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The quest for efficiency with lightness and reliability

The Gipsy Six I is a six-cylinder air-cooled motor

INVERTED. The Gipsy Six I is a six-cylinder air-cooled motor of 200 horse-power and weighs 468 lb. The cylinders are below the crankcase. Two Gipsy Six engines were used in the D.H. Comet, which won the England-Australia race in 1934 in 2 days 22 hours 58 minutes. A smaller model, the Gipsy Major I, is fitted in the Tiger Moth and similar aeroplanes.

THE majority of the aircraft engines in use today fall into one of three categories - the vertical engine, the V type and the radial engine. These three main types have established themselves after nearly forty years of research and experiment, and they all have a common origin, which is the light internal combustion engine, as used in the earliest motor cars.

When Dr. Otto introduced the light internal combustion engine in the seventies he laid the foundations of aircraft propulsion. His invention made controlled flight - as opposed to mere ascent in the air - a possibility.

Even the earliest internal combustion engines had a power-weight ratio which was much higher than that obtained from other forms of prime mover. It was this relatively high ratio that enabled the Wright brothers to make the first aeroplane flight in 1903.

The early motor cars played an important part in the development of the modern aircraft engine, although they were designed and used many years before the first aeroplane flight was made. It was the use of the internal combustion engine in these early cars that first led designers to attempt to decrease the weight of their engines and to increase the power output. The later history of the aircraft engine is an account of constant efforts to improve the power-weight ratio by every means within the designers’ reach.

All the early attempts to fly heavier-than-air machines, however, were carried out with aeroplanes equipped with adapted car engines, and no particular attempts were made to lighten them. Their power-weight ratio was generally approximately 8 lb per horsepower. The engine used for the first aeroplane flight was even less efficient; its weight was some 240 lb and it developed roughly 15 horse-power. It was a water-cooled four-cylinder engine mounted in the machine in such a way that its cylinders were horizontal.

The aircraft engine designer has from the first been faced with problems quite different from those of the automobile designer. In the motor car a heavy flywheel plays a most important part in the achievement of steady running, but a component as heavy as a flywheel cannot be used in an aero engine. The propeller has a flywheel action, but its effect at low speeds is not great, and the pioneers were dealing with speeds which are considered low today.

In their efforts to obtain smoother running without the use of a flywheel, the early designers increased the number of cylinders in their engines, thus securing a more even torque with an increased number of impulses for each revolution. The importance of producing an engine of light weight was apparent from the first, and this was achieved by shortening the length of the engine, by directly reducing the weight of the crankcase, and by using shorter and lighter crankshafts, fewer bearings, lighter bolts, liners and the like.

By energetic application of such methods, designers had succeeded, as early as 1909, in producing engines in which the power-weight ratio was far more favourable than that of motor car engines of the same period. For example, Green developed a six-cylinder vertical engine of 120 horse-power weighing only 3·66 lb per horse-power. It was in an aeroplane fitted with one of these engines that J. T. C. Moore-Brabazon won in 1909 a prize of £1,000 offered by a London newspaper. It was soon realized, however, that there was a limit to the number of cylinders that could be used with efficiency in the vertical type of engine with cylinders in line. The power-weight ratio increased rapidly after the first cylinder, but by the time four cylinders had been installed it was impossible to improve conditions greatly by adding more. This limitation led designers to break away from convention and to group their cylinders in two banks arranged in the form of a V. Such an arrangement, with banks of four or six cylinders each, is extensively used today for high-powered engines.

Pioneer designers were attracted also by the possibilities of mounting the cylinders radially round the crankshaft; this scheme reduced to a minimum the length of the crankshaft and crankcase and made possible further reductions in weight.

A logical development of the radial engine was the rotary engine, in which the crankshaft remained stationary, the cylinders and crankcase revolving about it. This type was largely developed in France, and the first Gnome rotary engines, exhibited at the Paris Salon in 1908, attracted much interest and promised to revolutionize the design of the aero engine. The early Gnome engines weighed little more than 3 lb per horse-power, which, at that time, represented an extraordinarily high degree of efficiency.

When the Gnome engines were introduced they were regarded as a permanent solution to many problems. When, however, they had been in practical use for some time their inherent disadvantages were realized. A large amount of power was wasted in rotating the cylinders, and various cooling problems arose. The gyroscopic effect of the heavy rotating parts had to be compensated for in the design of the aircraft in which rotary engines were used, and high fuel and oil consumption also began to worry the designers.

