Editing Radial engine

{{Short description|Reciprocating engine with cylinders arranged radially from a single crankshaft}}
{{About|the conventional radial engine with fixed cylinders and a revolving crankshaft|the otherwise similar engine with a rotating crankcase|rotary engine}}
[[File:Radial engine WACO QCF2.jpg|thumb|Radial engine in a [[biplane]] ]]

The '''radial engine''' is a [[reciprocating engine|reciprocating type]] [[internal combustion engine|internal combustion]] [[engine configuration]] in which the [[cylinder (engine)|cylinder]]s "radiate" outward from a central [[crankcase]] like the spokes of a wheel. It resembles a stylized [[Star polygon|star]] when viewed from the front, and is called a "star engine" in some other languages.

The radial configuration was commonly used for [[aircraft engine]]s before [[gas turbine]] engines became predominant.
[[File:Radial engine timing.gif|thumb|Moving parts showing operation of a typical small five-cylinder radial.<br>Pistons are in gold and valves in pink, master rod in pale purple, slaved connecting rods in blue, crankshaft / counterbalance in gray and timing ring and cams in red.]]
==Engine operation==
[[File:Radial engine.gif|thumb|Another example of the engine operation]]
[[File:TwinWaspConRods.jpg|thumb|Master rod (upright) and slaved connecting rods from a two-row, fourteen-cylinder [[Pratt & Whitney R-1535 Twin Wasp Junior]] ]]Since the axes of the cylinders are coplanar, the [[connecting rod]]s cannot all be directly attached to the [[crankshaft]] unless mechanically complex forked connecting rods are used, none of which have been successful. Instead, the [[piston]]s 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 rod]]s' 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 cycle|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.

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. Its cam lobes are placed in two rows; one for the intake valves and one for the exhaust valves. The radial engine normally uses fewer cam lobes than other types. For example, in the engine in the animated illustration, four cam lobes serve all 10 valves across the 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 valve]]s driven by [[pushrod]]s and [[Tappet|lifter]]s 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 use [[sleeve valve]]s such as the 14-cylinder [[Bristol Hercules]] and the 18-cylinder [[Bristol Centaurus]], which are quieter and smoother running but require much tighter [[manufacturing tolerance]]s.{{Citation needed|date=October 2014}}

==History==
===Aircraft===
[[File:Rotary Piston Engine 8b03632r.jpg|thumb|[[Continental engine|Continental]] radial, 1944]]
[[File:H19 showing engine.jpg|thumb|[[Pratt & Whitney R-1340]] radial 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 engine]]s, for [[Samuel Pierpont Langley|Langley]]'s ''Aerodrome'' aircraft. [[Manly–Balzer engine|Manly's engine]] produced {{convert|52|hp|kW|abbr=on}} at 950 rpm.<ref name=vivian>{{cite book | last = Vivian | first = E. Charles | title = A History of Aeronautics | publisher = Dayton History Books Online | year = 1920 | url = http://www.daytonhistorybooks.citymax.com/page/page/3259323.htm | access-date = 2008-07-05 | archive-date = 2009-05-23 | archive-url = https://web.archive.org/web/20090523051050/http://www.daytonhistorybooks.citymax.com/page/page/3259323.htm | url-status = dead }}</ref>

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.<ref>{{cite book | last = Day | first = Lance | author2 = Ian McNeil | title = Biographical Dictionary of the History of Technology | publisher = Taylor & Francis | year = 1996 | page = [https://archive.org/details/isbn_9780415060424/page/239 239] | url = https://archive.org/details/isbn_9780415060424/page/239 | isbn = 0-415-06042-7 }}</ref>

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 {{convert|200|hp|kW|abbr=on}}, with its cylinders arranged in four rows of five cylinders apiece.<ref name=vivian/>

Most radial engines are [[air-cooled]], but one of the most successful of the early radial engines (and the earliest "stationary" design produced for World War I combat aircraft) was the [[Salmson water-cooled aero-engines|Salmson 9Z series of nine-cylinder water-cooled radial engines]] that were produced in large numbers. Georges Canton and Pierre Unné patented the original engine design in 1909, offering it to the [[Salmson]] company; the engine was often known as the Canton-Unné.<ref name="Lumsden225">Lumsden 2003, p. 225.</ref>

