24 June, 1940 Compared with the old Supermarine assembly plant in Eastleigh, the white office building of de Havilland Aircraft Company in Hatfield was ...

24 June, 1940

Compared with the old Supermarine assembly plant in Eastleigh, the white office building of de Havilland Aircraft Company in Hatfield was a showpiece of modern functionalism. Spreading low behind a well-maintained lawn, the well-proportioned two-storey pavilion reflected the renewed prosperity and confidence of the 1930s. The style was “streamlined”, with curved corner windows surrounding the inset, centrally placed portico in art deco style. On both sides of the wide facade, enormous windows lit the modern management, design and drawing offices.

De Havilland ranked as one of the prime British aircraft companies, but contrary to most of their competitors, their success was mainly based on a civilian market. During the 1930s, the company established its reputation as a manufacturer of elegant, aerodynamic, light-weight aircraft with higher performance than suggested by their engine power. Their Moths and Dragons set the standards in recreational flying, business use, and light transport. The key to their success was the width of the company’s offering: de Havilland were unique in building their own engines and propellers, not only for their own aircraft, but as suppliers to other companies.

In the morning hours of 24 June 1940, a convoy of lorries departed from the factory yard behind the office. One by one, they proceeded along the driveway to the main gate. Each lorry, besides its cargo, carried a crew of men in grey and blue workshop coats. Once on the main road, the vehicles spread in different directions. The lorries and their crew were all civilian, but their assignment was highly secret.

Their mission was to bring the Spitfires’ performance on par with that of the German Messerschmitt Bf 109.

The actual story started two weeks previously, when RAF engineer officer telephoned Hatfield to ask whether “without a lot of paperwork and fuss”, de Havilland two-pitch propellers installed in the RAF Spitfires then in service could be converted to constant-speed operation, starting with a trial installation on one aircraft.

Back in 1935, following the success of their own Comet racer, de Havilland acquired a licence from Hamilton Standard in the United States for variable-pitch propeller mechanism.

de Havilland were the first to offer variable-pitch propellers on the British market

As opposed to the fixed propellers which were widely used since the beginning of aviation, variable-pitch propellers could take better advantage of the power supplied by an engine in much the same way that a transmission in a car takes better advantage of its power source. While the first variable-pitch propellers dated as far back as 1917 at Britain’s Royal Aircraft Factory, it was the development of electrically and hydraulically adjustable mechanisms in the 1930s which brought their wide adoption, starting with the Unites States. De Havilland was the first company to supply such propellers in Britain, which made it an obvious choice for the Spitfire and the Hurricane.

Constant-speed, fully feathering propellers were already widely employed on multi-engine aircraft, but not in fighters because of the weight penalty of the mechanism – the metal de Havilland propeller was already much heavier than the wooden two-bladers used on both the Spitfire and the Hurricane initially, causing some CG and torque-related problems. So the RAF had adopted a simpler two-pitch mechanism from de Havilland as an interim measure, relying on a home-designed Rotol (a joint venture between Rolls-Royce and Bristol) to produce a lightweight, constant-speed wooden propeller for fighters. The de Havilland model was of “bracket” type, allowing only two settings: fine for take-off and coarse for all conditions of flight.

Earliest production Hurricanes and Spitfires, including many of the Hurricanes sent to France, featured fixed two-blade propellers.

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Major Werner Mölders of Luftwaffe’s JG 51 had the opportunity to test captured British Spitfire and Hurricane against their own Bf 109E:

“It was very interesting to carry out the flight trials at Rechlin with the Spitfire and the Hurricane. Both types are very simple to fly compared to our aircraft, and childishly easy to take-off and land. The Hurricane is good-natured and turns well, but its performance is decidedly inferior to that of the Me 109. It has strong stick forces and is “lazy” on the ailerons. The Spitfire is one class better. It handles well, is light on the controls, faultless in the turn and has a performance approaching that of the Me 109. As a fighting aircraft, however, it is miserable. A sudden push forward on the stick will cause the motor to cut; and because the propeller has only two pitch settings (take-off and cruise), in a rapidly changing air combat situation the motor is either overspeeding or else is not being used to the full.”

A constant-speed propeller of the Bf 109 was a superior alternative as the pitch could be adjusted freely to match engine RPM, optimising the thrust for any given condition in flight. In Britain, Spitfires had been delivered with Rotol propellers since November 1939, but the numbers were insufficient to employ it in all aircraft. More importantly, the Rotol required a specially modified Merlin engine which made it suitable primarily for new production aircraft.

De Havilland’s reply on the phone line was well-informed and to the point. Yes, the mechanism was essentially the same as their more advanced Hydromatic propeller, so technically it should not pose much of a problem. The propeller pitch range would have to be extended, but it could be done without exchanging the blades. Connection to a secondary engine shaft would be needed to drive the hydraulic pump; then there would be some new external oil pipes and new propeller control unit in the cockpit. de Havilland could produce the necessary conversion parts on their own, including the accessories for the Merlin, in order to avoid disturbing Rolls-Royce, who were snowed under heavy demand for their engines. Paperwork would be inevitable to divert the necessary work force and materials if conversion of all aircraft in service was to follow.

The company set about producing the necessary parts for the prototype conversion, and, having received one Spitfire from No. 65 Squadron, completed the work in only four days. The modified Spitfire performed admirably, to everyone’s surprise reaching the same level of performance as the same aircraft with Rotol constant speed unit. The modified Spitfire had a shorter take-off run, was much faster in climb, and its maximum altitude increased by 7,000 feet. All these attributes were direly needed by Dowding, who was fully aware that every second to altitude mattered if his fighters were to meet the incoming enemy formations on anything resembling equal terms.

Lord Beaverbrook embraced the idea and on 22 June, verbally instructed de Havilland to convert in the field all the Spitfires, Hurricanes and Defiants in service, in the following priority: Spitfire squadrons first, then Hurricanes and Defiants, then aircraft in Maintenance Units. Resources were to be diverted as necessary, at the expense of other ongoing production. No formal contract was necessary; that could be taken care of later.

A de Havilland clerk remarked “We shall probably never be paid for this work” – and very nearly they weren’t. Even in 1943, with Beaverbrook no longer in position as Minister of Aircraft Production, the Air Ministry was asking in vein for a record that the work had been done. It it a fantastic testimony of Geoffrey de Havilland’s leadership that his company never hesitated to deliver.

On the aforementioned 24 June, de Havilland sent engineering parties to twelve Spitfire stations, each lorry carrying six conversion sets. The engineers were to perform the first conversion, educate a selected team of RAF fitters at each station, then proceed to the next location. This way, the time to convert an entire squadron was limited to ten days and performed mostly by the local staff. All Spitfire squadrons would be completed in a record time by 20 July.

By 16 August, every Spitfire and Hurricane in service had been converted with constant-speed propellers – 1,051 aircraft in all. Constant-speed propellers gave the RAF fighters a significant extra margin in performance just in time for their all-out battle, when an edge in altitude, speed or climb could mean all the difference to the fighting pilots.

A still from 1940 newsreel showing a mixture of Rotol (recognisable by their blunt spinners) and de Havilland propellers employed on first-line Spitfires during the Battle.