The Gnome was one of several rotary engines popular on
fighter planes during World War I. In this type of
engine, the crankshaft is mounted on the airplane, while the
crankcase and cylinders rotate with the propeller.
often attributed to the American F.D.Farwell the rotary engine may
have had an earlier beginning in a compressed-air engine worked out
by the Australian pioneer Lawrence Hargrave some eight or nine years
prior. It is certain, however, that the French brothers Seguin
brought the engine into commercial and mechanical life based on the
conceptions of one brother (Laurent). It was in 1907 that his 7-cyl
rotary was born - and it came to be known as the Gnome. This engine
was followed by a succession of designs by many manufacturers, most
of which were successful.
We are accustomed to seeing someone swing the prop to start an
engine. This was not always necessary because a crank could be
engaged to the gear on the rear of the thrust plate and enough
rotary motion could be generated to get the engine started. When you
compare this with the starting of a radial or an inline the reason
why rotaries started easier can be seen. In the case of the radial
or inline, it was necessary to set the innards of the engine in
motion. The rotary was started by rotating the engine. The mass of
the rotary added to the starting function and assisted the effort.
The rotary was its own inertial starter!
The rotary engine gained quick acceptance because of its remarkable
power to weight ratio. The only comparable ratios came from the
brilliant mind of an American, Charles Manly. He had, in the very
early years of the 1900s, achieved P/W ratios that even rotaries did
not match until 1916. His 5-cyl 4-stroke static radial gave a ratio
of 2.4 lb per hp dry and 4.0 with all of its plumbing attached. How
remarkable was his achievement can be seen in a comparison of the
Wright's engine which delivered one hp per 15 lbs and the 1912 Gnome
rotary of 80hp which had a 2.625 ratio. Manly did not produce his
engine commercially: the brothers Seguin did.
That the rotary engine dominated the early years of aviation is
evident - although there were some very fine engines extant such as
the twins of Duthiel-Chalmers and Darracq, the Antoinette by
Levavasseur, and those of Fiat.
The demise of the rotary came about for several reasons. Among the
most important of these was the large rotating mass of the engine
which produced gyroscopic forces. These forces had their useful
features - if the pilot could master them before something happened
to lessen his desire to fly. It provided the Sopwith Camel with
remarkable turning power. However, the engine also delivered sharp
torque reversals when the ignition was cut which was tough on the
engine mounts and the airframe.
Another problem encountered by rotary engine designers was met when
trying to meet the demand for greater power. The size of the engine
could be expanded in only two directions: make it larger in
circumference, make it more than one row (deeper). The problem with
the first solution was that this just made the gyroscopic forces
even more unmanageable. The second way out of the problem provided
much the same effect and the rear bank of cylinders were hard to
There are other reasons that would have tended against the use of
the rotary into more modern times and the greatest of these would be
its enormous appetite for oil. The fuel was mixed with air as it was
introduced through a primitive "carburettor" - usually in the tail
end of the crankshaft. Via this route it made its way to the
crankcase where is picked up all of the oil that was loose. When the
fuel mixture was introduced to the combustion chamber it was very
much a mix of fuel, air, and castor oil.
The imperfect combustion of any engine is not equalled by that of a
rotary. The castor oil, being the least combustible of the two
liquids, was spewed out into the atmosphere. It would be but a short
time before the whole of the slipstream area of the aeroplane would
be well coated with castor oil. The pilot would be soaking up oil at
a fairly rapid rate as well. It is arguable that the reason for
cowling the engine had as much to do with trying to control the
wildly spewing oil as it was to do with the concepts of
streamlining. The usual practice was to direct the oil underneath
the fuselage by opening up the bottom of the cowl.
However, a cowling is not a favourite item to a rotary. The
cylinders are air-cooled. As has been mentioned, the use of two
banks of cylinders caused trouble enough. The cowling made the
engine much hotter that it liked. The reason for the cutout in the
bottom of the cowl, then, was to direct the spray of oil as well as
to aid in cooling the engine. Some of the cowlings of WWI aeroplanes
show evidence of extra cooling openings being cut into them by
mechanics in the field.
