-shells together. The series of five circular clamps holding the two halves together were in fact each made from two laminated timber halves held together and thereby applying pressure by adjustable turnbuckles.
The leveling and drilling jigs in place for drilling the No. 6 bulkhead for the fin front pick-ups.
A point of interest in Mosquito fuselage construction was the simplification of assembly by arranging the control cable runs down the port side of the fuselage and the hydraulic plumbing as far as possible along the starboard side.The control column was also mounted on the port half-shell and connected to the rudder and elevator linkage before the joining stage was reached.
The bottom rear spar boom laminations in the cementing fixture.
The Mosquito wing was very distinctive. It was made in one piece from tip to tip and was based on conventional two-spar practice with the usual interspar rib structure. The stressed-skin covering, however, was a departure from the usual type of construction. The birch plywood skin was reinforced by closely-spaced, square-section, spanwise stringers of Douglas fir, which, over the upper surface of the wing were sandwiched between a double covering of skin. On the under surface, the outboard panels of the skin were of identical construction but with only one skin. Over the centre portion of the span, where the fuel tanks were housed between the spars, the wing under surface was completed by stressed covers to the tank bays. These tank doors were of balsa-plywood sandwich construction, with bolting edges of shear resistant material.
Rough machining a bottom boom on an overhead planer. Below right: Spindling lightening recesses in a top spar-boom extension.
The main fixtures for rear-spar assembly. For applying the skin to the forward face of the spar the inverted fixture in the foreground was used.
Assembly of the spars was done in large fixtures with sloped access platforms made necessary by the swept-forward outboard ends of the spars. The booms were very simply located between blocks on the base of the fixture at each side. Longitudinal locationwas given by the combined forward sweep and dihedral in conjunction with a chordwise taper on the top and bottom faces of the boom. A slight excess length was left on the tips of the booms and was trimmed off in the fixture. It is worthy of note that a tolerance of 0.020 inches was maintained on the overall length of each half of the boom.
The booms were secured in the fixture by wedges driven in against their inner faces and the locating blocks. Spruce spacing members between the booms were positioned on the fixture to templates located over the faces of the booms. Beetle cement - a form of Urea-Formaldehyde Resin glue -was applied to the booms and spacers and to the underside of the web, which was then laid in position and screwed down, the screws providing the pressure required for the bonding of the joint. Owing to the increased setting time required for this cement as compared with the adhesive formerly used, and the fact that no other work could be done on the spar during the setting period, it became necessary to devise a means of accelerating the setting of the cement in order to avoid creating a bottleneck at this stage.
It was well known that the setting of synthetic adhesives could be speeded up by raising their temperature and it was decided to make use of this fact and to adopt some form of electrical heating. The problem was not a simple one, as direct application of heat to the cement line was not possible and the area to be considered varied considerably across the span of the spar. The problem was solved by the application of wooden panels with heating elements embedded in their lower faces.
The complete drilling jig for the rear spar.
From the inverted fixture, the spars were transferred to the drilling jig, in which the holes were drilled for all the metal fittings that were mounted on the spar, such as the undercarriage and the hinge brackets for the ailerons on the rear face.
Bush plates were provided for drilling from both faces of the spar, which was supported and located between each pair of wooden pads below and on each side. The bottom pad could be raised or lowered by screw adjustment to set the height of the spar in the jig and the pads on one side of the fixture could be screwed in to clamp the spar in position. Setting was made to a longitudinal datum line and a vertical datum at the centre line, both of which were laid out on the spar web from templates in the main assembly fixture. The longitudinal datum was set to pointers mounted on the jig base.
Colour coding was used to distinguish the bushes for different drill diameters and a portable drill was used. The production of these large components in wood to a tolerance of ± 0.040 inches over a length of 50 feet was an achievement which reflected the greatest credit upon the sub-contractors.
A close-up view of the centre drill plates and locations on the spar-drilling jig for the rear spar.
An early stage of assembly in the wing fixture. Both spars were in position and most of the ribs have been fitted between them.
