De Havilland DH-98 B/TT Mk. 35 Mosquito

Officials in the British Air Ministry vehemently resisted building it, but from the day production finally began in 1941 until the war ended, the Royal Air Force never had enough Mosquitoes to perform the amazing variety of missions that air tacticians devised for this outstanding airplane. It excelled at day and night bombing from high or very low altitudes, long-range reconnaissance, air-to-air combat in daylight and darkness, and finding and striking distant targets at sea. No less than forty-two distinct versions of the D. H. 98 entered service. At extreme speeds, Mosquitoes carried heavy loads great distances because of two key design features: a lightweight, streamlined, wooden airframe propelled by powerful, reliable engines. The "Wooden Wonder" was constructed from Alaskan spruce, English ash, Canadian birch and fir, and Ecuadorian balsa glued and screwed together in new, innovative ways, and motivated by the world's finest reciprocating, liquid-cooled power plants, a pair of Rolls Royce Merlins. There has never been a more successful, combat-proven warplane made of wood.

Officials in the British Air Ministry vehemently resisted building it, but from the day production finally began in 1941 until the war ended, the Royal Air Force never had enough Mosquitoes to perform the amazing variety of missions that air tacticians devised for this outstanding airplane. It excelled at day and night bombing from high or very low altitudes, long-range reconnaissance, air-to-air combat in daylight and darkness, and finding and striking distant targets at sea. No less than forty-two distinct versions of the D. H. 98 entered service. At extreme speeds, Mosquitoes carried heavy loads great distances because of two key design features: a lightweight, streamlined, wooden airframe propelled by powerful, reliable engines. The "Wooden Wonder" was constructed from Alaskan spruce, English ash, Canadian birch and fir, and Ecuadorian balsa glued and screwed together in new, innovative ways, and motivated by the world's finest reciprocating, liquid-cooled power plants, a pair of Rolls Royce Merlins. There has never been a more successful, combat-proven warplane made of wood.

The Mosquito descended from civilian, not military designs. In 1934, de Havilland decided to construct a new airplane to compete in the England-Australia Air Race. In only ten months, the firm designed and built three D.H. 88 Comet racers. Pilots raced all three against sixty-four entrants from thirteen countries. A Comet won the 17,710-km (11,000-mile) race in 71 hours and another finished fourth but the third Comet dropped out because of engine trouble. An advanced plywood skin formed the wing and fuselage of these twin-engine airplanes and de Havilland used the same technology to build Mosquito wings.

A more direct ancestor to the D. H. 98 was the de Havilland D.H. 91 Albatross air transport. After much official delay, de Havilland built seven of these four-engine airliners and Imperial Airways bought five and began to fly them on scheduled routes in December 1938. World War II completely overshadowed the world-class speed and economical performance of the Albatross but its impact on Mosquito development was profound. In both airplanes, radiators mounted inside the wings cooled the engines. Thin slots cut into the wing leading edges allowed cooling air to flow through these radiators. This was a significant improvement because on older aircraft such as the Spitfire, the radiators hung beneath the lower wing surface and the drag generated by this arrangement robbed the Spitfire of precious speed. The Mosquito shared another Albatross trait. Design engineer Arthur E. Hagg conceived of a lightweight, strong composite wooden construction technique to build the Albatross fuselage. He left de Havilland in 1937 but the company used his composite construction methods again on Mosquitoes.

Nazi aggression escalated during the late 1930s. With every act of terror, Geoffrey de Havilland (founder and head of the firm) and his design staff became more convinced that they could create an exceptional warplane based on the Comet and Albatross. The members of the Mosquito design team included the chief designer and team leader, R. E. Bishop, Richard M. Clarkson, assistant chief engineer and Mosquito aerodynamicist, C. T. Wilkins, assistant chief designer and the fuselage specialist, W. A. Tamblin, senior designer and the wing specialist, and Fred Plumb who managed constructing the prototype. Their thoughts coalesced in 1938 to focus on the design of a high-speed, unarmed bomber. The new design would weigh thousands of pounds less than conventional bombers armed with enclosed, power-driven turrets and heavy machine guns, its finish smooth and streamlined enough to speed past all pursuers, even the most advanced, single-engine fighters. For two years, de Havilland and the Air Ministry argued over several different designs and government specifications for the new airplane. Doubts racked most ministry officials about committing national resources to build a small, unarmed bomber out of wood. The notion ran counter to aeronautical trends in every other nation worldwide but in addition to speed, de Havilland's idea had other advantages. Wood, and the skilled personnel required to work it, was plentiful while aluminum was in dangerously short supply and aircraft metalworkers were already consumed with producing Spitfires, Hurricanes, and other metal airplanes.

