Profile – DH Mosquito

The aircraft became equally effective as a night fighter, intruder, pathfinder, photo-reconnaissance, high-speed courier and target tug, to name some of its roles. It was also developed as a high-altitude fighter.

The Mosquito was also modified to carry two Highball weapons, similar to but smaller than the Barnes Wallis bouncing bomb used on the famous dam-busting raid. Although trials were successful and 618 Squadron was trained to use these revolutionary weapons, there was official reluctance to use them in case the idea was copied by the enemy.

Another remarkable feature of the Mosquito was its wooden structure, which many people predicted would be torn apart by the enormous power of the Merlins. It was more than strong enough and absorbed battle damage better than metal aircraft and was easier to repair. Timber was a non-strategic material and used labour from the furniture and allied trades which would otherwise have been unable to contribute to the war effort.

The stressed skin wooden sandwich structure had been developed by de Havilland for the Comet Racer of 1934. It was also used in the Dragonfly and scaled up for the Albatross airliner. The shell consisted of two layers of plywood separated by balsa wood. The whole section was formed over concrete moulds and glued to form a light, strong and very smooth structure, capable of using to full advantage every horsepower developed by the Merlin engines.

The Mosquito design team moved to the comparative seclusion of the moated Salisbury Hall near St Albans in October 1939 to begin work on the aircraft as a company-funded venture away from prying eyes. The team was gradually strengthened, filling every available space in the small stately home, and a hangar was built just across the moat.

The main problem was to obtain materials since none could be obtained without official approval. Only the backing of Sir Wilfred Freeman, an Air Council member for research and development, kept the project alive and allowed de Havillands to make progress. For his perseverance, the project became known as “Freeman’s Folly”, but this was soon forgotten when the true worth of the aircraft became known.

The first prototype was produced as an unarmed bomber, with the second similar airframe as a photo reconnaissance aircraft. Both were built in the hangar at Salisbury Hall and on completion were dismantled and taken the five miles to Hatfield by road.

Although it was produced in great secrecy there were no special security arrangements which might have alerted the local population and the new buildings were disguised as farm buildings.

During the peak period of development, 12 12-hour days were worked for six and a half days a week. Bombs were dropped on the surrounding fields and the first German land mine to be captured was recovered near to Salisbury Hall, when its parachute became caught in the branches of a tree. Had it exploded much of the Mosquito project would have been destroyed.

On the night of May 13, 1941 a German named Karl Richter landed by parachute about two miles from Salisbury Hall, but he turned out to be a courier carrying funds and he was ill-prepared for his stay in Britain. Not only had he been expected, but he had landed many miles from where planned and he was soon captured by a local policeman. Despite all the bombs and the landing of a potential spy, there was no evidence to suggest the enemy knew the true purpose of the activities at the moated stately home.

Salisbury Hall escaped damage by the enemy, but Hatfield was not so lucky. On October 3, 1940 a stray Ju 88 pilot missed his target of Reading and came across Hatfield aerodrome, dropping his bomb load on the factory. The Ju-88 was hit by the guns defending the airfield and crashed near the country town of Hertford.

This was the only direct hit of the war, but 21 people were killed and a further 70 injured. The bombs also destroyed about 80 per cent of the stock of raw materials and much of the work in progress on the Mosquitoes.

This resulted in the decision to disperse the production of the aircraft widely, so no enemy bombs could cause a major disruption.

A month after this raid, the yellow-painted Mosquito prototype was moved to Hatfield by road. The maiden flight was made on November 25, 1940 by Geoffrey de Havilland Jnr, the elder son of the founder of the company and chief test pilot since October 1937.

The prototype became the principal development of Mosquito. Its service trials at Boscombe Down were slow to begin, due to official apathy. But as the aircraft began to demonstrate its qualities, the interest in it grew. More and more visitors appeared from London to inspect this new aeroplane. There was a temporary setback when the tail wheel caught in a net during taxiing and the fuselage was fractured. It was soon demonstrated how easy it was to repair the wooden structure, and a strengthening patch was put over the port fuselage by the wing trailing edge as a repair. This patch can be seen on the prototype now preserved in its birthplace at the Mosquito Aircraft Museum, at Salisbury Hall.