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. It was similar in concept to the later radial, the main difference being 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 alleviated by the engine generating its own cooling airflow.<ref name="nahum">{{cite book| last = Nahum| first = Andrew| title = The Rotary Aero Engine| year = 1999| publisher = NMSI Trading Ltd| isbn = 1-900747-12-X }}</ref>

In [[World War I]] many French and other Allied aircraft flew with [[Gnome Engine Company|Gnome]], [[Le Rhône]], [[Clerget-Blin|Clerget]], and [[Bentley BR2|Bentley]] rotary engines, the ultimate examples of which reached {{cvt|250|hp}} although none of those over {{convert|160|hp|kW|abbr=on}} were successful. By 1917 rotary engine development was lagging behind new inline and V-type engines, which by 1918 were producing as much as {{convert|400|hp|kW|abbr=on}}, and were powering almost all of the new French and British combat aircraft.

Most German aircraft of the time used water-cooled inline 6-cylinder engines. [[Motorenfabrik Oberursel]] made licensed copies of the Gnome and Le Rhône rotary powerplants, and [[Siemens-Halske]] built their own designs, including the [[Siemens-Halske Sh.III|Siemens-Halske Sh.III eleven-cylinder rotary engine]], which was unusual for the period in being geared through a [[bevel gear]]train in the rear end of the crankcase ''without'' the crankshaft being firmly mounted to the aircraft's airframe, so that the engine's internal working components (fully internal crankshaft "floating" in its crankcase bearings, with its conrods and pistons) were spun in the opposing direction to the crankcase and cylinders, which still rotated as the propeller itself did since it was still firmly fastened to the crankcase's frontside, as with regular ''umlaufmotor'' German rotaries.
  
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 (aviation)|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 Aeroplane Company|Bristol]] and [[Armstrong Siddeley]] produced reliable air-cooled radials such as the [[Bristol Jupiter]]<ref>{{cite book |last=Gunston |first=Bill |date=1989 |title=World Encyclopedia of Aero Engines |location=Cambridge, UK |publisher=Patrick Stephens Ltd |pages=29, 31 & 44 |isbn=1-85260-163-9 }}</ref> and the [[Armstrong Siddeley Jaguar]].{{Citation needed|date=October 2014}}

In the United States the [[National Advisory Committee for Aeronautics]] (NACA) noted in 1920 that air-cooled radials could offer an increase in [[power-to-weight ratio]] and reliability; by 1921 the U.S. Navy had announced it would only order aircraft fitted with air-cooled radials and other naval air arms followed suit. [[Charles Lawrance]]'s [[Lawrance J-1|J-1 engine]] was developed in 1922 with Navy funding, and using aluminum 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.<ref>{{cite book |last=Bilstein|first=Roger E.|title=Flight Patterns: Trends of Aeronautical Development in the United States, 1918–1929|publisher=University of Georgia Press|year=2008|page=26|isbn=978-0-8203-3214-7}}</ref>

Wright's {{convert|225|hp|kW|abbr=on}} [[Wright J-5 Whirlwind|J-5 Whirlwind]] radial engine of 1925 was widely claimed as "the first truly reliable aircraft engine".<ref>{{cite book|last=Herrmann|first=Dorothy|title=Anne Morrow Lindbergh: A Gift for Life|publisher=Ticknor & Fields|year=1993|page=[https://archive.org/details/annemorrowlindbe00herr/page/28 28]|isbn=0-395-56114-0|url=https://archive.org/details/annemorrowlindbe00herr/page/28}}</ref> Wright employed [[Giuseppe Mario Bellanca]] to design an aircraft to showcase it, and the result was the [[Wright-Bellanca 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.<ref>"[http://www.charleslindbergh.com/plane/ The Spirit of St. Louis]". Charles Lindergh: An American Aviator, Retrieved 21 August 2015.</ref>

