Power Behind the Prop: A Look at World War 1 Aircraft Engines

In spite of recent technological advances in machinery, ground forces in the Great War still relied heavily on horses to transport men and material one horsepower at a time. By contrast, the gasoline engines used to power the various planes of the air war were each capable of generating hundreds of horsepower.

Aircraft design came a long way in the ten years between the Wright brothers’ first flight in 1903 and the outbreak of World War One in 1914.  And as a result of the increasingly competitive atmosphere brought on by war, even more radical changes continued to be made over the course of the next few years as well.

Figure 1. The French, who are seen in the background using horses for the ground war, captured this German Rumpler aircraft equipped with an inline engine capable of generating hundreds of horsepower.

This was especially true in the development of aircraft engines.  As a consequence, horsepower to weight ratios increased dramatically, yet engine design and performance was far from uniform.  Three basic engine designs were adapted for aviation use.  These were inline, radial and rotary.  Each had its unique advantages and disadvantages.

Figure 2. Members of an American aviation unit pack up their gear preparatory to being shipped out. The names of three of the unit’s members are visible on the trunks in the foreground; K.S. Stewart, U.S.A. Aviation Section; J.J. Quinn; and Wm. S. Petersen, Aviation Section, S.C. U.S.A.

By the time the United States entered the war in 1917, the European Allies had been at war for three years and had vastly surpassed the Americans in aircraft design and production.  Brigadier General George O. Squires, the U.S. Army’s chief Signal Service officer, predicted, “Airplanes will put the Yankee punch in the war.  The way to beat Germany is to flood the air with airplanes.  Take the war out of the trenches and put it into the air.” Yet initial threats to “darken the skies over Europe” with American-designed and built aircraft were groundless boasts considering that in 1916 there was a grand total of only two outdated aircraft in United States military service.  Yet by 1917 the United States was rapidly expanding its aircraft industry.

Figure 3. Forward America! A patriotic poster from 1917 depicting Lady Liberty riding on the back of an American Eagle, followed by an unending flight of biplanes. (Original poster on display in the Smithsonian Air and Space Museum, Washington, D.C.)

The American-made two-seat Curtis-JN biplane, which was powered by a 90-hp Curtiss OX-5 V-8 engine, was used primarily for training purposes.  Known as the Jenny, it became the most famous American airplane of World War I.

Figure 4. The Curtiss-JN Biplane, known as the Jenny, was the most famous American airplane of World War I. Although it never got into combat, the two-seater was used primarily for training purposes. Beginning in April 1917 when the United States entered the war, approximately 95% of all American and Canadian pilots received their flight training in Jennies. After the war, hundreds of these durable planes were sold to civilian markets.

As it turns out, however, the only American-made planes ever to see combat over the Western Front were exact copies the British two-seat Airco DH-4, known as the DH-4A “Liberty” model (See Figure 5 below.), and a handful of Curtiss flying boats that saw service with the Royal Navy. (See Figure 6 below.)  All other combat aircraft flown by American pilots during the Great War were of foreign manufacture, predominantly of French or British origin.  Perhaps America’s greatest contribution to aircraft design and production was the Liberty 12-A, a twelve-cylinder water-cooled V style inline engine installed in the American-made Airco DH-4A and the Curtiss “Large America” flying boats.  With over 400 horsepower, the Liberty surpassed similar European engines at the time. (See Figure 7  below.)

Figure 5. This Airco DH-4B was an improved version of this versatile aircraft that came too late to replace the DH-4A in France. Both of the American-made models, the DH-4A and DH-4B, were powered by 400 hp V-12 Liberty inline engines, while the British-built DH-4 models saw service with a variety of Rolls Royce and other engines capable of producing from 200 to 375 horsepower.
Figure 6. A “Large America” flying boat sits on display at the Philadelphia Naval Aircraft Factory in 1918. The plane here is either a Curtiss H-12 or, more likely, a Curtiss H-16. They were used for anti-submarine patrol by the U.S. Navy off the coast of North America and by the Royal Navy in Europe. Power was supplied by two V-12 Liberty engines for the U.S. Navy planes or two Rolls Royce Eagle VIII inline engines for those in British service.
Figure 7. This unidentified Army Air Corps pilot provided the brains to guide his Airco DH-4A nimbly through the skies, but it was the powerful 12-cylinder water-cooled Liberty Engine that supplied the muscle.

