While visiting your average aviation museum, you will likely encounter flying machines from the dawn of flight to the supersonic fighter aircraft of the 21st century. Most of these early aircraft are equipped with what might be called the standard piston engine, in that there are stationary cylinders which house pistons that turn a crankshaft, spinning a propeller at a high enough revolution to pull the machine through the atmosphere, resulting in powered flight. But take a closer look at those aircraft from at and before 1918… you may notice something very peculiar about their engines. That’s right, some of those machines may be equipped with a rotary engine, where the entire engine spins around WITH the propeller!
Developing an engine of that type comes with various issues regarding the distribution of the fuel/air mixture to the cylinders, lubrication with oil, massive amounts of torque for the pilot to juggle, and tedious maintenance procedures that often result in shorter times between major overhauls, among many other things. The rotary engines you will typically find on these old aircraft are the Gnome, Le Rhone, and Le Clerget engines. The Gnome rotary engine is the oldest of the bunch, being introduced in 1908 at the Paris Airshow. Later came the Le Rhone engine, specifically the 9C, being introduced in 1912 and powering such popular aircraft as the Nieuport Bebe, Sopwith Pup, Sopwith 1 ½ Strutter, and Sopwith Camel. Finally, the Clerget engine is a modification of the Le Rhone engine by W. O. Bentley and looks similar but was designed with some differences. Functionally, however, the two are nearly identical. These engines were extremely prevalent prior to and during the First World War, and their operation was of a very tedious nature. The general principle of their operation, lubrication, fuel delivery, and their significance to aviation as a whole will be discussed in this article.
Rotary Engines - How Do They Work and Why Do They Matter?
Rotary Engines- How Do They Work and Why Do They Matter?
Introduction
Sopwith Triplane fitted with a Le Clerget rotary engine
Why spin the entire engine?
how is power produced?
Lubrication and oil
Spark and
Ignition
Valves and Pushrods
Proceed directly to:
rotary vs. radial engines
By: Carson Hawkins
Where the radial engine is mounted firmly to the firewall and the crankshaft spins the propeller like the standard engine, the rotary engine’s crankshaft is mounted to the firewall and the entire engine spins with the propellor attached to the crankcase and cylinders. This means the cylinders, crankcase, spark plugs, pushrods, valves and every other part necessary to make a piston engine function are spinning ‘round and ‘round at over 1,000 revolutions per minute, driving an attached propeller to generate thrust. Fascinating, right?
It can be quite difficult to tell the difference between a radial engine and a rotary engine on a vintage aircraft. For the most part, rotary engines are found on aircraft from around 1908 to 1918. Around late 1918, the threshold of maximum power had been reached with rotary engines and stationary engines were becoming more popular due to their ease of maintenance, reliability, and power output. The RFC/RAFs training aircraft, the AVRO 504, for instance, was equipped with rotary engines throughout the first world war but with the advancement of technology, its rotary engine was replaced by the radial engine around 1919 and that remained so until the AVRO 504s retirement from military service.
Determining whether or not an aircraft has a radial engine can mostly be narrowed down to the aircraft's year of manufacture or when it was in service. If the aircraft is pre-WWI or during the WWI years, and its cowling is circular, there is a mighty chance that it has a rotary engine. Another way to tell if an aircraft has a rotary or radial engine is by sound! If you hear a 'brrrap! brrrrap! brrrap!" sound, that is a rotary engine with its ignition being momentarily cut to control engine speed.
Difference Between Rotary and Radial Engines
Sopwith Camel with rotary engine
Grumman FF with radial engine
The earliest engines used on flying machines struggled greatly with dependability and the lack of horsepower generated by what was usually two or three cylinders. The engine blocks were generally extremely heavy, the weight of the cooling system was excessive, and reliability was a constant issue. With these shortcomings in mind, the Gnome rotary engine was introduced in 1908. The issue of cooling was solved by rotating the entire engine which means there wasn't the added weight of plumbing and fluid for a cooling system. The rotary engine was also its own flywheel, significantly reducing issues related to vibration. Additionally, multiple cylinders could be fitted to the engine's casing and a greater horsepower could be achieved while still keeping a small and nimble profile. As early as the late 1890s, rotary engines can even be seen fitted to bicycle wheels, forming a type of motorcycle that is most unusual!
