The Heart of the Cobra

Development of the Allison V-1710 Engine

by Randy Wilson

Copyright � 1997 by the Confederate Air Force and Randy Wilson. All rights reserved.

Here are a couple of images of the engine and its workings. They are a bit large due to the detail in them.

James A. Allison was a wealthy sportsman and keen businessman, and his love of automobile racing in the first years of this century led him to build, along with others, the famous Indianapolis Speedway. In 1915, Allison founded a company to build and service racing cars and engines near the speedway, and in 1917 hired Norman Gilman as its superintendent and chief engineer.

When the US entered World War I in 1917, Gilman, at James Allison's urging, was part of the team that developed the Liberty engine, a water-cooled, V-12 of 1650 cu. in. displacement giving 400 horsepower. The first two production prototypes of the Liberty were, in fact built by Gilman and his workers in the Allison shop. Overhaul and modification of Liberty engines kept the Allison shop involved with aviation after the war, and in 1927 Gilman redesigned the Liberty's bearings, curing a chronic problem in the engine. Profits from the manufacture and sale of Gilman's patented bearings helped fund Allison's future engine developments.

In 1928 James Allison died, and the following year, General Motors purchased the company, as part of GM's bid to enter the quickly growing aviation marketplace. Gilman was named president but continued to act as the chief engineer. The US military remained a major customer, and in 1928 Allison tested the use of ethylene glycol as an improved engine coolant at the request of the Army.

The Navy was also interested in newer, more powerful engines for both aircraft and dirigibles, to replace existing German Maybach engines. In 1930 Allison began construction of a V-12 design of 1,710 cu. in. displacement, thus giving rise to the designation V-1710. Later, the engine was often just called the Allison.

The new engine was designed to use glycol for cooling, and was often described as being "chemically cooled". A mechanical supercharger was fitted to all but one version, but from its inception, the Allison V-1710 was designed to accept a turbo-supercharger. The V-1710B, built as a dirigible engine for the Navy was the only Allison without any type of supercharger, having to run both forward and backwards for reversing in airship use.

From 1932 until 1937, Allison worked to increase the reliability and power of the V-1710 from its initial 750 hp at 2400 rpm. In 1933, Gilman hired Ronald Hazen from Fairchild, where he had helped design the Ranger engine, to solve special problems with the new engine. In March 1936, Hazen became Allison's chief engineer. Gilman's confidence in Hazen was justified on April 23, 1937 when the Allison V-1710 completed a tough Army 150-hour type test, the first engine to do so with 1,000 horsepower.

An extract from an Allison press release following that successful test notes the importance of both chemical coolants and the turbo-supercharger:

The chemist has contributed to the new era of powerful aircraft engine designs by developing synthetic coolants that have high boiling points and low freezing points. These coolants have replaced water which has been generally used for liquid cooled engines in the past. Ethylene Glycol is the coolant used in the Allison engine. It has a boiling point of 387 degrees F. thus permitting more effective cooling radiators to handle the heat transfer from cylinders to atmosphere, than water which boils at 212 degrees F. and therefore requires larger radiator area.

This new engine is reported as especially adapted for high altitude flying as it permits a very practical installation of a turbo supercharger. The turbo supercharger is an American invention sponsored by the Army Air Corps… Up to twenty-five thousand feet for which level the turbo supercharger is designed, the Allison engine provides 1000 horsepower.

[Here for the entire text of the Allison press release.]

By the 1930s, military aircraft, both bombers and fighters, were being designed to operate at altitudes as high as 30,000 feet, but existing mechanical superchargers could only provide full power to the engine at about 12,000 feet or lower. At the General Electric Company, Dr. Sanford Moss had been developing turbo-superchargers for aircraft engines since World War I. His pioneering work and driving force for turbocharging was awarded with the Collier Trophy in 1940.

The basic concept of an exhaust-driven supercharger is simple, but its implementation proved to be difficult. A turbine wheel must be spun at 20,000 rpm by the white-hot (1500�F+) and highly corrosive exhaust gas, causing problems of metallurgy and lubrication. In addition, a sophisticated control system must keep the turbocharger from over-speeding the turbine wheel and over-boosting the engine, which could quickly destroy the power plant.

Prior to 1940, turbine blades were individually forged and machined, and it was not uncommon for early turbochargers to fail, often exploding and catching on fire. In 1940, however, a material similar to that used in dentures, Haynes-Stellite No. 21 (H-S 21), was introduced, allowing the casting of turbine blades, thus drastically reducing the failure rate in turbochargers. Later during the war, this experience with turbine blades was a major factor in General Electric being asked to build the first US jet engines.

The various models of GE turbochargers were designed for different horsepower ranges and configurations. Type B-22 equipped late model B-17s and B-24s, while B-11 models were fitted to B-29s, and B-13 and B-33 turbocharged the Allisons in later P-38s.

