Last updated Liberty L-12 engine, from which Hyper Engine No.1 was derived
The hyper engine was a 1930s study project by the United States Army Air Corps (USAAC) to develop a high-performance aircraft engine that would be equal to or better than the aircraft and engines then under development in Europe. The project goal was to produce an engine that was capable of delivering 1hp/in3 (46kW/L) of engine displacement for a weight of less than 1lb/hp delivered. The ultimate design goal was an increased power-to-weight ratio suitable for long-range airliners and bombers.
At the time, no production engine could come close to the requirements, although this milestone had been met by specially modified or purpose-built racing engines such as the Napier Lion and Rolls-Royce R. A typical large engine of the era, the Pratt & Whitney R-1830 Twin Wasp radial, developed about 1,200hp (895kW) from 1,830in3 (30L) so an advance of at least 50% would be needed. Simply scaling up an existing design would not solve the problem. While it would have increased the total available power, it would also increase the weight, and thus not have any significant effect on the power-to-weight ratio. To meet the goals, more radical changes were needed.[1]
Several engines were built as part of the hyper program, but for a variety of reasons none of these saw production use. Air-cooled engines from a variety of US companies were delivering similar power ratings by the early 1940s, and the licensed production of the Rolls-Royce Merlin as the Packard V-1650 provided hyper-like performance from an inline while the Allison V-1710 did the same from a US design, one produced as a private effort outside the hyper program.
Design and development
Improvements in construction and lighter materials had already delivered some benefits on the way to higher power-to-weight ratios. Aluminum was being introduced in place of steel as the quality and strength of aluminum alloys improved during the 1930s; this lowered engine weight noticeably, but not enough to achieve a 50% overall improvement. To reach that goal, the power of the engine would also need to be increased. Power is a combination of energy and the rate it is delivered, so to improve the power-to-weight ratio, one would need to increase the operating pressures of the engine, the operating speed, or a combination of both. Further gains could be made by eliminating losses like friction, combustion inefficiencies and scavenging losses, delivering more of the theoretical power to the propeller.[2]
The USAAC engineers determined that it would study all three improvements. Before long, they concluded that increasing the combustion temperature and scavenging efficiency promised the greatest increases of all of the possibilities. To meet that goal, increasing engine speed seemed to be the most attractive solution. However, there were a number of practical problems that were impeding progress in these areas.
Increasing the compression ratio is an easy change that improves the mean effective pressure (MEP), but leads to engine knocking from inconsistent detonation. Uncontrolled, knock can damage the engine and was a major block on the way to improved power settings. This change would also increase the operating temperatures, which presented a problem with the valves. Valves were already reaching temperatures that would cause pre-ignition of the fuel as it flowed past them.
Increasing operational speed is also, theoretically, a simple change to the engine design. However, at high operating speeds the valves do not completely close before the cam opens them again, a problem called "valve float". Valve float allows gases in the cylinder to escape through the partially open valve, reducing the engine efficiency. Increasing valve spring pressure to close the valves faster led to rapid cam wear and increased friction, reducing overall performance by more than any horsepower gained.[3]
As valves were a key issue in both approaches to improved performance, they had been a major area of research in the 1920s and 30s. In the UK, Harry Ricardo had written an influential paper on the sleeve valve system for exactly these reasons, claiming it was the only way forward. He had some success in selling this idea, most notably to Bristol Aeroplane Company Engines, where Roy Fedden became "a believer". Ricardo's friendly competitor, Frank Halford, designed his own sleeve valve engine with Napier & Son, another prominent British engine maker.[4]
The USAAC was not so convinced that the sleeve valve was the only solution. Ironically it was one of Ricardo's papers on the sleeve valve design that led to the USAAC's hyper engine efforts. In one late 1920s paper he claimed that the 1hp/in³ goal was impossible to achieve with poppet valve type engines. The USAAC engineering team at Wright Field decided to test this claim by beating it. They proposed an engine of about 1200 cubic inches (20 L), hoping the engine's smaller size would lead to reduced drag and hence improved range.
