YC-15 Advanced Medium STOL Transport (AMST)
McDonnell Douglas used the externally blown flap (EBF) concept with a four-engine configuration and large double-slotted flaps that extended over 75 percent of the total span for the YC-15 prototypes. The YC-15 already had a rear cargo ramp and soft-field landing gear. It also had unusually large tail surfaces, which increased the safety of the demonstration program.
John P. Campbell of NASA Langley had conceived the innovative externally blown flap concept in the mid-1950's as a relatively simple approach to augment wing lift for low-speed operations. In this concept, the exhaust from pod-mounted engines impinges directly on conventional slotted flaps and is deflected downward to augment the wing lift. The magnitude of lift augmentation is extremely large, and the resulting lift can be as much as twice the value for a conventional aircraft. However, no serious consideration was given to the EBF concept initially because of the severe high-temperature impingement on the wing and flap surfaces from the turbojet (no bypass or fan flow) engines used at that time. Also, the relatively small mass flow from such engines was a limiting factor for lift augmentation. In addition, considerable concern was expressed over potential control problems in the event that an engine became inoperative during flight at low speeds with high-power settings. With the advent of turbofan engines, however, the efflux from the engines was relatively cool, and large quantities of air became available for increased airflow through the flaps. The turbofan engine, therefore, provided the breakthrough mechanism that permitted researchers to evolve and mature the applications of the EBF concept.
Employing "under-surface blowing" to achieve STOL capability, the YC-15's wings were configured with sets of double-slotted flaps which could be extended downward directly into the jet flow from its four turbofan engines. Part of the exhaust was directed downward by the flaps while the rest passed through and then downward over the flaps by means of the "Coanda effect" (air turning on the convex side of an aerodynamic surface). The YC-15 convincingly demonstrated the feasibility of this concept which would later be incorporated into the design of the C-17 transport.
The YC-15 was the first military transport to use supercritical wings, a major innovative technology conceived and developed through wind-tunnel research by Richard Whitcomb at NASA Langley. Whitcomb's supercritical wings incorporate advanced airfoils that enhance the range, cruising speed, and fuel efficiency of aircraft by producing weaker upper-surface shock waves, thereby creating less drag and permitting higher efficiency. McDonnell Douglas subsequently incorporated supercritical wing technol-ogy in the C-17 design.
The McDonnell Douglas YC-15 Advanced Medium STOL (Short-Takeoff and Landing) Transport aircraft landed at Edwards at the end of its maiden flight on 26 August 1975, and was joined by the second prototype in December of that year. The YC-15 demonstrated exceptional STOL performance in its flight-test program with an approach speed of only 98 mph and a field length of 2,000 ft at a landing weight of 150,000 lb.
Though originally conceived as a potential replacement for the venerable C-130 Hercules, funding cuts limited the YC-15 to the role of an advanced technology demonstrator. A lack of money presented an insurmountable obstacle. Although the US Army and the Military Airlift Command backed procurement of the AMST, the escalation of costs, from $5 million per aircraft in 1970, to twice that much in 1977, and an estimate of $20 million by 1982, killed the production of either prototype.
One of only two YC-15 prototype aircraft that was produced in the 1970's had been in AMARC's desert storage for a period of 17 years. This aircraft was identified to play a new role as a test bed for advanced technology. In order to meet this need, the AMARC workforce in conjunction with Boeing worked on the YC-15 for several months on site. After more than 15 years in storage in the Arizona desert, in late 1996 the McDonnell Douglas YC-15 was brought out of mothballs to continue its mission as an advanced technology demonstrator. It was the first Air Force developmental aircraft leased back to a contractor under a Cooperative Research and Development Agreement. The primary reason for the agreement was to provide a prototype to explore new technology applications for the C-17 and other airlift aircraft. Without the use of the YC-15 for airlift testing to assist the lone C-17 test aircraft, the Air Force would have to rely on the operational C-17 fleet, which was already heavily tasked with global commitments. The YC-15, which first flew in 1975, had been stored at Davis-Monthan Air Force Base in Tucson, Ariz., the home to the Air Force's Aerospace Maintenance and Regeneration Center, an Air Force Material Command facility. Refurbishment was being done by both McDonnell Douglas and AMARC crews. The result: a successfully refurbished and flight ready aircraft departed AMARC in 1997. The YC-15 operated out of McDonnell Douglas facilities in Long Beach, CA.
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