A comparison of the Allison engines used in the first Bell P-39 Airacobras, shows the advantages of the turbocharged engines. The different versions were identified by the military with "dash numbers" after the basic engine designation, while Allison used company model numbers with both letters and number. Thus Allison's designation of the XP-39 Airacobra's engine was V-1710-E2, while the Army called it the V-1710-17. This turbocharged engine produced its full rated power of 1,150 hp up to 25,000 feet. But the V-1710-35 (Allison's -E4) which powered early production Airacobras had a rated altitude of only 11,800 feet, above which it lost power.

Thus, the Lockheed P-38 Lightning, which, with a few exceptions, was produced with turbocharged Allisons from the start, was an efficient high-altitude fighter and late model Lightnings could produce their full 1,425 hp as high as 30,000 feet.

Probably the most famous Allison-powered fighter was the Curtiss P-40 Warhawk, which like the P-39 suffered from the lack of efficient supercharging above 12-15,000 feet. Is there an explanation for the Army Air Corps' installation of turbochargers in the Boeing B-17 and all other long-range bombers but not in most fighters designed during the late 1930s? One answer may be the Army's continued belief, even after the initial reports of the air war in Europe, that America was protected from enemy fighters by its two vast oceans, and that long-range bombers were the real aerial defenders of the United States.

Great Britain had only the narrow English Channel separating her from Germany in mid-1940. There, the Rolls-Royce V-1650 Merlin engine, the other major Allied V-12 of the war, had been developed at the same time as the Allison, to power the legendary Hurricane and Spitfire, among others. In 1942, with the introduction of two-speed, two-stage mechanical superchargers, Merlin 60-series engines fitted in Spitfire VII and later marks produced up to 1,500 hp as high as 21,000 feet. Recognizing the benefits of two-stage supercharging, Allison developed an auxiliary engine-driven supercharger, which fitted onto the existing blower at the rear of the engine and first appeared on the V-1710-93 (-E11) engine in Bell P-63 Kingcobras.

From the start, the two-stage Merlins had both an intercooler and aftercooler, which lowered the temperature of the air coming out of each stage of the supercharger, thus increasing the density of the fuel and air mixture to the engine and reducing damaging detonation, or pre-ignition. However, the first two-stage Allison to incorporate an aftercooler was the V-1710-119 (-F32R) which powered North American's XP-51J to a speed of 471 mph at 27,400 feet in the spring of 1945. The same engine was intended to power the first production P-82A Twin Mustang, but the first Allison powered P-82E did not fly until 1947.

During the war, Allison had studied the use of turbocompounding, where the energy lost to the engine's exhaust was partially recovered by a gas turbine and fed directly back into the engine through reduction gearing. Allison proposed such a variant, the V-1710-127 (-E27), to power the Bell XP-63H, with a General Electric CT-1 turbine, based on the CH-5 turbocharger used in P-47M and Ns. However the project was cancelled in 1946. If it had been built, this turbocompound engine might have produced 3,000 hp with 115/145-PN (performance number) fuel, ADI (anti-detonation injection) and a manifold pressure of 100 inches Hg!

Allison also built a "hyper" engine, as some of the largest and highest-powered piston engines were called. The V-3420 was a 24-cylinder engine that was essentially two V-1710s mounted in a common crankcase. Power ranged from 2,300 to a maximum of 3,000 hp in various models, and the design powered only four prototype designs, the Douglas XB-19A, Boeing's XB-39, Lockheed's XP-58 Chain Lightning and the Fisher XP-75 Eagle.

The Allison V-1710 was America's only liquid-cooled engine to see wide use in the Second World War, powering tens of thousands of P-38, P-39, P-40, P-63 and other fighters. Although it proved to be a great engine, the Army's lack of interest and funding to develop better mechanical superchargers in the 1930s kept the engine from being developed to its full potential. The Allison was in many ways stronger and more rugged than the British Merlin, capable of withstanding operating speeds of 200-400 rpm higher than the Rolls-Royce design. One can only wonder what would have happened if Allison had adopted the Merlin's two-stage supercharger to the V-1710.

Allison V-1710 Engine Images

The following diagrams and images are from the Allison Service School Hand Book for V-1710 Models "E" and "F" Engines, dated April 1, 1943. This manual is from the AAH Museum archives and used with permission.

This figure shows the location of major accessories on a V-1710-35 engine, as used in early Bell P-39s. This is a view from the rear. Note the rounded supercharger housing and downdraft carburetor.

The basic operation of an engine-driven mechanical supercharger is shown in this diagram. The carburetor would be mounted above the air inlet. Note the fuel injector nozzle, which enriched or boosted the mixture upon accelleration. Since the late 1920s, mechanical superchargers had become common on most engines of 400 or more horsepower, especially radials, where the mixture needed to be evenly distributed to both upper and lower cylinders. Manifold pressure is indicated in inches of mercury (Hg), with 29-30 inches being the normal pressure at sea level. Early superchargers only boosted this by about 6-12 inches. Later models provided more than 30 inches of supercharge.

