Ever since man first experienced powered flight in 1903 the search has been on for light powerful engines to power flying machines. The radial engine has almost always been at the forefront of aircraft engine development even before the historic flight at Kitty Hawk.
In 1901 the radial motor made its first appearance Charles Matthews Manly, an American engineer, constructed a water-cooled five-cylinder radial engine he converted from one of Stephen Balzer's unsuccessful rotary engines. Manly later installed this engine in Langley's Aerodrome A aircraft and managed to produce 52 hp at 950 rpm, a considerable improvement on the 12 hp at 1,025 rpm of the Charles Taylor engine used on the Wright Flyer.
Between 1903 and 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. The engine was installed in his triplane and he successfully made a number of short free-flight hops.
Another early radial engine was the three-cylinder Alessandro Anzani, originally built as a W3 "fan" configuration, one of which powered Louis Blériot's Blériot XI across the English Channel. Before 1914 Anzani had developed radial engines ranging from 3 cylinders (spaced 120° apart), to a massive 200 hp 20-cylinder engine, with its cylinders arranged in four rows of five cylinders apiece.
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 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; the engine was often known as the Canton-Unné.
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.
In World War 1 many French and other Allied aircraft flew with Gnome, Le Rhône, Clerget, and Bentley rotary engines, the ultimate examples of which reached 250 hp although none of those over 160 hp was successful. By 1917 rotary engine development was lagging behind new inline and V-type engines, which by 1918 were producing as much as 400 hp 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 eleven-cylinder rotary engine, which was unusual for the period in being geared through a bevel geartrain 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 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 its rotary engines into stationary radial engines.
By 1918 the potential advantages of air-cooled radials over the water-cooled inline engine and an 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 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 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 of 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 J-5 Whirlwind radial engine of 1925 was widely acclaimed 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 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.
In 1925 the American Pratt & Whitney company was founded, competing with Wright's radial engines. Pratt & Whitney's initial offering, the 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.
In the United Kingdom, the Bristol Aeroplane Company was concentrating on developing radials such as the Jupiter, Mercury, and sleeve valve 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. France continued its development of various rotary engines but also produced engines derived from Bristol designs, especially the Jupiter.
Although other piston configurations and turboprops have taken over in modern propeller-driven aircraft, Rare Bear, which is a Grumman F8F Bearcat equipped with a Wright R-3350 Duplex-Cyclone radial engine, is still the fastest piston-powered aircraft.
In America 125,334 twin-row, 18-cylinder Pratt & Whitney R-2800 Double Wasp, with a displacement of 2,800 in³, that’s a whopping 46 litres, producing between 2,000 and 2,400 hp were manufactured. They 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), 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 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 43-litre displacement, 14-cylinder 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 Duplex-Cyclone powering the four-engine Boeing B-29 Superfortress and others.
The Soviet Shvetsov 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 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 1,540 and 1,970 hp, powered the German single-seat, single-engine Focke-Wulf Fw 190 Würger, and twin-engine Junkers Ju 88.
In Japan, most aircraft were powered by air-cooled radial engines like the 14-cylinder Mitsubishi Zuisei in the Kawasaki Ki-45, Mitsubishi Kinsei in the Aichi D3A, Mitsubishi Kasei in the Kawanishi H8K, Nakajima Sakae in the Mitsubishi A6M and Nakajima Ki-43, and 18-cylinder Nakajima Homare was used in the 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 D4Y3.
In Britain, Bristol produced both sleeve valved and conventional poppet valved 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 II and 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.
In the years leading up to World War II, as the need for armoured 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. However, this reliance had a downside: 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.
A number of companies continue to build radials today. Vedeneyev produces the M-14P radial of 360–450 hp 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, Pitts S12 "Monster" and the Murphy "Moose".
Rotec Aerosport’s 110 hp 7-cylinder and 150 hp 9-cylinder engines are freely available. Australia's HCI Aviation offers the R180 5-cylinder 75 hp and R220 7-cylinder 110 hp, 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 25 to 150 hp.
