Aircraft use reciprocating engines that follow a four-stroke cycle. Among these, a rotary engine has cylinders arranged radially around a central crankshaft, while a radial engine has cylinders arranged radially around a central crankcase. A rotary engine has fewer moving parts and is internally balanced with spinning counterweights that are phased to cancel out any vibrations. The Gnome Omega engine is a rotary engine. A radial engine is a type of internal-combustion engine and has odd-numbered cylinders. A V-type engine has cylinders arranged in a V shape.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a rotary engine in aircraft?
A rotary engine is an early type of internal-combustion engine in which the entire crankcase and its attached cylinders rotate as a single unit around a crankshaft that remains stationary and is fastened rigidly to the aeroplane. The propeller is bolted to the crankcase, so the whole engine spins with the propeller. The cylinders are arranged radially in a circular shape around the eccentric central crankshaft, and the engine is usually designed with an odd number of cylinders per row in a radial configuration. The rotary engine is a standard Otto cycle engine, and it transforms chemical energy into rotational energy. Its design provides a continuous rotational output while being air-cooled and internally balanced with spinning counterweights phased to cancel vibrations.
Do airplanes use rotary engines?
Yes, airplanes use rotary engines, but only in a narrow sense. Rotary engines powered many early aircraft and carried many WWI aces. The Fokker Dr.I triplane was powered by an 80 hp Le Rh ne rotary engine. Lighter engine weight meant rotary-powered airplanes were more maneuverable than planes with water-cooled inline or vee engines, yet rotary engines were not used on large aircraft, just on early fighters. Rotaries were tough on early pilots, who had to steer while dealing with the gyroscopic effects of a spinning motor and manipulate the fuel and air mixture manually. The British Royal Air Force used rotary engines for longer than most other operators, and the museum has 16 rotary engines attached to aircraft, planning to place one on a Nieuport 17. Today, the AE50R is the only rotary engine worldwide certified according to EASA Part 22 Subpart H, while the RT600-XE twin-rotor rotary engine delivers exceptional power density in a compact, lightweight package, and the RT600-HC is engineered for VTOL and rotorcraft missions.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
How does a rotary aircraft engine work?
A rotary aircraft engine works by converting the pressure produced by the combustion of an air-fuel mixture into rotational motion. It takes fuel, mixes it with air, compresses the mixture, ignites it with a spark plug, allows the expanding gas to perform mechanical work and turn a shaft, then ejects the spent gas from the combustion chamber. What distinguishes it from a conventional piston engine is that the crankshaft remains stationary while the entire cylinder block rotates around it. Pistons change their relative position in any given cylinder as a result of the offset crank on the end of the crankshaft, so each cylinder passes through intake, compression, power and exhaust strokes in the same space, while the whole assembly spins like an enormous flywheel. A firing order of 1, 3, 5, 7, 2, 4, 6 on seven-cylinder rotaries (or 1, 3, 5, 2, 4 on five-cylinder versions) gives three separate power strokes for each complete rotation, and the rotating mass provides momentum that smooths power pulses. Total-loss oil lubrication of the Gnome type means the aircraft's range is limited by the amount of oil it can carry as well as fuel.
To regulate speed, the pilot manipulates fuel and air levers that feed mixture into the hollow crankshaft of a Le-Rh ne, or blips the ignition on and off with a ‘coupe switch’, because these engines lacked conventional throttles. Engines like the Camel therefore gave partial throttle simply by not firing some cylinders. Early pilots had to steer while coping with the gyroscopic effects produced by a heavy spinning object that resists efforts to disturb its orientation. A counterbalance weight sits opposite the rod hub to prevent engine vibration, helping the pilot manage the rough power pulses at typical speeds of about 600 rpm idle and 1,200 rpm at maximum throttle.
What is the Wankel engine in aviation?
The Wankel engine is a type of internal-combustion rotary engine distinguished by an orbiting triangular rotor that functions as a piston. The rotor moves in an epitrochoidal housing and eliminates the need for conventional pistons and cylinders. Because it converts pressure into rotating motion instead of reciprocating motion, the engine is almost vibrationless and provides continuous rotational output, reducing wear and tear on aircraft components and contributing to flight stability.
