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The Douglas DC-4, – 6, and – 7 Piston Airliners

The Douglas DC-4, – 6, and – 7 Piston Airliners

1. Introduction

The Douglas DC-4, DC-6, and DC-7 represent the very development of the quad-engine piston airliner during the 1940s and 1950s.

2. DC-4

Ever increasing acceptance of air transportation sparked by the reduced travel times it offered over existing rail means, along with improvements in safety and reliability, sparked the need for larger-capacity, longer-range aircraft during the end of the 1930s, particularly to fulfill the still-elusive goal of providing nonstop transcontinental service.

Based, therefore, upon the foundation laid by its all-metal, twin-engine DC-3 monoplane, Douglas Aircraft Company commenced design studies of a larger successor after a consortium of five US carriers, inclusive of American, Eastern, Pan Am, TWA, and United, each contributed $100,000 toward the development of a prototype that would incorporate all current advancements, yet at the same time offer double the number of powerplants, rest on a tricycle undercarriage, and accommodate a greater number of passengers.

The conventionally configured result, sporting a low, straight wing, a triple vertical tail, four 1,400-hp Pratt and Whitney Twin Hornet engines, and seating for up to 52 in a pressurized cabin, first took to the sky on June 21, 1938. Designated DC-4E, it offered a 190-mph cruising speed and a 2,000-mile range.

Although the then-mammoth, 65,000-pound airliner incorporated considerable advancement, including a well-appointed galley and smoking and dressing rooms, it represented too much of a leap for the era, and the sponsoring carriers requested both modifications and a reduction in size.

Equally anticipating lower passenger demand because of the pending war, Douglas redesigned it with a shorter, unpressurized fuselage accommodating 42, a shorter span, tapered wing, and a more conventional single vertical fin, resulting in the definitive DC-4.

World War II, in the event, provided a far greater impact on the program than anticipated. Indeed, after the infamous, December 7, 1941 Japanese attack on Pearl Harbor, the 24 aircraft that had been intermittently ordered by American and United were requisitioned by the US Army Air Corps for use as transports and the first of these, draped in military livery, first flew on February 14 of the following year, now designated the C-54 Skymaster. Those operated by the Navy were identified as R5Ds.

The war, at least in terms of production, had a positive influence on the type, with 1,163 military versions ultimately built by 1945, while 79 additional, purely commercial counterparts were constructed after this time. The first, operated by American Overseas Airlines, inaugurated the first post-war transatlantic service with it from New York to Hurn, England, via Gander, Newfoundland, and Shannon, Ireland, in October.

Numerous ex-military C-54s were also converted to commercial standard and the type formed the backbone of many airlines’ post-war fleets.

Powered by four 1,450-hp Pratt and Whitney R-2000-2SD-13G Twin Wasp radial engines, the DC-4 featured a 93.5-foot overall length, a 117.6-foot wingspan, and a 1,462-square-foot area. Carrying a 14,200-pound maximum payload, the aircraft, with a 73,000-pound gross weight, could cruise at 207 mph and fly 1,150- to 2,180-mile nonstop sectors, depending upon its payload-to-fuel ratio.

Both profitable and flexible, it could accommodate up to 86 single-class passengers, making it particularly attractive to charter or nonscheduled carriers, and could alternatively carry main deck cargo after the installation of an upward-opening door.

3. DC-6

In order to cater to anticipated post-war demand and remain competitive with Lockheed’s own comparable, quad-engine L-649/-749 Constellation airliners, Douglas stretched the DC-4 by 81 inches, in the process confirming that its configuration would serve as the standard for long-range transports. Although it retained its wing, planform, span, area, and hydraulically actuated, double-slotted flaps, it re-introduced the pressurization briefly offered by the original DC-4E and increased its passenger capacity to match it.

Attempting to compete with TWA’s Constellations, both American and United respectively placed launch orders for 50 and 35 of what would become the DC-6 in late-1944.

Originally intended for the US Army Air Force, the type, initially designated XC-112, first flew in prototype form powered by four 2,100-hp Pratt and Whitney R-2800-34 engines on February 15. 1946, but, with resolution of World War II, they no longer needed a military version, leaving the first commercial variant to take to the sky four months later.

