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Radial engine

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Title: Radial engine  
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Subject: Wright R-975, Wright R-540, Bristol Neptune, British Salmson AD.9, Kinner C-5
Collection: Piston Engine Configurations, Radial Engines
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Radial engine

Radial engine timing and cam mechanism. Click here for full resolution animation.
Radial engine of a biplane
Master rod (upright), slaves and balances from a two-row, fourteen-cylinder Pratt & Whitney Twin Wasp
Mounting of a Wright R-2600 Twin Cyclone engine on a North American B-25 Mitchell bomber, 1942

The radial engine is a reciprocating type internal combustion engine configuration in which the cylinders "radiate" outward from a central crankcase like the spokes of a wheel. It resembles a stylized star when viewed from the front, and is called a "star engine" (German Sternmotor, French moteur en étoile) in some languages. The radial configuration was very commonly used for aircraft engines before gas turbine engines became predominant.


  • Engine operation 1
  • History 2
    • World War II: use in tanks 2.1
    • Modern radials 2.2
  • Comparison with inline engines 3
  • Other types of radial engines 4
    • Multi-row radials 4.1
    • Diesel radials 4.2
    • Compressed air radial engines 4.3
    • Model radial engines 4.4
  • See also 5
  • Notes 6
  • References 7
  • External links 8

Engine operation

Since the axes of the cylinders are coplanar, the connecting rods cannot all be directly attached to the crankshaft unless mechanically complex forked connecting rods are used, none of which have been successful. Instead, the pistons are connected to the crankshaft with a master-and-articulating-rod assembly. One piston, the uppermost one in the animation, has a master rod with a direct attachment to the crankshaft. The remaining pistons pin their connecting rods' attachments to rings around the edge of the master rod. Extra "rows" of radial cylinders can be added in order to increase the capacity of the engine without adding to its diameter.

Four-stroke radials have an odd number of cylinders per row, so that a consistent every-other-piston firing order can be maintained, providing smooth operation. For example, on a five-cylinder engine the firing order is 1, 3, 5, 2, 4 and back to cylinder 1. Moreover, this always leaves a one-piston gap between the piston on its combustion stroke and the piston on compression. The active stroke directly helps compress the next cylinder to fire, making the motion more uniform. If an even number of cylinders were used, an equally timed firing cycle would not be feasible.[1] The prototype radial Zoche aero-diesels (below) have an even number of cylinders, either four or eight; but this is not problematic, because they are two-stroke engines, with twice the number of power strokes as a four-stroke engine.

The radial engine normally uses fewer cam lobes than other types. As with most four-strokes, the crankshaft takes two revolutions to complete the four strokes of each piston (intake, compression, combustion, exhaust). The camshaft ring is geared to spin slower and in the opposite direction to the crankshaft. The cam lobes are placed in two rows for the intake and exhaust. For the example, four cam lobes serve all five cylinders, whereas 10 would be required for a typical inline engine with the same number of cylinders and valves.

Most radial engines use overhead poppet valves driven by pushrods and lifters on a cam plate which is concentric with the crankshaft, with a few smaller radials, like the Kinner B-5 and Russian Shvetsov M-11, using individual camshafts within the crankcase for each cylinder. A few engines utilize sleeve valves such as the 14-cylinder Bristol Hercules and the 18-cylinder Bristol Centaurus, which are quieter and smoother running but require much tighter manufacturing tolerances.


A Continental radial engine, 1944
A Pratt & Whitney R-1340 radial engine mounted in Sikorsky H-19 helicopter

C. M. Manly constructed a water-cooled five-cylinder radial engine in 1901, a conversion of one of Stephen Balzer's rotary engines, for Langley's Aerodrome aircraft. Manly's engine produced 52 hp (39 kW) at 950 rpm.[2]

In 1903–1904 Jacob Ellehammer used his experience constructing motorcycles to build the world's first air-cooled radial engine, a three-cylinder engine which he used as the basis for a more powerful five-cylinder model in 1907. This was installed in his triplane and made a number of short free-flight hops.[3]

Another early radial engine was the three-cylinder Anzani, originally built as a W3 "fan" configuration, one of which powered Louis Blériot's Blériot XI across the English Channel. Before 1914, Alessandro Anzani had developed radial engines ranging from 3 cylinders (spaced 120° apart) — early enough to have been used on a few French-built examples of the famous Blériot XI from the original Blériot factory — to a massive 20-cylinder engine of 200 hp (150 kW), with its cylinders arranged in four rows of five cylinders apiece.[2]

