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The radial engine is a reciprocating type internal combustion engine configuration in which the cylinders point outward from a central crankshaft like the spokes on a wheel. Multiple rows of radial cylinders can be used for increased capacity. The radial engine was very commonly used in large aircraft engines before most large aircraft started using turbine engines.
Since the axes of the cylinders are coplanar, the connecting rods cannot be attached to the crankshaft as usual (Inline-four_engine). 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.
Four-stroke radials always 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 5-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 compressing the next cylinder to fire, so making the motion more uniform. If an even number of cylinders was used, the equally timed firing cycle would not be feasible.  The protoype 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 uses very few cams compared to other types. As usual for a four-stroke, the crankshaft takes two revolutions to complete the four strokes of each piston (intake, compression, combustion, exhaust). The camshaft is coaxial with the crankshaft and is typically geared with a 1:(1-n) transmission ratio, where n is the number of cylinders. As shown in the animation, the crankshaft spins slower and in the opposite direction. The actual cams (cam lobes) are placed on two rows for the intake and exhaust. For the example, only 4 cams serve all 5 cylinders, whereas 10 would be required for a typical inline engine with the same number of cylinders.
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 five-cylinder Kinner B-5 and Russian Shvetsov M-11, using individual camshafts within the crankcase for each cylinder. A few engines utilize sleeve valves instead, like the very reliable 14-cylinder Bristol Hercules (built up to 1970 under licence in France by SNECMA) and the powerful 18-cylinder Bristol Centaurus.
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.
In 1903-04 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. During 1908-9, Ellehammer developed another engine, which had six cylinders arranged in two rows of three. His engines had a very good power-to-weight ratio, but his aircraft designs suffered from his lack of understanding of control. If he had concentrated on his engines, he might have become a successful manufacturer.
Another early radial engine was the three-cylinder Anzani, originally built as a "semi-radial" W3 configuration design, one of which powered Louis Blériot's Blériot XI in his July 25, 1909 crossing of the English Channel. By 1914 Anzani had developed their range, their largest radial being a 20-cylinder engine of 200 hp (150 kW), with its cylinders arranged in four groups of five. One of the three-cylinder "fully radial", 120° cylinder angle Anzani powerplants still exists today, in fully running condition, in the nose of Old Rhinebeck Aerodrome's restored and flyable 1909 vintage Blériot XI. There is also another running Anzani at Brodhead airfield to go on a replica Blériot XI.
Radial engines are regarded as being air-cooled almost by definition—so that it is interesting that one of the most successful of the early radial engines was the Salmson 9Z series of nine-cylinder water-cooled radial engines that were produced in large numbers during the First World War. Georges Canton and Pierre Unné patented the original engine design in 1909, offering it to the Salmson company—and the engine was often known as the Canton-Unné.
Little development of the radial engine was undertaken in Germany during World War I, where most aircraft used water-cooled inline 6-cylinder engines. Two radial engines were made there before the war but they were not proceeded with.
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 however the prop was bolted to the engine, and the crankshaft to the airframe. The primary reason for this was to ensure cooling of the cylinders, a notorious problem with all of the early radials.
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 240 hp (180 kW). The German Oberursel firm (who had originated the Gnom design) made licensed copies of the Gnome and Le Rhône powerplants while Siemens-Halske built a number of their own designs including the Siemens-Halske Sh.III eleven-cylinder rotary engine.
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 during the intake stroke due to the rotary motion, 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 although most of the other early radial engines were new designs.
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 well appreciated but remained unrealized. While British designers had produced the ABC Dragonfly radial in 1917, they were unable to resolve its cooling problems, and it was not until the 1920s that the Bristol Aeroplane Company and Armstrong Siddeley produced reliable British radials such as the Bristol Jupiter and the Armstrong Siddeley Jaguar.
In the US, NACA noted in 1920 that air-cooled radials could offer an increase in the power-to-weight ratio and reliability, and by 1921 the US 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.
Wright's 225 hp (168 kW) J-5 Whirlwind radial engine of 1925 was widely acknowledged as "the first truly reliable aircraft engine". Wright employed Giuseppe Mario Bellanca to design an aircraft to showcase it, and the result was the Wright-Bellanca 1, or WB-1, which was first flown in the latter part of that year. The J-5 was used on many advanced aircraft of the day, including Charles Lindbergh's Spirit of St. Louis with which he made the first solo trans-Atlantic flight.
In 1925, the American rival firm to Wright's radial engine production efforts, Pratt & Whitney, was founded. The P & W firm's initial offering, the Pratt & Whitney R-1340 Wasp, test run later that year, began the evolution of the many models of Pratt & Whitney radial engines that were to appear during the second quarter of the 20th century, among them the 14-cylinder, twin-row Pratt & Whitney R-1830 Twin Wasp, the most-produced aviation engine of any single design, with a total production quantity of nearly 175,000 engines.