Although outwardly similar to the DC-4 upon which it was based, the DC-6 offered several design variations. Powered itself by 2,100-hp, 18-cylinder, two-row, air-cooled Pratt and Whitney R-2800-CA15 Double Wasp engines featuring three-bladed, electrically-deiced propellers, for instance, it introduced rectangular passenger windows and a squared-off tail. Its wing, empennage, and windshield were provisioned with thermal anti-icing.

Internally, it featured an electronic autopilot system, an automatic pressure control system, air (temperature and humidity) conditioning, radiantly heated floors and walls, and the ability to accept pre-loaded cargo containers.

The 8.10-foot-wide by seven-foot-high cabin accommodated 46 20-inch-wide, Douglas designed, four-abreast seats installed at a 40-inch pitch, along with six in an aft lounge.

First flying on July 10, 1946 and delivered to launch customers American and United during a joint ceremony on November 27, it was inaugurated into service on April 27 of the following year, affording ten-hour eastbound and 11-hour westbound transcontinental schedules.

Cabin configurations varied according to carrier. American, for example, accommodated 50 four-abreast day passengers or 24 night sleeper ones in ceiling extendible upper berths with their own slender, upper fuselage windows and 42-by-76-inch lower ones created by the seats themselves.

Sabena, the type’s first foreign operator, offered mixtures of both.

Other DC-6 operators included Braniff, Delta, KLM, Linee Aerea Italiane, National, Panagra (Pan American Grace Airways), and SAS.

Of the aircraft’s several records, that on March 2, 1950, entailing a three-hour, eight-minute flight from Chicago to Miami operated by Delta at a 376-mph speed, was notable and indicative of the improving performance of the modern pistonliner.

Analysis of the basic DC-6 design indicated that it was structurally capable of carrying additional payload when coupled with a modest, five-foot fuselage stretch to increase its center-of-gravity range and higher horsepower engines, resulting in two further versions.

The first, a dedicated freighter designated the DC-6A Liftmaster, featured port, 7.7-by-5.7- forward and 10.4-by-6.6-foot aft main deck, upward-opening cargo doors. First flying on September 29, 1949, it entered service with Slick Airways, which ultimately flew 13, on April 16, 1951.

Eighteen carriers placed 67 orders for the type.

The second, an all-passenger version dubbed the DC-6B, featured 54 four-abreast seats, along with a six-place aft lounge, although high-density arrangements permitted accommodation of up to 102.

First flying on February 2, 1951 and entering service with United Airlines two months later, on April 11, the type, powered by four 2,500-hp R-2800-CB17 Double Wasp engines, featured a 105.7-foot overall length and 117.6-foot wingspan. Carrying a 24,565-pound payload and offering a 107,000-pound all-up weight, it could cruise at 316 mph and serve 3,000- to 4,720-mile sectors.

Operated by American, Continental, National, Northeast, Northwest, Pan Am, Trans American, United, and Western in the US, and Alitalia, KLM, Linee Aerea Italiane, Olympic, Sabena, SAS, and Swissair in Europe, it enjoyed a 288-aircraft production run.

Offering the lowest seat-mile costs of any then-current piston airliner, it facilitated considerable interior and route flexibility, proving particularly attractive to all-tourist North Atlantic operators. Northeast, for example, operated its DC-6Bs along the East Coast, funneling passengers from its once traditional New England route system-encompassing Montreal, New York, and Washington-to Florida sunspots, while Northwest served the Pacific with its dozen aircraft. Western operated the type with an eight-seat forward, 38-seat main, and 14-seat aft cabin, in addition to the standard six-place lounge.

The type enabled SAS to inaugurate service on the Great Circle route, from Copenhagen to Los Angeles, over the North Pole in November of 1954, reducing travel time by ten hours over the traditional transatlantic journey via New York.

Replacing its DC-4s with DC-6Bs, Olympic served European and Eastern Mediterranean destinations with mixed, forward-day and aft-night sleeper configurations.

Ethiopian Airlines operated its DC-6Bs in a high-density arrangement, carrying 102 passengers between its Khartoum base in Africa and Athens and Frankfurt in Europe.

LAN-Chile connected the North and South American continents with its aircraft, linking Miami with Lima, Peru.

Australian National operated four aircraft to cities within its home country.

