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Overview
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.
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