In 1925 the American [[Pratt & Whitney]] company was founded, competing with Wright's radial engines. Pratt & Whitney's initial offering, the [[Pratt & Whitney R-1340|R-1340 Wasp]], was test run later that year, beginning a line of engines over the next 25 years that included the 14-cylinder, twin-row [[Pratt & Whitney R-1830 Twin Wasp]]. More Twin Wasps were produced than any other aviation piston engine in the history of aviation; nearly 175,000 were built.<ref>[https://web.archive.org/web/20131111025015/http://www.pw.utc.com/R1830_Twin_Wasp_Engine - Archived (Nov. 11, 2013) manufacturer's product page, R-1830] Retrieved: 7 February 2019</ref>

[[File:RAR2009 - Rare Bear makes emergency landing.jpg|thumb|[[Rare Bear]] ]]
In the United Kingdom the [[Bristol Aeroplane Company]] was concentrating on developing radials such as the Jupiter, [[Bristol Mercury|Mercury]], and [[sleeve valve]] [[Bristol Hercules|Hercules]] radials. Germany, Japan, and the Soviet Union started with building licensed versions of the Armstrong Siddeley, Bristol, Wright, or Pratt & Whitney radials before producing their own improved versions.{{Citation needed|date=October 2014}} France continued its development of various rotary engines but also produced engines derived from Bristol designs, especially the Jupiter.

Although other piston configurations and [[turboprop]]s have taken over in modern [[Powered aircraft#Propeller aircraft|propeller-driven aircraft]], [[Rare Bear]], which is a [[F8F Bearcat|Grumman F8F Bearcat]] equipped with a [[Wright R-3350 Duplex-Cyclone]] radial engine, is still [[Fastest propeller-driven aircraft#Piston engines|the fastest piston-powered aircraft]].<ref>Lewis Vintage Collection (2018), [http://www.lewisairlegends.com/aircraft/rare-bear "'Rare Bear' web site."] {{Webarchive|url=https://web.archive.org/web/20131027142821/http://www.lewisairlegends.com/aircraft/rare-bear |date=2013-10-27 }}. Retrieved: 6 January 2018.</ref><ref>Aerospaceweb, [http://www.aerospaceweb.org/question/performance/q0023.shtml "Aircraft speed records."] ''AeroSpaceWeb.org''. Retrieved: 6 January 2018.</ref>

125,334 of the American twin-row, 18-cylinder [[Pratt & Whitney R-2800 Double Wasp]], with a displacement of 2,800 in<sup>3</sup> (46 L) and between 2,000 and 2,400&nbsp;hp (1,500-1,800&nbsp;kW), powered the American single-engine [[Vought F4U Corsair]], [[Grumman F6F Hellcat]], [[Republic P-47 Thunderbolt]], twin-engine [[Martin B-26 Marauder]], [[Douglas A-26 Invader]], [[Northrop P-61 Black Widow]], etc. The same firm's aforementioned smaller-displacement (at 30 litres), [[Pratt & Whitney R-1830 Twin Wasp|''Twin Wasp'']] 14-cylinder twin-row radial was used as the main engine design for the [[B-24 Liberator]], [[PBY Catalina]], and [[Douglas C-47]], each design being among [[List of most-produced aircraft|the production leaders]] in all-time production numbers for each type of airframe design.

The American [[Wright Cyclone series]] twin-row radials powered American warplanes: the nearly-43 litre displacement, 14-cylinder [[Wright R-2600|''Twin Cyclone'']] powered the single-engine [[Grumman TBF Avenger]], twin-engine [[North American B-25 Mitchell]], and some versions of the [[Douglas A-20 Havoc]], with the massive twin-row, nearly 55-litre displacement, 18-cylinder [[Wright R-3350|''Duplex-Cyclone'']] powering the four-engine [[Boeing B-29 Superfortress]] and others.