Often pioneering routes, SAS inaugurated the first scheduled, round-the-world service on May 29, 1953, requiring just over 83 hours for the global circumnavigation.

A succeeding, convertible DC-6C variant, although only garnering seven orders, incorporated both DC-6A and -6B features, facilitating main deck mixtures of passengers and cargo, ranging from 76 of the former, their baggage, and 2,400 pounds of general freight to an all-cargo load of almost 13 tons, by means of a moveable, four-position bulkhead. First delivered to Hunting Clan Airways on August 7, 1958, it was subsequently joined by existing aircraft converted to this standard either by Douglas or its approved modification center, Pacific Aeromotive.

Sabena produced two aircraft with swing tails to enable it to accommodate outsize cargo pieces as long as 60 feet.

Several military versions were ultimately also built. The first of 100 ordered C-118s, based upon the DC-6A, flew on July 15, 1962 and was operated by the Military Air Transport Command (MATS), which was later renamed the Military Airlift Command, while the Navy ordered 65 R6D-1s, which themselves were redesignated C-118Bs. A singular VC-118, utilizing a DC-6 production slot reserved for American Airlines, became President Truman’s aircraft named the “Independence” when it was delivered on July 1, 1947.

4. DC-7

Keeping pace with unrelenting passenger increases, yet remaining competitive with Lockheed’s own, multiply stretched Constellation that would soon appear in L-1049 and L-1649 Starliner models, Douglas examined a still-larger DC-6B derivative capable of offering nonstop westbound transcontinental service and potentially transoceanic ones, once again based upon impetus from American Airlines, which placed its initial contract in December of 1951 for what would become the ultimate evolution of the quad-engine pistonliner.

First flying two years later, on May 18, and naturally designated DC-7, it introduced a 42-inch forward-of-the-wing fuselage stretch to either accommodate an additional seat row or more cargo, and resulted in a 62-passenger interior subdivided into eight-forward, 38-main, and 16-aft seat cabins, respectively separated by lavatories and a galley/buffet. The latter, as with all previous models, was provisioned with an aft, port passenger door. Optionally higher densities entailed between 80 and 99. Sleeper or day and night mixed arrangements could also be chosen, with the passengers accommodated in four aft upper and lower berths.

Delta Air Lines, for example, carried eight passengers in a forward “Sky Room,” 42 in the main cabin, 14 in the rear section, and five in the tail on the DC-7s it operated between Chicago and Miami.

Although the type employed the same wing introduced by the DC-4 with its maximum, 50-degree, trailing edge flap travel, the previously reliable Pratt and Whitney R-2800 engines had reached their peak of development, thus requiring a new type to cater to the higher gross weights needed for increased range and payload. In this case, the 18-cylinder, 3,250-hp air-cooled Wright R-3350 Turbo Compound driving four-bladed, governor-controlled, hydraulically positioned Hamilton Standard propellers was selected. Because of the inner engines’ proximity to the fuselage, they were restricted to 13.5-foot diameters, and because of their increased size, they, along with the outer ones, were housed in redesigned nacelles. Although they offered a 20-percent increase in output in comparison to the Pratt and Whitney engines powering the DC-6, they offered comparable fuel consumption.

The hydraulically actuated, forward-retracting tricycle undercarriage, consisting of a single nose wheel and dual main wheels, was retained, although the main gear could be independently extended to serve as a speed brake at 300 mph IAS or 410 mph at 20,000 feet to augment rapid, but controlled descents.

CAA certified on November 12, 1953 after a 204-flgiht test program conducted over a six-month period, the DC-7 was inaugurated into service 17 days later with American Airlines, for the first time permitting bi-directional, east- and westbound transcontinental operations.

Three other carriers, including Delta, National, and United, placed orders, bringing total production to 105.

In order to avoid confusion with its existing DC-6A Liftmaster, Douglas used the “B,” or “DC-7B,” designation to denote the first variant of the baseline model, in this case a still higher gross weight and longer-range aircraft that was sparked by Pan Am’s late-1953 order for seven with intercontinental range.

Introducing optional, external, range-extending saddle fuel tanks installed in the upper portion of the engine nacelles, provision for propeller spinners, improved wing flaps, and gross weights as high as 126,000 pounds, the type first flew on April 21, 1955. It was certified the following month, on May 27, and entered service with Pan Am on the transatlantic, New York-London route one month after that, on June 13.