Allison Press Release
on the Certification of its V-1710 Engine by the Air Corps

The following memorandum is released for immediate publication by Mr. O. T. Kreusser, General Manager of the Allison Engineering Division of General Motors Corporation at Indianapolis, Indiana:

New Aeroplane Engine

From the Air Corps at Wright Field, Dayton, Ohio, comes the news that the U. S. Army aircraft engine type test approval has been granted to the Allison V-1710 aircraft engine which completed the necessary tests April 23, 1937. This is the first engine to pass the Army 150 hour type test for engines of one thousand horsepower, or larger.

For the past several years, intensive development work by different aircraft engine manufacturers has been under way to meet the Army Air Corps requirements and to pass the 150 hour type tests for aircraft engines of 1000 horsepower normal rating.

It is interesting to note that the Allison engine is a Vee type twelve cylinder design, chemically cooled instead of the widely used radial air cooled design that has done so much to date to enhance the progress of American aviation.

The chemist has contributed to the new era of powerful aircraft engine designs by developing synthetic coolants that have high boiling points and low freezing points. These coolants have replaced water which has been generally used for liquid cooled engines in the past. Ethylene Glycol is the coolant used in the Allison engine. It has a boiling point of 387 degrees F. thus permitting more effective cooling radiators to handle the heat transfer from cylinders to atmosphere, than water which boils at 212 degrees F. and therefore requires larger radiator area.

This new engine is reported as especially adapted for high altitude flying as it permits a very practical installation of a turbo supercharger. The turbo supercharger is an American invention sponsored by the Army Air Corps comprising a turbine compressor driven by the waste exhaust gases of the engine. It compresses and delivers to the carburetor at sea level pressure the rarefied atmosphere at higher altitudes to provide the air required for combustion in the engine cylinders. When the engine is supplied with air at sea level pressure it maintains sea level power output. Up to twenty-five thousand feet for which level the turbo supercharger is designed, the Allison engine provides a 1000 horsepower. With such equipment a plane is capable of very rapid climbing and very high speeds. Both of these qualifications are very valuable for a military aeroplane. The chemically cooled engine has a very small frontal area - less than six and one quarter square feet. Air cooled radial engines now in general use but of lesser power output have a frontal area of sixteen and three quarters square feet. The small frontal area of the chemical cooled engine provides for ideal streamlining of the plane which in turn adds five or more per cent to the increased speed of the plane for the same expenditure of engine power.

While high output chemically cooled engines and the higher speed planes will definitely cost more than planes and engines now in use, it seems likely that the advantages both as to speed and the superior strategy of high altitude flying and rapid climbing for military and the economy of long distance transport will more than offset the higher first cost.

It is generally agreed that in aeroplanes, greater useful and available engine power and economical higher speeds are important factors contributing to increased safety of flying. Both factors offset some of the hazards of bad weather either as to ice formation and arriving at the destination before marked weather changes take place.

Thirty-five years ago the Wright brothers at Dayton, Ohio, designed and built the first engine that flew an aeroplane. It was a four cylinder water cooled engine built of aluminum, cast iron and steel. It developed 30 horsepower and weighed 180 pounds. Today we have the last word for the moment from the Air Corps - a 1000 horsepower aircraft engine weighing 1275 lbs. The engineer, the metallurgist and the chemist refuse to admit that the future is more venturesome as to further improvement than the past - tomorrow we may hear of further accomplishments of the engines and planes for new eras of safe flying.

For several years many aviation technicians have felt that some of the European nations had the more modern aircraft engines particularly for military purposes. The Rolls-Royce Merlin engine has demonstrated extraordinary performance as to speed for British military aircraft. The U.S. Army rating of 1000 horsepower normal for the Allison engine brings back the lead to America for the most powerful lightest weight aircraft engine.

The development and construction of this new engine has been under way for almost five years by the Allison Engineering Company at Indianapolis, a division of General Motors, devoted to development work. The Air Corps Power Plant Section at Wright Field has purchased a number of these engines and carried through the intensive testing of the different designs that led to the engine specification now being released for production.

In addition to the test work carried on in the engine laboratories, Wright Field installed a test engine in an experimental test aeroplane. This test plane with the thousand horsepower Allison engine to date has flown over 100 hours with satisfactory performance as to power and smoothness in take-off and in flight and particularly good starting in cold weather. The ceiling or maximum altitude reached by this test aeroplane which is not equipped with turbo supercharging was made by Lieut. B. S. Kelsey of the Flight Section at Wright Field. He reached an altitude of 26,400 feet.

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