An “Ideal” Specification

Despite all these disadvantages the Gnome engine was extensively used during the war of 1914-18, but it was finally superseded by improved forms of the older types of engine, with cylinders in line or with the radial arrangement.

The Gnome engines of 1908 used seven cylinders and developed 50 horse-power. They were followed by an 80 horse-power Le Rhone engine and by a 100 horse-power Gnome, these engines being of the nine-cylinder type. A Clerget engine, also with nine cylinders, developed 80 horse-power, and the three types were all in use during the war period. In Great Britain Captain W. O. Bentley designed two rotary engines, the B.R.1 and B.R.2. These were the last types produced before the design became obsolete.

Meanwhile the radial type of engine was undergoing development, and French designers were among its champions. This type, however, declined in popularity from 1915 onwards, until fresh impetus was given to its development by the appearance of new engines of high efficiency. Two such engines were the A.B.C. Wasp and Dragonfly, both of which had the remarkable power-weight ratio of 1·75 lb per horsepower. The Dragonfly engine was adopted by the Air Ministry for many types of Service machines.

The Dragonfly was a nine-cylinder air-cooled engine with a bore of 5·5-in and a stroke of 6·5-in, and it developed 340 horse-power at 1,650 revolutions a minute. The cylinders were machined from steel forgings and had radiating fins over most of their length, the outer surface being coated with copper to assist radiation. The pistons were of the aluminium alloy “slipper” type, with two piston rings above the gudgeon pins.

The next radial engine to be developed was the Bristol Jupiter, which appeared in 1921. This also was a nine-cylinder engine, of somewhat greater capacity than the A.B.C. Dragonfly. Its bore was 5·75-in, its stroke 7·5-in, and it developed 400 horsepower at 1,550 revolutions a minute.


V-TYPE RENAULT OF 1913, developing 80 horse-power. This French engine had eight air-cooled cylinders, arranged in two banks of four. The earliest V-type engines used in aircraft were small two-cylinder and four-cylinder motor car engines. Today the V-type is considered one of the best layouts for liquid-cooled engines, whereas air cooling finds favour in vertical or radial designs.

In 1917 the British Air Ministry issued an “ideal” specification to encourage research. This led to the designing of another radial engine, the Armstrong Siddeley Jaguar of 1924. This fourteen-cylinder engine developed 385 horsepower at 1,700 revolutions and incorporated several features of design that were before their time. Power-weight ratios were not being improved solely by the reduction of weight; by this time carburation and ignition systems had been greatly improved. Oil and fuel consumption and general reliability were considerably improved also.

Some idea of the enormous technical advances of twenty-one years may be gained from the fact that the Bristol Jupiter engine weighed slightly more than three times as much as the original Wright engine, but developed twenty-six times the power.

The Jupiter and Jaguar engines were extensively used in Service aircraft, and they reigned supreme among radial engines for some years.

Introduction of Aluminium

Rapid as development seemed to be during the period between 1914 and 1924, it continued apace, and to such good effect that the Bristol Pegasus X, produced in 1936, had a power-weight ratio of 1·09 lb per horse-power. This engine was superseded in 1937 by the Pegasus XXII, and two entirely new engines, the Pegasus XVII and XVIII, were developed with two-speed superchargers.

The range of Pegasus engines in 1937 included the Pegasus Xc, which was used by Imperial Airways to equip the Empire flying boats. The increased power outputs and decreased weights were obtained in several ways. Improved fuels have contributed largely to the achievement of higher power outputs. Higher degrees of supercharging, higher compression ratios and increased crankshaft speeds have also been important lines of development. At the same time, weight has been reduced by refinement in the design of various components, and by the introduction of improved steels of greater strength and durability. The judicious use of magnesium castings has made a valuable contribution to development.

Vertical engines have reached their present standard of efficiency by a continuous line of development which began with the first aircraft engines. Early engines always used cast iron for their cylinder blocks. It was an ideal material in several ways; it had excellent wearing qualities under friction, it was easy to mould, cast and machine. It was, however, a weak material in tension and bending, and thick, heavy sections had to be used. The rotary type of engine was the first to use steel cylinders, but steel was soon used for vertical engines also. By 1919 cast iron was obsolete as a material for cylinder walls and bearing surfaces. Hispano-Suiza engines were the first to use cast aluminium cylinder blocks with steel liners, and aluminium was used in a British engine in 1909. In 1911 this 60 horse-power Green engine won a prize of £1,000, offered by Mr. Patrick Alexander.