Only operated by a single foreign carrier-namely, South African Airways on intercontinental sectors, such as from Johannesburg to London-the DC-7B was primarily ordered by US operators, including American, Continental, Delta, Eastern, National, Pan Am itself, and Panagra, which served South American destinations with it. One hundred twelve were built.

Along with four standard DC-7s, National was able to satisfy passenger demand on its Florida routes with an additional four DC-7Bs.

Eastern, also serving Florida destinations and hitherto operating earlier Constellation versions, ordered 50 DC-7Bs, configuring the first 30 with 64 seats, but introducing high-density, all-coach, 93-passenger interiors with the last 20.

Delta equally operated its 11 aircraft with varying densities, from 69 to 90.

Although the DC-7B offered the required transatlantic range, as evidenced by Pan Am’s own operations, winds sometimes dictated the need for refueling stops in Gander, once again prompting Douglas to design a version with even greater capability, and this became the DC-7C, the definitive and ultimate expression of piston airliner development.

Aside from incorporating another fuselage stretch-in this case, of 42 inches-it was the first to employ a modification of the basic DC-4 wing. Entailing a ten-foot extension by means of five-foot plugs inserted between the inner engine nacelles and the fuselage, it featured a 127.6-foot span, a 1,637-square-foot area, an increased, 9.93 aspect ratio, and could accommodate 1,000 additional, range-translating pounds of fuel. Varying in capacity from 4,512 to 6,378 US gallons carried in both the greater volume wing and in the enlarged engine nacelles’ saddle tanks, it permitted 3,610- to 5,642-mile ranges to be flown. Because of the five-foot greater distance between the inner powerplants and the fuselage, the cabin, able to accommodate up to 105 passengers, was quieter, while the clearance also facilitated the use of 14-foot-diameter propellers.

Powered by four 3,400-hp Wright R-3350-EA1 or -EA4 piston radials, the DC-7C could carry a 21,500-pound payload and had a 143,000-pound gross weight. Cruise speed was 345 mph and service ceiling was as high as 21,700 feet.

Serving as a true intercontinental airliner, it was capable of full-payload, east- and westbound transatlantic operations against any wind condition.

According to Harry Gann in the Douglas DC-6 and DC-7 (Specialty Press, 1999, p.74), “In a superb marketing ploy, the DC-7C was named the ‘Seven Seas.’ The DC-7C was the ultimate development of the Douglas four-engine transports and it carried the bulk of the North Atlantic and polar traffic until the introduction of the Boeing 707 in 1959.”

First flying on December 20, 1955, it entered service with Pan Am the following year, on June 1. Although it became its largest operator, with 26 aircraft, and three other US carriers ordered the type, it saw extensive foreign airline service with the likes of Alitalia, BOAC, Mexicana de Aviacion, Japan Air Lines, KLM, Panair do Brasil, Sabena, SAS, and Swissair.

SAS achieved a record flight with the type when it flew it 6,005 miles on November 17, 1956, requiring 21 hours, 44 minutes for the trip.

5. Threshold to the Jet Age

Synonymous with stretched-fuselage aircraft, which resulted in the DC-1 to DC-3 and then the DC-4 to DC-7 series of piston airliners, Douglas next considered a powerplant type change instead in order to remain competitive with the emerging turboprop Vickers V.700 and.800 and Lockheed L-188 Electra aircraft, conducting a number of both DC-6 and DC-7 airframe studies that incorporated this technology. The most significant was the proposed DC-7D.

Powered by four Rolls Royce RB.109s, it would have introduced yet another, 40-inch fuselage stretch and a swept vertical tail.

Despite strong interest from long loyal American Airlines, Douglas could not prove the economic feasibility of the project, although it would most likely have been the first long-range turbine-powered airliner to have appeared, and elected instead to bypass this technology and immediately produce a pure-jet design, the DC-8, considered the DC-7’s next-generation successor.

Based upon the overwhelming passenger acceptance of, and therefore demand for, this vibrationless, speed-fostering powerplant, Douglas-along with Boeing and, to a lesser degree, de Havilland with their respective 707 and DH.106 Comet aircraft–provided the threshold to this next stage of airliner development, accelerating the replacement of both piston and turboprop aircraft on all but short-range routes.

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