Most inline engines, with the cylinders lined up one behind the other, had to be water cooled, thus increasing their weight and reducing their power to weight ratios.  In addition to the Liberty 12-A, other inline engines used by the Allies included those built by Hispano-Suiza and Rolls Royce.  Inline engines could have any number of cylinders aligned along the length of the crankshaft in either a single or double row.  Double rows could be aligned in either a V configuration or flat with the rows opposing each other.  Each cylinder or cylinder pair was connected to a different point along the crankshaft.  Air had to flow through the radiator to cool the water which circulated through the engine block, however the radiator created wind resistance, or drag.  This could be compensated for somewhat by building a larger engine capable of producing more horsepower, but at the expense of adding yet more weight.  Water-cooled engines, nonetheless, had one big disadvantage.  They were very vulnerable to combat damage.  One small leak caused by a single bullet could cause complete engine failure.  Still, the engines were widely used because of their ability to produce high horsepower and fast aircraft.  Captaine Georges Guynemer, one of the leading Allied fighter aces and a French national hero with 53 confirmed victories over German aircraft, accumulated most of his kills while flying a SPAD S.VII (See Figure 8 below).  Eddie Rickenbacker, America’s top ace of WWI with 26 confirmed kills, flew most of his missions (and got 20 of his aerial victories) in a SPAD S.XIII equipped with an inline 235-hp Hispano-Suiza engine. (See Figure 9 below.)

Figure 8. Quoted as saying, “Until one has given all, one has given nothing,” French air hero Captaine Georges Guynemer shocked the French public the when he failed to return from a combat mission on 11 September 1917. France’s leading ace with 53 aerial victories at the time of his death, Guynemer is pictured here flying a SPAD S.VII. Only 22 when he was shot down over no man’s land, Guynemer’s body was never recovered and he was declared missing in action.
Figure 9. Captain Eddie Rickenbacker, America’s top ace, leans on the tire of his SPAD S.XIII late in 1918. Note the manifold along the side of the fuselage from the fixed inline V-8 235-hp Hispano-Suiza engine that discharged exhaust aft of the pilot; something not possible in rotary engines.

Radial and rotary engines had cylinders radiating out from the engine axis like spokes.  With all the cylinders exposed equally to the airflow, they were efficiently air-cooled.  All the cylinders were connected to a single point on the crankshaft.  This meant that there had to be an odd number of cylinders in these engines.  As the crankshaft made one rotation, the pistons in the even-numbered cylinders would each, in turn, go through the power and exhaust strokes while the odd-numbered cylinders would go through the intake and compression strokes.  By the time the crankshaft had made one complete revolution, the even-numbered cylinders would be ready for intake and compression while the odd numbered ones would be firing and expelling the exhaust.  The primary difference between radial and rotary engines was that the radial engine was fixed to the airframe and the crankshaft turned the propeller, whereas the entire cylinder block of a rotary engine spun with the propeller while the crankshaft was fixed to the airframe.

Few radial engines were used during WWI.  The Anzani, ABC WASP and the Salmson Z-9 (the latter unusual because it was water-cooled) were probably the three most common.  (See Figure 10 below.)

Figure 10. The two-seat Salmson 2 was designed as a “heavy” observation airplane but also saw service as a light bomber and ground attack plane. The plane was powered by the unique water-cooled Salmson Z-9 radial engine and had a distinctive louvered engine cowling. Note the American roundel with the white center spot at the bottom of the wing.

Of all the engine types, rotary engines produced the highest power to weight ratios and were widely relied upon in the race to give lightweight WWI fighter planes an advantage over their opponents.  But rotary engines also had significant operational drawbacks that limited their use and their size (dictating their maximum horsepower), and eventually led to their eclipse – primarily by their cousin, the radial engine.  The principal Allied rotary engines were the 130-hp Clerget (See Figure 11 below.), several types of Gnome including the 160-hp Gnome Monosoupape (Figure 12.), and those made by Le Rhone. (Figure 13.)

