December | 2013 | Old Machine Press

By William Pearce

Albert (Al) S. Menasco got his start in the aircraft engine business in 1926 when he was tasked with settling the estate of his friend Art Smith. Before he passed, Smith had purchased 250 Salmson Z-9 water-cooled 9-cylinder radial engines. Waldo Waterman approached Menasco with an idea to convert the Salmson engines to air-cooling. Menasco ended up progressing on his own with the project and made further modifications to the Salmsons. Despite his best efforts, Menasco could not make the Menasco-Salmson B-2 engine reliable enough to pass the new Department of Commerce’s 50-hour test.

This front view of the Unitwin illustrates the slight contrary angle of the Buccaneer engine sections. Visible are the brackets that bolt the two crankcases together to make the Unitwin more rigid. Note the oil supply lines on each side of the gear case for spraying oil on the freewheeling clutches.

In 1928, Menasco found himself with a good shop and a good crew but no engine to manufacture. Jack Northrop was visiting Menasco one day and suggested he should build an inverted, in-line, air-cooled, four-cylinder aircraft engine. This was the start of the 90 hp (67 kW) four-cylinder Menasco Pirate engine and quickly led to the 160 hp (119 kW) six-cylinder Menasco Buccaneer in 1931. Remembering the troubles with the Salmson, Menasco designed these engines to run at 125% rated power for 100 hours straight. The Pirate and Buccaneer were very popular and very successful engines with air racers. Over years of continued development, the engines were supercharged and their output increased to 150 hp (112 kW) and 260 hp (194 kW) respectively.

In mid-1935, Robert Gross, Cyril Chappellet, and Hall Hibbard discussed with Menasco the possibility of coupling two six-cylinder Buccaneer engines together side-by-side to make an inverted U-12 engine. Gross, was the President of the Lockheed Aircraft Corporation, Chappellet its corporate secretary, and Hibbard its chief engineer. Menasco thought the Buccaneer could be developed to produce 350 hp (261 kW), giving the coupled engine 700 hp (522 kW). Through freewheeling (or overrunning) clutch mechanisms, the two engines would power a single propeller. If one engine were to fail, the other would be unaffected and continue to power the constant-speed propeller. The propeller’s pitch would change to compensate for the decrease in power. This arrangement would give twin-engine reliably without the drag of a conventional twin-engine installation or the asymmetric thrust during an engine failure. The men from Lockheed thought the engine would be well suited to power personal aircraft or small feeder aircraft for airline service.

The Menasco Unitwin engine with the air-cooling baffles in-place. Note the intake manifold: on a standard Buccaneer engine, this manifold was on the opposite side. On the Unitwin, both engine sections had the intake manifold on the outside of the engine. Note the revised oil lines to the gear case compared with the previous image.

In 1936 and with Lockheed sponsorship, Menasco coupled two 90 hp Wright Gipsy four-cylinder engines to test the feasibility of a freewheeling gear case. This coupled engine endured 300 hours of tests and paved the way for the Menasco Unitwin. Lockheed formed a new subsidiary, the Vega Airplane Company (originally the AiRover Company), to manufacture an aircraft powered by the Unitwin. In addition, Lockheed explored the possibility of installing Unitwin engines in its Model 12 Electra Junior. By 1937, Vega had taken over the Unitwin project which was behind schedule. Issues were encountered with the Buccaneer engine being able to produce the desired 350 hp (261 kW) output. In addition, the Menasco Manufacturing Company was in poor financial health. Lockheed’s Gross and Chappellet personally invested in Menasco to keep the company going.

The Menasco 2-544 Unitwin was comprised of two 544 cu in (8.9 L) C6S-4 Super Buccaneer six-cylinder engines. The Buccaneer engines were positioned side-by-side and coupled together by a common gear case. Each engine was canted out 10-degrees from vertical. The gear case housed hydraulic freewheeling clutches that allowed the engines to be operated completely independent of each other. In addition, the rear of the gear case had provisions to run essential accessories, including the prop governor and an oil pump. The gear case had its own oil supply, independent from the engine sections. The engine sections were modified to rotate opposite the normal direction, and once through the gear case, rotation returned to normal for the use of a standard propeller. The gear case also incorporated a 0.667 (may have been 0.6375) propeller gear reduction.