Towards the end of 1940 official interest in the Mosquito had grown so much that orders were placed for over 200 aircraft, including the important initial batch of 50 aircraft which had been secured by de Havilland against all the difficulties. However, the demand was more for photo reconnaissance aircraft and fighters, the latter in particular becoming important. So urgent was the fighter requirement, that it was decided to save a month of dismantling and re-assembly by flying the prototype out of the fields adjacent to Salisbury Hall.

Therefore, on May 15, 1941, Geoffrey de Havilland Jnr flew the lightly loaded aircraft out of a 450-yard field adjacent to its assembly hangar.

Although visually similar, the fuselages of the bomber/PR and fighter variants differed in some major details. The bomber versions had a Vee windscreen, a bomb aimer’s clear blister in the nose and a crew entry in the cockpit floor. The fighter versions had a flat bulletproof windscreen for the gunsight, and had four 303-inch machine guns in the unglazed nose in front of an armoured bulkhead, with four 20 mm cannons under the cockpit floor, which meant the re-location of the crew entry door to the fuselage side.

Five Mosquitoes were built at Salisbury Hall, three of which were flown out before production of all versions became established at Hatfield. Another line of fighter types was set up at a new de Havilland shadow factory at Leavesden which is now the Rolls-Royce factory producing helicopter engines. The Standard Motor Company at Coventry later produced fighter bomber Mosquitoes and Percivals at Luton concentrated on bomber types.

Meanwhile, production had been set up in the Dominions with de Havilland Canada and de Havilland Australia, the Canadian production designed to supplement the UK aircraft, particularly with bombers, and Australia to supply the needs in the Far East. All the overseas production was powered by Merlin engines licence-built in the USA by Packard. A total of 7619 Mosquitoes were built, requiring some 20,000 Merlin engines. The Mosquito and the Merlin combined to make a unique partnership. It was one of the few airframe/engine partnerships which continued from the prototype, right through to the last one built.

The first Mosquito flew with a pair of Merlin 21s developing 1460 hp each and the ultimate production NF Mk 38 was powered by the Merlin 114As developing 1710 hp. This added power did not increase the top speed, but advantage was gained by increasing the all-up weight from 19,670 lb to 23,000 lb and the ceiling from 35,000 feet to 43,000 feet. One refinement that did increase. The top speed of the night fighters was the injection of nitrous oxide into the inlet manifold. For this purpose, fighter bomber Mosquitoes were used and the gas cylinders were carried in the bomb bay. The gas gave an additional 30 kt for five minutes and increased the ceiling by 3000 ft.

The Mosquito commenced operations on unarmed photo reconnaissance with No. 1 PRU (Photo Reconnaissance Unit) at Benson in Oxfordshire on September 17, 1941, following delivery of the second aircraft W4051 on July 13, only eight months after the maiden flight of the prototype. It had greater endurance and speed than previous PR aircraft, plus the security of two engines and a second crew member who could help pinpoint the target areas more accurately. The Mosquito ranged far and wide over Europe, photographing prospective targets, and then going over the bombers to assess the damage.

The security of the two Merlin engines was always appreciated, as if one stopped the aircraft could maintain height until over a friendly airfield, and make a safe landing. In general, it was not expected that height could be gained, but this was proved wrong during a PR sortie with 60 Squadron SAAF from Italy.

While flying in the Salzburg-Innsbruck area, a Me 262 jet fired from behind a Mosquito causing damage to the port propeller and reduction gear which had to be feathered. The aircraft dived on the remaining engine to escape the Me 262, which it shook off in a cloud. The crew then realised that their altitude of 9000 feet (2743 m) was insufficient to clear the Alpine barrier between themselves and their base. It was necessary to clear 14,000 feet (4267 m), so the pilot selected a maximum boost for 30 minutes, well above the recommended time, and climbed successfully over the Alps. Despite bad weather, they made a safe return to base where the shot-up engine was changed. Not surprisingly, the punishment engine had to be changed not long after.

Mosquito bomber operations began from Horsham St Faith, near Norwich in May 1942, their main use being in the precision high-speed, low-level bombing of small targets, such as Gestapo-occupied buildings, which were often in the midst of friendly occupied territory. Mosquito bombers were used on spoof raids to divert the attention from the main bomber force and with the bulged bomb bay of the B Mk XVI, a 4000 lb (1814 kg) bomb could be carried, which was the same load as the four engine, USAAF B-17 Flying Fortress.

