Radial engine

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. This configuration was very commonly used in large aircraft engines before most large aircraft started using turbine engines. Radial engines of 100 to 165 hp were popular during the 1930s, before the introduction of modern-type opposed-cylinder engines.

In a radial engine, 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. Most radial engines use overhead poppet valves driven by pushrods and lifters on a cam plate which is concentric with the crankshaft. A few engines utilize sleeve valves instead.

History
In 1903-04 Jacob Ellehammer used his experience constructing motorcycles to build the world's first air-cooled radial engine, a 3-cylinder engine which he used as the basis for a more powerful 5-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 3-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, with its cylinders arranged in four groups of five. One of the three-cylinder "fully radial", 120º 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.

The radial engine was not developed in Germany: two radial engines were made there before World War I, but the Germans seemed to lose faith in the type under war conditions, or it may have been that insistence on standardization ruled out any but proven engine types.

During WWI, many of the French and other Allied aircraft flew with Bentley, Clerget, Gnome and Le Rhone rotary radial engines, the ultimate examples of which produced 240hp, with the Germans either making close copies of the Gnome and Le Rhone powerplants built by the Oberursel firm, or very late in the war, using the unique Siemens eleven-cylinder rotary engine.

Radial versus inline debate
By 1918 the potential advantages of air-cooled radials over the water-cooled inline and air-cooled rotaries that had powered World War I aircraft were well appreciated. While British designers had produced the ABC radial in 1917, they were unable to resolve its cooling problems, and it was not until the 1920s that the Bristol Aircraft Company produced reliable British radials.

In the US, NACA noted in 1920 that air-cooled radials could offer ship-based aircraft less weight per horsepower plus increased reliability, and by 1921 the US Navy had announced it would only order aircraft fitted with air-cooled radials. Charles Lawrance's J-1 engine, 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 acceptable for liquid-cooled engines. At the urging of the Army and Navy the Wright Aeronautical Corporation bought Lawrance's company, and subsequent engines were known as Wright Radials. The radial engines gave confidence to Navy pilots performing long-range overwater flights, and their increased performance meant that carrier-based aircraft could hold their own against land-based aircraft in combat.

The Wright company's 225 hp 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 Ryan NYP with which he made the first solo Atlantic flight.

By 1929 it was considered by some that inline engines would completely displace air-cooled radials, and the Bristol Aeroplane Company was considered to be falling behind in engine production, as they had not produced an inline engine, concentrating instead on radials. At that time inline engines were mostly air-cooled, and presented some cooling problems.

Rolls Royce introduced the Merlin engine in 1933, which eventually powered the Spitfire and Hurricane fighters and the Lancaster heavy bomber, amongst others; the Merlin was also built in the US as the Packard V-1650. By 1938 liquid-cooled inline aircraft engines had been successfully developed in the US by the automobile industry, with the backing of the US Army. Many notable fighter aircraft of World War II were powered by inline engines, including the Supermarine Spitfire, P-51 Mustang, P-38 Lightning and Messerschmitt Bf 109, but radial engines also saw service in the successful Mitsubishi Zero, P-47 Thunderbolt, F4U Corsair, F6F Hellcat, and Focke-Wulf Fw 190, while the late-war Hawker Sea Fury, one of the fastest production single piston-engined aircraft ever built, used a radial engine. Until the development of the jet engine, bombers, transport aircraft, and airliners commonly used radial engines. Factors influencing the choice of radial over inline were the larger radial engine displacements available, the reliability of the engine, and the maintenance simplicity. Additionally, the larger total frontal area of these aircraft meant the radial engine's large frontal profile was less detrimental in proportion to smaller aircraft designs.

The radial was popular largely due to its simplicity, and most navy air arms had dedicated themselves to it because of its improved reliability for over-water flights and better power/weight ratio for aircraft carrier takeoffs. Being liquid-cooled, inline engines require the added weight and complexity of cooling systems and are generally more vulnerable to battle damage. Damage to an inline engine could result in a loss of coolant and consequent engine seizure, while an air-cooled radial could take damage but continue to operate. Additionally, radials offered higher mechanical efficiency than inline engines, as they had shorter and stiffer crankshafts, a five-cylinder radial needing only two crankshaft bearings as opposed to the seven required for a six-cylinder inline engine. The shorter crankshaft also produced less vibration and hence higher reliability. Another advantage of the air-cooled radial is that all cylinders receive equal cooling airflow, and most radial-engined aircraft designed since the 1930s were fitted with NACA cowlings to further improve cooling and reduce drag. Also, by being flat, radial engines resulted in shorter aircraft with better landing visibility (very important for carrier landings), and with a smaller moment of inertia that were able to turn more tightly. The latter was especially important in fighter aircraft in general.

The inline engine's major advantage was a smaller frontal area compared to radial engines. This made it possible to build more streamlined designs and, for single-engine aircraft, could improve the pilot's forward visibility. In addition, being liquid-cooled offered greater options for both engine and radiator placement. For example, the P-39 Airacobra mounted the engine behind the pilot to allow the large M4 cannon to be mounted in the front of the aircraft, while the Spitfire incorporated an underwing radiator design which offset cooling drag by using the cooling air to generate thrust.

Multi-row radials
Originally radial engines had one row of cylinders, but as engine sizes increased it became necessary to add extra rows. Most did not exceed two rows, but the largest radial engine ever built in quantity, the Pratt & Whitney R-4360 Wasp Major, nicknamed corncob, 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 kilowatts). 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. 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 at 2,000 rpm. They were used on fast attack craft, such as Osa class missile boats.

Modern radials
At least five companies build radials today. Vedeneyev engines produces the M-14P model, 360 hp (up to 450hp) radial used on Yakovlevs, and 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". 110hp 7-cylinder and 150hp 9-cylinder engines are available from Australia's Rotec Engineering. HCI Aviation offers the R180 5-cylinder (75hp) and R220 7-cylinder (110hp), 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 to 172 HP. Miniature radial engines for model airplane use are also available from 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.

Diesel radials
While most radial engines have been produced for gasoline fuels, there have been instances of diesel fueled engines. The Bristol Phoenix of 1928-1932 was successfully tested in aircraft and the Nordberg Manufacturing Company of the US developed and produced a series of large radial diesel engines from the 1940s.

To reduce the danger of engine fires, in 1932 the French company Clerget developed the 14D, a 14-cylinder 2-stroke diesel radial engine. After a series of improvements, in 1938 the 14F2 model produced 520 hp at 1910 rpm cruise power, with a power-to-weight ratio near that of contemporary gasoline engines and a specific fuel consumption of 166g/hp/hour. During WWII the research continued, but no engines were mass-produced because of the Nazi occupation, and by 1943 the engine had grown to produce over 1000 hp 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 4000 hp, but in 1947 the company abandoned piston engine development in favor of work on the emerging turbine engines.

The Nordberg engines were initially designed for electricity production in aluminium smelters. They differed from the norm of radial design by using two opposite cylinders as a double master instead of the more usual single master rod, and managed to run perfectly circular. The engine design also permitted even numbers of cylinders in a single row with the cylinders being fired in consecutive order. The engines were a two-stroke design and were also available in a dual-fuel gas/diesel model. A number of powerhouse installations utilising large numbers of these engines were made in the US.

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 to produce 225 horsepower. 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.

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.