Rear view of the Menasco Unitwin showing the separate superchargers, carburetors, magnetos, and starters.

Rigid supports connected the two Buccaneer crankcases together to utilize one engine mount. The two rows of cylinder shared baffles to direct the cooling air through their fins. Later in development, the intake and exhaust on the left engine section were reversed; this allowed the intake manifolds to travel along the outside of both engine sections and for the exhaust manifolds to be located in the middle, between the sections.

The inverted U-12 had a 4.75 in (121 mm) bore and a 5.125 in (130 mm) stroke. Total displacement was 1,090 cu in (17.9 L). Each cylinder had one intake and one exhaust valve actuated by pushrods. Two spark plugs per cylinder were fired by Scintilla magnetos, two on each engine section. The compression ratio was 5.5 to 1, and each engine section had its own supercharger. The engine produced 580 hp (433 kW) at 2,400 rpm and had a maximum output of 660 hp (492 kW) at 2,700 rpm. The Unitwin was 80.0 in (2.0 m) long, 38.0 in (1.0 m) wide, and 30.5 in (0.8 m) tall. The engine weighed 1,380 lb (626 kg).

Other Unitwin gear case and engine configurations were considered. One gear case design raised the propeller shaft above the engine. With a hollow propeller shaft, this would allow a machine gun or cannon to be mounted above the engine with its barrel projecting through the propeller shaft. This design was intended for military use, but it is unlikely that it was ever built. A different engine configuration arranged the cylinders horizontally, to create a “flat” engine for installation buried in an aircraft’s wing. This engine configuration was never built.

Two views of the Vega-built Altair serving as a testbed for the Manasco Unitwin.

While the Unitwin was undergoing stringent bench tests to prove its reliability, Vega (AiRover) assembled a Lockheed Altair 8G (registered NX18149) from spare parts to serve as a flying testbed for the engine. This aircraft was the AiRover/Vega Model 1 and was also called the Flying Test Stand. The Unitwin-powered Altair first flew on 29 June 1938. In this aircraft, the Unitwin was put through its paces. One engine section would run at a steady power while the other was throttled quickly between full open and closed. With one engine’s throttle sealed closed, the Altair took off and climbed to 12,000 ft (3,658 m) without aircraft stability or engine cooling issues. However, the Menasco Manufacturing Company’s financial issues continued to worsen, and Gross and Chappellet essentially took over the company. This resulted in Al Menasco leaving the company in June 1938.

Encouraged by the results of the Unitwin in the Altair, Vega built a five to six place aircraft around the engine. Vega president, Mac Short, oversaw the design of this aircraft, known as the Vega Model 2 Starliner (registered as NX21725). The Starliner was meant to appeal as a feeder airliner and an executive and personal transport aircraft. Its first flight, on 22 April 1939, ended in an emergency landing when the propeller slipped into fine pitch. The pilot (Vern Dorrell) and observer (J. B. Kendrick) were uninjured, and the Starliner escaped with only minor damage.

The two-piece windscreen and twin tails of the Vega Starliner gave it a similar appearance to contemporary Lockheed transports.

The aircraft was originally built with twin tails. While it was being repaired, a single, conventional tail replaced the twin fins, and the rear of the fuselage was modified. The Starliner was soon back in the air but was later damaged again when the nose gear would not extend for landing. The aircraft was repaired again, but Vega was becoming increasingly occupied with government contracts. In addition, Vega realized the Unitwin would never be able to produce 700 hp (522 kW) and that the Starliner was too small for feeder line service. One aircraft was ordered by Mid-Continent Airlines, but this order was later cancelled.

After amassing 85 hours of flight time, Starliner development was discontinued. The airframe was sold off as a Hollywood prop. Although the Unitwin engine operated without issues and performed as designed, it had no prospects beyond the Starliner. Menasco Manufacturing Company, like Vega, was receiving a flood of government contracts; therefore, Unitwin development was halted in 1940. There are no known surviving examples of this engine.