 ROLLS-ROYCE KESTREL. This power unit has twelve water-cooled cylinders, arranged in two banks of six at an angle of 60. It is in striking contrast to the air-cooled V-type Renault of 1913, illustrated above. The cylinder capacity is 21 litres and the engine develops, in unsupercharged form, 570-625 horse-power; its weight is 900 lb. Rolls-Royce Kestrel engines are extensively used by the Royal Air Force. A development of the Kestrel is the Merlin, introduced in 1937. This fully supercharged engine, rated at 970 horse-power, with a maximum of 1,040 horse-power, weighs about 1,325 lb.

Further saving of weight, as well as improvement in mechanical efficiency, was effected by eliminating the valve tappet rod system and operating the valves directly by overhead camshafts. The overhead camshaft was introduced into automobile engine design, but even at the time of writing its use is far from general. The aero engine and the motor car engine sprang from the same source, but their requirements are so different that each has developed in its own way.

The use of pistons made of aluminium or of one of its light alloys has had an effect far in excess of mere reduction of weight, for such pistons have been responsible for an enormous improvement in mechanical efficiency.

During the period 1909-14 at least four types of engine were in regular production - the vertical, the V type, the radial and the rotary. France and Great Britain favoured the eight-cylinder V engine, but Germany was markedly in favour of the four- or six-cylinder vertical type. A famous German engine was the Mercedes, which superseded the German Argus engine of 1910 and was used in the Taube aircraft during the war of 1914-18. The Mercedes, with another engine, of Benz design, had by 1912 established itself as the most favoured type in Germany. At the same time the Green engine won favour in Great Britain, and in its 35 horse-power size this engine was fitted to the first Avro biplane of 1911. Other vertical engines were developed in Great Britain by the A.B.C. and Beardmore companies, and a 200 horse-power Beardmore-Halford-Pullinger engine was produced by the Siddeley Deasy Motor Company in 1917. This was a six-cylinder water-cooled engine with two separate cylinder blocks, each comprising the cylinder heads, water jackets, valve passages and inlet manifold of three cylinders. This engine developed 246 horse-power at 1,400 revolutions a minute and weighed 625 lb.

Modified Car Engines

German engine of the the 300 horse-power Maybach, which also had six water-cooled cylinders. The Maybach engine developed 294 horse-power at 1,400 revolutions a minute and weighed 911 lb - a less favourable power-weight ratio than that of the British engine. Today the vertical engine is most extensively used in light aircraft. The two great rival designs for high-powered aero engines are the air-cooled radial type and the water-cooled V type. The Y design, although one of the earliest of all, has now been accepted as one of the best layouts for liquid-cooled engines. The first V engines to be used in aeroplanes were small two- and four-cylinder types built for motor cars. Among the first types with eight cylinders in two banks of four was a 200 horse-power Darracq engine of 1905, built for a racing car. The first V engine which had a sufficiently good power-weight ratio for use in an aircraft was the French Antoinette engine of 1905, which had eight cylinders and developed 50 horse-power.

This engine used steam cooling in conjunction with an air-cooled condenser arranged along the sides of the fuselage. Its first recorded appearance was in a racing launch on the River Seine in April 1905, and the first aeroplanes to which it was fitted were the Bleriot-Yoisin biplane (1906) and the Santos Dumont “canard” type. The other engines available were mostly modified car engines, but none of them had the weight per horse-power figure of the Antoinette. In America, the Curtiss V-type engine, developed from a motor-cycle engine, came into use in 1905-6, and the first water-cooled example of this type was used in the Curtiss biplane which won the Rheims speed contest in 1909, although its air-cooled form had previously been used with success in the U.S.A.

The Bristol Hercules is one of the most powerful of British aero engines

WITH FOURTEEN CYLINDERS, the Bristol Hercules is one of the most powerful of British aero engines. Air-cooled, radial and provided with sleeve valves, the Hercules was first tested early in 1936. The total cylinder capacity of the engine is 38·7 litres. In medium supercharged form it has a maximum take-off power of 1,240-1,290 brake horse-power at 2,650 revolutions and an international rating of 1,100-1,150 brake horse-power at 2,400 revolutions at 5,000 feet.