In a separate area skin panels were bring drilled from wooden templates, as seen in the left of this picture. In the centre, stringers have been laid out in the slots of the asssembly fixture while on the right, ply skin is being applied over a similar set. Cement was applied to the top faces of the stringers and the underside of the skin which was then laid over the stringers, located against metal stops from the edge of the tables and screwed down by pump-action screwdrivers. The screws provided the required pressure to make the joint, thus forming the inner surface of the wing skin.
The inner shell in position. The centre section stringer scarf joints have been fitted and the wooden wedge cramps were in place to hold them during the setting of the cement.
A close-up of the centre section of the inner shell, showing the wooden wedge cramps in place on the stringers.
Drilling the walnut panels for the attachment of the forward fuselage pick-ups on the upper wing surface.
Applying the upper surface skin to the wing.
The wing assembly shop - fitting and installation work is proceeding in the foreground.
Engine and radiator fairings were fitted to the top wing surface and the aileron and flap-control cables were assembled, but not connected up. For transport to the doping shop, the wing was slung on the overhead crane in the vertical position and mounted on two bogies which picked-up on the engine fittings. The usual red dope, madapolam covering and camouflage were applied in the dope shop, from which the wing emerged into the installation section. One small doping operation on the back of the shroud was done off the bogies as access could not be obtained to it in the shop.
Installation of electrical and hydraulic services was then completed and the engine support struts assembled. Tanks were also installed. The wing was then ready for transport to final assembly.
Doping a completed wing. All the lamps in the dope shop were shielded by glass from the dope fumes. The wings were transported from the assembly shop on wheeled bogies.
Mosquito wings passing through the final stages of equipment installation before despatch to the final assembly area. The flaps and ailerons were already in position, but the wing-tips were yet to be installed.
The assembly fixture for the Mosquito wing-tip, the edges of which were made from multiple laminations. For attachment to the wing a Bakelite-reinforced strip was fitted to the inboard edges and screws inserted through to a series of anchor-nuts around the inside of a corresponding flange on the wing.
Setting up for drilling the strap plate and radius rod fittings on the Mosquito undercarrage leg.
The building of the undercarriage leg was a matter of straightforward assembly. As received from the press-shop, the joint flanges and ends of the half-casings were pierced ready for assembly, which commenced with the temporary joining of a pair of pressings by the insertion of service bolts or rivets through the flanges.
A close-up of the jig showing the clamping and bushings for drilling the radius rod fittings on a radial drill.
Riveting the two halves together was then completed on a bench-mounted squeeze-riveting machine.
As small variations in form were almost inevitable on an assembly built up in this way from pressings, a sizing operation was carried out on the interior of the completed casing. This operation, performed on a horizontal broaching machine, was rather burnishing than broaching, as metal was not actually removed from the interior of the casing.
Sizing the internal form of the undercarriage leg casing. This was performed on a horizontal broaching machine by pulling a die of the correct shape and size through it.
The piston tube, piston, rebound rubber and guide-block assembly of a Mosquito undercarriage leg.
The complete leg assembly with external casing.
Inserting the leg assembly into the casing. The pack of rubber blocks and their associated spacers were temporarily held in place by an assembly rod that was removed though the top cap after final assembly.
Screwing the casing to the guide-block with the rubbers held under compression in a screw press.
The first stage of tailplane assembly - front spars, nose ribs and leading edges were built up as a separate unit.
Assembly of the tailplane was done in two stages in vertical fixtures. The front spar had already been drilled and the fuselage pick-up fittings were first assembled to the existing holes. The spar was located to datum centre lines on the front web. Locations were also provided on the fixture for the outboard end ribs, to control the overall length. This was important, as the horn balances of the elevators overlapped the tips of the tailplane and proper clearance for their working was essential.
Rib posts were already in position on the spar faces and the nose ribs were positioned from them and from slot locations formed by two sections of angle iron mounted on the fixture. The spar was supported from beneath at several points. Alignment of datum centre lines on ribs and spar determined the final setting of the ribs and leading edge bend were then cemented and pinned in position. Final shaping of this edge and fairing in of the rib profiles was done by hand on the fixture, templates being used to check the profile at each rib station. As with the front spar, the fittings were first mounted on the rear spar out of the fixture. The centre hinge fittings, incorporating the elevator trimming adjustment, was mounted, followed by the outer tailplane hinge brackets.
