In the early-1970s, five American companies submitted proposals to the U.S. Air Force after it issued its Advanced Medium STOL Transport (AMST) requirement for a new jet-powered tactical airlifter to replace the venerable Lockheed C-130 Hercules. In 1972, two proposals were accepted for construction as the Boeing YC-14 and McDonnell Douglas YC-15 prototypes.
Both test aircraft were designed to a common cargo specification and utilized off-the-shelf engines to achieve the "Coanda Effect" (air turning on the convex side of an aerodynamic surface) to maximize lifting capability during STOL (Short Takeoff and Landing) operations.
The Boeing Model 953 design for STOL performance was based on the use of a supercritical wing, developed by NASA from the wind-tunnel research of Dr. Richard Whitcomb, which provides highly efficient performance from the wing at high subsonic speeds. To this wing Boeing added an advanced wing upper-surface blowing concept, mounting the twin turbofan engines forward and above the wing so that their efflux was exhausted over the wing (this location also gave the airplane a quieter noise footprint.). With the wing's leading-edge flaps and Coanda-type trailing-edge flaps extended, the high-speed airflow from the engines tended to cling to the upper surface of the wing/flap system and was thus directed downward to provide powered lift. It was the most efficient powered-lift system ever developed.
The YC-14's basic mission was to carry large, bulky payloads into and out of short, rough dirt fields that were less than 2,000 feet (610m) long, even if an engine failed. It had a large fuselage to accommodate most tanks, trucks and armored personnel carriers used by the U.S. Army.
The first Boeing YC-14 (#72-1873) took to the skies on 9 August 1976 and soon proved to have admirable performance. Two aircraft were built, the second being tail number 72-1874.
At the completion of testing in the late summer of 1977, the YC-14 prototypes were returned to Boeing for continuing development if the company so wished. Due to budget restrictions, no further government funding for development or procurement was made.
The McDonnell Douglas YC-15, like Boeing's YC-14 prototype, had a high-set wing, fuselage blister fairings for the main landing gear units, and an upswept T-tail above the rear ramp/door arrangement. The primary difference between the two aircraft was the way each achieved STOL performance. Unlike the YC-14, the YC-15 wings were configured with sets of double-slotted flaps which could be extended downward directly into the jet flow from its four under-wing turbofan engines. Employing "under-surface blowing" to achieve STOL capability, 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".
Two YC-15s (#72-1875 and #72-1876) were built with two different size wingspans, 132 feet (40.42m) and 110 feet (33.6m), respectively. Both aircraft are 124 feet (37.86m) in length.
First flown on 26 August 1975, a 600-hour test program followed. Funding cuts eventually cancelled the AMST program in 1979. Both the YC-14 and the YC-15 satisfied the AMST performance requirements, which would later be incorporated into the design of the larger C-17 Globemaster III transport.
Returning to Service
In 1996, after more than 15 years in storage in the Arizona desert, the McDonnell Douglas YC-15 (N15YC / 72-1875) was being brought out of mothballs to continue its mission as an Advanced Technology Demonstrator (ATD). McDonnell Douglas Military Transport Aircraft, a division of Boeing, will be operating the YC-15 on an eight-year no-cost lease from the U.S. Air Force. It was the first Air Force developmental aircraft to be leased back to a contractor under a Cooperative Research and Development Agreement.
The primary reason for the agreement is to provide a prototype to explore new technology applications for the C-17 and other airlift aircraft. Boeing will pay all costs for refurbishment and testing. The government retains license to use for government purposes the technology developed. The contractor can obtain title to resulting technology for commercial use.
Using the YC-15 will reduce the risk and span time for developing and using new aircraft technologies for both the Air Force and Boeing. Some technologies will be directly related to the C-17 Globemaster III and some will affect possible future advanced airlift aircraft.
Back to top