The Soviet [[Shvetsov]] [[OKB|''OKB-19'' design bureau]] was the sole source of design for all of the Soviet government factory-produced radial engines used in its World War II aircraft, starting with the [[Shvetsov M-25]] (itself based on the American [[Wright R-1820|Wright ''Cyclone 9'']]'s design) and going on to design the 41-litre displacement [[Shvetsov ASh-82]] fourteen cylinder radial for fighters, and the massive, 58-litre displacement [[Shvetsov ASh-73]] eighteen-cylinder radial in 1946 - the smallest-displacement radial design from the Shvetsov OKB during the war was the indigenously designed, 8.6 litre displacement [[Shvetsov M-11]] five cylinder radial.

Over 28,000 of the German 42-litre displacement, 14-cylinder, two-row [[BMW 801]], with between 1,560 and 2,000 PS (1,540-1,970&nbsp;hp, or 1,150-1,470&nbsp;kW), powered the German single-seat, single-engine [[Focke-Wulf Fw 190]] ''Würger'', and twin-engine [[Junkers Ju 88]].

In Japan, most airplanes were powered by air-cooled radial engines like the 14-cylinder [[Mitsubishi Zuisei]] (11,903 units, e.g. [[Kawasaki Ki-45]]), [[Mitsubishi Kinsei]] (12,228 units, e.g. [[Aichi D3A]]), [[Mitsubishi Kasei]] (16,486 units, e.g. [[Kawanishi H8K]]), [[Nakajima Sakae]] (30,233 units, e.g. [[Mitsubishi A6M]] and [[Nakajima Ki-43]]), and 18-cylinder [[Nakajima Homare]] (9,089 units, e.g. [[Nakajima Ki-84]]). The [[Kawasaki Ki-61]] and [[Yokosuka D4Y]] were rare examples of Japanese liquid-cooled inline engine aircraft at that time but later, they were also redesigned to fit radial engines as the [[Kawasaki Ki-100]] and [[Yokosuka D4Y]]3.

In Britain, Bristol produced both [[sleeve valve]]d and conventional [[poppet valve]]d radials: of the sleeve valved designs, more than 57,400 Hercules engines powered the [[Vickers Wellington]], [[Short Stirling]], [[Handley Page Halifax]], and some versions of the [[Avro Lancaster]],  over 8,000 of the pioneering sleeve-valved [[Bristol Perseus]] were used in various types, and more than 2,500 of the largest-displacement production British radial from the Bristol firm to use sleeve valving, the [[Bristol Centaurus]] were used to power the [[Hawker Tempest|Hawker Tempest II]] and [[Hawker Sea Fury|Sea Fury]]. The same firm's poppet-valved radials included: around 32,000 of [[Bristol Pegasus]] used in the [[Short Sunderland]], [[Handley Page Hampden]], and [[Fairey Swordfish]] and over 20,000 examples of the firm's 1925-origin nine-cylinder Mercury were used to power the [[Westland Lysander]], [[Bristol Blenheim]], and [[Blackburn Skua]].

===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.{{Citation needed|date=October 2014}}

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]], and [[Landing Vehicle Tracked|LVT-2 Water Buffalo]].{{Citation needed|date=October 2014}}

The [[Guiberson T-1020]], a 9-cylinder radial diesel aero engine, was used in the [[M1 Combat Car|M1A1E1]], while the [[Wright R-975|Continental R975]] saw service in the [[M4 Sherman]], [[M7 Priest]], [[M18 Hellcat]] [[tank destroyer]], and the [[M44 self propelled howitzer]].{{Citation needed|date=October 2014}}

===Modern radials===
[[File:Scarlett mini 5.png|thumb|Four-stroke aircraft radial engine Scarlett mini 5]]