Hyper No.1
Sam Heron, head of development at Wright Field and a former colleague of Ricardo while Heron had been working at the Royal Aircraft Factory, Farnborough, started working on the problem with a single-cylinder test engine that he converted to liquid cooling, using a Liberty L-12 engine cylinder. He pushed the power to 480psi Brake Mean Effective Pressure, and the coolant temperature to 300°F (149°C) before reaching the magic numbers. By 1932, the USAAC's encouraging efforts led the Army to sign a development contract with Continental Motors Company for the continued development of the engine design. The contract limited Continental's role to construction and testing, leaving the actual engineering development to the Army.[5]
Starting with the L-12-cylinder, they decreased the stroke from 7in to 5in in order to allow higher engine speeds, and then decreased the bore from 5in to 4.62in, creating the 84in³ cylinder. This would be used in a V-12 engine of 1008in³ displacement.[6] They used the L-12's overhead camshaft to operate multiple valves of smaller size, which would improve charging and scavenging efficiency. Continental's first test engine, the single-cylinder Hyper No.1, first ran in 1933.
They eventually determined that exhaust valves could run cooler when a hollow core filled with sodium is used — the sodium liquefies and considerably increases the heat transfer from the valve's head to its stem and then to the relatively cooler cylinder head where the liquid coolant picks it up.[6]
Liquid cooling systems at that time used plain water, which limited operating temperatures to about 180°F (82°C). The engineers proposed using ethylene glycol, which would allow temperatures up to 280°F. At first they proposed using 100% glycol, but there was little improvement due to the lower specific heat of the glycol (about 2/3 that of water). They eventually determined that a 50/50 mixture (by volume) of water and glycol provided optimal heat removal.[6]
Hyper No.2
A second cylinder was added to Hyper No. 1 to make a horizontal opposed engine for evaluation of an horizontal opposed 12-cylinder engine. After running the modified engine with different combinations of cylinder bore and stroke, it was found that the high coolant temperature required to maintain the required output was impractical. A third high-performance single-cylinder engine was then constructed with lower operating parameters. This engine was designated "Hyper No. 2", and became the test bed for developing the cylinders that would become the O-1430-1.[6]
The Army apparently became concerned about the development of a suitable supercharger for high-altitude use, and for further development in 1934 they asked for a newer cylinder with slightly less performance and an increased volume of 118.8in3 from its 5.5in (140mm) bore and 5.0in (130mm) stroke. This size cylinder would then be used in a 1,425in3 12-cylinder engine, delivering the same 1,000hp, with a performance of 0.7hp/in3. This placed its performance on a par with newer experimental engines from Europe like the Rolls-Royce PV-12, at least when running on the higher-octane fuels the Army planned to use.[7]
Another change was to the engine layout. The Army, convinced that future aircraft designs would use engines buried in the wings for additional streamlining, asked Continental to design a full-sized flat-horizontally opposed engine for installation inside a wing. The resulting engine was the Continental O-1430, which would require a ten-year development period which changed the layout to first an upright V-12 engine and later, an inverted V-12 engine before becoming reliable enough to be considered for full production as the Continental IV-1430 in 1943. By then other engines had already passed its 1,600hp (1,200kW) rating, and although the IV-1430 had a better power-to-weight ratio, there was little else to suggest setting up production in the middle of the war was worthwhile.[7]
The project was eventually guided by the requirements in the "Request for data R40-C", which was included as a part of the Financial Year (FY) 1940 aircraft procurement program.
Request for data R40-C
As 1938 came to an end, war in Europe was imminent. At this point, European aircraft had greatly surpassed US designs.[8] The two top USAAC fighters, the Seversky P-35 and the Curtiss P-36A, were just able to hit 300mph (480km/h). Against the 340+ mph Messerschmitt Bf 109 they would be completely outclassed. One of America’s answers to this issue, the twin-engined Lockheed XP-38, was entering an extended test program.
Although the XP-38 was able to fly at speeds in excess of 413mph, its twin engines and relatively large frame meant it was large and heavy. This, in turn, meant the XP-38 was not as maneuverable as most single-engine fighters.[9] The XP-38 also had a newly introduced liquid-cooled engine, the Allison V-1710. The Allison's in-line vee cylinder arrangement allowed for a narrow aerodynamic shape that had less drag than the air-cooled radial engine fighters that predominated in America at the time.[10]
The fighter aircraft procurement program for FY 1940 was contained in a document that was approved by Assistant Secretary of War Louis K. Johnson on 9 June 1939. That document was the "Request for Data R40-C", and unlike previous aircraft procurement requests, it was sent to only a limited number of aircraft manufacturers. The original document was to be sent to:[11]
These companies had only ten days to agree to the terms of the document, and only 30 days to submit their designs.