The Mosquito bombers were also ideal for pathfinder operations, where, using the skilled crews and the latest in navigational aids, they could mark the target areas accurately, even when obscured by smoke or cloud.

The most dramatic and risky Mosquito operations were with the fighter bombers, flying at low levels. The aircraft could drop a significant load in a pin-point precision attack, . such as the famed Amiens prison raid. If there were defending guns, the offensive armament of the Mosquito could keep the enemy heads down. Similar intruder operations, using guns only, were effective at train busting and attacking German aircraft at night when they were landing at their bases.

The fighter bombers were also extremely effective on coastal strike operations, particularly around the Norwegian coastline. A small number of Mosquitoes were fitted with a six-pounder 57 mm anti-tank gun, in place of the normal four 20 mm cannons. This could punch fairly large holes in the sides of ships.

Although these were very effective, no squadron was fully equipped with this version, as by dispersing them amongst various units, a greater capability could be achieved during any sortie. Rocket projectiles were also fitted on underwing racks; they had no recoil action and were highly destructive.

A salvo from 60 lb semi armour-piercing explosive rocket projectiles was equivalent to a broadside from a cruiser, but the crews preferred the solid heads because they did not throw up exploding debris which was a hazard when flying low over the target. Anti-shipping operations were more dangerous than many others; they were usually in daylight and attacking well-defended targets. U-boats were also a much sought-after target, and the climax came on April 9, 1945, when the Banff Strike Wing sank three U-boats in the Kattegat.

The Mosquito night fighter operations perhaps became most well known due to the exploits of John Cunningham, who gained the reputation of having excellent night vision, in an effort to conceal the use of radar. Many of his early successes were in Beau fighters, but with improvements in enemy aircraft, an aircraft with the speed and manoeuvrability of the Mosquito was needed.

The Merlins could push the Mosquito close to 500 mph in a dive after a night enemy raider, but the radiators in the inner wing leading edge were vulnerable to damage from pieces of wreckage of a destroyed aircraft. It was not uncommon to return on one engine, but at least there was one enemy aircraft less.

Mosquito night fighters were originally equipped with a very basic radar set with a fixed arrow-head aerial in the nose between the four machine guns. As radars improved, so the machine guns were removed from the nose. They were replaced with a radome covering radar scanners that could search from side to side through 180 degrees and tilt up or down. Targets always had to be identified visually to avoid destroying an allied aircraft. Mosquito night fighters continued to fly with the RAF on peacetime operations until May 1952, when the last was replaced by the jet-powered night fighters.

Mosquitoes also flew high-speed, unarmed, courier services, operating as civil aircraft. They were flown by airline crews from Leuchars in Scotland to Stockholm in Sweden. Not only could urgent cargo be carried, including ball bearings required to keep Britain’s industry moving, but on occasions passengers were carried in the bomb bay, lying on a mattress with oxygen mask, radio communications with the crew and a reading lamp. There were more comfortable ways to travel, but this was the quickest and the least risky.

Mosquitoes were used by the Royal Navy and on March 25, 1944 the first multi-engined landing was made on the carrier HMS Indefatigable by a Mosquito. The aircraft were later fitted with folding wings and each could carry a torpedo in addition to its other offensive loads.

Mosquitoes were used for meteorological reconnaissance, training, target towing, gust research and electronic countermeasures. In addition to being built overseas, they were exported widely during and after the second World War.

Of the thousands of Mosquitoes built, only a handful still exist. Some 31 have survived in various stages of preservation throughout the world but sadly perhaps, only one (operated by British Aerospace to give flying demonstrations at air shows) is regularly flying.

Philip Birtles was a student of the de Havilland Aeronautical Technical School and joined the de Havilland Aircraft Company at Hatfield in 1957. In 1962 he was appointed personal assistant to John Cunningham, chief test pilot. He became, in 1969, deputy public relations manager, at Hawker Siddeley Aviation at Hatfield. Since 1979 he has been public relations manager, at British Aerospace Dynamics Group, Stevenage division, and has recently had a book – Mosquito, A Pictorial History – published by Jane’s.