Figure 11. Introduced to combat in 1917, the early Sopwith Camels were powered by 130 hp Clerget 9B Rotary engines and armed with dual forward-firing synchronized Vickers machine guns.
Figure 12. Later variants of the famous Sopwith Camel were fitted with a more powerful Gnome Monosoupape 9 Type N rotary engine capable of producing 160 horsepower. Though universally considered difficult to fly, the Camel owed its extreme maneuverability to the strong gyroscopic effect of the rotary engine and the fact that 90% of its weight was within seven feet of the nose of the aircraft.
Figure 13. Lieutenant LeMaitre of the French Aviation Forces in America, explains the rotary engine mechanism of a French-built Nieuport 17 to an American Captain at Fortress Monroe, Virginia in 1917. This model aircraft, equipped with a 110-hp Le Rhone rotary engine, was a first-class dog-fighter. The plane was quick and climbed “like a witch.” Some Nieuport 17s were later equipped with the 130-hp Clerget engine; but not this one.

The German-built 110-hp Oberursel (Figure 14.) was almost an exact copy of the 110-hp Le Rhone, but frequent engine failures may have been the result of German substitution of mineral oil for the harder to obtain castor oil necessary for proper lubrication of rotary engines.  (Figures 15.)  As a consequence most German aircraft utilized water-cooled in-line engines.   (Figure16.)

Figure 14. The Germans utilized rotary engines in a few of its fighters. Here a Fokker D.VIII monoplane sports a 110-hp Oberursel rotary engine. Note the open cowl only on the bottom to keep the engine exhaust and oil fumes spewing from the rapidly whirling engine away from the pilot’s face.
Figure 15. A shortage of castor oil in Germany led to the substitution of less satisfactory mineral oil that was blamed for numerous rotary engine failures. This Pfalz D.VIII fighter plane, equipped with a rotary engine, was abandoned by the Germans late in the war.
Figure 16. Most German planes used in-line engines, such as this Albatros D.IV equipped with a water-cooled Mercedes D.IIIa engine that produced between 160 and 175 horsepower. Notice the fixed exhaust manifold carrying the smoke and fumes off to the side and away from the pilot.

The mass of the spinning engine was one of the factors that helped give rotary engines such a high power to weight ratio, however, because the entire engine rotated, it had to be precisely balanced.  Mechanically, this also meant that rotary engines, with the cylinders spinning around the engine axis, had to feed the fuel through a port in the central crank shaft where it would get mixed with the castor oil used to lubricate the moving parts.  In addition, with the engine spinning there could be no fixed exhaust manifold on a rotary engine.  As a result, exhaust was discharged directly from the cylinder heads as they rotated.

Castor oil, chosen because of its specific lubricating qualities, was less ignitable than the gasoline used for fuel.  As a result, unburned and partially burned castor oil spewed out of the rapidly spinning rotary engine along with the engine exhaust and was carried back over the rest of the plane in a fine mist.  The need for a good pair of goggles for the pilot was not entirely due to the force of the wind coming across the cockpit.  Castor oil is probably best known for its foul tasting medicinal use and the fact that it is a strong purgative.  Too much exposure to it by a pilot during a long flight could have uncomfortable, if not dire, side effects.  In an effort to help control the spray of exhaust, engine cowlings were installed on aircraft equipped with rotary engines. (Figure 17.)  Cowling flaps were kept closed except for those on the bottom of the fuselage.  This reduced the cooling effect of air flowing over the engine but it helped somewhat in deflecting the spray of castor oil away from the pilot.

Figure 17. A French and American Officer stand by the cowling of a biplane fighter equipped with a rotary engine. In the spring of 1918 the American Expeditionary Force bought over 500 French manufactured Sopwith 1 1/2 Strutters, primarily for use as trainers. Note the American roundel with white center spot on the bottom of the wing. A pencil note on the back of this photo says, “This is what we call a Camel — real name (Sopwitch)” [sic]. Yet, in spite of the note, this is actually a Sopwith 1 1/2 Strutter powered by a Clerget rotary engine. An extremely versatile and effective airplane, the 1 1/2 Strutter was significant as the first British aircraft to enter service with a synchronized machine gun.