De Havilland finally won a contract to build a prototype five months after Hitler invaded Poland but the Air Ministry, and many people in the British aircraft industry, remained skeptical right up until March 3, 1941. On that day government test pilots, conducting official trials with the Mosquito prototype, published a favorable report on the aircraft. From this time forward, official doubt turned to quiet confidence.

Like the Comet and Albatross wings, de Havilland built Mosquito wings out of shaped pieces of wood and plywood cemented together with Casein glue. Approximately 30,000 small, brass wood screws also reinforced the glue joints inside a Mosquito wing (another 20,000 or so screws reinforced glue joints in the fuselage and empennage). The internal wing structure consisted of plywood box spars fore and aft. Plywood ribs and stringers braced the gaps between the spars with space left over for fuel tanks and engine and flight controls. Plywood ribs and skins also formed the wing leading edges and flaps but de Havilland framed-up the ailerons from aluminum alloy and covered them with fabric. Sheet metal skins enclosed the engines and metal doors closed over the main wheel wells when the pilot retracted the landing gear.

To cover the wing structure and add strength, de Havilland woodworkers built two top wing skins and one bottom skin using birch plywood. The top skins had to carry the heaviest load so the designers also beefed them up with birch or Douglas fir stringers cut into fine strips and glued and screwed between the two skins. The bottom skin was also reinforced with stringers. Together the top and bottom skins multiplied the strength of the internal spars and ribs. A Mosquito wing could withstand rigorous combat maneuvering at high G-loads when the airplane often carried thousands of additional pounds of fuel and weapons. To maintain strength, trim weight, and speed fabrication time, the entire wing was finished as a single piece, wingtip to wingtip, with no break where the wing bisected the fuselage. A finished and painted wing was light and strong with a smooth surface unblemished by drag-inducing nail or rivet heads.

De Havilland engineers and technicians used generally the same techniques to build the Comet, Albatross, and Mosquito wings out of wood and plywood. When they designed and built the fuselage, however, they copied the methods and materials employed to build the Albatross fuselage. This airliner was the product of the brilliant mind of Arthur E. Hagg, de Havilland's Chief Draftsman in 1937. He left the company that same year but his ideas lived on in the Mosquito. Hagg created a light, strong, very streamlined structure by sandwiching 9.5 mm (three-eighths inch) Ecuadorian balsa wood between Canadian birch plywood skins that varied in thickness from 4.5 mm to 6 mm (about ¼ inch). The plywood/balsa/plywood sandwich was formed inside concrete molds of each fuselage half, and each mold held seven birch plywood formers reinforced with spruce blocks, plus bulkheads, floors, and other structural members. As the glue cured, metal clamps held the skin layers tight to the mold. Technicians finished the edge of each half of the fuselage with male and female wedge joints as fitters attached wiring and other equipment to the inner walls. Final fuselage assembly was reminiscent of a typical plastic model airplane kit as the two halves were glued and screwed together. Fabricators completed the final step in building the fuselage when they covered it with Mandapolam.

To build the empennage, workers framed the rudder and elevator out of aluminum and covered them with fabric but they built the vertical and horizontal stabilizers from wood. Although the materials are different, Hagg's composite sandwich construction material is similar to the foam and fiberglass composite sandwich developed by Burt Rutan during the 1970s. Rutan revolutionized the design and construction of homebuilt aircraft when he marketed kits and plans to build the Rutan VariEze (see NASM collection).

