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Developed from the motor car engine of thirty years ago, the aircraft engine of to-day is a miracle of efficiency and compactness. For high-powered engines the water-cooled “V” type is rivalled by the air-cooled radial type; for smaller engines the air-cooled “in line” arrangement is popular


THE ROLLS-ROYCE KESTREL has twelve water-cooled cylinders arranged in two banks of six at an angle of 60°. Each block carries a camshaft driven by bevel gears to operate the overhead valves. The engine, which may be either supercharged or unsupercharged, is made in various forms to suit specific requirements. With a swept volume of 21 litres, the “Kestrel”, in unsupercharged form, develops 570-625 horse-power and weighs 900 lb. The normal speed is 2,500 revolutions a minute; the propeller is geared down by a special type of reduction gear.

THE evolution of the aircraft engine extends over a period of more than thirty years. During this period an extraordinary amount of ingenuity has been exercised to bring about the advance that so far has been achieved.

In the beginning the early designers were faced with great difficulties, the chief of which concerned the matter of weight. It was of great importance in those pioneer days that the engine weight per horse-power should be kept at the lowest value possible because, with the inefficiency of contemporary airscrews and airplane wings, every ounce saved in dead weight made a difference.

Engineers therefore constantly tried to lower the weight of their motors without lessening the power output. They generally did this by reducing the mass of metal in such parts as the cylinder and crankcase castings and by copiously drilling internal parts such as the pistons and connecting rods.

These straightforward methods, although fairly successful in one way, frequently brought trouble in their train, as the lightened parts were liable to fail. Many of the early airplane engines were built on car-engine lines, with four or six water-cooled cylinders in line and the airscrew hub fitted to one end of the crankshaft. There were, on the other hand, some novel designs such as the famous “Gnome” rotary motor.

Invented by M. Laurent Sequin, the “Gnome” was one of the most ingenious internal combustion engines ever conceived. It was more than this, however, for it met the urgent demand of the time for a light, smooth-running power unit for aircraft. The “Gnome” engine gave a great impetus to aviation and, though engines working on its unique principle have long been obsolete, rotary-engined aeroplanes were used with the greatest success right up to the close of the war of 1914-18.

The early “Gnome” had seven symmetrically disposed air-cooled cylinders arranged round a crankcase and the whole engine, with the propeller, rotated about the axis of the fixed crankshaft. The supply of gas from the carburettor reached the combustion chambers through the crankshaft (which was hollow) and, passing into the crankcase, entered the cylinders through automatically operated inlet valves set in the crown of the pistons. The exhaust gases were expelled through exhaust valves located in the cylinder heads, the valves being operated by rockers and push-rods.

Bristol Pegasus aircraft engineThe rotary action was caused by the side pressure of the pistons on the cylinder barrels; this pressure was produced from the obliquity of the connecting rods. The engine, which was rated at 50 horse-power, ran at a speed of 1,200 revolutions a minute, and the total weight, including magneto, carburettor and oil pump, was 172 lb. The good power-to-weight ratio and the uniformity of motion due to the large rotational inertia were the salient features of the “Gnome”.

A NEW HEIGHT RECORD of 49,944 feet was made on September 28, 1936, by Squadron Leader F. R. D. Swain, flying a monoplane fitted with a Bristol “Pegasus” engine, of the type illustrated. This is a radial engine with nine air-cooled cylinders having a total capacity of 28·7 litres and weighing 1,010 lb. The normal rated brake horse-power is 740 at 3,500 feet and the normal speed is 2,475 revolutions a minute. A reduction gear is fitted to lower the airscrew speed.

There were, however, some disadvantages; the fact that the cylinders and crankcase revolved complicated the matter of lubrication because of the centrifugal action and, with so large a rotating mass, the balance had to be finely arranged or serious vibration ensued. Further, the delicate mechanism of the inlet valves and other parts necessitated a complete stripping down of the engine after about every sixteen hours to maintain satisfactory running. To keep the weight down, the various parts were made to fine limits which not only reduced the safety factor but also made the manufacture costly. The cylinders, for instance, were made from solid nickel-chrome steel ingots and were machined with the cooling fins so that the finished thickness of the barrels was but 1½ millimetres, that is to say, something under 1/16 in.

As time went on the “Gnome” and other types of rotary engine were steadily developed until the design culminated in the Bentley B.R.2, a nine-cylinder engine of 200 h.p.

