The Convair XC-99 began life as an experimental transport aircraft and is perhaps one of the most unique designs of a larger plane by using the pusher formation. Designed in the late 1940s as a variant of the Convair B-36 Peacemaker bomber, the XC-99 held the record as the largest piston-driven propeller plane constructed in the United States.
Only one was produced and it served in a limited capacity towards the end of its life, but it also achieved remarkable flight records during its time in operation.
The origins of the XC-99 can be traced back to the earlier stages of the Second World War at a time when the Allied forces were facing heavy defeats inflicted by Axis powers in all global theatres of the conflict.
In 1941, the American government became deeply concerned that successive Nazi German and Japanese victories would cut off the US mainland from Allied countries and overseas US military bases in other continents.
There was also a fear that should the British forces lose the Battle of Britain raging at the time, and eventually succumb to a full-scale Nazi invasion, the United States would lose both an important ally and a strategic military platform in Europe to launch aerial bombing attacks on mainland Germany.
The United States Army Air Corps (USAAC) responded by sending out a proposal for an ultra-long range, intercontinental bomber and transport aircraft that could fly a range of up to 8,000 miles and carry a payload of up to 10,000 lbs of bombs or alternatively, 100,000 lbs worth of cargo or 400 troops.
The proposed design would mean that America could still perform missions over Germany and Europe, even if Allied countries were lost to invasions.
A contract to manufacture a prototype was awarded to the Consolidated Aircraft Corporation, which subsequently became Convair in 1943 following a merger with Vultee.
Much of the proposed XC-99 design was based on the existing B-36 Peacemaker bomber layout, which had also been designed by Convair. The B-36 was the largest strategic bomber to be built in the United States and Convair’s designers felt that it would provide a solid and existing platform on which to create a giant transport and bomber aircraft to meet the needs of the specification.
The design team decided that the XC-99 would keep the same wings, engines, and tail of the B-36 format, but these would be combined with a completely different fuselage design designed from scratch to allow for more cargo space.
Convair also proposed the XC-99 as a six-engine pusher design with rear-facing propellers mounted at the back of the wings which they felt would maximize the speed and efficiency of a bigger plane.
The new aircraft was provisionally named the XB-36 in 1942. The USAAC approved the idea and asked Consolidated to go ahead with producing a more detailed concept in 1943 for testing.
However, subsequent changes in the demands for the type of aircraft needed by the US military and combined Allied victories against Axis forces in Europe and the Pacific shifted priority away from building a larger intercontinental plane.
The threat of a Nazi invasion of the British mainland had waned and the Allies subsequently secured greater air superiority over Europe before launching a liberation campaign on the ground with D-Day.
In response, the US government limited Consolidated to building one example of an XC-99 which would be designated as an experimental aircraft.
Construction of the new plane was delayed and did not officially start until the end of the war.
Despite the setback, Convair pressed ahead with the design and construction process. A finished airframe of the XC-99 was completed in 1947 based on the initial XB-36 design. The result was the largest transport plane in the world in terms of the payload it could carry.
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The new prototype had a total wingspan of 230 feet and a length of 185 feet. The maximum cargo payload was 101,000 lbs, just above what the original specification called for. The XC-99 also had 28 wheels on its landing gear, which Convair designed in a way to better distribute the weight of the plane and make it easier to pilot during take-off and landing.
Most notably the XC-99 had a double-decker passenger space interior and an entirely pressurized cabin, something of a rare feature for aircraft of the era. The intention was to make it more comfortable for its passengers and to allow the plane to cruise at higher altitudes.
The pusher engine configuration was also kept and consisted of six piston-driven radial engines. The sheer size of the aircraft crowned it as the largest piston-driven propeller plane to be built in the United States.
The XC-99 completed its maiden flight in San Diego, California on the 23rd of November, 1947.
Following further testing and research, the XC-99 was presented to the newly formed United States Air Force (USAF) in 1949 for acceptance trials and proving. Unfortunately, the XC-99’s late development found it a competition with already established bombers and transport aircraft in service with the USAF, including the Douglas C-74 Globemaster, the Boeing C-97 Stratofreighter and the Douglas C-124 Globemaster II.
As such, there was no real demand from the government to mass produce an entirely new transport aircraft at great expense and no further working examples of the XC-99 were built.
