The early DC-7s were only purchased by U.S. carriers. European carriers could not take advantage of the small range increase in the early DC-7, so Douglas released an extended-range variant, the DC-7C (Seven Seas) in 1956. Two 5 ft (1.5 m) wing-root inserts added fuel capacity, reduced interference drag, and made the cabin quieter by moving the engines further outboard; all DC-7Cs had the nacelle fuel tanks previously seen on Pan American's and South African's DC-7Bs. The fuselage, which had been extended over the DC-6B's with a 40 in (100 cm) plug behind the wing for the DC-7 and −7B, was lengthened with a similar plug ahead of the wing to give the DC-7C a total length of 112 ft 3 in (34.21 m).
Since the late 1940s Pan Am and other airlines had scheduled some nonstop flights from New York to Europe, but westward nonstops against the wind were rarely possible with an economic payload. The 1049G and DC-7B that appeared in 1955 could make the trip if the headwinds weren't bad, but in summer 1956 Pan Am's DC-7C finally started making the westward trip fairly reliably. BOAC was forced to respond by purchasing DC-7Cs rather than wait on the delivery of the Bristol Britannia. The DC-7C found its way into several other overseas airlines' fleets, including SAS, which used them for cross-polar service to North America and Asia. The DC-7C sold better than its rival, the Lockheed L-1649A Starliner, which entered service a year later, but sales were cut short by the arrival of Boeing 707 and DC-8 jet aircraft in 1958–60.
Starting in 1959, Douglas began converting DC-7 and DC-7C aircraft into DC-7F freighters. The airframes were fitted with large forward and rear freight doors and had some cabin windows deleted. This modification extended the life of the aircraft past its viability as a passenger transport.
The predecessor DC-6, especially the DC-6B, had established, for its time, a reputation for straightforward engineering and reliability. Pratt & Whitney, manufacturer of the DC-6's Double Wasp engines, did not offer an effective larger engine apart from the Wasp Major, which had a reputation of poor reliability. Therefore Douglas turned to Wright Aeronautical for a more powerful engine. The Duplex-Cyclone had reliability issues of its own, and this affected the DC-7's service record and usage. Carriers which had both DC-6s and DC-7s in their fleets usually replaced the newer DC-7s first once jets started to arrive. Some airlines had to scrap their DC-7s after little more than five years of service, whereas the vast majority of DC-6s lasted longer and then sold more readily on the secondhand market.
Basic price of a new DC-7 was around £570,000.
Price of a DC-7B was around £680,000 in 1955, rising to £820,000 in 1957.
Similarly, the price of a DC-7C was £800,000 in 1956, increasing to £930,000 by 1958.
Cost of the DC-7F "Speedfreighter" conversion was around £115,000 per-aircraft.
Read more about this topic: Douglas DC-7
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