The first Mosquito prototype flew on November 24, 1940. Flight trials revealed only minor development problems and de Havilland finished twenty production aircraft before 1941 ended. A photo-reconnaissance D. H. 98 flew the first operational Mosquito sortie to survey the western part of the border between France and Spain on September 17, 1941. Bomber and fighter versions began operating in early 1942 and Mosquitoes soon swept across the length and breadth of Western Europe.

As a bomber, the Mosquito was fast enough to excel at precision attacks against heavily defended targets. Courageous crews often flew these raids at altitudes of 15-50 meters (49-164 feet). Flying this type of raid in a single-engine airplane would border on suicide but the Mosquito's twin Merlins doubled the crew's chances of surviving engine failure. Nonetheless, on numerous occasions, anti-aircraft gunfire or patrolling German fighters splintered Mosquito airframes. Men of No. 105 Squadron set the tone for these pinpoint raids when they attacked Nazi Gestapo headquarters in Oslo, Norway, on September 26, 1942. Four crews flew their Mosquito B. Mk. IV bombers a roundtrip total of 1,770 kilometers (1,100 miles) and the mission lasted four hours and 45 minutes. A BBC news broadcast that followed this raid marked the first official confirmation that the Mosquito existed.

Former Royal Air Force mechanic and Mosquito test technician, David van Vlymen, in his article "Un-Gluing the Mosquito," wrote vividly of his experiences with the Wooden Wonder. Van Vlymen's account illustrates the dangers routinely faced by men operating high-performance aircraft during wartime. After volunteering for the RAF in 1940, van Vlymen became a certified airframe mechanic. In 1943, Mosquitoes built at the Hatfield factory began "piling up waiting for their test flight which de Havilland was unable to perform quickly enough. So large numbers were sent to Henlow for us to pass as serviceable. No. 2 Repair Section got the job and being in the "test flight" section I was lucky, I got to fly!"

"Of course I was fully aware that at some time while flying I may have to bail out and the possibility of a parachute jump was something I should have liked to experience. It was getting out of the Mosquito that was the problem. We were testing the fighter version. The pilot could jettison the panel above his head and then somehow get out, probably breaking his back in doing so, there was no such thing as an ejector-seat in those days. For me in the Navigators seat I had to jettison the side door and dive out head first, right into the propeller, so it was first necessary to feather the prop, an action that took several seconds that seemed like an age! I used to just sit in a Mosquito on the ground and practicing how to get out in a hurry, but fortunately I never had to do so."

"Now the Mosquito had a maximum speed of a little over 400 mph which was moving in those days, and when it went into a vertical dive speed increased dramatically. At around 460 mph a shudder often started, and remember this was an all wooden aircraft. It was my belief that this shudder allowed the metal undercarriage doors to slightly open (they were held shut only by strong springs) and then be whipped off by the air stream and smash into the tail unit demolishing the elevators, thus making it impossible to pull out of the dive. I mentioned this to the powers that be and a positive uplock latch was designed and installed after which, I am pleased to report, our loss of aircraft was substantially reduced."

Copyright 2000, David van Vlymen, Portland, Oregon, e-mail: able2@att.net, fax: 413-383-3877, from The Mosquito Page website at http://www.mossie.org/mosquito.html.

Unlike Allied heavy and medium bomber crews, Mosquito men routinely operated in daylight at extremely low altitudes. They used this tactic to minimize exposure to anti-aircraft defenses and to insure precise accuracy during bombing and strafing attacks. For their trouble, they experienced a sweeping, personal view of the war in Europe that was not available to any other group of combatants. Wing Commander John Wooldridge, writing in his book "Low Attack," summed up the experience this way:

"It would be impossible to forget … the sensation of looking back over enemy territory and seeing your formation behind you, wing-tip to wing-tip, their racing shadows moving only a few feet below them across the earth's surface; or that feeling of sudden exhilaration when the target was definitely located and the whole pack were following you on to it with their bomb doors open, while people below scattered in every direction and the long streams of flak came swinging up; or the sudden jerk of consternation of the German soldiers lounging on the coast, their moment of indecision, and then their mad scramble for the guns; or the memory of racing across The Hague at midday on a bright spring morning, while the Dutchmen below hurled their hats in the air and beat each other on the back. All these are unforgettable memories. Many of them will be recalled also by the peoples of Europe long after peace has been declared, for to them the Mosquito came to be ambassador during their darkest hours."

