By William Pearce
In April 1926 the Curtiss Aeroplane and Motor Company initiated a design study for a 600 hp (447 kW), air-cooled aircraft engine. The engine was to have minimal frontal area while keeping its length as short as possible. Configurations that were considered but discarded were a 9-cylinder single-row radial, a 14-cylinder two-row radial, a 12-cylinder Vee, and a 16-cylinder X. The selected design was a rather unusual 12-cylinder engine that Curtiss referred to as a “hexagon” configuration. This engine was built as the Curtiss H-1640 Chieftain.
The Curtiss H-1640 was designed by Arthur Leak and Arthur Nutt. The Chieftain’s “hexagon” design was a combination of a radial and Vee engine. The intent was to combine the strengths of both engine configurations: the light and short features of a conventional radial with the narrow and high rpm (for the time) of a conventional Vee engine.
The Chieftain was arranged as if it were a 12-cylinder Vee engine cut into three sections, each being a four-cylinder Vee. The Vee engine sections were then positioned in a radial form 120 degrees apart (each cylinder bank being 60 degrees apart). The end result was a two-row, twelve-cylinder, inline radial engine. The H-1640 resembled a conventional radial engine except that the second cylinder row was directly behind the first.
Each four-cylinder Vee section had the cylinder exhaust ports on the inside of the Vee and the intake ports on the outside. Each inline cylinder pair had its own intake runner and dual-overhead camshafts that were enclosed in a common valve cover. The camshafts were driven via a single vertical shaft from the front of the engine. There were four valves per cylinder.
Cooling air was directed through each four-cylinder section’s exhaust Vee; here it met a baffle fitted to the rear of the engine and attached to the cowling. This baffle deflected the air and forced it to flow between the inline cylinders and behind the rear cylinder. The air then flowed into the intake Vee that was blocked off at the front. The air exited the cowling via louvers over the intake Vee.
The pistons were aluminum and operated in steel cylinder barrels that were screwed and shrunk into cast aluminum cylinders with integral cooling fins. From U.S. patent 1,962,246 filed by Leak in 1931, it appears that the Chieftain’s connecting rods consisted of two halves that were bolted together. Each half was made up of one master rod and two articulating rods.
The H-1640 Chieftain had a bore of 5.625 in (143 mm) and a stroke of 5.5 in (140 mm), giving a total displacement of 1,640 cu in (26.9 L). The engine’s maximum diameter was 45.25 in (1.15 m). However, a special cowling was used, cut to allow the valve covers and exhaust stacks to protrude through, reducing the diameter of the cowling to 39 in (0.99 m). The engine was 52.3 in (1.33 m) long and weighed 900 lb (408 kg). The Chieftain had a 5.2 to 1 compression ratio and was rated at 600 hp (447 kW) at 2,200 rpm but developed 615 hp (459 kW). When the engine was pressed to 2,330 rpm, it produced 653 hp (487 kW). It was equipped with a centrifugal-type supercharger that allowed the engine to maintain sea-level power up to 12,000 ft (3,658 m). All Chieftain engines built were direct drive but geared versions had been planned. In addition, some design work on a four-row, 24-cylinder version of 1,200 hp (895 kW) had been done.
Because the engine had an even number of cylinders per each row, a unique firing order was developed that alternated between the front and rear rows. When the engine was viewed from the rear, the cylinders were numbered starting with the cylinder bank at the 9 o’clock position and proceeding clockwise around the engine. The rear cylinder row had odd numbers, and the front cylinder row was even so that the rear cylinder of the cylinder bank at 9 o’clock was number 1 and the front was number 2. The firing order was initially 1, 10, 5, 7, 4, 11, 8, 3, 12, 2, 9, 6 but was later changed to 1, 10, 5, 2, 9, 11, 8, 3, 12, 7, 4, 6 in an effort to smooth out the engine.
The H-1640 Chieftain was first run in 1927 and flown in a modified Curtiss O-1B Falcon, redesignated XO-18, in April 1928. The Chieftain-powered test-bed aircraft was found to out-climb and have a higher ceiling than the standard liquid-cooled Curtiss D-12-powered Falcon. In addition, the top-speed of the two aircraft was the same, which was unheard of for that time period when liquid-cooled aircraft were faster than their air-cooled counterparts. However, the engine suffered cooling issues, and the aircraft was modified back to an O-1B in July 1930.
Thomas-Morse also responded to the Army’s interest in using the Curtiss H-1640. The company’s Viper fighter prototype was built to use the Chieftain engine. This aircraft was tested at Wright Field in June 1929 and given the designation XP-13. Engine overheating was encountered, and a revised cowling was tried in an effort to provide adequate cooling for the H-1640. The new cowling had enlarged openings, and the blocked off sections were angled to force more air into the openings. However, over-heating persisted. The XP-13 was tested until September 1930, when a Pratt & Whitney R-1340C engine was installed and the aircraft redesignated XP-13A. Even though this engine was not as powerful, it was lighter and did not suffer the cooling issues present with the Chieftain. The XP-13A was found to be 15 mph (24 km/h) faster than the Chieftain-powered XP-13. Curtiss had planned to produce the Viper under the designation XP-14, but the H-1640 engine was lacking support so no aircraft were built.
Another Chieftain was installed in the Navy’s second Curtiss XF8C-1 prototype in 1930. The H-1640-powered aircraft was known as the Curtiss XOC3. It too suffered from engine over-heating. The Chieftain engine remained installed in the XOC3 until the aircraft was removed from the Navy’s inventory in April 1932.
In October 1928, the Army ordered three Curtiss P-6 Hawk aircraft to be powered by the H-1640 engine and designated them XP-11. However, shortly after the order was placed, the engine’s cooling trouble became known and the engine’s development ceased. The aircraft were never built with the Chieftain engine.
A total of eight H-1640 engines were made with six going to the Air Corps and two to the Navy. While the Chieftain’s design may have been problematic, the event that directly led to its lack of support and ultimate abandonment was the merger of Curtiss Aeroplane and Motor Company with Wright Aeronautical in July 1929. After the merger, the liquid-cooled engines were provided by Curtiss and the air-cooled engines from Wright. There was no longer a need for the Chieftain, an air-cooled engine of rather dubious design. However, the concept of a hexagonal engine would be revisited with the Wright H-2120, and other hexagonal engines include the SNCM 137, the Junkers Jumo 222, and the Dobrynin series of aircraft engines..
Reportedly, at least one Curtiss H-1640 Chieftain survives and is in storage at the National Air and Space Museum’s Garber Facility in Silver Hill, Maryland.
– Modern Aviation Engines, Volume 2 by Victor Page (1929)
– “The Curtiss ‘Chieftain’ Engine,” Flight by Erik Hildeshim (14 June 1928)
– Dyke’s Aircraft Engine Instructor by A.L. Dyke (1929)
– Aerosphere 1939 by Glenn Angle (1940)
– Curtiss Aircraft 1907-1947 by Peter Bowers (1979/1987)
– American Combat Planes of the 20th Century by Ray Wagner (2004)
– Fighters of the United States Air Force by Dorr and Donald (1990)
– General Dynamics Aircraft and their Predecessors by John Wegg (1990)