Radial engines are regarded as being Salmson company — and the engine was often known as the Canton-Unné.[4]

From 1909 to 1919 the radial engine was overshadowed by its close relative, the rotary engine — which differed from the so-called "stationary" radial in that the crankcase and cylinders revolved with the propeller. Mechanically it was identical in concept to the later radial except that the propeller was bolted to the engine, and the crankshaft to the airframe. The problem of the cooling of the cylinders, a major factor with the early "stationary" radials, was solved by the engine generating its own cooling airflow.[5]

In World War I, many French and other Allied aircraft flew with Gnome, Le Rhône, Clerget and Bentley rotary engines, the ultimate examples of which reached 250 hp (190 kW) although none of those over 160 hp (120 kW) were successful. By 1917 rotary engine development was lagging behind new inline and V type engines which by 1918 were producing as much 400 hp (300 kW) and were powering almost all of the new French and British combat aircraft.

In Germany during the same time frame, most aircraft used water-cooled inline 6-cylinder engines. Oberursel made licensed copies of the Gnome and Le Rhône rotary powerplants while Siemens-Halske built their own designs including the Siemens-Halske Sh.III eleven-cylinder rotary engine, which was unusual for the period in being geared so that the engine spun at a higher speed and in the opposite direction to the propeller.

By the end of the war the rotary engine had reached the limits of the design, particularly in regard to the amount of fuel and air that could be drawn into the cylinders through the hollow crankshaft, while advances in both metallurgy and cylinder cooling finally allowed stationary radial engines to supersede rotary engines. In the early 1920s Le Rhône converted a number of their rotary engines into stationary radial engines.

By 1918, the potential advantages of air-cooled radials over the water-cooled inline engine and air-cooled rotary engine that had powered World War I aircraft were appreciated but were unrealized. British designers had produced the ABC Dragonfly radial in 1917 but were unable to resolve the cooling problems, and it was not until the 1920s that Bristol and Armstrong Siddeley produced reliable air-cooled radials such as the Bristol Jupiter and the Armstrong Siddeley Jaguar.

In the United States, the National Advisory Committee for Aeronautics (NACA) noted in 1920 that air-cooled radials could offer an increase in the power-to-weight ratio and reliability, and by 1921 the U.S. Navy had announced it would only order aircraft fitted with air-cooled radials while other naval air arms followed suit. Charles Lawrance's J-1 engine was developed in 1922 with Navy funding, and using aluminium cylinders with steel liners ran for an unprecedented 300 hours, at a time when 50 hours endurance was normal. At the urging of the Army and Navy the Wright Aeronautical Corporation bought Lawrance's company, and subsequent engines were built under the Wright name. The radial engines gave confidence to Navy pilots performing long-range overwater flights.[6]

Wright's 225 hp (168 kW) J-5 Whirlwind radial engine of 1925 was widely claimed as "the first truly reliable aircraft engine".[7] Wright employed Giuseppe Mario Bellanca to design an aircraft to showcase it, and the result was the Wright-Bellanca 1, or WB-1, which first flew later that year. The J-5 was used on many advanced aircraft of the day, including Charles Lindbergh's Spirit of St. Louis in which he made the first solo trans-Atlantic flight.[8]

In 1925, the American rival firm to Wright's radial engine production efforts, Pratt & Whitney, was founded. Pratt & Whitney's initial offering, the Pratt & Whitney R-1340 Wasp was test run later that year, beginning a line of engines over the next 25 years that would include the most-produced aviation engine of any single design, the 14-cylinder, twin-row Pratt & Whitney R-1830 Twin Wasp with nearly 175,000 built.

In the United Kingdom the Bristol Aeroplane Company was concentrating on developing radials such as the Jupiter, Mercury and sleeve valve Hercules radials. Germany, Russia and Japan started with building licensed versions of the Armstrong Siddeley, Bristol, Wright, or Pratt & Whitney radials before producing their own improved versions. France continued its development of various rotary engines but also produced engines derived from Bristol designs, especially the Jupiter.

