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AIR FORCE/MILITARY HISTORY

YC-15 STOL TRANSPORT

While the USAF moved away from its AMST program which begat the YC-15, the design - in modified form - reemerged to become the C-17 Globemaster III transport.


The McDonnell Douglas YC-15 is a prototype four-engine short take-off and landing (STOL) tactical transport. It was McDonnell Douglas' entrant into the United States Air Force's Advanced Medium STOL Transport (AMST) competition to replace the Lockheed C-130 Hercules as the USAF's standard STOL tactical transport. In the end, neither the YC-15 nor the Boeing YC-14 was ordered into production, although the YC-15's basic design would be used to form the successful McDonnell Douglas (later Boeing) C-17 Globemaster III.


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 Request for proposal (RFP) was issued in January 1972, asking for operations into a 2,000-foot (610 m) semi-prepared field with a 27,000-pound (12,000 kg) payload and a 400-nautical-mile (460 mi; 740 km) mission radius.  For comparison, the C-130 of that era required about 4,000 feet (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 two top 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 components from other McDonnell Douglas aircraft, with its nose gear coming from the Douglas DC-8 and the nose section and cockpit being derived 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.


Two YC-15s were built, one with a wingspan of 110 feet (34 m) (#72-1876) and one of 132 feet (40 m) (#72-1875). Both were 124 feet (38 m) long and powered by four Pratt & Whitney JT8D-17 engines, each with 15,500 pounds-force (69,000 N) of thrust.

Specifications

Primary Function                                                                                                                           Navigator trainer

Builder                                                                                                                                               Boeing Co.

Power Plant                                                                                                                                      Two Pratt & Whitney JT8D-9A engines

Thrust                                                                                                                                                14,500 pounds (6,525 kilograms) each engine

Length                                                                                                                                                100 feet (30.3 meters)

Height                                                                                                                                                37 feet (11.2 meters)

Maximum Takeoff Weight                                                                                                            115,000 pounds (67,500 kilograms)

Wingspan                                                                                                                                          93 feet (28.2 meters)

Speed                                                                                                                                                  535 mph (Mach 0.72) at 35,000 feet

Ceiling                                                                                                                                                37,000 feet (11,212 meters)

Range                                                                                                                                                 2,995 miles (2,604 nautical miles)

Armament                                                                                                                                         None

Crew 12                                                                                                                                              navigator students, six instructor navigators, pilot and co-pilot

Date Deployed                                                                                                                                 September 1973

Unit Cost                                                                                                                                           $5,390,000

Inventory                                                                                                                                           Active force, 13; ANG, 2; Reserve, 0

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