November 25 / de Havilland Mosquito first flight
de Havilland Mosquito
deHavilland Mosquito FB.26 KA114 at Oshkosh 2022
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.
Originally conceived as an unarmed fast bomber, the Mosquito's use evolved during the war into many roles, including low- to the 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.
deHavilland Mosquito FB.26 nose
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 the speed of prototyping, rapid development, minimisation of jig-building time, and employment of a separate category of the 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 2,000 lb (910 kg) of bombs, or eight RP-3 unguided rockets.
KA114 first took to the skies after restoration in 2013 in Ardmore, New Zealand
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 200 mph (320 km/h), and rpm set to 2,650. The speed could be reduced to an acceptable 150 mph (240 km/h) for low-speed flying. For cruising, the optimum speed for obtaining maximum range was 200 mph (320 km/h) at 17,000 lb (7,700 kg) weight.
The Mosquito had a high stalling speed of 120 mph (190 km/h) with undercarriage and flaps raised. When both were lowered, the stalling speed decreased from 120 to 100 mph (190 to 160 km/h). Stall speed at normal approach angle and conditions was 100 to 110 mph (160 to 180 km/h). Warning of the stall was given by buffeting and would occur 12 mph (19 km/h) 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.
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 10 to 13 mph (16 to 21 km/h).
7,781 Mosquito’s were built
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. 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.
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. 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 22 in (56 cm) 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 280 lb (130 kg).
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 attacks.
The investigators concluded that construction defects occurred at the two plants. They found that the "... standard of glueing ... left much to be desired." Records at the time showed that accidents caused by "loss of control" were three times more frequent on Mosquitos 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.
deHavilland Mosquito FB.26 Fighter-bomber
Fuselage
The oval-section fuselage was a frameless monocoque shell built in two vertically separate halves formed over a mahogany or concrete mould. The 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 0.55 in (14 mm) 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.
Work on the separate half-fuselages included the 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.
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 support during this complex glueing work. The resulting large structural components had to be kept completely still and held in the correct environment until the glue cured.
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.
The Mosquito’s were produced from 1940 until 1950
Systems
The fuel systems gave the Mosquito good range and endurance, using up to nine fuel tanks. Two outer wing tanks each contained 58 imp gal (70 US gal; 260 L) of fuel. These were complemented by two inner wing fuel tanks, each containing 143 imp gal (172 US gal; 650 L), 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. In the FB.VI, these tanks contained 25 imp gal (30 US gal; 110 L) each, while in the B.IV and other unarmed Mosquitos each of the two centre tanks contained 68 imp gal (82 US gal; 310 L). Both the inner wing, and fuselage tanks are listed as the "main tanks" and the total internal fuel load of 452 imp gal (545 US gal; 2,055 L) was initially deemed appropriate for the type. In addition, the FB Mk. VI could have larger fuselage tanks, increasing the capacity to 63 imp gal (76 US gal; 290 L). Drop tanks of 50 imp gal (60 US gal; 230 L) or 100 imp gal (120 US gal; 450 L) could be mounted under each wing, increasing the total fuel load to 615 or 715 imp gal (739 or 859 US gal; 2,800 or 3,250 L).
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 100 imp gal (120 US gal; 450 L) drop tank under each wing supplementing the main fuel cells. The FB.VI had a wingspan of 54 ft 2 in (16.51 m), a length (over guns) of 41 ft 2 in (12.55 m). It had a maximum speed of 378 mph (608 km/h) at 13,200 ft (4,000 m). Maximum take-off weight was 22,300 lb (10,100 kg) and the range of the aircraft was 1,120 mi (1,800 km) with a service ceiling of 26,000 ft (7,900 m).
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 15 imp gal (18 US gal; 68 l) oil tank, including a 2.5 imp gal (3.0 US gal; 11 l) 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 radiator 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.
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.
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. The Dunlop-Marstrand anti-shimmy tailwheel was also retractable.
The last Mosquito was retired in 1963