The Menasco Unitwin-powered Vega Starliner with a single tail. Note the updated paint job with “Starliner” on the engine cowl.

Sources:
– “Unitwin Power Plant” by Mac Short, Aero Digest (February 1939)
– Aerosphere 1939 by Glenn Angle (1940)
– Race With the Wind by Birch Matthews (2001)
– Jane’s All the World’s Aircraft 1940 by C. G. Grey and Leonard Bridgman
– Multiple Motor Drive for Aircraft Propellers US patent 2,284,473 by Albert S. Menasco and Hall L. Hibbard (granted 26 May 1942)
– Multiple Motor Drive US patent 2,180,599 by Albert S. Menasco (granted 21 November 1939)
– Lockheed Aircraft since 1913 by Rene J. Francillon (1982/1987)
– The Menasco Story by Ralph J. Schmidt (1994)
– “Menasco Aircraft Engines and their Air Racing Heritage, Part 1” by Larry M. Rinek, Torque Meter Vol. 2 No. 4 (Fall 2003)
– Waldo: Pioneer Aviator by Waldo Dean Waterman with Jack Carpenter (1988)

By William Pearce

In mid-1942, Republic Aviation Corporation initiated a design study to lighten their P-47 Thunderbolt fighter for improved performance. The Thunderbolt had been steadily gaining weight as the design matured, while comparative enemy aircraft, like the Focke-Wulf FW 190A, were much lighter. Republic officially proposed a light-P-47 to the Army Air Force (AAF) on 22 November 1942. On 1 April 1943, the AAF gave Republic a letter of intent to purchase two light-weight P-47s, and the contract was officially approved on 18 June 1943. This new aircraft was designated the XP-47J.

An early image of the Republic XP-47J before the Superman nose art was applied. Note the cooling fan vanes around the spinner inside the cowling.

As with all P-47s, Alexander Kartveli was the main designer of the XP-47J, and he was assisted by Murray Burkow. The XP-47J was similar in appearance to a P-47B, but it was a completely new aircraft. The XP-47J had a close-fitting cowl installed around its Pratt & Whitney (P&W) R-2800 engine of increased power output. A large spinner was added, along with a fan to aid engine cooling. The turbosupercharger’s intake had been refined, and the flow of its exhaust was directed to provide additional thrust. Two of the .50-cal machine guns were deleted (leaving six) in the XP-47J’s lightened wing, and the rounds per gun were reduced to 267. Other weight-saving measures were the omission of radio equipment and the aft fuel tank. To keep the aerodynamics clean, the XP-47J had no provisions for external stores under the wings or fuselage.

Originally, the AAF wanted the XP-47J to have contra-rotating propellers and a bubble canopy, like a late P-47D. To expedite the XP-47J, the decision was made for the first prototype to be a razorback version and the second prototype would include a bubble canopy. The six-blade contra-rotating propeller was test-flown on a P-47B but showed no increase in performance. This, combined with delays at P&W on the R-2800-61 engine with a contra-rotating gear reduction, resulted in the substitution of a R-2800-57 engine with a standard gear reduction. Both the -57 and -61 engines were rated at 2,100 hp (1,566 kW) and had a War Emergency Power (WEP) rating of 2,800 hp (2,088 kW). The -61 engine would be supplied later, when it was available. The engine, cowling, and cooling fan were installed and test flown on a P-47D-15.

This side view of the XP-47J reveals its distinct intercooler air and exhaust exits under the rear fuselage.

The XP-47J had a wingspan of 40 ft 10 in (12.4 m) and a length of 33 ft 3 in (10.1 m). The aircraft had an empty weight of 9,663 lb (4,383 kg) and a design gross weight of 12,400 lb (5,625kg). Its service ceiling was 45,000 feet (13,716 m). The XP-47J had a range of 765 mi (1,231 km) at a cruising speed of 400 mph (644 km/h) and a range of 1,070 mi (1,722 km) at optimum cruising speed. Fuel capacity was 287 US gal (1,086 L).