World War II: use in tanks

In the years leading up to World War II, as the need for armored vehicles was realized, designers were faced with the problem of how to power the vehicles, and turned to using aircraft engines, among them radial types. The radial aircraft engines provided greater power-to-weight ratios and were more reliable than conventional inline vehicle engines available at the time. This reliance had a downside though: if the engines were mounted vertically, as in the M3 Lee and M4 Sherman, their comparatively large diameter gave the tank a higher silhouette than designs using inline engines.

The Continental R-670, a 7-cylinder radial aero engine which first flew in 1931, became a widely used tank powerplant, being installed in the M1 Combat Car, M2 Light Tank, M3 Stuart, M3 Lee, LVT-2 Water Buffalo.

The Guiberson T-1020, a 9-cylinder radial diesel aero engine, was used in the M1A1E1, while the Continental R975 saw service in the M4 Sherman, M7 Priest, M18 Hellcat tank destroyer, and the M44 self propelled howitzer.

Modern radials

Four-stroke aircraft radial engine Scarlett mini 5

A number of companies continue to build radials today. Vedeneyev produces the M-14P radial of 360–450 hp (270–340 kW) as used on Yakovlev and Sukhoi aerobatic aircraft. The M-14P is also used by builders of homebuilt aircraft, such as the Culp Special, and Culp Sopwith Pup,[9] Pitts S12 "Monster" and the Murphy "Moose". 110 hp (82 kW) 7-cylinder and 150 hp (110 kW) 9-cylinder engines are available from Australia's Rotec Aerosport. HCI Aviation offers the R180 5-cylinder (75 hp (56 kW)) and R220 7-cylinder (110 hp (82 kW)), available "ready to fly" and as a build-it-yourself kit. Verner Motor of the Czech Republic now builds several radial engines ranging in power from 25 to 150 hp (19 to 112 kW).[10] Miniature radial engines for model airplanes are available from O. S. Engines, Saito Seisakusho of Japan and Shijiazhuang of China, and Evolution (designed by Wolfgang Seidel of Germany, and made in India) and Technopower in the USA.

Comparison with inline engines

A 1935 Monaco-Trossi, a rare example of automobile use.[11]
Weight: Air-cooled radial engines often weigh less than equivalent liquid-cooled inline engines.
Damage tolerance: Liquid cooling systems are generally more vulnerable to battle damage. Minor shrapnel damage easily results in a loss of coolant and consequent engine seizure, while an air-cooled radial might be largely unaffected by minor damage.[12]
Simplicity: Radials have shorter and stiffer crankshafts, a single bank radial needing only two crankshaft bearings as opposed to the seven required for a liquid-cooled six-cylinder inline engine of similar stiffness.[13]
Reliability:The shorter crankshaft also produces less vibration and hence higher reliability through reduced wear and fatigue.
Smooth running: It is typically easier to achieve smooth running with a radial engine
Cooling: While a single bank radial permits all cylinders to be cooled equally, the same is not true for multi-row engines where the rear cylinders can be affected by the heat coming off the front row, and air flow being masked.[14]
Drag: Having the cylinders exposed to the airflow increases drag considerably. The answer was the addition of specially designed cowlings with baffles to force the air between the cylinders. The first effective drag reducing cowling that didn't impair engine cooling was the British Townend ring or "drag ring" which formed a narrow band around the engine covering the cylinder heads, reducing drag. The National Advisory Committee for Aeronautics studied the problem, developing the NACA cowling which further reduced drag and improved cooling. Nearly all aircraft radial engines since have used NACA-type cowlings.[Note 1] Because radial engines are often wider than similar inlines or vees, it is more difficult to design an aircraft to minimize cross sectional area, a major cause of drag, although by the beginning of the Second World War, this disadvantage had largely disappeared as aircraft sizes increased, and multi-row radials increased the power produced in relation to the cross sectional area.
Power: Because each cylinder on a radial engine has its own head, it is impractical to use a multivalve valvetrain on a radial engine. Therefore, almost all radial engines use a two valve pushrod-type valvetrain which may result in less power for a given displacement than multi-valve inline engines. The limitations of the poppet valve were largely overcome by the development of the sleeve valve, but at the cost of increased complexity, maintenance costs and reduced reliability.
Visibility: Pilot visibility may be poorer due to the width of the engine on single-engine aircraft, although tight fitting cowlings helped reduce this factor somewhat. Equivalent inline engines often resulted in overly long noses, which similarly impaired visibility directly forward.
Installation: It is more difficult to ensure adequate cooling air in a buried engine installation or with pusher configurations.
Size: The smallest classes of radial engines, with three and five cylinders are very rough running and unreliable when compared to equivalent four cylinder inline or horizontally opposed engines which later became more popular for light aircraft as a result.