The only example of the XC-99 was put into service and made its first operational flight in July 1950.
It was used as part of Operation Elephant in which it broke a record by transporting 101,266 lb of cargo, a majority of which included engine parts and propellers for the B-36, from San Diego to Kelly Air Force Base in Texas.
The XC-99 would go on to break its own record in August 1953 when it transported 104,000 lb worth of cargo from Kindley Air Force Base in Bermuda to Rhein-Main Air Force base in Germany via the Azores islands. The unusual design features of the XC-99 prompted many public spectators to turn out to watch it during the journey.
The sole XC-99 saw active combat service during the Korean War in which it transported goods for US and United Nations forces. It also continued to shuttle parts for the B-36 between Kelly Air Force Base and McClellan Air Force Base in California on a twice-a-week schedule.
The size and space of the XC-99 prompted some civilian commercial interest, with Pan American Airways discussing the possibility of ordering fifteen XC-99 units for transatlantic passenger and air freight service.
However, the XC-99 was also hampered by expensive operating and maintenance costs in its military service. While the size allowed for mass transportation, it also limited the plane to military and civilian airfields with facilities and runways large enough to accommodate it.
The incoming jet age also rendered the XC-99’s piston propeller design old-fashioned and unviable from a commercial standpoint. Proposed designs for air force jet transport aircraft such as the Lockheed C-141 Starlifter indicated the future direction of aircraft technology.
At the same time, many commercial airlines had started to explore incorporating proposed and faster jet airliner examples into their fleets for long-distance flights, with aircraft such as the Boeing 707 and the De Havilland Comet being tested and marketed towards civilian airlines as a means of transporting passengers to far flung destinations in a shorter time.
Ultimately, the civilian orders came to nothing and the single XC-99 remained in US Air Force service as a domestic transporter shuttling supplies between bases.
In 1957, the USAF announced the XC-99’s service would come to an end and the aircraft was grounded having flown a substantial 7,400 hours and carried around 60 million pounds worth of cargo.
After the XC-99 was retired, there was talk of transferring the aircraft from California to the National Museum of the United States Air Force in Dayton, Ohio, however, the USAF did not want to cover the expensive cost of transferring the plane.
The XC-99 initially remained on display at Kelly Air Force Base where it began to rust due to the weather conditions in the area. In 2004, the proposed closure of Kelly AFB and the state of the XC-99 prompted calls to preserve the aircraft and move it to a safer location.
In 2008, the XC-99 was disassembled by a team of engineers and moved to the National Museum of the United States Air Force for restoration and preparation for public display.
However, issues with rust were found to be worse than initially thought, with some of the wing components in need of a total replacement.
The XC-99 was moved to await reconstructive work in a hanger belonging to the 309th Aerospace Maintenance and Regeneration Group in Tucson, Arizona but the difficulty was found in sourcing parts that could replace those that had completely rusted.
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In 2012, it was returned to Kelly AFB to await further restoration although the cost of the project has cast doubt on whether it can be completed.
Although the timing of the of the Convair XC-99’s production may not have worked in its favour, the prototype plane has remained fascinating piece of aviation history that is often overlooked. Despite its commercial failure, the XC-99 played an important role in the development of aviation technology and set several world records thanks to its impressive design features and space that was unmatched by other aircraft.
While it may not have achieved commercial success, the Convair XC-99 will always be remembered as an impressive and awe-inspiring example of human engineering and achievement.
- Crew: 5 (with a relief crew of 5)
- Capacity: up to 400 troops / 100,000 lb (45,000 kg) cargo
- Length: 182 ft 6 in (55.63 m)
- Wingspan: 230 ft 0 in (70.10 m)
- Height: 57 ft 6 in (17.53 m)
- Empty weight: 135,232 lb (61,340 kg)
- Max takeoff weight: 320,000 lb (145,150 kg)
- Powerplant: 6 × Pratt & Whitney R-4360-41 Wasp Major 28-cylinder air-cooled radial piston engine, 3,500 hp (2,600 kW) each
- Maximum speed: 307 mph (494 km/h, 267 kn)
- Range: 8,100 mi (13,000 km, 7,000 nmi) with 19,112 US gal (15,914 imp gal; 72,350 l) and 10,000 lb (4,500 kg) payload