A number of early designers favoured the “Y” type of engine, mainly because, being compact and short, it could easily be installed in the airframe. As a rule, engines of this type had eight cylinders and were either air- or water-cooled.

On the whole, though their power-to-weight ratio was not exceptionally good, these power units were fairly satisfactory and some of the finest engines of the present time are based on the “V” design.

The radial type of engine which is so extensively used to-day is also of early origin but, in contrast to the “V” type, its evolution was not consistently maintained. For a considerable period it was virtually abandoned in favour of other types, and its success and wide adoption for use in all types of airplanes is really the result of a concentrated development that has occurred only in the past two decades.

The wonderful advance that aircraft engine manufacturers have made is largely attributable to the growth of metallurgical knowledge that has taken place in the last twenty years. The pre-war manufacturer was greatly limited in the material at his disposal for the construction of his engine, but various new alloys and light metals that are available to-day give the most valuable aid to the designer in the vital matter of weight reduction. In every other direction, too, progress has been made. By the accumulation of knowledge now at his disposal the engineer can calculate his stresses to the closest limits and parts can be made with the finest accuracy without their strength or efficiency being impaired.

The solution of problems relating to fuel and carburation and to cylinder cooling and general heat dissipation is another factor which has had a most marked influence on the rapid improvement of the aircraft engine.

Increase of power has been obtained by raising the compression, by the use of superchargers which force the mixture into the cylinders at high pressure, and by increasing the speed of the engine. The last-named feature has necessitated the incorporation of a reduction gear between the crankshaft and airscrew, because above a certain speed the airscrew becomes inefficient. This added complication, however, has not detracted from the reliability of the modern engine and the general tendency is towards higher engine speeds.

AN AIRCRAFT ENGINE BEING TESTED at the Royal Aircraft EstablishmentSo far as high-powered aircraft engines are concerned there are at present two rival designs - the water-cooled “V” type and the air-cooled radial type. As an example of the former the Rolls-Royce “Kestrel” engine, which is fitted to a large number of R.A.F. machines, is outstanding. This power unit has twelve cylinders arranged in two banks of six at an angle of 60° and each block carries a camshaft driven by bevel gears to operate the overhead valves. With a swept volume of 21 litres, the engine in unsupercharged form develops 570-625 horse-power; the weight is 900 lb. The normal speed is 2,500 revolutions a minute and the propeller is geared down by a special type of reduction gear.

AN AIRCRAFT ENGINE BEING TESTED at the Royal Aircraft Establishment, Farnborough, Hants. The engine is set on a frame in front of an enormous wind tunnel specially designed for the testing of aircraft equipment. The tunnel is built of steel and reinforced concrete and is capable of dealing with aircraft having a span of up to 56 feet. Air is driven through the tunnel by a fan with a diameter of 30 feet.

(This colour plate previously appeared as the cover design for part 18.)


The ignition system is duplicated and embodies two magnetos driven off the rear end of the camshafts, and there are two carburettors, each feeding six cylinders. Throughout its construction light metals are extensively used in this engine, especially the alloy known as “Hiduminium”, which was evolved by Rolls-Royce, Limited. The “Kestrel” engine is produced in various forms for specific duties in military aircraft and it is supplied with and without superchargers according to requirements.

The engine used in the Schneider Trophy S.6 seaplane which won the trophy outright for Great Britain in 1931 was of Rolls-Royce manufacture, and was a supercharged racing version of the company’s standard twelve-cylinder type. The bore and stroke dimensions were 152·5 mm. by 169 mm.

In building this engine it was highly important that the bulk and frontal area should be reduced to the lowest limits to cut down as far as possible the resistance to the air. All the auxiliaries had to be “squeezed” into the smallest possible space: this was no simple task, especially in the installation of the supercharger. In its final form the entire power unit measured only 7 ft. 7¾ in. in length; it had an overall width of but 2 ft. 6 in., and was 3 ft. 4 in. high. The greatest triumph of the designers, however, was in the fact that the engine developed no less than 2,350 horsepower and yet weighed only 1,630 lb.

The Gnome rotary aircraft engineThe Rolls-Royce “Merlin”, introduced in 1937, is a development of the “Kestrel”. The newer engine is a fully supercharged liquid- cooled twelve-cylinder engine, rated at 970 horsepower, with a maximum of 1,040 horsepower. Its weight is 1,318½ lb. and the weight per horse-power is only 1·25 lb. Cooled by the ethylene glycol, the “Merlin” can run at much higher temperatures than that of boiling water.