Why Spin The Entire Engine?
The delivery of the fuel/air mixture to the combustion chamber on a rotary engine is a unique operation due to the nature of constantly rotating cylinders. For all types of rotary engines, the fuel and air enters the crankcase through a hollow crankshaft located just behind the pilot's instrument panel. Fuel is able to reach the engine from the tank by way of an air pump located at the rear of the engine and driven by a gear but, in the event this air pump fails, a hand pump is available to the pilot in the cockpit to build up sufficient pressure to maintain operation of the engine. Air reaches the engine through air scoops extending out of the side of the aircraft where it then proceeds to the crankshaft and into the engine for combustion. The fuel, air, and castor oil are then distributed to the cylinders by different means according to what the engine type is. Other specifics and the way power output is managed differ slightly between the Gnome engines and the Le Rhone/Clerget engines. These differences are outlined below.
Delivery of Fuel/Air Mixture and Controlling Power Output
GNOME 9N MONOSOUPAPE:
The Gnome Monosoupape is called such a name because it only has one valve that acts as the intake valve and exhaust valve. The 'Mono', as it became known, is the most popular of the Gnome series and is not equipped with a carburetor. Instead, fuel passes from the tank through a regulating valve, available to the pilot in the cockpit, and is then routed via copper piping through the crankshaft to a jet located at the very tip of the crankshaft inside the engine. The pilot can meter the fuel being sprayed into the engine by adjusting the regulating valve inside the cockpit, however the pilot has no amount of control over the air entering the engine and must regulate the fuel at different altitudes so as to avoid bogging down or starving the engine of fuel.
Once the fuel and air mix in the crankcase with the castor oil, they pass into the cylinder for combustion. The cycle works like this: as the piston travels downward towards the inside of the engine, the valve at the top of the cylinder remains open momentarily to draw air into the cylinder. The valve then closes as the piston is still traveling downwards which creates a vacuum effect. This vacuum effect is what helps draw the fuel/air/castor oil mix into the cylinder for combustion.
Gnome Monososupape from 'Aviation Engines' by Victor W. Page (1918)
Fuel regulating valve in a Nieuport 28
At the very bottom of this intake stroke, the fuel/air/castor oil mixture enters the combustion chamber through 36 circular ports drilled into the bottom of the wall of the cylinder. These ports are then closed off by the wall of the piston as the piston begins travelling upwards which compresses the mixture and prepares it for combustion. As the piston nears the top of the stroke, the mixture is ignited by the singular spark plug located at the top of the cylinder. The valve then opens again (now acting as an exhaust valve), expels the exhaust gasses and castor oil, remains open momentarily to allow fresh air to be drawn into the cylinder as the piston begins its downstroke, and the process starts over again!
Excerpts from 'The Care of the 100 h.p. Monosoupape Engine' that describe the fuel jet at the tip of the crankshaft inside the engine. The drawing to the left represents the view of the engine with the propellor and front covering plate removed if looking at it from the front.
So how does the pilot control power output on the Gnome Mono? Actually, for the most part, the ignition system of the Gnome is what is relied upon to change the power output. The pilot doesn't have a throttle to control the Mono like he does with the Le Rhone or Clerget engine. As stated above, to control the engine by means other than the ignition, the pilot only has a fuel regulating valve that he adjusts with change in altitude since the engine breathes the same air as the pilot. To avoid bogging down or starving the engine of fuel, the pilot can only adjust the fuel output at the jet in the crankcase with the regulating lever in the cockpit.
Controlling power output on a Gnome Mono is mostly done with a 'blip switch' on the control stick which, when pressed, cuts off the spark going to the spark plugs. If you have ever heard a rotary engine run and noticed the "brrrap! brrrrap!" noises, you are hearing the pilot pressing the blip switch! More on this later, under the Spark and Ignition topic.