FY 1940
A total of 26 designs, with a mix of 16 engine models from six engine companies, were submitted by seven of the selected companies. These engines became known as the "Hyper Engines", a contraction of High-performance engines. The submitted designs were graded using a "Figure of merit" (FOM) rating system, and then, using the FOM results (which ranged from 444.12 for the Allison V-1710-E8 to 817.90 for the Pratt and Whitney X-1800-A4G), they were separated into one of three groups.
Those placed in the first group were little more than modifications to existing designs. They were not considered to be sufficiently advanced.
Those placed in the third group proposed using an engine that was unlikely to be developed into flying condition by the time the airframe was ready to fly. They were not considered to be viable in the time frame allowed.
The remaining ten designs were placed in the second group: those that were an advancement in aeronautical engineering, with an engine that would be ready to fly, when needed.
Only three of these ten designs were approved, and contracts were made for a limited prototype run of three aircraft for each.[12]
The three aircraft/engine combinations that were selected:[13]
Bomber B, the German Luftwaffe's advanced medium bomber program that used similar high-output aviation powerplants of over 1,500 kW output apiece.
Related Research Articles
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The Wright R-3350 Duplex-Cyclone is an American twin-row, supercharged, air-cooled, radial aircraft engine with 18 cylinders displacing nearly 3,350 cubic inches (54.9 L). Power ranged from 2,200 to over 3,700 hp, depending on the model. Developed before World War II, the R-3350's design required a long time to mature before finally being used to power the Boeing B-29 Superfortress.
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The Curtiss-Wright XP-55 Ascender is a 1940s United States prototype fighter aircraft built by Curtiss-Wright. Along with the Vultee XP-54 and Northrop XP-56, it resulted from United States Army Air Corps proposal R-40C issued on 27 November 1939 for aircraft with improved performance, armament, and pilot visibility over existing fighters; it specifically allowed for unconventional aircraft designs. An unusual design for its time, it had a canard configuration, a rear-mounted engine, swept wings, and two vertical tails. Because of its pusher design, it was satirically referred to as the "Ass-ender". Like the XP-54, the Ascender was designed for the Pratt & Whitney X-1800 engine, but was redesigned after that engine project was canceled. It was also the first Curtiss fighter aircraft to use tricycle landing gear.
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The Curtiss P-60 was a 1940s American single-engine single-seat, low-wing monoplane fighter aircraft developed by the Curtiss-Wright company as a successor to their P-40. It went through a lengthy series of prototype versions, eventually evolving into a design that bore little resemblance to the P-40. None of these versions reached production.
The Lycoming XR-7755 was the largest piston aircraft engine ever built in the United States, 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.
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The Continental XI-1430 Hyper engine was a liquid-cooled aircraft engine developed in the United States by a partnership between the US Army Air Corps and Continental Motors. It was the "official" result of the USAAC's hyper engine efforts that started in 1932, but never entered widespread production as it was not better than other available engines when it finally matured. In 1939, the I-1430-3 was designated as the engine to power the Curtiss XP-55, an extremely radical pusher-engine fighter design that would not reach production.
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The Pratt & Whitney X-1800 was an H-block aircraft engine project developed between 1938 and 1940, which was cancelled with only one example being built.
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The Lycoming O-1230 was a flat-twelve engine for aircraft designed and developed by Lycoming Engines in the 1930s. Although the engine was flown in an aircraft, it was not fitted to any aircraft selected for production. It later served as the basis for the Lycoming H-2470 engine.
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The P-47 Thunderbolt was a World War II fighter aircraft built by Republic Aviation from 1941 to 1945.
Balzer, Gerald H. (2008). American Secret Pusher Fighters of World War II. Specialty Press. ISBN978-1-58007-125-3.
Biermann, Arnold E, Corrington, Lester C. and Harries, Myron L. (1942). Effects of Additions of Aromatics on Knocking Characteristics of Several 100-octane Fuels at Two Engine Speeds. Cleveland, Ohio, May: Aircraft Engine Research Laboratory.{{cite book}}: CS1 maint: multiple names: authors list (link)
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Schlaifer, Robert and Herron S.D. Development of Aircraft Engines and Development of Aviation Fuels. Harvard University.
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Gunston, Bill (2006). World Encyclopedia of Aero Engines, 5th Edition. Phoenix Mill, Gloucestershire, England, UK: Sutton Publishing Limited. ISBN0-7509-4479-X.
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