Old Airliners, Douglas DC-6

While the DC-3’s and DC-4’s civilian careers were interrupted by WW2, the opposite applies to the DC-6, which started off in response to a military airlift requirement, and went on to become Douglas’ most successful four engined piston airliner. During the latter stages of WW2 Douglas began work on a developed DC-4 for postwar commercial use. However the improved DC-4 (which would feature a 2.11m/6ft 11in fuselage stretch and P&W R-2800 Double Wasp engines) soon attracted the attention of the US Army Air Force, which devised a requirement which the new transport was developed against. A prototype was built, designated XC-112, but it did not fly until February 15 1946, by which time the war was over and the military requirement no longer stood.

Instead Douglas continued development of the type as a long range airliner, resulting in the DC-6. The XC-112 served as the prototype for the DC-6 program. US airlines had already shown strong interest in the new transport, with launch orders for the DC-6 placed in September 1944. The first production DC-6 first flew in June 1946 and service entry, with United Air Lines, occurred on April 27 1947. However early service was not smooth with the fleet grounded for four months from November that year after two internal fuselage fires in flight, one being fatal, caused by fuel venting entering the cabin heater ram air intake.

The availability of the more powerful R-2800 engines with water/methanol injection prompted Douglas to develop the further stretched DC-6A Liftmaster freighter (first flight September 29 1949) and the equivalent passenger DC-6B (first flight February 2 1951). The DC-6C, the last DC-6 model to be developed, was a convertible passenger/freight version of the DC-6A. Meanwhile renewed military interest in the DC-6 was sparked by the Korean War, with the result that large numbers of USAF C-118s and USN R6D-1s were built. Many of these were later sold to civilian operators.

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Old Airliners, Douglas DC-4

The history of the DC-4 dates back to when United Airlines devised a requirement for a four engine long range airliner.

United looked to Douglas to fulfil the requirement, who devised the highly ambitious DC-4E (where the E stood for experimental). This four engined behemoth was flight tested in 1939. It was roughly three times the size of the DC-3 (its wingspan was 42.17m/138ft 3in, and length 29.76m/97ft 7in), had triple tail surfaces, tricycle undercarriage, was pressurised and potentially could fly nonstop from Chicago to San Francisco.

However all the ground breaking new technology on the DC-4E meant that it was costly, complex and had higher than anticipated operating costs, so Douglas thoroughly revised the design, resulting in the smaller and simpler definitive DC-4.

The new DC-4 was developed under the darkening clouds of WW2, and upon the USA’s entry into war all DC-4s then on the production line were requisitioned for the US military. The result was that the first DC-4 flew for the first time on February 14 1942 in military markings (as the C-54 Skymaster). The DC-4 was found to admirably suit the USAAF’s requirement for a long range cargo transport, and 1162 were built through the war years.

As was the case with the DC-3, the end of war meant that much of that number were surplus and sold to the world’s airlines. Further to this Douglas built an additional 78 DC-4s to new orders. Over the years the survivors have been passed down to charter and freight airlines, and today small numbers survive in service as freighters.

Notable developments of the DC-4 include Aviation Trader’s much modified Carvair freighter (described separately) while Canadair built a number with RollsRoyce Merlin engines and pressurised fuselages. The DC-4 also formed the basis for the larger DC-6 and DC-7 which are described separately (the DC-4 was the first airliner to introduce a circular section, constant diameter fuselage which made stretching the basic aircraft relatively simple).

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