LE RHONE/CLERGET:
Unlike the Gnome, the Le Rhone and Le Clerget rotary engines are equipped with a basic type of carburetor called the 'Bloc-Tube' carburetor that the pilot uses to adjust the ratio of fuel and air entering the engine through the crankshaft. The carburetor is located at the very entrance of the hollow crankshaft, just behind the instrument panel, and is manipulated by two levers located on the "Tampier" quadrant in the cockpit. The Bloc-Tube carburetor consists of a rectangular metal frame that houses a fuel nozzle at one end and a sliding metal square wicket with a needle at its head at the other end.
One lever on the Tampier quadrant is linked to the sliding square wicket and, when adjusted, moves the square over the entrance of the crankshaft and therefore adjusts the amount of air being permitted. Because the needle is attached to this square piece at its head, movement of the lever also adjusts the amount of fuel coming from the fuel entrance nozzle and into the crankshaft. The other lever on the quadrant controls the amount of fuel directed into the fuel nozzle at the Bloc-Tube. This means the pilot can change the engine's power in two different ways: by moving the first lever, which controls both air and fuel going through the carburetor simultaneously, or by moving the second lever which controls the overall amount of fuel being sent to the fuel entrance at the carburetor.
Once the fuel and air mixture passes through the carburetor, it enters the engine's crankcase where it mixes with the castor oil and is passed to the head of the cylinders by way of induction tubes from the centrifugal force of the spinning motor. On the Le Rhone engine, the induction tube is a copper pipe that runs vertically up to the top of the cylinder from the crankcase and on the Clerget, the induction tube is located behind the cylinders and is matte silver in color. Since the fuel/air/castor oil mixture enters the combustion chamber from the top of the cylinder; ports are not drilled into the pistons of these engines. The Le Rhone and Clerget engines have two valves unlike the 'Mono', so the combustion cycle is practically the same as the average 4-stroke engine: As the piston travels downwards, the intake valve allows the mixture to enter the cylinder for combustion. The valve is then closed and the mixture is compressed before being ignited by the spark plug located at the top of the cylinder. This explosion forces the piston downward and the exhaust valve opens so when the piston begins rising again, the exhaust gasses and excess castor oil leave the cylinder and vaporize in the atmosphere. The exhaust valve closes, the intake valve opens, and the cycle repeats itself!
Le Rhone 9C
Edited excerpt of the Bloc-Tube carburetor or Tampier Valve ('Les Carburateurs et la Carburation', 1920)
Tampier quadrant in a Sopwith Camel
Rotary engine air scoop on the Sopwith Pup
Rotary engines use castor oil instead of other mineral oils for lubrication because of its insolubility with gasoline and resistance to the higher temperatures associated with air-cooled engines. In this case, the issue of insolubility is of special importance due to the fuel/air mixture entering the crankcase, where the castor oil spends most of its time, before entering the cylinders. If any other oil was chosen, the fuel entering the crankcase would eliminate the oil's lubricating properties as metal thrashes around in the crankcase and the engine would be susceptible to failure or damage almost instantaneously. Although castor oil was selected as the preferred lubricant for these rotary engines, it wasn't without its drawbacks.