While inline liquid-cooled engines continued to be common in new designs until late in World War II, radial engines dominated afterwards until overtaken by jet engines, with the late-war Hawker Sea Fury and Grumman Bearcat, two of the fastest production piston-engined aircraft ever built, using radial engines.

Other types of radial engines

Multi-row radials

The Wasp Major, a four-row radial.

Originally radial engines had one row of cylinders, but as engine sizes increased it became necessary to add extra rows. The first known radial-configuration engine to ever use a twin-row design was the 160 hp Gnôme "Double Lambda" rotary engine of 1912, designed as a 14-cylinder twin-row version of the firm's 80 hp Lambda single-row seven-cylinder rotary. Only the German Oberursel U.III clone of the Double Lambda reproducing the Gnome Double Lambda's twin-row design before the end of World War I.

Two-row designs began to appear in large numbers during the 1930s, when aircraft size and weight grew to the point where single-row engines of the required power were simply too large to be practical. Two-row designs often had cooling problems with the rear bank of cylinders, but a variety of baffles and fins were introduced that largely eliminated these problems. The downside was a relatively large frontal area that had to be left open to provide enough airflow, which increased the amount of drag. This led to significant arguments in the industry in the late 1930s about the possibility of using radials for high-speed aircraft like modern fighters.

The solution was introduced with the BMW 801 14-cylinder twin-row radial. Kurt Tank designed a new cooling system for this engine that used a high-speed fan to blow compressed air into channels that carry air to the middle of the banks, where a series of baffles directed the air over all of the cylinders. This allowed the cowling to be tightly fit around the engine, reducing drag, while still providing (after a number of experiments and modifications) enough cooling air to the rear to keep the engine working well. This basic concept was soon copied by many other manufacturers, and many late-WWII aircraft returned to the radial design as newer and much larger designs began to be introduced. Examples include the Bristol Centaurus on the Hawker Sea Fury or the Shvetsov ASh-82 in the Lavochkin La-7.

For even greater power, adding further rows was considered too difficult to consider practically due to the problems of providing the required airflow to the rear banks. Larger engines were designed, but these generally used water cooling to address these problems, although this greatly increased the complexity of the designs and eliminated some of the advantage to the radial air-cooled design. One example of this concept is the BMW 803, which never entered service.

A major study into the airflow around radials using wind tunnels and other systems was carried out in the US, and demonstrated that ample airflow was available through careful design. This led to the R-4360 which has 28 cylinders arranged in a 4 row corncob configuration. The R-4360 saw service on large American aircraft in the post-World War II period. The US and Soviet Union continued experiments with larger radials, but the UK abandoned such designs in favour of newer versions of the Centaurus and rapid movement to the use of turboprops, which easily exceeded radials in power without the weight or complexity.

Large radials continued to be built for other uses though, although they are no longer common. One example is the like the Zvezda diesel boat engine series with 56 cylinders or 112-cylinders in 8 or 16 rows of 7 cylinders each displacing 383 litres (23,931 in3) and producing 10,000 hp (7,500 kW). These were used on fast attack craft, such as Osa class missile boats.

Diesel radials

Packard DR-980 diesel radial aircraft engine.
A Nordberg Manufacturing Company two-stroke diesel radial engine for power generation and pump drive purposes.

While most radial engines have been produced for gasoline, there have been diesel radial engines. Two major advantages favour diesel engines — lower fuel consumption and reduced fire risk.


Packard designed and built a 9-cylinder 980 cubic inch (16,000 cm3) displacement diesel radial aircraft engine, the 225 horsepower (168 kW) DR-980, in 1928. On 28 May 1931, a DR-980 powered Bellanca CH-300, with 481 gallons of fuel, piloted by Walter Edwin Lees and Frederick Brossy set a record for staying aloft for 84 hours and 32 minutes without being refueled.[17] This record stood for 55 years until broken by the Rutan Voyager.[18]


The experimental Bristol Phoenix of 1928–1932 was successfully flight tested in a Westland Wapiti and set altitude records in 1934 that lasted until World War II.


