Alcor Duo-6 Lockheed

Alcor Duo-4, Duo-6, and C-6-1 Transports

By William Pearce

In 1929, the Lockheed Aircraft Corporation was bought by the Detroit Aircraft Corporation. Lockheed’s founder, Allan H. Loughead (phonetically pronounced Lockheed) was unhappy with the acquisition and had voted against it. Allan left and formed a new company in 1930 with his brother Malcolm. The pair had worked together in aviation before pursuing separate interests in the 1920s. The new company was known as the Lockheed Brothers Aircraft Corporation.

Alcor Duo-4 front

The Duo-4 with “Olympic” written on the nose. Note the cooling slits for the Menasco Pirate engines.

Their first aircraft was the Olympic Duo-4, and its fuselage was similar to the Lockheed Vega 5. In place of the Vega’s single radial engine were two Menasco C4 Pirate engines. These in-line, four-cylinder engines were air-cooled and produced 125 hp (92 kW). The engines were positioned in the nose of the Duo-4 so that the tips of the propellers cleared each other by about 3 in (76 mm). The engines were laid on their sides so that their heads were close together and the crankshafts were farthest apart and canted out at a slight angle. The Duo-4’s engine arrangement had less air resistance than a normal twin-engine plane. In addition, when one engine was shut down, the Duo-4 behaved much like a single-engine aircraft.

The four to six passenger Duo-4 was a high-wing cantilever monoplane. The monocoque fuselage had a wooden structure and was covered with a plywood skin that was molded under pressure. The wings also had a wooden structure and were covered with plywood. The aircraft (registered as NX962Y) was first flown by Frank Clarke in 1930. In March 1931, the Duo-4 was damaged when a sudden gust of wind caused it to nose-over and then collide with a vehicle during a landing at Muroc (now Edwards Air Force Base), California. Unfortunately, this incident caused investors to back away from the Lockheed Brothers Aircraft Corporation, and funds were not available to quickly repair the Duo-4.

Alcor Duo-4 Pancho Barnes

The Olympic Duo-4 at Muroc Dry Lake with Florence “Pancho” Barnes. Note that “Olympic” no longer appears on the nose and the propeller tip clearance.

Over the next few years, the Duo-4 was slowly repaired and modified. The four-cylinder Pirate engines were replaced by six-cylinder Menasco B6S Buccaneer engines. The supercharged, 230 hp (171 kW) Buccaneers were in-line, air-cooled engines and turned 7 ft 6 in metal propellers. After the modifications, the aircraft was renamed the Duo-6 (some sources refer to it as the Loughead Alcor). It flew again in early 1934.

Allan Loughead officially changed his name to Allan Lockheed in February 1934. Also in 1934, the Lockheed Brothers Aircraft Corporation went out of business, but Allan continued with the Duo-6. In May 1934, one propeller was removed to demonstrate the Duo-6’s single engine performance. At Mines Field (now Los Angeles International Airport), the Duo-6 took off in 1,200 ft (366 m) and attained 130 mph (209 km/h) on just one engine. Reportedly, with one engine shut down, the aircraft handled with little yaw, much like a single-engine plane. In May, Allan flew the Duo-6 back east to demonstrate it to the Navy and Army. However, nothing came from this exposure.

Alcor Duo-6 Lockheed

The Duo-6 on its trip back east with Allan Lockheed in front. Note that “Alcor” is written on the tail and the changes to the engine cowling from the Duo-4 image above.

In October 1934, the United States placed operating restrictions on single-engine transports carrying passengers. This regulation marked a permanent shift to multi-engine transports for passenger service. Presumably, the twin-engine Duo would have done well under the new regulations with its ability to perform like a conventional single-engine aircraft in the event of one engine being shut down. Unfortunately, the Duo-6 crashed in late 1935 and was not repaired.

The Duo-4 and Duo-6 had a 42 ft (12.80 m) wingspan and were 28 ft 6 in (8.69 m) in length. The Duo-4 had an empty weight of 2,265 lb (1,027 kg). The aircraft had a max speed of 140 mph (225 km/h) and a landing speed of 47 mph (76 km/h). The Duo-6 had an empty weight of 2,885 lb (1,309 kg) and a gross weight of 5,090 lb (2,309 kg). The aircraft had a max speed of 183 mph (295 km/h), a cruise speed of 157 mph (253 km/h), and a landing speed of 57 mph (92 km/h). The service ceiling was 18,500 ft (5,639 m) and its range was 660 mi (1,062 km). The single engine performance of the Duo-6 was a max speed of 125 mph (201 km/h), a cruise speed of 100 mph (161 km/h), and a ceiling of 6,400 ft (1,951 m).

Alcor C-6-1 top

This unique top view of the C-6-1 doing an engine run shows how the engine nacelles were blended into the nose and wings.

In February 1937, Allan started a new aviation company: the Alcor Aircraft Corporation. The “Alcor” came from Allan Lockheed Corporation. Alcor’s first official aircraft (the Duo-6 had been built before the company was formed) was the C-6-1 Junior Transport. It was designed to carry six to eight passengers. The C-6-1 used the engine installation of the Duo but with improved C6S-4 Super Buccaneer engines that produced 275 hp (205 kW) at 2,400 rpm for takeoff. Each engine was canted out 4 degrees and the propellers cleared each other by 12 in (0.3 m).