Castor oil is of vegetable base and originates from the seeds of the castor plant. Because of this, carbon deposits and gummy accumulations form more frequently in the engine which demands frequent overhauls to scrape these buildups out of the engine casing and combustion chambers. This buildup of gummy deposits is a chief issue on early Gnome engines that were built with the intake valve in the piston itself, and it was very common for that valve to gum up and malfunction or to catch fire completely due to the vegetable oil properties of castor oil. Another drawback worth mentioning is not regarding the oil itself, but the oil usage of a rotary engine. Due to the nature of a rotary engine, the recovery or recycling of oil is nearly impossible and oil loss is far more excessive than that of a typical stationary engine. The oil system of rotary engines is a total-loss system, as the spent oil is flung out of the engine after combustion. This is largely the reason for the existence of the cowling surrounding early aero engines, and the reason for oil tanks that are almost the size of fuel tanks on aircraft that are equipped with rotary engines. A detailed look into the oiling of these engines is as following:
Lubrication and Oil
Lubrication of the Gnome, Le Rhone, and Clerget is accomplished by combining pressure from an oil pump and the centrifugal force of the rotating engine. In most cases the oil is fed from the tank to the pump by gravity, but a pressure pump can also be used. The thrust plate, also referred to as an anchor plate (Le Rhone/Clerget) or a Main Bearer Plate (Gnome), is a stationary metal plate that mounts the engine to a test stand or the firewall of an aircraft and has a hole drilled into it which allows for the gears of the oil pump to mesh with a gear fixed onto the rotating engine. Coming off the oil pump are two distributing copper pipes of equal size that run through the hollow crankshaft. As the engine spins, so too does the gear attached to it which in turn drives the oil pump, pushing oil through the crankshaft. The two distributing pipes have different lubricating responsibilities as they proceed through the crankshaft and into the moving parts of the engine, but oil from both pipes ultimately ends up in the crankcase and cylinders. The oil mixes with the fuel and air in the combustion chamber and is ejected as the exhaust valve opens.
Profile diagram of the Gnome Monosoupape from 'Aviation Engines' by Victor W. Page, 1918
Oil pulsator in a Sopwith Camel
Oil pump on the anchor plate of a Le Rhone 9C
Branching off the two oil pipes going into the crankcase are two more pipes that lead to pulsators in the cockpit. Being glass bulbs with oil lines protruding from the bottom, these pulsators provide the pilot with an indication of the status of the oil pump. While the engine is running, the pilot can look down and see the oil pulsating in the glass bulbs, providing him with an indication that the oil is flowing as it should to the necessary parts of the engine. In certain cases, engine RPM can be determined by studying the pulses of the oil in the bulb and applying a mathematical equation! A very easy thing to do when you are surrounded by the Hun...
"Contact!" - A phrase that is known throughout aviation circles worldwide and is nearly impossible to forget when in conversation about rotary engines. But what exactly does the saying mean? How is spark delivered to spark plugs that are rotating faster than the eye can decipher? Well, the question of providing spark to spark plugs spinning at nearly 1,500RPM is answered with the typical genius that most issues regarding rotary engines are tackled. The Gnome Mono, Le Rhone, and Le Clerget ignition systems are all quite similar in operation with the exception of the Clerget which has two spark plugs per cylinder. The Gnome and Le Rhone engines only have one spark plug and one magneto to provide ignition.
Spark and Ignition
Along with the oil pump and air pump on the anchor/main bearer plate of the rotary engine lies the magneto. On the Gnome and Le Rhone engines, there is only one magneto that powers one spark plug per cylinder but on Le Clerget engines, there are two magnetos powering two spark plugs per cylinder. This engine, and its Bentley counterparts, were built with redundancy in mind; a feature that nearly every modern aviation piston engine is equipped with today. The magneto is built with a protruding gear that, when the magento is installed on the anchor/main bearer plate, meshes with the larger gear attached to the spinning engine which also powers the oil pump and air pump. The spinning of the larger gear spins the gearing of the magneto which generates an electrical current.
Just beyond the gear that spins the aforementioned equipment is a distributer in the shape of a solid ring that is also firmly attached to the engine and therefore spins with it. As a reminder, the anchor/main bearer plate does does not rotate with the engine and is what mounts the entire engine to a test stand or the firewall of the aircraft. The gear and distributer ring spin with the engine as a single unit. Electrical current from the magneto is transferred to the distributer via an electrical cable that connects a brush holder which is also mounted to the anchor plate/main bearer plate, just above the crankshaft. The electrical brush in the holder extends through the anchor/main bearer plate and contacts the distributer, transferring the electrical charge for distribution to the spark plugs. Nine brass segments (one per cylinder) are planted in the distributer ring. Attached to these brass segments are the spark plug wires that extend from the segments to the spark plug at each cylinder. There, the spark is carried out and the fuel/air mixture is detonated! But... what control does the pilot have over this operation? Where does "Contact!!" come into play?
