Engines of the Principal Military Aircraft of the Great War

War has long been known as the 'Mother of Invention' and the Great War proved the truth of this in a quite exceptional way. One of the areas of the most spectacular and rapid technical development was in the war planes, and in the engines that propelled them above the battlefields.

In 1903, Orville and Wilbur Wright successfully achieved the vital equation essential for manned flight with their 12 horsepower (hp) engine of four in-line cylinders, i.e.

Weight of pilot + weight of aircraft + weight of engine and fuel.
= Less than
Aerodynamic lift of wing of aircraft and airframe design + propulsion power of engine.
=
Controlled manned flight.

The duration and speed of their flight was directly related to the capacity of their engine to drive the aircraft through the air in forward motion, the ability of the wings to provide the aerodynamic 'Lift' and the design of the airframe that allowed directional control.

Within 11 years of the Wright brothers' breakthrough, aviators were flying military aircraft above the Great War battlefields of France and Belgium as purposeful machines of war. Remarkably, these Great War aircraft were powered by petrol internal combustion engines of an astonishing range and variety.

Background information

In-line engine

All the cylinders, i.e. 4, 6, 8 or 12 in number, were ranged in a single row in the engine block above the crankshaft that was driven by the reciprocating pistons. The airscrew (propeller) was fixed to one end of the crankshaft and was rotated by it.

V engine

Two rows of cylinders were arranged in a V shaped engine block set at an angle that on different models varied from 45 to 90 degrees and numbered from 6 to 12. The pistons drove the crankshaft that was located at the base of the V and the airscrew was fixed to the crankshaft and was rotated by it.

Radial engine 

There was no crankshaft. The cylinders were arranged around the hub of the master-rod like the arms of a starfish. The pistons jointly drove the hub, and the airscrew, which was fixed to the hub, and was rotated by it.

Rotary engine

The cylinders with the pistons, and the entire engine block, rotated around a fixed crankshaft. The airscrew was fixed to the engine block and rotated with it. It had a very high power-to-weigh ratio and produced a pronounced gyroscopic effect that increasingly affected the handling of the aircraft the heavier and more powerful the engine was. Most modern pilots would look askance at having to deal with this potentially dangerous gyroscopic effect, and rotary engines went out of favour after the Great War.

A further disadvantage inherent in rotary engines was its need for additional lubrication - usually Castor Oil. The aviation fuel was mixed with air and fed into the engine via the crankshaft housing where it picked up any excess Castor Oil. The enriched mixture was fed into the cylinders for combustion. Unburnt Castor Oil was ejected from the engine exhaust. A cowling was usually fitted to contain this excess oil but inevitably both pilot and fuselage received a constant spray of it to the discomfort of the former. Hence the need for the famous fliers' muffler and goggles.

Means of aircraft propulsion

The internal combustion engine was uniformly used in the Great War to turn the airscrew(s) that provided the thrust to force the aircraft through the air. Most airscrews were two bladed helices - usually carved from wood - and connected together at 180 degrees by a hub that attached them to the engine of the aircraft. They were based on the principle of the Achimedes Screw.

Exceptionally, four-bladed airscrews were widely used by the Royal Airforce Factory (RAF) on their light aircraft and less frequently by other manufacturers. However, they were more generally employed on larger aircraft such as bombers.

Contra-rotating propellers (two airscrews turning simultaneously in clockwise and anticlockwise directions on co-axial shafts) were not used in the Great War although they had been invented 40 years previously.

The pitch (or, more correctly, the effective pitch) of a airscrew blade is the distance that the blade moves forward in one revolution in air. This is determined by the 'angle of attack' of the airscrew - normally around 2 to 4 degrees - and in the Great War this angle was fixed by the manufacturer of the engine. The variable pitch propeller was not used on military aircraft until much later.

The faster the airscrew turns the higher speed that the aircraft is propelled through the air. Subject, of course, to wind resistance, or drag, created by the aerodynamics of the plane itself.

Aircraft engine systems employed in the Great War

Two engine systems were employed:

• The Traction Engine
• The Pusher Engine

The Traction Engine pulled the aircraft through the air. Accordingly, the airscrew and the engine were mounted on the front of the aircraft, and usually in front of the pilot. This reduced the forward vision of the pilot. It also meant that any forward firing gun had to be situated to fire outside of the arc of the airscrew, making aerial ballistics difficult to master. This problem was resolved in 1915 by the invention of the machine-gun interrupter gear. This device synchronised the timing of the firing of the bullets so each passed between the airscrew blades without hitting them. The synchronised interrupter gear was the key that made possible the genesis of the purpose built fighter aircraft that became such a critical factor in the air war of 1915-1918.