Many people remark about the pleasantness of the odour of burnt
castor oil. Out in the open where one's exposure is contrasted with
other scents, it can be an enjoyable sensation. It is still nice if
you are saying, "bye-bye" to the pilot before you go back to your
mechanic's tasks. But to sit behind an engine that is spraying you
with un-burnt - as well as burnt - castor oil is quite another
matter after a few hours. The oil is known for its purgative
qualities. It would be impossible to expose oneself to such an
atmosphere and not experience certain difficulties.
It is the need for cooling that is part of the reason that pilots
'blipped' their engines. One could not use a throttle on them
because they had such great need of motion to keep them cool. That
they were allowed to stop to descend is true but the combustion had
ceased during that time. (Of course, starting them up again could be
an exciting experience. If they were not too loaded with the
explosive fuel mixture - they might do just that: explode. If badly
loaded in one or two cylinders, the rough running could cause
considerable concern before it cleared.)
Although the cowling did cause them to overheat, It also allowed
them to produce greater power as the air trapped within the cowl was
easier to "stir" with the cylinders than would be a stream of high
velocity air directed at the front of the engine.
They were easy to start by diving to turn the prop - which turned
the engine. And they have been known to run with the most awesome
damage inflicted on one or more cylinders.
There are many stories about the gyroscopic forces and their ability
to turn a sorely pressed pilot out of danger. The most engaging
terms used to describe the turn of a Camel was said by Dick Day:
"Why, it puts both eyes on the same side of your nose!"
Gnome differed from other engines in that the intake valve was
located in the piston itself as was opened by a vacuum being formed
in the cylinder during the intake cycle. The fuel gas mixture was
admitted through the crankcase and sucked in by the vacuum as the
piston moves downwards. Power was regulated by 'blipping' the
As the piston moves to the bottom of the cylinder, vacuum is lost
and the inlet valve closes. The piston then moves upwards thus
compressing the fuel air mixture. The ignition spark occurs before
top dead centre.
The power stroke now
begins, the piston being forced downwards by the pressure of the
expanding gasses. The exhaust valve opens well before bottom dead
This engine has a fairly
long exhaust stroke. In order to improve power or efficiency,
engine valve timing often varies from what one might expect.
A number of engines were
designed this way, including the Gnome, Gnome Monosoupape,
LeRhone, Clerget, and Bentley to name a few. It
turns out there were some good reasons for the configuration:
Balance. Note that the crankcase and cylinders revolve in
one circle, while the pistons revolve in another, offset circle.
Relative to the engine mounting point, there are no reciprocating
parts. This means there's no need for a heavy counterbalance.
Air Cooling. Keeping an engine cool was an ongoing challenge for
early engine designers. Many resorted to heavy water cooling
systems. Air cooling was quite adequate on rotary engines,
since the cylinders are always in motion.
No flywheel. The crankcase and cylinders provided more than
adequate momentum to smooth out the power pulses, eliminating the
need for a heavy flywheel.
these factors gave rotary engines the best power-to-weight ratio of
any configuration at the time, making them ideal for use in fighter
planes. Of course, there were disadvantages as well:
Gyroscopic effect. A heavy spinning object resists efforts to
disturb it's orientation (A toy gyroscope demonstrates the effect
nicely). This made the aircraft difficult to manoeuvre.
Total Loss Oil system. Centrifugal force throws
lubricating oil out after it's first trip through the engine.
It was usually castor oil that could be readily combined with the
fuel. (The romantic-looking scarf the pilot wore was actually a
towel used to wipe the slimy stuff off his goggles!)
aircraft's range was thus limited by the amount of oil it could
carry as well as fuel. Most conventional engines continuously
re-circulate a relatively small supply of oil.