Magneto on the anchor plate of a Le Rhone 9C
Main Bearer Plate of the Gnome Monosoupape. The assembly on the left is the oil pump, the air pump is in the low center, and the magneto is on the right. Excerpts from 'The Care of the 100 h.p. Monosoupape Engine'
Notice how the electrical brush is mounted to the anchor plate but protrudes through it to contact the distributer. From 'Aviation Engines' by Victor W. Page, 1918
Circled in red are the two magneto switches of this Clerget-powered Sopwith Camel. In blue is the 'blip' switch
Power to the magnetos is controlled by switches in the cockpit that are immediately available to the pilot on the instrument panel. On most rotary-powered machines, this control takes the form of a gold switch with two positions: on and off. When a switch is placed into the on position, the pilot calls... get ready for it... "Contact!" With a quick flip of the switch into the 'contact' position, the circuit is completed and power will be sent to the spark plug as the engine, and therefore the distributer ring, spins when the maintainer on the ground pulls the propeller through. On aircraft powered with Gnome, Le Rhone and Clerget engines, the gold magneto switches are standard. In addition to the magneto switches, the 160hp Gnome-powered Nieport 28 has what is called a 'selector' switch. The selector switch on these Nieuports allows the pilot to throttle the engine by adjusting the number of cylinders that will fire on each revolution of the engine and has five positions: 0, 1, 2, 3, and 4. When the selector switch is set to 4, the engine is making full power. Conversely, when the switch is set to 0, the engine is off; no spark is being delivered. When set to position 3, the engine skips every other cylinder in the firing order; this is considered half power. One quarter of the engine's power is achieved at position 2, and one eighth of the engine's power is developed at position 1 where it takes 16 revolutions of the engine for all cylinders to fire.
Nieuport 28 ignition selector switch
Another way, and certainly the most known way among vintage aviation enthusiasts, of controlling the power output of a rotary engine is by way of a 'blip' switch on the pilot's control stick. By pressing this switch with a simple movement of the thumb, the pilot interrupts the circuit of the ignition system, thereby cutting power to the spark plugs and windmilling the engine while the button is depressed. The blip switch is a spring loaded button so when the pilot releases his thumb, power is immediately restored to the spark plugs and power output is returned to normal. This effect can be seen and heard quite plainly when viewing a rotary engine in action; when the pilot presses and releases the blip switch, a distinct 'brrrap! brrrap! brrrap!" sound is heard and the wings rock back and forth from the inconsistent torque. This level of torque presents quite the challenge to a pilot flying and landing the machine, as the gyroscopic force of the engine is interrupted and restored and interrupted and restored.
The operation of the valves on a rotary engine is not dissimilar to other engines with overhead valves. Perhaps the main difference between a stationary engine and a rotary engine, as far as valve operation is concerned, is the inconsistent number of pushrods and valves across the spectrum of rotary engines. The Gnome Monosoupape only has one valve at the head of the cylinder which acts as the intake valve and the exhaust valve. Early Gnome engines did have an exhaust valve at the head of the cylinder but the intake valve was located in the head of the piston, causing numerous issues for pilots and maintainers alike. On Le Rhone engines, one pushrod controls the operation of two valves and on Le Clerget engines, a common ground is found with modern aero engines in that there are two pushrods per cylinder: one that controls the intake valve and one that controls the exhaust valve.
Valves and Pushrods
The mechanisms that are responsible for operating the valves of a rotary engine are found in the cam box, an illustration of which can be seen above and circled in blue. What the cam box contains differs between rotary engines but many pieces are the same; cams, tappets to move the pushrods, and gears that determine camshaft timing. As the engine rotates, cams that rotate with the engine move the pushrods which actuate the valves, allowing air and exhaust to enter and exit the cylinder. Not too different!
Gnome Mono camshaft operation from 'The Care of the 100 h.p. Monosoupape Engine'. Circled in red is the camshaft
Basic illustration of the parts that make up a rotary engine from 'Rotary Engines, Magnetos, and Carburetors' by Harold Pollard
We'll be in touch soon!
Thank you!