In the main assembly fixture, the rear spar was located on these hinge fittings, while the leading-edge was located from the fuselage attachment fittings on the front spar. Box-type interspar ribs were assembled by sliding them in over the rib posts mounted on the inner web of each spar and gluing and pinning them in place. Fairing of the rib profiles was completed by hand shaping.
The skin - complete with handholds, inspection hatches and strengthening - was applied by Beetle cement and was also screwed and pinned to the structure.
In the main tailplane assembly fixture, the rear spar, leading edge and interspar ribs were brought together to make the complete unit.
Rows of forward sections of the nose shells awaiting scarfing to the tail sections of Mosquito supplementary fuel tanks.
Some later marks of Mosquito carried wing mounted overload or ‘slipper’ fuel tanks. Very little has ever appeared in print about these devices, indeed, many do not realise that they were moulded out of plywood veneers. This type of moulding represents probably the first application of this production technique to the manufacture of plywood units of compound curvature in the UK.
For ease of moulding, the Mosquito tank was divided into nose and tail sections, the joint being made roughly midway in its length. Two similar half-mouldings were made at each pressing in the autoclave. The inside jacket was first placed in position and to give stability to the whole assembly was stapled around its open edges to the mould. The other two were then dropped over it, followed by a rubber bag. A clamping frame was lowered over the rubber to secure it to the platform, and create an airtight joint.
Initial atmospheric pressure on the mould was then obtained by pumping out the air from beneath the rubber bag, after which the mould on its mobile platform was pushed into the autoclave. Steam and compressed air were used together to give the necessary heat and pressure. While the moulds were in the autoclave another two were being prepared on a track outside. There were several variants of this process, among which were Duramold, Vidal and Timm. In the Duramould process a female mould was used and in the Timm and Vidal methods a male former.
The scarf joint between the forward and aft sections of the tank was bonded under the pressure maintained by a series of screw jacks.
Drilling the tank shell for the internal structure from a basket-type jig.
The first stage of assembling the internal structure: applying adhesive to the bulkhead baffles.
The inner bulkhead baffles were secured to the shell by countersunk screws as well as adhesive.
A reinforcing structure was fitted inside the nose half of the tank, consisting of a small laminated bulkhead or frame with two stringers or longerons. This structure was made up as a separate unit before being both glued and screwed to the tank shell.
It was at this stage that the panel which formed the upper portion of the nose was assembled. For this operation the nose section of the tank was located in a wooden fixture. Adhesive was applied to the nose reinforcing structure and this also was inserted into the nose shell.
When it was in its correct position, the two stringers were centrally disposed over the joints between the separate panel and the main nose section. With the panel in position the plywood was marked out drilled, then screwed and glued into position before trimming.
To seal the screw holes and to make the tank completely fuel-proof, the interior was subjected to a sloshing treatment. In order to cover the whole of the interior the tank was held in two rotatable cradles (left) which were carried in a frame.
This permitted the tilting of the tank to any angle. A sequence of positions was followed with two cycles of operation - a slow slosh and a quick slosh.
In the slow slosh, the tank was left for five minutes in each position, in the quick slosh it was moved continuously from one position to the next. An ICI zinc-chromate compound was used, about 20 gallons being placed in the tank, after which the air pressure was pumped up to 1 Ib./sq. in. for the first slush and to 2 Ib./sq. in for the second one.
After the sloshing operation the tank was placed in a drying chamber where a stream of air was passed through the fuel pipe into the interior and out through the filler hole. Drying requires a period of four hours, during which excess compound was also drained from the tank.
A pressure test for leaks was next performed (right) on the tanks, air being pumped in to a pressure of 2.25Ib. /sq. in., which had to be maintained for a period of twenty minutes without falling below 2 .Ib./sq. in. The tanks were then covered in madapolam and doped, after which the pressure test was repeated.