In the United Kingdom the Bristol Aeroplane Company was concentrating on developing radials such as the Jupiter, Mercury and sleeve valve Hercules radials. France, Germany, Russia and Japan largely built licenced or locally improved versions of the Armstrong Siddeley, Bristol, Wright, or Pratt & Whitney radials.
Radial versus inline debate
Liquid-cooled engines often weigh more and their cooling systems are both more complex and 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 would be unaffected. Additionally, radials offer higher mechanical efficiency than inline engines, as they have shorter and stiffer crankshafts, a single bank radial needing only two crankshaft bearings as opposed to the seven required for a six-cylinder inline engine of similar stiffness. The shorter crankshaft also produces less vibration and hence higher reliability through reduced wear.
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 are affected by the heat coming off the front row, and air flow being masked. Additionally, having the cylinders in the airflow increases drag considerably, adding turbulence that destroys the laminar airflow over the fuselage and adjacent wings. The answer to both these problems was the addition of specially designs cowlings with baffles to force the air over 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 ring around the engine covering the cylinder heads, not only reducing drag, but adding a small amount of thrust. NACA then studied the problem further and developed the NACA cowling which further reduced drag, increased thrust and improved cooling. Nearly all aircraft radial engine installations since have used NACA type cowlings. Several aircraft with liquid-cooled engines in service until the end of the Second World War, such as the Spitfire and the P-51 took advantage of the Meredith Effect to generate thrust, which worked in a similar manner to the NACA cowling.
The radial engine typically has a larger frontal area, and is less amenable to streamlining and drag reduction than an inline engine. Pilot visibility is often sacrificed due to the greater width of the engine, and the designer is more limited in engine placement as greater care must be taken to ensure adequate cooling air, either in a buried engine installation or in a pusher configuration.
While inline liquid-cooled engines continued to be common until the end of World War II, radial engines saw widespread service in the successful Mitsubishi Zero and Focke-Wulf Fw 190, while the late-war Hawker Sea Fury and Grumman Bearcat, two of the fastest production piston-engined aircraft ever built, used radial engines. Until the development of the jet engine, large aircraft commonly used radial engines. Factors influencing the choice of radial over inline were reliability, simplicity in maintenance, and the ability to package a radial as a power egg, readily removable from the aircraft with the disconnection of only a few lines. Additionally, the large frontal area of these aircraft meant the radial engine's own frontal profile was a less significant factor when it came to drag.
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, with 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. Most stationary radial engines did not exceed two rows, but the largest displacement radial engine ever built in quantity, the Pratt & Whitney R-4360 Wasp Major, with cylinders in corncob configuration, was a 28-cylinder 4-row radial engine used in many large aircraft designs in the post-World War II period. The Lycoming R-7755 was the largest piston-driven aircraft engine ever produced; with 36 cylinders totaling about 7,750 in3 (127 L) of displacement and a power output of 5,000 horsepower (3,700 kW). It was originally intended to be used in the "European bomber" that eventually emerged as the Convair B-36. Only two examples were built before the project was terminated in 1946. The USSR also built a limited number of 'Zvezda' engines with up to 56 cylinders, which were even larger in displacement than the Lycoming R-7755. The 112-cylinder diesel boat engines featuring 16 rows with 7 banks of cylinders, bore of 160 mm (6.3 in), stroke of 170 mm (6.7 in), and total displacement of 383 liters (23,931 in3). The engine produced 10,000 hp (7,500 kW) at 2,000 rpm. They were used on fast attack craft, such as Osa class missile boats.
At least five companies build radials today. Vedeneyev engines produces the M-14P model, 360 hp (270 kW) (up to 450 hp (340 kW)) radial used on Yakovlev, Sukhoi Su-26, and Su-29 aerobatic aircraft. The M-14P has also found great favor among builders of experimental aircraft, such as the Culp's Special, and Culp's Sopwith Pup , 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 Engineering. 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, from the Czech Republic, now builds several radial engines. Models range in power from 71 hp (53 kW) to 172 hp (128 kW).  Miniature radial engines for model airplane use are also available from Seidel in Germany, OS and Saito Seisakusho of Japan, and Technopower in the USA. The Saito firm is known for making three different sizes of 3-cylinder radials, as well as a 5-cylinder example, as the Saito firm is a specialist in making a large line of miniature four-stroke engines for model use in both methanol-burning glow plug and gasoline-fueled spark plug ignition engine formats.
While most radial engines have been produced for gasoline fuels, there have been instances of diesel fueled radial engines. Two major advantages favour diesel engines - reduced fuel consumption and reduced risk of fires - however they have all the disadvantages to which diesel engines have been prone.