A number of companies continue to build radials today. [[Vedeneyev]] produces the M-14P radial of {{convert|360|-|450|hp|kW|abbr=on}} 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]],<ref>{{cite web|url=http://culpsspecialties.com/site_files/suppages/specs.html |title=Aircraft |publisher=Culp Specialties |access-date=2013-12-22}}</ref> [[Pitts Special|Pitts]] S12 "Monster" and the [[Murphy Moose|Murphy "Moose"]]. In Poland, WSK PZL Kalisz produces the [[Shvetsov ASh-62]] engine. A version with direct injection has also been developed<ref>https://www.wsk.kalisz.pl/produkty-i-uslugi/silniki-lotnicze/</ref>. [[Rotec R2800|{{convert|110|hp|kW|abbr=on}}]] 7-cylinder and [[Rotec R3600|{{convert|150|hp|kW|abbr=on}}]] 9-cylinder engines are available from Australia's [[Rotec Aerosport]]. HCI Aviation<ref>{{cite web|url=https://aeroenginesaz.com/en/brand_hci |title=HCI (USA) |publisher=Aerospace Engines A to Z |access-date=2023-02-11}}</ref> offers the R180 5-cylinder ({{convert|75|hp|kW|abbr=on}}) and R220 7-cylinder ({{convert|110|hp|kW|abbr=on}}), available "ready to fly" and as a build-it-yourself kit. [[Verner Motor]] of the Czech Republic builds several radial engines ranging in power from {{convert|25|to|150|hp|kW|abbr=on}}.<ref>{{cite web|title=Verner Motor range of engines|url=http://vernermotor.eu/engines/|work=Verner Motor|access-date=23 April 2013|url-status=dead|archive-url=https://web.archive.org/web/20141006154103/http://vernermotor.eu/engines/|archive-date=6 October 2014}}</ref>  Miniature radial engines for [[Radio-controlled aircraft|model airplane]]s 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 US.{{Citation needed|date=October 2014}}

==Comparison with inline engines==
[[File:Monaco-Trossi1935.jpg|thumb|The 1935 Monaco-Trossi race car, a rare example of automobile use<ref>{{cite web|title=MONACO - TROSSI mod. da competizione|url=http://www.museoauto.it/website/en/component/content/article/40-monaco-trossi/57-monaco-trossi-mod-da-competizione|work=museoauto.it|access-date=10 November 2016}}</ref>]]

Liquid cooling systems are generally more vulnerable to battle damage. Even minor shrapnel damage can easily result in a loss of coolant and consequent engine overheating, while an air-cooled radial engine may be largely unaffected by minor damage.<ref>{{cite book |last=Thurston|first=David B.|author-link=David Thurston|title=The World's Most Significant and Magnificent Aircraft: Evolution of the Modern Airplane| publisher=SAE| year=2000|page=155| url=https://books.google.com/books?id=7HTPRym0iYIC&pg=PA155| isbn =0-7680-0537-X}}</ref> Radials have shorter and stiffer crankshafts, a single-bank radial engine needing only two crankshaft bearings as opposed to the seven required for a liquid-cooled, six-cylinder, inline engine of similar stiffness.<ref>Some six-cylinder inline engines used as few as three bearings, but at the cost of heavier crankshafts, or crankshaft whipping.</ref>

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.<ref>{{Cite journal|last=Fedden|first=A.H.R.|author-link=Roy Fedden|title=Air-cooled Engines in Service|journal=Flight|volume=XXI|issue=9|pages=169–173|date=28 February 1929|url=http://www.flightglobal.com/pdfarchive/view/1929/1929%20-%200433.html}}</ref>

A potential disadvantage of radial engines is that having the cylinders exposed to the airflow increases [[drag (physics)|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.{{refn|group=Note|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.<ref name=becker>Becker, J.; [http://www.hq.nasa.gov/pao/History/SP-445/ch5-5.htm ''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]</ref> The effect ''was'' put to use in the radiators of several mid-1940s aircraft that used liquid-cooled engines such as the [[Supermarine Spitfire|Spitfire]] and [[North American P-51 Mustang|Mustang]],<ref name="document p24">Price 1977, p. 24.</ref> and it offered a minor improvement in later radial-engined aircraft, including the [[Focke-Wulf Fw 190|Fw 190]].}}

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 F8F Bearcat]], two of the fastest production piston-engined aircraft ever built, using radial engines.

