Before the start of the Second World War, Pan American Airways was the world's biggest operator of large ocean-going flying boats with the Boeing 314, Martin M-130 and Sikorsky S-42 in the fleet that spanned Pan American's worldwide network. However, the airline recognized that the pace of development in aviation technology meant that landplanes would be the dominant airline aircraft of the future. Pan Am worked with Boeing to bring the Boeing 307 Stratoliner to fruition (the world's first pressurized airliner). But with the start of the war for the United States in 1941, Pan American's operations were shifted to support the war effort and in this capacity the airline solicited the US Navy for the construction of a true heavy-lift landplane transport. This was finalized with the US Navy, Lockheed, and Pan American in November 1942 with what became the Lockheed Model 49 Constitution.
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The R6V Constitution on final approach at Moffett Field in California
(Wikipedia/US Navy) |
The requirements issued by the Navy as suggested by Pan American were for a range of 5,000 miles, 17,500 lb payload at 255 mph at 25,000 feet cruising altitude. A year later on 1 November 1943 the contract was formally issued to Lockheed. Pan American's engineers led by their head engineer, Andre Priester, worked alongside Lockheed's engineers and their head, Willis Hawkins (he also designed the Constellation and later on would work on the F-80 Shooting Star, F-104 Starfighter, and the C-130 Hercules). The fuselage of the Constitution was a double deck, double lobed cross-section design with the large wing passing through the mid-fuselage between decks. With a fully-pressurized double-deck, the Constitution could carry up to 204 military passengers but the normal complement would be 168 passengers. Pan American's plans were for 51 passengers on the lower deck and 58 passengers on the top deck. Cargo doors were installed on the lower deck and the wings were deep enough to allow mechanics to access the four radial engines in flight for maintenance. The Constitution was also the first large transport aircraft to have multi wheel main landing gear bogies (four wheels to each main landing gear).
The wing itself was based on the layout and structure of the wings used on the Constellation and the P-38 Lightning. Four 3,000 horsepower Pratt & Whitney R-4360 28-cylinder Wasp Major engines drove four bladed props. Unusually, the upper surface trailing edge root of the wings could hold RATO units to shorten takeoff runs. There were three units in each wing- they were fired when the landing gear retraction sequence started. As the landing gears took 14 seconds to retract, the RATO units burned for 15 seconds.
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Takeoff using the overwing integral RATO units
(San Diego Air & Space Museum) |
Since the Constitution was a low-priority project during the war, it wasn't until well after the war ended in August 1945 that the aircraft was completed. Though standard for today's design work on modern airliners, Lockheed used a full-scale hydraulic and electrical systems test rig that today would be known as an "iron bird". The system was loaded so the hydraulics and flight control systems would "experience" loads similar to what would be found inflight and were invaluable in letting the Constitution's test pilots get familiar with the large aircraft. The first flight came on 9 November 1946 and after the first 44 flight hours of testing the Constitution was found to be significantly underpowered. More powerful versions of the R-4360 Wasp Major were installed that theoretically produced 3,500 horsepower, but in practice even these engines could only garner 2,900 to 3,300 horsepower and that was with water injection and bypassing the superchargers on takeoff. As a result, use of the integral RATO units was commonplace.
To keep Pan American interested in the project, Lockheed proposed the civilian version of the Constitution be powered by Wright 5,500 horsepower Typhoon turboprop under development, but by this point Pan American had thrown its lot with the Boeing Stratocruiser and bowed out of the Constitution program. Designated XR6O-1 by the US Navy, the first Constitution underwent a full year of flight testing at NAS Patuxent River, Maryland. The second XR6O-1 made its first flight on 9 June 1948 and unlike the first aircraft, the upper deck was fitted out for VIP passenger service with 92 seats while the lower deck was fitted out to carry as much as 40,000 lbs of cargo. Dual spiral staircases at each end of the cabin provided access to the upper deck from the lower deck and passenger entry was via the nose gear well which was large enough to allow airstairs to be pulled up just in front of the nose gear.
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The Constitution on static display during an open house at San Francisco Airport
(San Francisco International Airport/FlySFO.com) |
In February 1949 the second R6O (the X prefix was dropped) was commissioned into service at NAS Moffett Field, California, with the fleet logistics support squadron VR-44. Soon joined by the first R6O, the Navy embarked on a series of publicity flights across the country, using the Constitution to not only transport personnel and materiel, but also to stimulate interest in naval aviation. The R6O carried one and a half times more cargo than the next biggest Navy transport, the Douglas R5D (C-54 Skymaster/DC-4). In 1950, the two R6Os were redesigned R6V; in the Navy's aircraft designation system used prior to 1962, "V" stood for the Vega Division of Lockheed that had built the PV-1 Ventura and PV-2 Harpoon in the Second World War- the "O" of Lockheed was dropped as it could be confused with the number zero and "V" took it's place as the Lockheed designator code. They were reassigned to VR-5 for expanded operational duties that included flights to Hawaii and Alaska. With a total of 3,760 flight hours between the two aircraft, in 1953 the R6O Constitutions were retired and placed in storage at NAF Lichtfield Park, Arizona. The aircraft were offered to the airlines on a proposed five-year lease, but no interest came about.
The first Constitution ended up in Las Vegas as a promotional billboard for Alamo Airways at McCarran Airport and plans were floated to move the aircraft to the Strip to be part of a casino. However, the plans were never materialized and when Howard Hughes acquired the property that the aircraft sat on, he also gained ownership of the aircraft and had it scrapped in 1970. The second Constitution ended up in Opa Locka, Florida, where it was to be sold to a German businessman who wanted to use it for a restaurant in Barcelona, Spain. The deal fell through and the aircraft mysteriously caught fire which gutted the interior but spared the exterior. After several years of legal wrangling, the aircraft was scrapped in 1979.
Further reading:
The Convair Model 6: A Jumbo Jet Before Its Time
Pan American and the Boeing 314 Toilet Scandal
The Cadillac of the Constellation Line
by Steve Ginter. Ginter Books, 2009.