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Authors: James Hamilton-Paterson
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practically certain that not a single British military aircraft that flew in the first batch to France had a British engine. Initially, our development of both aero engines and aircraft was seriously hampered by a chronic lack of machine tools, ball bearings and magnetos, as well as steel and alloys of sufficient quality. Britain, the erstwhile cradle of the Industrial Revolution, now had only a single ball bearing factory capable of bulk output and supplies had to be imported urgently from Sweden and the United States. As for magnetos, home-grown production proved equally inadequate and until mid-1916 the RFC relied largely on a pre-war stockpile of German-made magnetos to enable its aircraft to fight Germany. 27 By 1918 the desperate modernization of British industry had gathered considerable pace and things had much improved. Even research into new alloys had become advanced.
For the first two years of the war, however, the RFC was almost entirely reliant on French-designed rotary engines. In fact, in August 1914 there were only two British-designed aero engines being built, the 60 h.p. Wolseley that powered the earliest B.E.1 and Sunbeam’s 120 h.p. Crusader. Both were V-8s and as such were bulky and heavy for their output. Rotaries were the obvious choice: it was a capable and ingenious design. Tens of thousands were built by all sides throughout the war, and yet they virtually disappeared the moment the Armistice was signed. By then their drawbacks had exceeded their usefulness.
While the rotating engine did indeed provide smoothness, it also produced a powerful gyroscopic effect that could make anaircraft easy to turn in one direction but less so in the other. This feature became notorious in Sopwith Camels, which were typically powered by the 130 h.p. Clerget engine. That aircraft also had a marked tendency to swing on take-off and landing, one of several tricky features that led to countless crashes in training. Because rotaries lacked carburettors they were tricky to control with a throttle. They tended to run ‘full on’, and the normal way to reduce power was by using a cut-out switch that prevented every other cylinder from firing and required repeated ‘blipping’ of the engine. This – together with a fuel-air mixture control that demanded constant monitoring – made flying all rotary-engined aircraft a handful, and the Camel most of all. Rotaries also worked on a ‘lost oil’ principle that used great quantities of castor oil, much of which was sprayed back half-burnt over the pilot.
None of this was ideal, although it had to be lived with at the time. The real reason why the engines fell out of fashion so quickly after the war was because aircraft designers wanted more and more power. Rotaries were comparatively slow-revving and the propeller could only turn as fast as the engine, unlike stationary engines where the power could be greatly increased and the propeller geared for maximum efficiency. (The underlying problem is that petrol engines reach their maximum efficiency at relatively high speeds, whereas propellers are more efficient at lower speeds.) During the First World War rotaries were developed as far as they could be, even acquiring a second bank of cylinders ‘staggered’ with respect to the first. The high point was probably reached with the Bentley BR.2, a magnificent 25-litre rotary engine whose single bank of nine cylinders produced 250 h.p. But rotaries had reached their limit for reasons of simple physics. The faster the cylinders whirled, the more the drag on them increased (since drag in air increases with the square of the velocity). But the power needed to overcome drag is the cube of speed, and very soon a point was reached when much of a rotary engine’s power was spent in making itself turn.
*
Given the various armies’ more or less dismissive attitudes to aviation at the outbreak of war, it is ironic how quickly they came to rely on aircraft as engines and airframes
For the first two years of the war, however, the RFC was almost entirely reliant on French-designed rotary engines. In fact, in August 1914 there were only two British-designed aero engines being built, the 60 h.p. Wolseley that powered the earliest B.E.1 and Sunbeam’s 120 h.p. Crusader. Both were V-8s and as such were bulky and heavy for their output. Rotaries were the obvious choice: it was a capable and ingenious design. Tens of thousands were built by all sides throughout the war, and yet they virtually disappeared the moment the Armistice was signed. By then their drawbacks had exceeded their usefulness.
While the rotating engine did indeed provide smoothness, it also produced a powerful gyroscopic effect that could make anaircraft easy to turn in one direction but less so in the other. This feature became notorious in Sopwith Camels, which were typically powered by the 130 h.p. Clerget engine. That aircraft also had a marked tendency to swing on take-off and landing, one of several tricky features that led to countless crashes in training. Because rotaries lacked carburettors they were tricky to control with a throttle. They tended to run ‘full on’, and the normal way to reduce power was by using a cut-out switch that prevented every other cylinder from firing and required repeated ‘blipping’ of the engine. This – together with a fuel-air mixture control that demanded constant monitoring – made flying all rotary-engined aircraft a handful, and the Camel most of all. Rotaries also worked on a ‘lost oil’ principle that used great quantities of castor oil, much of which was sprayed back half-burnt over the pilot.
None of this was ideal, although it had to be lived with at the time. The real reason why the engines fell out of fashion so quickly after the war was because aircraft designers wanted more and more power. Rotaries were comparatively slow-revving and the propeller could only turn as fast as the engine, unlike stationary engines where the power could be greatly increased and the propeller geared for maximum efficiency. (The underlying problem is that petrol engines reach their maximum efficiency at relatively high speeds, whereas propellers are more efficient at lower speeds.) During the First World War rotaries were developed as far as they could be, even acquiring a second bank of cylinders ‘staggered’ with respect to the first. The high point was probably reached with the Bentley BR.2, a magnificent 25-litre rotary engine whose single bank of nine cylinders produced 250 h.p. But rotaries had reached their limit for reasons of simple physics. The faster the cylinders whirled, the more the drag on them increased (since drag in air increases with the square of the velocity). But the power needed to overcome drag is the cube of speed, and very soon a point was reached when much of a rotary engine’s power was spent in making itself turn.
*
Given the various armies’ more or less dismissive attitudes to aviation at the outbreak of war, it is ironic how quickly they came to rely on aircraft as engines and airframes
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