==Hydrolock==
{{Main|Hydrolock}}

Whenever a radial engine remains shut down for more than a few minutes, oil or fuel may drain into the combustion chambers of the lower cylinders or accumulate in the lower intake pipes, ready to be drawn into the cylinders when the engine starts. As the piston approaches [[top dead center]] (TDC) of the compression stroke, this liquid, being incompressible, stops piston movement. Starting or attempting to start the engine in such condition may result in a bent or broken connecting rod.<ref>{{Cite book|url=https://books.google.com/books?id=-kpRAQAAMAAJ&dq=The%20US%20Air%20Force%20Powerplant%20Maintenance%20Manual&pg=PP4|title=Powerplant Maintenance for Reciprocating Engines|publisher=[[Department of the Air Force]]|year=1953|pages=53–54}}</ref>

==Other types of radial engine==

===Multi-row radials===
[[File:Pratt & Whitney R-4360 Wasp Major 1.jpg|thumb|The [[Pratt & Whitney R-4360 Wasp Major|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 radial-configuration engine known to use a twin-row design was the 160&nbsp;hp Gnôme "Double Lambda" rotary engine of 1912, designed as a 14-cylinder twin-row version of the firm's 80&nbsp;hp [[Gnome Lambda|Lambda]] single-row seven-cylinder rotary, however reliability and cooling problems limited its success.

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 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.{{Citation needed|date=October 2014}}

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 fitted around the engine, reducing drag, while still providing (after a number of experiments and modifications) enough cooling air to the rear. 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.{{Citation needed|date=October 2014}} Examples include the [[Bristol Centaurus]] in the [[Hawker Sea Fury]], and the [[Shvetsov ASh-82]] in the [[Lavochkin La-7]].{{Citation needed|date=October 2014}}

For even greater power, adding further rows was not considered viable due to the difficulty of providing the required airflow to the rear banks. Larger engines were designed, mostly using water cooling although this greatly increased complexity and eliminated some of the advantages of the radial air-cooled design. One example of this concept is the [[BMW 803]], which never entered service.{{Citation needed|date=October 2014}}

A major study{{which|date=October 2014}} into the airflow around radials using [[wind tunnel]]s and other systems was carried out in the US, and demonstrated that ample airflow was available with careful design. This led to the [[Pratt & Whitney R-4360|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 [[turboprop]]s such as the [[Armstrong Siddeley Python]] and [[Bristol Proteus]], which easily produced more power than radials without the weight or complexity.{{Citation needed|date=October 2014}}

Large radials continued to be built for other uses, although they are no longer common. An example is the 5-ton [[Zvezda M503]] diesel engine with 42 cylinders in 6 rows of 7, displacing {{convert|143.6|L|cuin}} and producing {{convert|3942|hp|kW|abbr=on}}. Three of these were used on the fast [[Osa class missile boat]]s.{{Citation needed|date=October 2014}} Another one was the [[Lycoming XR-7755]] which was the largest piston aircraft engine ever built in the United States with 36 cylinders totaling about 7,750 in<sup>3</sup> (127 L) of displacement and a power output of 5,000 horsepower (3,700 kilowatts).

===Diesel radials===
[[File:Packard DR-980 USAF.jpg|thumb|Packard DR-980 diesel radial aircraft engine]]
[[File:Nordberg radial engine 648.JPG|thumb|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 engine]]s — lower fuel consumption and reduced fire risk.{{Citation needed|date=October 2014}}

;Packard
Packard designed and built a 9-cylinder 980 cubic inch (16.06 litre) displacement diesel radial aircraft engine, the {{convert|225|hp|kW}} [[Packard DR-980|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.<ref>[http://www.enginehistory.org/Diesels/CH1.pdf Chapter 1: Development of the Diesel Aircraft Engine"] {{webarchive|url=https://web.archive.org/web/20120212213152/http://www.enginehistory.org/Diesels/CH1.pdf |date=2012-02-12 }} Aircraft Engine Historical Society — Diesels p.4 Retrieved: 30 January 2009.</ref> This record stood for 55 years until broken by the [[Rutan Voyager]].<ref>[http://www.aerofiles.com/chrono.html Aviation Chronology] Retrieved: 7 February 2009.</ref>

;Bristol
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.{{Citation needed|date=October 2014}}