The aircraft had a low-wing, and the main gear retracted back into the wing with the wheels turning 90 degrees to lay flat. The wings and fuselage had a structure made mostly of wood. However, there were some components in high-stress areas that were made of metal. The fuselage had a circular section and was made up of laminated spruce framework with a two-piece plywood skin that was molded under pressure. The engines were closely cowled and faired into the nose and wing. The C-6-1 was a streamlined aircraft that was very efficient and had excellent flight characteristics.

Alcor C-6-1 side

Side view of the Alcor C-6-1 Junior Transport complete with spinners.

The Junior Transport had a wingspan of 49 ft (14.94 m) and a length of 31 ft 8 in (9.65 m). The aircraft had an empty weight of 4,141 lb (1,878 kg) and a gross weight of 6,200 lb (2,812 kg). The aircraft had a max speed of 211 mph (340 km/h) at 5,500 ft (1,676 m) and a cruise speed of 190 mph (306 km/h) at 5,500 ft (1,676 m) and 200 mph (322 km/h) at 10,000 ft (3,048 m). The service ceiling was 24,000 ft (7,315 m) and its range was 835 mi (1,344 km). On one engine, the C-6-1 had a top speed of 147 mph (237 km/h), could cruise at 129 mph (208 km/h), and had a ceiling of 12,600 ft (3,840 m).

The C-6-1 (registered as NX15544) was first flown on 6 March 1938. On a test flight over San Francisco Bay on 27 June 1938, the C-6-1 went out of control during a high-speed dive. The dive test was instigated by the pilot and not part of the flight schedule. Unable to regain control, the pilot and observer bailed out, leaving the sleek C-6-1 to crash into the bay. The aircraft was insured, but the funds were only sufficient to pay off Alcor’s debts. With no capitol, Allan closed out Alcor. Allan continued to be involved in aviation for the rest of his life, but he did not build any further aircraft of his own design.

Even though the Duo-4 and Duo-6 were built under Lockheed Brothers Aircraft Corporation name, they are often referred to as the Alcor Duo-4 and Alcor Duo-6. In addition, the Alcor C-6-1 is often incorrectly referred to as the Lockheed Alcor.

Alcor C-6-1 flight

Alcor C-6-1 on a fight over San Francisco Bay. The San Francisco Bay Toll-Bridge (now San Mateo–Hayward Bridge) can be seen in the background. Note the absence of spinners on the otherwise sleek aircraft.

Jane’s All the World’s Aircraft 1932 by C.G. Grey
Jane’s All the World’s Aircraft 1934 by C.G. Grey
Jane’s All the World’s Aircraft 1938 by C.G. Grey and Leonard Bridgman
Lockheed Aircraft since 1913 by Rene J. Francillon (1982/1987)
– “Commercial Aviation: An American Feeder-Line Machine,” Flight 6 July 1934
– “A ‘Flat’ Engined Transport,” Flight 12 May 1938
Brief Allan Lockheed 1910-1942 Autobiography

2 thoughts on “Alcor Duo-4, Duo-6, and C-6-1 Transports

  1. Allan Lockheed Jr

    Hi Bill,Great to run across your footprints and fine work here, after Reno. The work you have done with the Olympic Duo cum Alcor is excellent, and also great to see Pancho Barnes with the Duo. Turns out Dad retained her to fly tests at Muroc of the Vega 5c, last and most upgraded version.

    I had understood that the Alcor engines (there is a drawing I have somewhere) were canted out 6 deg’s each, included angle = 12 deg’s. That looks to be more extreme than reality. I like the 3.5 deg’s/each better – don’t know…

    I was told the Olympic had a steel tube inner fuse structure for the faint of heart who were beginning to prefer metal to wood. The low-wing Alcor was classic Loughead = all-wood monocoque with strength to weight ratio = mild steel verified by Richard Von Hake.

    Tony Stadlman told me how Dad tricked the shell parts from the Olympic to comply with a round and larger fuse. Tony told him you could not use the skins from 3 smaller oranges to fit over a bigger orange. Dad told him you could within limits, did it, and Tony agreed it worked perfectly. The circular fuse was planned for mild pressurization. Hell, the perfect bonding of the Lockheed patent process with spruce laminae would have handled it easily. There are also drawings of the planned Alcor C6-2, which would have packaged 2 of the proposed Menasco V-12’s in place of the I6’s. The C6-1 exceeded 400 MPH in the power dive from which it recovered and flew graceful circles over Oakland Bay–without its witless pilots! So, the uprated performance with V-12’s would have been incredible.

    Again, bless you for the GREAT job here. Hope you may keep it up and reap deserved recognition and compensation!

    Allan H. Lockheed Jr.

    1. William Pearce Post author

      Hello Allan,

      Great to hear from you! I’m very glad you found the article and that it earned your stamp of approval. And thank you very much for the great additional information. I did not know the twin recovered by itself and continued to fly; what a shame there was no one onboard to land it.

      Best regards,



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