Packard designed and built a diesel radial aircraft engine, the DR-980, in 1928. It was a 9-cylinder radial engine displacing 980 cubic inches and rated at 225 horsepower (168 kW). On 28 May 1931, a Bellanca CH-300 fitted with a DR-980, piloted by Walter Edwin Lees and Frederick Brossy, set a record for staying aloft for 84 hours and 32 minutes without being refueled. This record was not broken until 55 years later by the Rutan Voyager.
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 engines were mass-produced 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 favor of work on the emerging turbine engines.
The Nordberg Manufacturing Company of the US 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 using having an even number of cylinders in a single bank (or row), thanks to an unusual double master connecting rod that allowed the engine to be timed so the cylinders fired in consecutive order. 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 US.
The Zoche in Germany have produced a prototype range of radial air-cooled two-stroke diesel aircraft engines, comprising a V-twin, a single-row cross-4 and a double-row cross-8. A Zoche engine has run successfully in wind tunnel tests, but Zoche seem barely closer to production than they were a decade ago. Experimental engine manufacturers often experience difficulties in proceeding beyond the prototype stage due to high development and certification costs particularly with markets dominated by relatively cheap WWII era engines.
Compressed air radials
A number of radial motors operating on compressed air have been designed, mostly for use in model airplanes, or simply as a building project. Radial compressed air engines include Liney Machine Halo 5, Mike Smyth SF376, Taiyo UTAM4, ... They have also been used in gas compressors.
Use in tanks
In the years leading up to WWII, 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, M2, and M3, while the Continental R975 saw service in the M4 Sherman, M7 Priest, M18 Hellcat tank destroyer, and the M44 self-propelled howitzer.
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 cu.in. (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 aeromodeling 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 cu.in. (15 cm3) to 4.50 cu.in. (75 cm3) in displacement. The German Seidel firm has made both seven- and nine-cylinder "large" (starting at 70 cm3 displacement) radio control model radial engines, mostly for glow plug ignition, with an experimental fourteen-cylinder twin-row radial being tried out.
- Kinner B-5
- List of aircraft engines
- Rotary engine
- Warner Scarab
- Zvezda M503, a 42-cylinder Soviet missile boat diesel radial engine.
- ↑ "Firing order: Definition from". Answers.com. 2009-02-04. http://www.answers.com/topic/firing-order. Retrieved 2011-12-06.
- ↑ 2.0 2.1 2.2 Vivian, E. Charles (1920). A History of Aeronautics. Dayton History Books Online. http://www.daytonhistorybooks.citymax.com/page/page/3259323.htm.
- ↑ Day, Lance; Ian McNeil (1996). Biographical Dictionary of the History of Technology. Taylor & Francis. p. 239. ISBN 0-415-06042-7.
- ↑ Bilstein, Roger E. (2008). Flight Patterns: Trends of Aeronautical Development in the United States, 1918–1929. University of Georgia Press. p. 26. ISBN 0-8203-3214-3.
- ↑ Herrmann, Dorothy (1993). Anne Morrow Lindbergh: A Gift for Life. Ticknor & Fields. p. 28. ISBN 0-395-56114-0.
- ↑ Thurston, David B. (2000). The World's Most Significant and Magnificent Aircraft: Evolution of the Modern Airplane. SAE. p. 155. ISBN 0-7680-0537-X. http://books.google.com/?id=7HTPRym0iYIC&pg=PA155.
- ↑ Some six-cylinder inline engines used as few as 3 bearing but at the cost of heavier crankshafts, or crankshaft whipping.
- ↑ Fedden, A.H.R. (28 February 1929). "Air-cooled Engines in Service". Flight XXI (9): 169–173. http://www.flightglobal.com/pdfarchive/view/1929/1929%20-%200433.html.
- ↑ Price 1977, p. 24.
- ↑ Aircraft Engine Historical Society - Diesels Retrieved: 30 January 2009
- ↑ Aviation Chronology Retrieved: 7 February 2009
- ↑ "Nordberg Diesel Engines". OldEngine. http://www.oldengine.org/members/diesel/Nordberg/Nordmenu.htm. Retrieved 2006-11-20.
- ↑ "zoche aero-diesels homepage". Zoche.de. http://www.zoche.de/. Retrieved 2011-12-06.
- ↑ "Prospekt 2007.indd" (PDF). http://www.zoche.de/zoche_brochure.pdf. Retrieved 2011-12-06.
- ↑ "zoche aero-diesels testbench video". Zoche.de. http://www.zoche.de/Zoche_video.html. Retrieved 2011-12-06.
- ↑ "Bock radial piston compressor". Bock.de. 2009-10-19. http://www.bock.de/en/Product_overview.html?ArticleSizesGroupID=169. Retrieved 2011-12-06.
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