McDonnell Douglas YC-15 - Design and Development

Design and Development

In 1968 the USAF started work on a series of prototype proposals, which would lead to both the AMST project and the Light Weight Fighter. The official RFP was issued in January 1972, asking for operations into a 2,000 feet (610 m) semi-prepared field with a 27,000 lb (12,000 kg) payload and a 400 nmi (740 km) mission radius. For comparison, the C-130 of that era required about 4,000 ft (1,200 m) for this load. Proposals were submitted by Bell, Boeing, Fairchild, McDonnell Douglas and the Lockheed/North American Rockwell team at this stage of the competition. On 10 November 1972, the top two bids from Boeing and McDonnell Douglas were selected. The companies were awarded development contracts for two prototypes each. McDonnell Douglas' prototype was designated YC-15.

McDonnell Douglas's design incorporated a supercritical wing, the result of NASA research carried out by the already famous Richard Whitcomb. This wing design dramatically lowers transonic wave drag by as much as 30% compared to more conventional profiles, while at the same time offering excellent low-speed lift. Most contemporary aircraft used swept wings to lower wave drag, but this led to poor low-speed handling, which made them unsuitable for STOL operations.

The design team also chose to use externally-blown flaps to increase lift. This system uses double-slotted flaps to direct part of the jet exhaust downwards, while the rest of the exhaust passed through the flap and then followed the downward curve due to the Coandă effect. Although the effects had been studied for some time at NASA, along with similar concepts, until the introduction of the turbofan the hot and concentrated exhaust of existing engines made the system difficult to use. By the time of the AMST project, engines had changed dramatically and now provided larger volumes of less-concentrated and much cooler air. For the YC-15 four engines were used, versions of the Pratt & Whitney JT8D-17 widely used on the Boeing 727 and Douglas DC-9 The YC-15 "borrowed" from other McDonnell Douglas aircraft, with its nose gear coming from the Douglas DC-8 and the cockpit from the Douglas DC-10. Parts borrowed from other aircraft included the Universal Aerial Refueling Receptacle Slipway Installation (UARRSI), taken from a Fairchild A-10, anti-tipover stabilizer struts from the Lockheed C-141 Starlifter, pumps taken from the McDonnell Douglas F-15 Eagle, Lockheed C-5 Galaxy, DC-9 and C-141 and actuators taken from the C-5 Galaxy and DC-10. In addition, the environmental cooling system was composed of components taken from the DC-9, C-141 and Boeing KC-135.