Wankel engines are light, compact, and offer a high power-to-weight ratio - exemplified by the AE50R model with 2 hp per kilogram - making them well suited for light aircraft, ultralight aircraft, homebuilt experimental aircraft like the ARV Super2, and auxiliary power units. The same qualities explain why the Wankel engine is used in uncrewed aerial vehicles and UAV applications, including loitering munitions, and why it is an excellent choice for powering model airplanes. A 13.4-ounce (380-gram) Wankel engine for models has been in production unchanged since 1970.
The first Wankel engine to power an airplane was an RC2-60 automobile engine installed in a Cessna Cardinal. Wright Aeronautical adapted three such engines for flight testing in 1970. Another RC2-60 powered a Lockheed Q-Star to demonstrate quiet flight, while a third powered a Hughes TH-55 helicopter. Curtiss-Wright and John Deere each developed lines of Wankel prototypes for aviation, and British MidWest aero-engines have also been flown. Despite these successes, economic considerations including fuel burn and emissions later diminished interest in the Wankel concept, yet its smooth, compact, and lightweight character keeps it present in niche aviation sectors.
What is a radial engine in an airplane?
A radial engine is a type of internal-combustion engine used mainly in small airplanes. In this engine the cylinders are mounted in a circle around the crankshaft, with the cylinders ranging from five to as many as 28 depending on engine size. When viewed from the front it resembles a stylized star. Circular symmetry allows engineers to arrange the cylinders in two or more rows, and because every row must have an odd number of cylinders, pistons are connected to the crankshaft with a master-and-articulating-rod assembly.
Why do planes use radial engines?
Planes use radial engines because their short crankshaft, few moving parts, and simple construction make them remarkably robust and resistant to damage. By arranging the cylinders around a central hub, the design distributes loads equally around the shaft, reduces vibration, and removes the need for radiators and their glycol coolant, in turn lowering both weight and maintenance. The large frontal area that houses the cylinders doubles as a shroud directing ample airflow, so the engines remain air-cooled even under heavy loads.
These characteristics yield a superior power-to-weight ratio, allowing designers to fit compact yet potent packages into fighters, naval reconnaissance machines, long-range bombers, and early cargo planes. Reliability was paramount for Navy pilots on over-water flights, and the consistent operation of Wright Cyclone, Pratt & Whitney, Bristol, and Shvetsov radials gave crews confidence in twin-row, fourteen-, or even eighteen-cylinder configurations. Power generators and farming equipment still employ similar layouts today, testimony to the enduring strength born from the same straightforward architecture that once drove B-17s, P-51s, and the Sikorsky S-55 skyward.
How does a radial aircraft engine work?
A radial engine works like any other four-stroke internal combustion engine, operating on the four-stroke principle of intake, compression, power and exhaust. Cylinders are arranged in a circle around the crankshaft, and the crankshaft absorbs the power or work from all the cylinders. An odd number of cylinders is required by the combination of the single-crank radial design and the desire to keep the power strokes evenly spaced in time, so an even number of cylinders in one bank of a radial engine does not work. Pistons are connected to the crankshaft with a master-and-articulating-rod assembly. The first cylinder's connecting rod is the master rod, and other cylinders attach to pivot points on the master rod.
A true four-stroke radial fires every other cylinder in order of crankshaft rotation, creating a firing sequence that follows motion of crank throw during rotation. For example, the firing order of a nine-cylinder radial is 1 3 5 7 9 2 4 6 8, while the firing order of a seven-cylinder radial is 1 3 5 7 2 4 6. Each cylinder has intake, compression, power and exhaust stroke. Overhead poppet valves are driven by pushrods and lifters on a cam plate, with the cam ring driven by crankshaft at reduced rate of speed through cam intermediate drive gear assembly. The cam ring is mounted concentrically with the crankshaft. When a cylinder is beginning its power stroke, the cylinders on either side of it are near to beginning their intake strokes, assuring smooth production of power and balanced operation. All pistons can be air-cooled, with cool air passing between outer casing and liner. However, rear rows are masked by the front rows, and air is already hot after passing the first set of cylinders, limiting cooling.
What is an inline engine in aviation?