IN THE EARLY DAYS of aircraft, the “Gnome” rotary engine was famous. The air-cooled cylinders were arranged round a crankcase and the whole engine, with the propeller, rotated about the axis of the fixed crankshaft. The rotary action was caused by the side pressure of the pistons on the cylinder barrels. The example shown has fourteen cylinders.

The radial aircraft engine - that is, the type which has its cylinders arranged spoke-fashion round the crankshaft and crankcase - is nearly always air-cooled. The advantages of the radial engine are numerous.

Great accessibility of all the parts is one excellent quality of the radial engine and this facilitates maintenance work. Another feature is that it is much shorter than the “V” engine and for this reason it simplifies the installation, and enables the airplane designer to reduce the overall length of his machine. This has an important influence on the flying qualities of an aircraft, making it more manoeuvrable - a vital point so far as a fighting machine is concerned.

To these advantages may be added the light weight of the radial engine, which is due to its short crankshaft and to the absence of water jackets, leads, pumps and radiator which are required when liquid cooling is incorporated.

THE NAPIER-HALFORD DAGGER aircraft engineThere are disadvantages, however, inherent in the design. The frontal area of a radial engine of high horse-power is necessarily considerable and the resistance presented to the air tends to detract from the performance of an aircraft to which such an engine is fitted. To a large extent this difficulty has been overcome by fitting specially shaped cowlings round the engine to give a streamline effect. These cowlings take the form of a ring of aerofoil section and are so formed as to deflect the air inwards and so reduce the drag.

THE NAPIER-HALFORD “DAGGER” has no fewer than twenty-four cylinders. These are arranged in four banks of six in “H” formation - that is, in two parallel rows above and below the crankcase. The horse-power developed is over 700 and the weight 1,280 lb. Special features are the low weight, the compactness and the small frontal area.

Among British manufacturers who have concentrated their efforts on the radial engine is the Bristol Company, and the Bristol “Pegasus” is an outstanding example of the type. It was with a Pegasus-engined monoplane that the height record of 49,944 feet was gained on September 28, 1936, by Squadron Leader F. R. D. Swain. The Bristol “Pegasus” engine is widely used in R.A.F. machines and in commercial aircraft, including the Short “Empire” flying boats belonging to Imperial Airways.

This power unit has nine air-cooled cylinders and is of 28·7 litres capacity. It weighs 1,010 lb., and measures 4 ft. 7 in. in diameter. The normal rated brake horse-power is 810-850 at 4,000 feet and the normal speed is 2,250 revolutions a minute. A reduction gear is built into the front of the engine.

Sleeve Valves

The valves are disposed in the head of the cylinders and are operated by push-rods and rockers. The timing gear is in the front portion of the crankcase immediately behind the reduction gear and the induction system and magnetos are compactly arranged behind the engine so that there are as few excrescences as possible. The supercharger also is housed behind the engine.

A recent development of the air-cooled radial engine is to be found in another type of Bristol motor, known as the “Aquila”. In place of the usual cam-operated poppet valves, sleeve valves are used on this engine, which has nine cylinders. The use of sleeves is accompanied by numerous advantages. The number of working parts is reduced, maintenance and the manufacturing process are simplified, the running is quieter than with a poppet valve engine, and there is a marked economy in fuel consumption. The Bristol “Perseus” sleeve-valve engine develops 710 horsepower at 2,500 revolutions at 14,500 feet.

One of the most powerful radial engines so far made is the Armstrong-Siddeley “Tiger”, which develops 880 horse-power. It has fourteen cylinders disposed in two radial rows, one behind the other, and staggered so that those in the front row do not blank off those behind and so affect their cooling. Other typical examples of the multi-row radial engine are the 500 horsepower Hispano-Suiza and the fourteen-cylinder “Mistral Major”, which weighs 1,196 lb. and yields nearly 900 horsepower.

Although the majority of high-powered, air-cooled engines are of radial design, there are some notable “in-line” types in which air-cooling has been successfully incorporated. One of these is known as the Napier-Halford “Dagger”, and has no fewer than twenty-four cylinders. These are arranged in four banks of six in “H” formation - that is, in two parallel rows above and below the crankcase. This engine is capable of developing over 700 horsepower, and weighs 1,280 lb. Its special features, besides it low weight, are its remarkable compactness and its small frontal area - only 7 square feet.