The Pusher Engine pushed the aircraft through the air. To achieve this, the engine and airscrew were mounted in the rear, usually behind the pilot. This arrangement gave the pilot and any observer an excellent forward view. It also meant that a machine gun(s) could be mounted on the very front of the aircraft giving a clear field of fire. The arrival of the interrupter gear largely cancelled out any limited advantage that pusher planes might have had, and they were phased out of the war.

On the heavier aircraft, such as the large bombers, one, two, or four engines were used to power multiple airscrews. A single engine mounted on the fuselage would have separate drive trains to two engines; or two or four separate engines could be mounted outboard of the fuselage either on the wing(s) or on a bi-plane, sandwiched between the wings. One of these heavier aircraft, the Handley Page V 1500, had two tractor and two pusher engines mounted in tandem.

A final means of propulsion was the glide. Here, an aircraft that had lost the power of its engine, whilst at sufficient altitude, could be flown in a controlled dive to provide a speed sufficient for the aircraft to glide to a soft landing on the ground. Of course, the aircraft had to be still structurally sound and air-worthy and a suitable landing site fortuitously located nearby.

Production of aircraft engines used in the Great War

The authoritative publication, Jane's Fighting Aircraft of World War I, lists a total of 65 national producers of aero-engines during the Great War. Inexplicably at least one, possibly more, manufactures was omitted e.g. Peugeot. The total listed national manufacturers by country are: 

Germany - 18

Britain - 17

USA - 15

France - 6

Italy - 3

Austria - 3

Holland/Denmark/Spain - 1 each

From these companies the following engines were used in the corresponding national aircraft that went into service during the months of war in 1914 -18.

The supply of aero-engines to the principal new aircraft of the major airforces, 1914-1918.

Austria/Hungary

* = Number of cylinders.
** = Number of engines, 

Like the Austro-Hungarian aircraft manufacturers, the three national engine makers relied on German technology and stuck to the German philosophy of in-line engines totally eschewing both radial and rotary engines. The locally produced Hiero and Daimler engines were of high power and durability, although some problems with over-heating with the more powerful Daimler engines were never fully resolved.

France

* = Number of cylinders.
** = Number of engines,

In terms of commercial activity and innovation, France was indubitably at the forefront of military aviation when the Great War began. This momentum and domination continued throughout the early years of the war until British and German aviation industries got into their stride and surpassed French production. However, it was the French who mainly supplied the needs of both Belgium and Russia and met the early requirements of the Americans when they entered the War. Also in times of crisis the British also sought French help.

It will be seen that the majority of French Great War aircraft were fitted with aero-engines by the manufacturers: Gnome/Le Rhone; Renault, and Hispano-Suiza. Gnome/Le Rhone were rotary engines, whilst Renault and Hispano-Suiza were V-engines.

Uniquely, Canton-Unné radial engines were also widely used.

Many French engines were also supplied to other manufacturers of Allied aircraft: mainly under licence or produced by sister companies.

Germany

* = Number of cylinders.
** = Number of engines,

The German aviation industry got off to a slow start. However, their major source of engines for the entire Great War was almost exclusively the Mercedes-Daimler Company - the company had won national prizes for excellence of their aero-engines in 1911 and 1913. The figure of between 85 to 90% is quoted as the level of domination of the market by Mercedes. Benz and a few other manufacturers had a sizeable production but a relatively small overall output.

From the start the Germans chose the 6 cylinder in-line engine as their standard and this format proved to be both highly reliable and durable. A few rotary engines were used by the Germans, but nowhere near the scale of the deployment by the Allies. In all 48,537 aircraft were supplied with German engines of which something in excess of 40,000 were Mercedes.

Great Britain

* = Number of cylinders.
** = Number of engines, 

The British aviation industry produced a large number of indigenous aircraft and engines to match. Other marques were produced under licence. Large numbers of rotary engines, mainly of French design, were used, e.g. in the highly successful Sopwith F.1 Camel. But only one radial entered service on the Western Front - the British Aerial Transport Company's B.A.T. Bantam, Mk. 1 - of which only 12 were constructed.

The Rolls Royce series of V engines, were extremely successful during the latter part of the War. And it is interesting to note how the output of the most powerful Rolls Royce engine used in the Great War - the Eagle VIII at 360hp- compared with that of the Merlin Mark III engine which powered both the Battle of Britain Mark IA Spitfire and Hurricane I, and delivered 1,030hp.

One British aircraft type was fitted with the new American Liberty engine in 1918 and, had the War continued into 1919, no doubt many more would have been supplied.