On 31 July 1943, Republic issued a report comparing the estimated performance of the XP-47J with the P&W R-4360-powered XP-72 that was under development. The report concluded that the Republic XP-72 had more potential and recommended resources be focused on that aircraft. In addition, 70% of the P-47 production line needed to be re-tooled in order to produce the P-47J. Republic called for the cancellation of the second XP-47J prototype but encouraged the completion and testing of the first prototype. The AAF approved Republic’s plan and cancelled the second XP-47J.

The sole XP-47J prototype (serial number 43-46952) was completed in mid-November 1943 and made its first flight on the 26th of that month. The aircraft was quickly dubbed Superbolt and wore nose art on both sides of the cowling of Superman holding a lightning bolt. After about 10 hours of flying time, the R-2800-57 engine was making metal and was replaced by another engine of the same type in February 1944.

The factory-fresh Superbolt is being run-up outside of Republic’s plant in Farmingdale, New York. Unlike a standard P-47, the intake under the engine was separate and pushed back from the front of the cowling.

A short time later, P&W informed the AAF and Republic that the R-2800-57 engine was not compatible with the 2,800 hp (2,088 kW) WEP rating. A water-injected R-2800-14W was substituted in the XP-47J in April. Water injection is a system that sprays anti-detonation fluid (a mixture of alcohol and water) into the induction system to lower the charge temperature and prevent detonation in the cylinders. This allowed the -14W to produce 2,800 hp (2,088 kW). In March, P&W informed Republic that it was still having difficulty with the R-2800-61’s contra-rotating gear reduction and did not know when the engine would be available. This effectively put an end to the possibility that the XP-47J would have contra-rotating propellers anytime in the near future.

On 11 July 1944 and equipped with a General Electric CH-3 turbosupercharger, the XP-47J achieved 493 mph (793 km/h) at 33,350 feet (10,165 m). Although the engine was producing 2,800 hp (2,088 kW), Republic believed the aircraft had more potential. At its own expense, Republic installed a CH-5 turbosupercharger and a larger 13 ft (3.96 m) Curtiss propeller. The propeller was an experimental unit with 2 in (51 mm) added to its trailing edge to increase its width. With the changes, the engine producing 2,730 hp (2,036 kW), and 400 lb (1.78 kN) of jet thrust from the exhaust, Mike Ritchie flew the XP-47J over a calibrated course at 34,450 (10,500 m) feet on 4 August 1944* and achieved 505 mph (813 km/h). This is the highest speed recorded in level flight by any propeller-driven aircraft during World War II.

The Republic XP-47J, now with the Superman nose art. Some differences from a standard P-47 are the additional plexiglass panel behind the pilot and the lack of intercooler exhaust doors on the sides of the rear fuselage.

The XP-47J was handed over to AAF and arrived at Wright Field, Ohio on 9 December 1944. During flight tests, the AAF was unable to get maximum power from the R-2800 engine. The AAF recorded a speed of only 484 mph (779 km/h) at 25,350 feet (7,727 m) and with the engine producing 2,770 hp (2,066 kW). Near the end of flight testing, the exhaust manifold system had a serious failure while the aircraft was at 36,000 ft (10,943 m). The cause of the failure was the increase in pressure and temperature from the CH-5 turbosupercharger acting upon the unstrengthened exhaust system. The Official Performance Summary report states the XP-47J had a max speed of 507 mph (816 km/h) and a 4,900 fpm (24.9 m/s) initial rate of climb. Republic’s Test Report No. 51 (27 January 1945) lists the max speed as 502 mph (808 km/h).

While the XP-47J Superbolt had remarkable performance, it was overshadowed by other aircraft, like the XP-72, that were under development. Work on the XP-72, which first flew on 2 February 1944, was not far behind that of the XP-47J, but as the war progressed and with the advent of jet fighters, neither aircraft were needed.

*Mike Ritchie’s report recording the 505 mph (813 km/h) speed was dated 5 August 1944, but it is believed the flight actually occurred on 4 August 1944.