The Gipsy Major four-cylinder inverted aircraft engine

UPSIDE DOWN. The “Gipsy Major” is a four-cylinder inverted engine, with the cylinders below the crankcase. This air-cooled “inline” engine is used with great success in the smaller types of aircraft, such as the “Tiger Moth”. The “Gipsy Major” has a single carburettor, two magnetos and overhead valves. The normal horse-power at 2,100 revolutions a minute is 120; the weight is 300 lb. A development of this engine is the “Gipsy Six”, a six-cylinder motor of 200 horse-power.

In smaller categories the air-cooled in-line arrangement has been used with the greatest success, and no better example of this type of engine could be found than in the D.H. “Gipsy” series. The “Gipsy Major”, which is fitted to the “Tiger Moth” and many other types of aircraft, is a four-cylinder inverted engine - that is, the cylinders are below the crankcase. It has a single carburettor, two magnetos and overhead valves. The normal brake horsepower is 120 at 2,100 revolutions a minute, and the weight is 300 lb.

A development of the “Gipsy Major” is the “Gipsy Six”, a motor of 200 horsepower. It is similar in design to the smaller engine, but has six cylinders and weighs 468 lb. Two “Gipsy Six” engines were used in the famous D.H. “Comet” which won the England-Australia race in 1934 by covering the 11,300 miles in two days twenty-two hours fifty-eight minutes.

The great expansion of commercial aviation has placed new demands upon the ingenuity of the aircraft engine constructor, and durability and running costs have to be studied alongside the matter of power output. In the past petrol and oil consumption were of secondary importance so long as the necessary power was available, but to-day in designing an engine the closest attention must be given to the fuel consumption.

Increase of power has been secured by increasing the crankshaft speed and by raising the compression ratio in the cylinders; but these methods have generally entailed the use of special fuels with “anti-knock” qualities so that the engine shall run smoothly and without overheating and detonation. The speeds of a modern engine make the use of special lubricating oils necessary and, as with all types of internal combustion engines, high speed is accompanied by an increase in the amount of fuel and oil consumed.

It has been considered for a long time that the true solution of economical commercial flying rests with the compression-ignition engine, which has the advantage of a low consumption and at the same time runs on crude oil costing a great deal less than more volatile fuels.

Two-Stroke Diesel

Compression-ignition, or so-called diesel aero engines, have been built by several manufacturers. The Bristol Company built an air-cooled radial engine of this type in 1932. This four-stroke engine had nine cylinders. Its total weight was 990 lb. and the brake horse-power was 415-470.

Another engine, which was used with success some years ago, was produced by the Packard Company of U.S.A. As with the Bristol engine, it was an air-cooled radial and yielded 225 horsepower. Its weight-power ratio was 2¼ lb. per horse-power. Other diesel-type aircraft engines have been evolved by Rolls-Royce and by the German Junkers concern.

THE NAPIER-JUNKERS CULVERIN aircraft engineThe Junkers engine has long passed the experimental stage, and is being successfully used in commercial aircraft. An engine of this type is built by the Napier Company and is known as the Napier-Junkers “Culverin”. It works on the two-stroke principle and it has six cylinders, in which are two opposed pistons. There are two crankshafts, one arranged in a crankcase on the top of the engine and one beneath the cylinders.

These two crankshafts are coupled together by a train of gear wheels, and incorporated in this train is the reduction gear and drive for the airscrew. The engine, which is water-cooled, gives 720 horse-power and weighs about 1,785 lb. It is 7 feet long, 1 ft. 11 in. wide and 5 ft. 7 in. high.

THE NAPIER-JUNKERS “CULVERIN” is a compression-ignition engine using crude oil instead of petrol. With this type of engine the risk of fire is largely eliminated. The “Culverin” is a two-stroke six-cylinder water-cooled engine, with opposed pistons. There are two crankshafts, one above and one below the cylinders. The horse-power is 720 and the weight 1,785 lb.

[From part 21, published 20 July 1937]

You can read more on “Airship Design & Equipment”, “Empire Flying Boats” and “Flights in the Stratosphere” on this website.

You can read more on the “Evolution of the Aero Engine” in Wonders of World Aviation

Aircraft Engines