Italy

* = Number of cylinders.
** = Number of engines, 

The construction of purely Italian aircraft for the Great War, only started around the time Italy entered the war in 1915. Up to then French Gnome rotary engines were used.

All three Italian aero-engine manufactures made contributions and uniformly followed the 6 in-line cylinder format except Fiat which produced a V12 engine in 1918. At 700hpthe Fiat A14, V12 was the most powerful aero-engine of the War.

In 1917 alone, 6,726 Italian engines were produced.

Russia.

* = Number of cylinders.
** = Number of engines,

Russia had a weak aviation industry that was ultimately seriously affected by the disorganisation wreaked by the October Revolution in 1917. According, throughout the War it depended for its supply of engines from its European Allies, France and Britain.

United States of America.

* = Number of cylinders.

The input of American planes and engines was relatively small during the Great War, although very large orders of American aircraft and engines were in the pipe-line at the time of the Armistice. Once the Americans arrived on the Western Front the interim was filled with exclusively French aircraft of the types Nieuport and Spad. The earlier American manufactured aircraft on the Western Front were fitted with rotary engines by Le Rhone.

The 1917 Liberty V12 engines were exceptionally powerful, delivering 400hp.

If one were to choose a small number of aero-engines which best chart this rapid development, perhaps a reasonable selection would be:

1. 1906: Antoinette, 50hp French

Although this French engine was designed long before the Great War, it had some original features that greatly affected the design of the Great War engines.

It was a robust eight-cylinder 'V' engine with the banks of four cylinders set at 90 degrees. It offered exceptional power for the time and refinements such as direct fuel injection.

2. 1909: Anzani, 25hp French.

Also a pre-war designed engine. Although lacking in power, it had design features that greatly influenced the early engines used in the war. It had three cylinders, was air-cooled, and was a semi-radial with automatic inlet and outlet valves.

2, 3. 1909: Gnome, 50hp French.

A completely revolutionary design of rotary engine with seven cylinders, air-cooled and a single valve per cylinder - the famous Monosoupape. The design was developed in ever more powerful versions throughout the Great War; particularly for use in Allied fighter aircraft. As the engine size and power increased, so did the gyroscopic effect of the rotating engine and inexperienced pilots did get into difficulties. More experienced pilots used the effect to their advantage to produce tighter turns in aerial combat.

3. 4. 1913: Le Rhone, 80hp French.

From the same stable as the Gnome, this was a revolutionary rotary engine of nine cylinders with good reliability, although the gyroscopic effect could, and did, cause difficulties in inexperienced hands. It was used throughout the war in many Allied aircraft types. It had outstanding engine torque (traction power) and a smooth running action.

5. 1915: Rolls Royce Eagle 360hp(and Falcon 280hp), British.

A huge V12 (angle = 60 degrees) water-cooled power-plant of impeccable performance, with a excellent mechanical balance and a superior power-to-weight ratio.

6. 1917: Mercedes, 180hp German.

Highly reliable and durable six cylinder vertical in-line water cooled engine. The chosen power-plant for many German aircraft types throughout the duration of the war.

7. 1917: Liberty, 400hp American

A wholly American government sponsored aero-engine - with the active co-operation of Britain, France and Italy - for which the highly standardised parts were made by every US motor car manufacturer. One of the most powerful aero-engines of the Great War it was a V12 (angle = 45 degrees) and water-cooled. Although a thousand Liberty engines were manufactured in the USA, it arrived too late on the front-line to have the significant effect expected of it.

The Liberty aero-engine was the first truly international model and had provision for four standardised versions with from four to twelve cylinders.

Conclusions

The further development of the aircraft engine that took place during the Great War was an incredible technical achievement, particularly so in France where an important part of the munitions infra-structure was in German occupied territory. Some of the more important technical innovations were made before the Great War began, but they greatly influenced the design of the engines that were developed during the Great War.

The number of engines produced for each type of aircraft obviously varied enormously. Some had extremely long runs, like the Gnome and Le Rhone rotaries and the Mercedes six cylinder in-lines. But others had quite short production runs as a result of curtailment by operational failure or the unanticipated rapid ending of the War.

As far as increased engine performance was concerned, a big factor was the improvement in metallurgy that amongst other advances produced aluminium alloys that hardened with age. An alloy of aluminium, copper, magnesium and magnesium was patented pre-War by the Germans as Duralumin, or Dural. The British discovered another alloy - aluminium, copper, nickel and magnesium - with similar strengthening properties, and called it 'Y Alloy'. These and other alloys provided engines with enhanced performance and durability characteristics, although the major developments with this technology only came at the end of the Great War and afterwards.