The V engine underwent worldwide development until 1918. Prominent manufacturers of the type were Fiat and Isotta-Fraschini in Italy, Curtiss in America and Wolseley, Sunbeam, Napier and Rolls-Royce in Great Britain. By logical stages of development the V type of engine reached the high pitch of efficiency typified by the Napier Lion and the Rolls-Royce R type.

Present-day examples of the type are the Rolls-Royce Kestrel and Merlin, extensively used in R.A.F. machines. These power units have twelve cylinders in two banks of six, mounted at an angle of 60° to each other. The Merlin, introduced in 1937, is fully supercharged and may be cooled by ethylene glycol, the use of which makes it possible to reduce the size of the radiator - an important point with the streamlined aeroplanes of today.

Power units with horizontally-opposed cylinders found favour for a time, but at present they have only a limited application, most light aeroplanes using vertical air-cooled engines of 80 horse-power and upwards. An example of this type of engine is the De Havilland Gipsy Major, used in the Tiger Moth and other aircraft, British and foreign. This inverted vertical engine weighs only 300 lb and develops 120 horse-power at 2,100 revolutions a minute.

A notable “in line” arrangement is that of the Napier-Halford Dagger, which uses twenty-four cylinders in four banks of six in H formation, with two banks above the crankshaft and two below. This engine is air-cooled and supercharged, and has a normal output (Series III) of 700-725 horse-power at 3,500 revolutions.

Today there is little difference between the power, weight and fuel consumption of the three main types of engine. The type selected for a particular aeroplane is dictated by the conditions in which it has to operate - duration of flight, rate of climb desired, total weight allowed for power plant and fuel, and length of periods between overhauls. Service single- and two-seater fighters are always fitted with a single engine, of high power, in the nose of the fuselage. Service aircraft designed to carry heavy loads over long distances are fitted with two, four or six engines mounted on the wing structure. Large private and commercial machines are generally equipped with two or more engines on the wing structure, and. light aeroplanes with a single engine of medium power in the nose of the fuselage.

Compression-ignition engines will undoubtedly have an important bearing of the design of future aircraft, although they are seldom used at the time of writing. The fuel used in such engines is almost immune from the risk of fire, and the compression-ignition engine has the further advantages of simplicity, reduced liability to valve failure and a cool exhaust. Their weight is their chief disadvantage.

Compression Ignition

Engines of this type have recently been developed by the Bristol Company, by the Royal Aircraft Establishment, Farnborough (Hampshire), which has converted a Rolls-Royce Condor, and by the German Junkers firm. A Junkers engine is built by Napier and is known as the Napier-Junkers Culverin. It is a six-cylinder two-stroke engine, with two crankshafts which are coupled together by a train of gearwheels. Incorporated in this train is the drive for the airscrew. The engine is water-cooled and develops 720 horse-power, its weight being 1,785 lb. In Germany this type of engine is known as the Jumo.

Many advantages are claimed for the sleeve valve engine. The Bristol Company has developed the single sleeve valve engine with conspicuous success. In 1933 appeared the Bristol Perseus, of 24·9 litres capacity. This was followed by the Aquila, a radial air-cooled sleeve valve engine of about 15½ litres capacity. One of the most powerful British aero engines is the fourteen-cylinder double-row Bristol Hercules, first tested in 1936. Its total capacity is 38·7 litres and the international rating of the medium supercharged model is 1,100-1,150 brake horse-power at 2,400 revolutions a minute at 5,000 feet.


ROTARY ENGINE OF 1908. This seven-cylinder Gnome, of French design, developed 50 horse-power and revolved round a stationary crankshaft. The engine weighed little more than 3 lb per horse-power - a remarkable ratio for the period. The early Gnomes were followed by larger rotary engines having nine cylinders and developing 80 and 100 horse-power. The rotary engine is now obsolete.

You can read more on “Bristol Pegasus and Mercury”, “Mercedes Benz” and “Rolls-Royce Merlin and Kestrel” on this website.

You can read more on “Aircraft Engines” in Wonders of World Engineering

Evolution of the Aero Engine