Like the bombers, de Havilland built sub-variants of the Mosquito adapted for day and night fighter operations. A Mosquito crew claimed the first air-to-air victory over a Dornier 217 twin-engine bomber on May 29, 1942. Many German fighters were also destroyed. From June 1944 to March 1945, Mosquitoes crews worked to defeat a menace hitherto unseen in warfare, mass attacks by low-flying, robot flying bombs propelled by pulse jet engines, the German V-1 'buzz bomb' vengeance weapons. In operations against shipping, Mosquitoes sank supply ships, and at least ten German U-boats along the French and Norwegian coasts. Mosquito crewmen flew many other unique missions including an unarmed, scheduled airline service between Scotland and Sweden. After the war, Mosquitoes laden with cameras surveyed all of India, Cambodia, and Australia. The last operational combat mission ended on December 21, 1955, when a Mosquito PR. 34A conducted a reconnaissance mission above suspected communist strongholds hidden in the jungles of Malaya.

More than 400 subcontractors built Mosquito components in England. The main factory at Hatfield and several other co-producers assembled these components into finished Mosquitoes. The aircraft was also produced in Canada and Australia and a United States production program was discussed but dropped when Canadian Mosquitoes began to fly. The U. S. did contribute by diverting production of Packard-built Merlin engines to Canada for the D. H. 98. Local deficiencies in labor and materials, and delayed shipments of critical components from England, slowed efforts to set up Mosquito production in Australia and the Royal Australian Air Force did not receive the first airplanes until 1944.

During the war, the United States, South Africa, and the Soviet Union also operated Mosquitoes. After the war ended, Belgium, France, Turkey, Czechoslovakia, Yugoslavia, Norway, the Dominican Republic, and Israel flew the D. H. 98. Canada sold 200 Mosquitoes to Nationalist China in 1947. They were disassembled, shipped, and reassembled at Chinese factories.

It often happens that when large numbers of successful aircraft fly, over time, individual airframes stand out. One particular Mosquito is noteworthy because it completed more combat missions than any other Allied aircraft. An unarmed, high-altitude Mosquito B. IX bomber, de Havilland serial number LR503 and Royal Air Force serial number GB-F, flew combat missions first in No. 109 Squadron beginning on May 28, 1943, and later in 105 Squadron. Pathfinder Fighter pilots in these two units flew a total of 213 missions in this aircraft but during a goodwill tour of Canada two days after V-E Day, GB-F was destroyed when it crashed at Calgary Airport, May 10, 1945 (see NASM Martin B-26 "Flak Bait" that completed 207 missions).

The National Air and Space Museum Mosquito, Royal Air Force (RAF) serial number TH 998, was built by the main de Havilland factory at Hatfield as a B. Mk. 35 bomber version in 1945 under Contract number 555/C.23(a). It was part of a batch numbered TH 977-999. On August 24, 1945, the Air Ministry took charge of this airplane and transferred it to No. 27 Maintenance Unit (MU) at RAF Shawbury, Shropshire. Nearly seven years later, TH 998 moved went to Brooklands Aviation Co., Ltd., at Wymeswold Aerodrome, Leicestershire, on May 14, 1952, for conversion to TT. Mark 35 standards.

Conversion was completed four months later and the Mosquito began to tow aerial gunnery targets. On September 30, 1952, TH 998 went to No. 3 Civilian Anti-Aircraft Cooperation Unit (CAAC) at RAF Exeter in Devon and received a new code letter assignment 'AT.' This civilian contractor unit performed target-towing duties for the RAF and had the distinction of operating the last Mosquitoes in Britain. After towing targets for ten years, the type was ready for retirement and ultimately, the scrap yard, but TH 998 was chosen for presentation to the Smithsonian Institution. On March 20, 1962, No. 60 MU at RAF Dishforth received the aircraft and overhauled and painting it for exhibition. On August 17, 1962, TH 998 was transported to the United States. The Mosquito is now in storage at the Museum's Paul Garber Facility and it is slated for display at the new Steven F. Udvar-Hazy Center at Dulles International Airport.