In addition to the difficulty of cooling and the embarrassing laxative effect of the exhaust, rotary engines had even bigger and deadlier drawbacks to their use.  The Sopwith Camel, equipped with a powerful 160 hp Gnome Monosoupape 9 Type N rotary engine and perhaps the most famous Allied fighter of WWI, needed to take off with full left rudder to counter the engine torque or else the plane would ground loop, crashing on its starboard wingtip.  And once in the air, a sudden loss of power in the engine could result in subjecting the airframe and engine mounts to a sharp reverse torque.  But by far the biggest problem was the gyroscopic force generated by the spinning mass of the engine.  This created hellish aircraft handling characteristics that could give a pilot fits until he learned to cope with them, or cause a fatal accident if he did not.  In fact the Camel, credited with shooting down 1,294 enemy aircraft (more than any other Allied fighter), killed 385 of its own pilots in non-combat accidents. (Figure 18.) This figure closely approaches the 413 Camel pilots who were killed in combat.

Figure 18. A soldier stands by the ruined remains of an unidentified plane wreck. Such wreckage is indicative of the hazards faced by WWI aviators in both combat and non-combat situations.

But if a skilled pilot learned to anticipate and work with the gyroscopic force created by the engine, he could get his flying machine to do amazing things.  With the propeller spinning clock-wise when viewed from the cockpit, a plane equipped with a rotary engine could make extremely tight evasive maneuvers by turning to the right.  A Camel could make a ¾ turn to the right in the same amount of time it would take to make a ¼ turn to the left.  Captain Ray Collishaw said, “we all knew the importance of doing our utmost to engage German pilots in right-hand waltzing matches.  On the other hand, the German…pilots…liked turning to the left because of their stationary engines.  In combat I had often seen the expression of despair on the face of an adversary, as he perceived that my rotary engined aircraft was gradually drawing on to his tail!”

To some extent, the firepower of a fighter could be improved by adding a swivel-mounted machine gun that could be aimed separately from the direction of flight. (Figure 19.)  But the added complication of operating the gun while maneuvering the aircraft probably negated much of its effectiveness.  Thus, a fixed mounted gun on a highly maneuverable airplane remained preferable.

Figure 19. The airplane, as an engine of war, was initially used solely as an observation platform. But quickly it was adapted to carry weapons in both an offensive and defensive capacity. In addition to a single belt-fed fuselage-mounted synchronized Vickers machine gun, this Nieuport chase plane was also equipped with a wing-mounted Lewis machine gun with drum magazine. Normally water-cooled, the Lewis was often mounted as an air-cooled weapon on Allied aircraft.

The Sopwith Camel was probably the most maneuverable fighter of WWI.  The gyroscopic effect of the rotary engine is something not normally experienced by modern pilots. After test-flying a Camel, Richard E. “Dick” Day, a latter-day test pilot, declared “Why, it puts both eyes on the same side of your nose!”  Of course attempting a maneuver counter to the gyroscopic force of the engine could lead to disaster.  According to veteran WWI fighter pilot Norman Macmillan of the RFC, “The Camel was a buzzing hornet, a wild thing….This fierce little beast answered readily to intelligent handling, but was utterly remorseless against brutal or ignorant treatment.”  Macmillan went on to say, “let the ham-fisted or inexperienced pilot pull the stick just a little too far back while turning all out, and the Camel would flick quickly into a spin, which was the pitfall of many a novice and, could be a death-trap at low heights.”

The gyroscopic force generated by rotary engines was the ultimate cause of their demise.  Larger and more powerful engines could not be built without either increasing the size or number of cylinders.  Both approaches drastically increased engine mass and caused a corresponding increase in gyroscopic force to unacceptable levels.  Thus the maximum useful size of rotary engines was self-limiting.  Following the war, inline and radial engines, with their fixed engine mass, completely replaced rotaries as larger and more powerful designs were needed for larger and heavier aircraft.  Converting from rotary to fixed cylinders reduced the oil and fuel consumption, allowed the engine to run faster and directed more of the horsepower to the propeller.  It also allowed more precise carburetion of the fuel-air mixture delivered to the cylinders.

Figure 20. A flight of rotary-engined scout planes returns from patrol.

The unusual rotary engines of the Great War are gone now from the skies (Figure 20.), replaced by larger and more powerful and more conventional engines.  Even many reproductions or restored WWI fighters being flown today use fixed radial engines in place of the original rotaries.  But looking back, one can’t help but wonder, what would it have been like to fly in combat sitting in a fragile fabric and wood kite fastened behind the madly whirling mass of a rotary engine.

(All photos in author’s collection except as noted.)