Only in 1939 was it decided that the RAF fighters must be able to fire a continuous burst of 300 rounds (from each gun). This required a complete redesign of the muzzle attachment. This modification was not available to all units until as late as May 1940. It was almost too late.

The RAF incendiary bullet was almost too late for the Battle of Britain too. In January 1939 the De Wilde bullet design was purchased for 30,000 in spite of its defects (which were not mentioned to the Belgian inventor for fear of telling the Germans about the test results). Modified by British experts, this De Wilde bullet (its name was unchanged) was the best of its type and contributed to RAF success in 1940.

Wing armament, in fighter aircraft of all nations, usually gave rise to two major problems: the positioning of the ammunition (and its feed to the gun) and the cold condensation of high-altitude flight. This was particularly troublesome when aircraft climbed through cloud to heights where the condensation froze. These problems had not been solved by 1939, because in peacetime the RAF seldom did much flying in bad weather. The first winter of war was severe enough to make the freezing Brownings a major problem. As an emergency measure, oil diluted with paraffin was applied to the guns, to cure the problem of dry cold. However, it gave the armourers a great deal of work, for it was ineffective as a rust-preventative.

The problem of wet cold encountered in cloud was solved, up to the moment of the first shot, by sealing over the gun muzzles with stretched pieces of fabric. As a longer-term solution, engine heat was fed to the guns.

Any comparison of the Merlin engine and the Daimler-Benz DBA must begin by mentioning the latter's fuel-injection system. The work of Rudolf Diesel, with Karl Benz, Gottlieb Daimler, and Nicholaus Otto, had made the modern internal-combustion engine virtually a German invention. Diesel's work gave German designers a big start when they applied the fuel-injection method to petrol engines.

Fuel injection, which puts a measured amount of fuel into each cylinder according to temperature and engine speed, etc. was demonstrably superior to the carburettors that the Merlins used. Carburettors are, at best, subject to the changes of temperature that air combat inevitably brings. At worst they bring a risk of freezing or catching fire. And with such large, high-performance engines, the carburettor system seldom delivers exactly the same amount of fuel simultaneously to each cylinder. Worst of all, the carburettor was subject to the centrifugal effect, so that it starved, and missed a beat or two, as it went into a dive.

The RAF pilots learned how to half-roll before diving, so that fuel from the carburettor was thrown into the engine instead of out of it, but in battle this could be a dangerous time-wasting necessity.

It was a legacy from the biplane age that pilots, feeling defensive about their equipment, claimed that they had the advantage of manoeuvrability. Pilots of Spitfires, Hurricanes, and Bf 109s all claimed that their aircraft had the tightest turning circle, but the 32-foot wingspan of the Bf 109 gave it the advantage over its rivals. In spite of its high wing loading, it had a turn radius of only 750 feet (Spitfire 880 feet and Hurricane 800 feet), and this could be a vital factor in air fighting.

German fighter pilots envied the sheet of armour plate that was behind the seat of the RAF fighters. Seat armour for fighters had been rejected by British officialdom because it would 'spoil the balance' of the aircraft. Ignoring this, 1 Squadron, in France early in the war, took a sheet of back armour from a wrecked Fairey Battle light bomber and fitted it to a Hurricane. To convince all concerned that it didn't spoil the balance, one of the pilots returned to England and performed a hair-raising series of aerobatics before an audience at Farnborough. After that it became a standard fitting. As we shall see, this proved a vital factor in the 1940 fighting.

Another semi-official modification that had an important effect on the Battle resulted from a phone call to de Havilland's propeller division. It came from Flight Lieutenant McGrath and it came as late as 9 June 1940. He asked if it would be possible to try out a constant-speed propeller on a Spitfire "without a lot of paperwork and fuss." The propeller added 7,000 feet to the service ceiling and so transformed the aircraft's performance that the Air Ministry authorized all Hurricanes and Spitfires to be changed to constant speed props (which the Bf 109 had as standard equipment). Each squadron selected its finest fitters, and the de Havilland experts went from airfield to airfield demonstrating the first conversion, supervising the second conversion, and watching the third, before handing over to the home-team and travelling on to the next squadron. A test pilot followed them. A total of 1,050 props were fitted by 15 August. All without a price being agreed, a contract written, or any paper signed. More than one de Havilland executive was convinced that they would never get paid for the work but they continued with it anyway.

Considering that the constant-speed unit was a British invention, it was remarkable that the Air Ministry sent its Hurricane and Spitfire to war with fixed-pitch, two-bladed wooden props, or at best with two-pitch (variable-pitch) propellers. The constant-speed unit was a simple governor that kept the propeller's speed (and the engine's speed) constant by varying the angle at which propeller blades bit into the air. For take-off, the blades were turned to bite very little air which, with the engine at full power, gave take-off thrust. For full speed, the constant-speed unit took over to give the maximum size of bite and so maximum forward speed. The angle of the old fixed-pitch prop was a compromise for all these situations. The constant-speed propeller's value to the squadrons was not only in the improved performance it conferred but in the much reduced wear and tear on the engines.

The flying characteristics of the Spitfire and Bf 109E-3 were only marginally different, and some of the differences were simply a matter of personal preferences. Both machines came off the ground very quickly the Messerschmitt had a very short run. Both swung on take-off, but in straight and level flight the Spitfire could be trimmed to fly feet off. The Messerschmitt required a boot on the rudder bar all the time. The German pilots tolerated it and soon got used to it, but Allied pilots who flew captured machines usually complained about this.

Visibility — life and death for combat flyers — was incomparably better in the Spitfire. The bubble-shaped hood gave head room and a chance to see down, up, and around. The 109 hood had thick bars like a prison and it sometimes touched the top of the helmet. (Later this hood was changed on the advice of Galland.) The 109 cockpit was very small indeed but there again it was what one got used to; some German pilots liked it. Both aircraft were light and delightful to fly at medium speeds but both became hard work at high speeds. This was a telling fault because in a diving attack this meant a great deal of muscle was required to bring the target into the gun-sights. Eventually both air forces discovered that this problem was due to the effect of high-speed airflow over the fabric-covered ailerons. When the ailerons were metal covered, like the wings, high-speed flying became much lighter.

[Turning Circles

(Drawn to Scale)

Figure 12

These curves are drawn to scale for the three single-seat fighters flying at 300 mph at 10,000 feet. This is for a vertically banked turn, in which there is no aerofoil providing lift. Such a turn is theoretically possible by inclined thrust (tilting the nose slightly skywards) but more or less impossible in practice, which is why all dog-fights moved downwards (and fighting at low altitudes carried a real risk of aircraft hitting the ground by accident).