In 1932 the French company Clerget developed the 14D, a 14-cylinder two-stroke diesel radial engine. After a series of improvements, in 1938 the 14F2 model produced 520 hp (390 kW) at 1910 rpm cruise power, with a power-to-weight ratio near that of contemporary gasoline engines and a specific fuel consumption of roughly 80% that for an equivalent gasoline engine. During WWII the research continued, but no mass-production occurred because of the Nazi occupation. By 1943 the engine had grown to produce over 1,000 hp (750 kW) with a turbocharger. After the war, the Clerget company was integrated in the SNECMA company and had plans for a 32-cylinder diesel engine of 4,000 hp (3,000 kW), but in 1947 the company abandoned piston engine development in favour of the emerging turbine engines.


The Nordberg Manufacturing Company of the United States developed and produced a series of large two-stroke radial diesel engines from the late 1940s for electrical production, primarily at aluminium smelters and for pumping water. They differed from most radials in that they had an even number of cylinders in a single bank (or row) and an unusual double master connecting rod. Variants were built that could be run on either diesel oil or gasoline or mixtures of both. A number of powerhouse installations utilising large numbers of these engines were made in the U.S.[19]


Electro-Motive Diesel (EMD) built the "pancake" engines 16-184 and 16-338 for marine use.[20]

Compressed air radial engines

A number of radial motors operating on compressed air have been designed, mostly for use in model airplanes and in gas compressors.[21]

Model radial engines

A number of multi-cylinder 4-stroke model engines have been commercially available in a radial configuration, beginning with the Japanese O.S. Max firm's FR5-300 five-cylinder, 3.0 (50 cm3) displacement "Sirius" radial in 1986. The American "Technopower" firm had made smaller-displacement five- and seven-cylinder model radial engines as early as 1976, but the OS firm's engine was the first mass-produced radial engine design in aeromodelling history. The rival Saito Seisakusho firm in Japan has since produced a similarly sized five-cylinder radial four-stroke model engine of their own as a direct rival to the OS design, with Saito also creating a trio of three-cylinder radial engines ranging from 0.90 (15 cm3) to 4.50 (75 cm3) in displacement, also all now available in spark-ignition format up to 84 cm3 displacement for use with gasoline. The German Seidel firm formerly made both seven- and nine-cylinder "large" (starting at 35 cm3 displacement) radio control model radial engines, mostly for glow plug ignition, with an experimental fourteen-cylinder twin-row radial being tried out - the American Evolution firm now sells the Seidel-designed radials, with their manufacturing being done in India.

See also


  1. ^ It has been claimed that the NACA cowling generated extra thrust due to the Meredith Effect, whereby the heat added to the air being forced through the ducts between the cylinders expanded the exhausting cooling air, producing thrust when forced through a nozzle. The Meredith effect requires high airspeed and careful design to generate a suitable high speed exhaust of the heated air - the NACA cowling was not designed to achieve this, nor would the effect have been significant at low airspeeds.[15] The effect was put to use in the radiators of several mid-1940s aircraft that used liquid-cooled engines such as the Spitfire and Mustang,[16] and it offered a minor improvement in later radial-engined aircraft, including the Fw-190.


  1. ^
  2. ^ a b
  3. ^
  4. ^ Lumsden 2003, p. 225.
  5. ^
  6. ^
  7. ^
  8. ^ "The Spirit of St. Louis". Charles Lindergh: An American Aviatior, Retrieved 21 August 2015.
  9. ^
  10. ^
  11. ^
  12. ^
  13. ^ Some six-cylinder inline engines used as few as 3 bearing but at the cost of heavier crankshafts, or crankshaft whipping.
  14. ^
  15. ^ Becker, J.; The high-speed frontier: Case histories of four NACA programs, 1920- SP-445, NASA (1980), Chapter 5: High-speed Cowlings, Air Inlets and Outlets, and Internal-Flow Systems: The ramjet investigation
  16. ^ Price 1977, p. 24.
  17. ^ Chapter 1: Development of the Diesel Aircraft Engine" Aircraft Engine Historical Society — Diesels p.4 Retrieved: 30 January 2009.
  18. ^ Aviation Chronology Retrieved: 7 February 2009.
  19. ^
  20. ^
  21. ^

External links

  • Cutaway radial engine in operation video on You Tube
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