;Clerget
In 1932 the French company Clerget developed the 14D, a 14-cylinder [[two-stroke diesel engine|two-stroke diesel]] radial engine. After a series of improvements, in 1938 the 14F2 model produced {{convert|520|hp|kW|abbr=on}} at 1910 rpm cruise power, with a power-to-weight ratio near that of contemporary gasoline engines and a [[Brake specific fuel consumption|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 {{convert|1000|hp|kW|abbr=on}} with a [[turbo-supercharger|turbocharger]]. After the war, the Clerget company was integrated in the [[SNECMA]] company and had plans for a 32-cylinder diesel engine of {{convert|4000|hp|kW|abbr=on}}, but in 1947 the company abandoned piston engine development in favour of the emerging turbine engines.{{Citation needed|date=October 2014}}

;Nordberg
The [[Nordberg Manufacturing Company]] of the United States developed and produced a series of large [[two-stroke engine|two-stroke]] radial diesel engines from the late 1940s for electrical production, primarily at [[aluminum]] 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.<ref>{{cite web|publisher=OldEngine|url=http://www.oldengine.org/members/diesel/Nordberg/Nordmenu.htm|title=Nordberg Diesel Engines|access-date=2006-11-20|archive-date=2018-09-19|archive-url=https://web.archive.org/web/20180919132942/http://www.oldengine.org/members/diesel/Nordberg/Nordmenu.htm|url-status=dead}}</ref>

;EMD
[[Electro-Motive Diesel]] (EMD) built the "pancake" engines 16-184 and 16-338 for marine use.<ref>{{cite web|url=https://oldmachinepress.com/2014/08/17/general-motors-electro-motive-16-184-diesel-engine/|title=General Motors / Electro-Motive 16-184 Diesel Engine|first=William|last=Pearce|date=18 August 2014|work=oldmachinepress.com|access-date=30 May 2016}}</ref>

;Zoche
[[Zoche aero-diesel]]s are a prototype radial design that have an even number of cylinders, either four or eight; but this is not problematic, because they are [[two-stroke engine]]s, with twice the number of power strokes as a four-stroke engine per crankshaft rotation.<ref>{{cite web|url=http://www.zoche.de|title=zoche aero-diesels homepage|work=zoche.de|access-date=30 May 2016}}</ref>{{third-party inline|date=November 2023}}

===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.<ref>{{cite web |url=http://www.bock.de/en/Product_overview.html?ArticleSizesGroupID=169 |title=Bock radial piston compressor |publisher=Bock.de |date=2009-10-19 |access-date=2011-12-06 |archive-date=2011-10-08 |archive-url=https://web.archive.org/web/20111008145502/http://www.bock.de/en/Product_overview.html?ArticleSizesGroupID=169 |url-status=dead }}</ref>

===Model radial engines===
A number of multi-cylinder 4-stroke [[model engine]]s 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&nbsp;cm<sup>3</sup>) 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 [[Flying model aircraft|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 series of three-cylinder methanol and gasoline-fueled model radial engines ranging from 0.90 cu.in. (15&nbsp;cm<sup>3</sup>) to 4.50 cu.in. (75&nbsp;cm<sup>3</sup>) in displacement, also all now available in spark-ignition format up to 84&nbsp;cm<sup>3</sup> displacement for use with gasoline.<ref>[http://www.saito-mfg.com/e-book/_SWF_Window.html Saito Seisakusho Worldwide E-book catalog, pages 9, 17 & 18]</ref> The German Seidel firm formerly made both seven- and nine-cylinder "large" (starting at 35&nbsp;cm<sup>3</sup> 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.{{Citation needed|date=October 2014}}

==See also==
* [[List of aircraft engines]]
* [[Swashplate engine]]
* [[Wankel engine]]

==Notes==
{{Reflist|group=Note}}

==References==
{{Reflist|30em}}

==External links==
{{commons|Radial engine}}
*[https://www.youtube.com/watch?v=Y3DHKyiXKyE Cutaway radial engine in operation video on You Tube]

{{Piston engine configurations}}
{{Authority control}}

[[Category:Radial engines| ]]
[[Category:Piston engine configurations]]
[[Category:Engines by cylinder layout]]