An inline engine is a reciprocating internal-combustion powerplant whose cylinders are aligned in a single straight row along one side of the crankshaft. Because all cylinders are lined up in a row it is often called a straight engine. In the aircraft context the word ‘inline’ is also applied more loosely to V, H, or horizontally opposed configurations, but the classical inline engine has only one bank of cylinders and therefore only one crankshaft. Straight engines are usually built with 3, 4, 5, or 6 cylinders. More than six cylinders is uncommon because an 8-cylinder row is too long for most engine bays. They are air-cooled or liquid-cooled, yet the air-cooled inline type is now confined to the low- and medium-horsepower engines that powered very old light aircraft. The design gives a small frontal area that reduces drag and is better adapted to streamlining, but the engine tends to be longer and taller than a V configuration. Many inline aero engines are inverted so that the 1-3 cylinders hang below the crankshaft, a layout that lowers the thrust line and further improves streamlining although it complicates installation on nose-wheel aircraft.
What is the difference between an aircraft radial engine and an inline?
Radial engines differ from inline types in both firing order and the path their connecting rods take to reach the single crankshaft. Where inline engines place cylinders in a straight bank and pass power through a long, slender crankshaft, radials arrange cylinders in a circle around a short, stiff crankshaft that accepts rods directly from every cylinder. This compact radial crankshaft is stiffer and lighter, yet demands an injection system for the lower cylinders because gravity works against fuel distribution in the circular layout.
Inline layouts, being liquid-cooled, can be run more intensively as the coolant carries heat away faster than the airflow that cools an air-cooled radial. Because of that superior cooling, inline engines tolerate higher boost pressures and compression ratios without detonation, giving the same power output from a smaller displacement or allowing smaller engines to match the power of larger radials. Radials are generally lighter overall, trading the weight of radiators, coolant, and long crankshafts for the simplicity of abundant cooling fins and a short crankshaft, advantages that let air-cooled radial engines compete effectively with their inline cousins.
What is a V-type engine in aircraft?
A V-type engine in aircraft is the equivalent of two in-line engines joined in a V configuration by a common crankshaft. The cylinders are aligned in two banks at an angle to each other, forming a V, with the angle between the two banks usually 60° or 90°.
The best-known examples are the supercharged Rolls-Royce Merlin and the Allison V-1710, a large American V-12 aviation powerplant whose name indicates a V-type layout and a displacement of 1710 cubic inches (27.9 L); this engine, 86 inches long (218.4 cm) and weighing 1400 lbs (635 kg), has a 60-degree vee angle, four valves per cylinder, and a single camshaft for each bank.
What are the advantages and disadvantages of V-type aircraft engines?
The advantages of V-type aircraft engines include a higher thrust line, less noise, increased ground clearance for the propeller, and better balance and less vibrations than other engine configurations. V-type aircraft engines produce more torque at lower RPM than other models and allow more cylinders in a smaller space. Their compact nature allows more cylinders in a smaller space, shortening the length of the crankshaft and resulting in small dimensions and low weight. V engines produce more torque at lower RPM and deliver good horsepower, enhancing power transfer to the crankshaft. The configuration also allows increased ground clearance for the propeller and provides a higher thrust line, giving these aircraft engine types high static performance and enabling them to climb to extremely high altitudes.
Disadvantages of V-type aircraft engines include that they are less fuel efficient than inline engines Cooling problems are a major drawback of the V-type aircraft engine layout. V-type aircraft engines have a higher weight because they have additional parts. V-type aircraft engines can cause mechanical stress and vibration if there is imbalance and are challenging for automakers to develop because of intricate design and assembly. Higher cost is another disadvantage of V-type engines, incurring higher servicing and manufacturing prices.
What is the relationship between an airplane and a V engine?
The relationship between an airplane and a V engine extends beyond raw power. An inverted V arrangement lowers the crankshaft and raises the cylinder heads, giving better centre of gravity and allowing a longer propeller without longer landing-gear legs. The same inversion reduces glare from the exhausts during night flights and provides the pilot a clearer view over the cowling. For these reasons, V engines have remained the preferred layout when frontal area, weight and cooling drag must all be kept small while still delivering the horsepower that pulls the airplane through the air.A V engine is a compact, high-output powerplant whose pairs of cylinders form a narrow V, giving the configuration a small frontal area that minimizes drag when mounted on an airplane. Because V engines provide more horsepower than inline engines of comparable displacement, they let designers meet thrust requirements without lengthening the cowling or widening the fuselage. This advantage first appeared in older V engines like the Curtiss OX-5 V-8, the Wright Hispano-Suiza H and the RAF-1a, and it continues in the most notable V-style powerplant of the post-war era, the Orenda V8 conversion installed on the Aero Commanders.
