During the early 1970s, the United States Air Force (USAF) actively sought a replacement for its excellent and highly-capable, yet-aging, Lockheed C-130 "Hercules" high-winged transports. These aircraft were introduced in the mid-1950s and went on to see extensive adoption and service across the globe. The USAF developed the "Advanced Medium STOL Transport" (AMST) program around the new requirement seeking an all-modern, medium-lift, tactically-minded form with better short-field / rough-field performance than what the in-service prop-driven C-130 could offer. The official requirement was drawn up in 1968 and the Request For Proposal (RFP) followed in 1972.
In the early stages, responses were had from industry players in Bell, Boeing, Lockheed (partnered with North American Rockwell) and McDonnell Douglas. Boeing and McDonnell Douglas ultimately advanced from this phase and each was granted a contract to cover two flyable prototypes for testing and head-to-head competition. They were designated as the Boeing "YC-14" (detailed elsewhere on this site) and the McDonnell Douglas "YC-15".
Unlike the Boeing submission, which relied on two engines overslung on the wing mainplanes, the YC-15 made use of a conventional four-engined layout with two nacelles paired under each wing element. Like the YC-14 (and the C-130 for that matter), the YC-15 too utilized a high-wing, straight-edged mainplane arrangement (of super-critical airfoil) for the needed lift, drag and short-field performance required by the USAF. Large double-slotted, externally-blown flaps made up a good length of the mainplane's span at the wing trailing edges and were intended to increase Short Take-Off and Landing (STOL) performance. The engines selected for the design were 4 x Pratt & Whitney JT8D-17 turbojets of 15,500lb thrust each which were already in production and service with civilian market airliners at the time.
The fuselage was large, relatively short and tubular in its general shape. The raised empennage would allow a cargo ramp to be lowered to accept cargo loads rather easily. The tail unit was configured in a typical "T-style" arrangement which saw the horizontal planes seated high atop the vertical fin. A tricycle undercarriage (retractable) was used for ground-running with the nosewheel being a revised version of the same steering leg used in the DC-8 narrow-body passenger airliner. Internally, the cockpit was taken from the proven McDonnell Douglas DC-10 wide-body passenger airliner with changes made to suit military service. The air-fueling receptacle came from the Fairchild A-10 "Thunderbolt II" attacker and other components were equally-borrowed from existing aircraft to speed development and keep costs at a minimum.
The two contracted-for YC-15 aircraft arrived in prototype form in 1975 with a first-flight recorded on August 26th, 1975 (the second prototype went into the air for the first time before year's end). Both aircraft were ahead of the competing Boeing design which did not make it airborne until the following year and the formal competition between the two began in late 1976.
Because of the changing requirements of the USAF, the AMST program suffered despite the progress being shown. Authorities eventually settled on continued support of the C-130 product for the foreseeable future and renewed a commitment to expanding the strategic transport capabilities of the USAF and this allowed the Hercules aircraft to see an extended service life that continues even today (2018).
The AMST program was ended in December of 1979 with no winner selected but the "C-X" program was drawn up in its place to now find a strategic-minded airlifter - this eventually led to a dimensionally larger version of the McDonnell Douglas YC-15 being built and flown - influencing what would become the swept-wing Boeing C-17 "Globemaster III" transport still to come. The C-17 entered service in 1995 and remains active today (2018) with nearly 300 units built. McDonnell Douglas eventually became a subsidiary of the Boeing Company brand, hence the C-17's official product name.
As built, the YC-15 utilized an operating crew of three. It held a capacity to ferry up to 150 combat-ready troops or up to 78,000lb of cargo. Dimensions included a length of 124.2 feet with a wingspan of 110.3 feet and a height of 43.2 feet. Empty weight was 105,000lb against an MTOW of 216,680lb. Performance specs included a maximum speed of 590 miles per hour with a cruising speed of 543mph, a range out to 3,000 miles and a service ceiling up to 30,000 feet.
- X-Plane / Developmental
124.34 ft (37.9 m)
132.55 ft (40.4 m)
43.31 ft (13.2 m)
104,940 lb (47,600 kg)
216,681 lb (98,285 kg)
590 mph (950 kph; 513 kts)
30,003 feet (9,145 m; 5.68 miles)
2,989 miles (4,810 km; 2,597 nm)
YC-15 - Base Prototype Designation; two examples completed.
C-15 - Assumed in-service designation.