The invention of the petrol engine had enabled Charles M. Manly to design a radial engine many years ahead of its time for a flying machine built by Professor Samuel Pierpont Langley. This machine crashed just a few days before the Wright brothers' famous flight, and the US authorities officially recognized Langley's aircraft as the first one capable of sustained man-carrying free flight. This so angered Orville Wright that he put his 'Flyer' into London's Science Museum. When war began, to safeguard it from bombs it was stored in a London subway. It stayed there until, in 1942, the USA officially conceded that the Wrights had made the first powered flight. It now has pride of place in the National Air and Space Museum (Smithsonian Institution), Washington, D.C.

The biplane configuration suited the early pioneers. These 'bird-cages' could be made from wire, wood, and fabric, with no other tools than were found in the average home. And the biplane did have certain advantages, not the least of which was its small turning circle (because of its smaller wingspan for any given wing area). It was this that encouraged some fighter pilots to believe that even if monoplanes would be better as transports or bombers, the biplane would remain pre-eminent as a fighter. It could turn, dive, and loop to avoid monoplane fighters, which would just go racing past them, and have to make a long gradual turn before returning for another attack. Fighter pilots were heard to declare that monoplane fighters would always be at the mercy of biplanes.

Perhaps some RAF pilots still believed these theories when they found themselves fighting against the German jet fighters in the final months of the war. Such combats proved beyond doubt that the ability to out-turn, out-dive, or out-loop one's opponent counted for nothing against superior speed. The very-high-speed jets could literally fly rings round their slower opponents, and simply place themselves in a favourable position to shoot them down. And so it would have proved had the RAF gone to war in biplanes against the monoplane-equipped German air force.

One of the limitations of the wood-and-fabric biplane fighters had been the positioning of the armament. Only the engine was mounted firmly enough to support machine guns (guns positioned over the upper wing were usually braced from the engine cowling). Because of this, the armament of the Hawker Fury biplane fighter was just two Vickers machine guns. But the new sort of metal monoplane was to have wings strong enough to hold guns. Instead of the Fury's two guns, the Hurricane had eight.

Now the fragile biplane's manoeuvrability counted for nothing, for a two-second burst of fire could shatter it to pieces.

But in spite of the better aerodynamics that the monoplane promised and its ability to carry heavier armament and all sorts of other equipment, it would fail unless it had an engine that could provide enough thrust.

Right from the beginning, the pioneers of flight understood that they were engaged in a contest between weight (plus drag), on the one hand, and thrust (plus the lift provided by the aerofoil) on the other. So the experimenters jumped to the conclusion that lightness was the secret of success. They were wrong! Only gradually did it become obvious that power was the key to flight. Men realized that they could build aircraft of almost any size and weight, providing that the power of the engine was high in proportion to the aircraft's weight.

Bleriot, the first man to fly across the Channel, climbed out of his machine saying, "More power, more power… I'm going to get a Gnome [engine]." The same motor-car engines that had seemed too heavy to power the Wrights' Flyer were now dismissed as too small.

The internal-combustion engine was a German invention, but the French were the first to build engines specifically for aircraft. The remarkable Le Rhone Gnome rotary, to which Bleriot aspired, set a pattern for aircraft engines, and like many other French aeronautical inventions was used by German, British, American, and French air forces throughout the First World War. The progress of aviation was no longer centred upon lightness. Even the refinement of the aerofoil was set aside. By the end of the First World War, aviation became a race for horsepower per unit of weight. (See Table 1.)

In spite of many other scientific factors, it has been calculated that streamlining so improved the performance of aircraft that it required four times the power to push a 1920 biplane as was required to push a streamlined 1950 aircraft of the same size and weight.

It was natural that British engineers aimed for an engine that would produce one horsepower for every pound of weight, although designers using metric measures had no such convenient target. The remarkably successful American 'Liberty' engine of 1918 had shown what a vee-shaped twelve-cylinder design could do. And this was the type of engine that Rolls-Royce had ready for the Supermarine S.6. In 1931 this engine and airframe combination won the Schneider Trophy outright with a speed of 340.8 mph. The engine had produced one horsepower for each eleven ounces of weight (that is, 0.68 of a pound per horsepower). But this had been done with the aid of special chemical fuels, in a machine designed like that of the Wright brothers — solely for brief flights. Moreover it was designed for highly skilled pilots. The new fighter would have to be an entirely different machine.

The original designation of the Merlin engine was PV 12, for 'private venture'. Its name came from that of the pigeon-hawk, a small falcon, at a time when Rolls-Royce were naming engines after birds of prey.

The design was made possible only by the large number of technological breakthroughs from research in the 1920s. The development of fuels in the USA resulted in the discovery that tetraethyl lead suppressed detonation. In 1925 came the iso-octanes. >From now on, fuel quality was measured by means of an octane scale. In the late 1930s came all the work on blending agents, so that by 1939 most military aircraft were using 100-octane fuel (compared to the earlier 87 octane).

Fuel development mostly done in the USA spurred engine designers to improve super-chargers (to use the engine's hot, high-pressure exhaust gas). Now it was the exhaust valve seatings (which became overheated) that most concerned the engine designer, and soon the use of sodium-cooling and stellite valve seatings enormously improved performance.

While the engine designers were concentrating upon power, and chemists upon detonation, the metallurgists were making significant changes in the weight of the power units. Not only was the range of alloy steels extended to chromium, nickel, manganese, tungsten, silicon, etc., but new lightweight alloys were made. A Duralumin crank-case, for instance, was not only one tenth of the weight of aluminium alloy, but almost twice as strong.

The super-charger was vital to the high-performance fighter. It blew air into the engine and increased combustion as does blowing into a fire. A luxury on motor cars or for the low-level flying of racing aircraft, it was a necessity for aircraft that must have constant peak performance while climbing high into thin air. A typical engine lost 45 per cent of its power in the low pressure air of 15,000 feet but with a super-charger it gained an extra 5 per cent.

At first the engine designers had been attracted to using the otherwise wasted energy of the exhaust gases to drive the blowers. But by the end of the 1920s these turbo-superchargers had given place to a more conventional design driven from the crankshaft.

The new power units were the heart of the all-metal monoplanes but there were a thousand new airframe problems that would also have to be solved.