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Aug. 26, 1975

To commemorate the  Air Force Flight Test Center, which was established June 25, 1951, the AFFTC History Office recalled some of the milestones in flight that took place here during the last half century.

Title: A Pair of Most Unusual Transports

By Dr. Raymond L. Puffer
Air Force Flight Center historian

On Aug. 26, 1975, a stubby new jet transport touched down on the Air Force Flight Test Center's main runway. It proved to be a highly unusual bird.

The Boeing YC-14 (pictured here) was a rival to the McDonnell Douglas YC-15.At first glance it seemed conventional, but a closer look revealed four jet engines projecting an unusual distance ahead of its high wing, and a very tall horizontal tail. This was the McDonnell Douglas YC-15, and on that day, just 26 years ago, the aircraft had just completed its maiden flight from Long Beach to take part in the Advanced Medium STOL Transport (AMST) program. Over the next two years, the AFFTC and contractor pilots would fly the YC-15 repeatedly to assess its performance potential and to compare it with an equally unusual rival, the Boeing YC-14.

The AMST was a concept demonstration program designed to evaluate the applications of advanced thrust technologies and their potential value for military tactical airlift. The medium-capacity C-130 Hercules had first flown in 1954, and in the early 1970s the Director of Defense Research and Engineering announced that tactical airlift should gain the same efficiencies that the airlines experienced when they made the transition from propellers to turbofan engines. Hence, the need to prove that the new technologies could produce a new medium transport with greater load capacity and the ability to operate in and out of very short, 2000-foot runways.

The YC-15 that arrived at Edwards that day was the first of two prototypes that would be jointly tested here and at the McDonnell Douglas flight test base in Yuma, Ariz.

The McDonnell Douglas YC-15 is pictured here in this aerial view.The most innovative design feature of the YC-15 was an externally blown flap (EBF) powered lift system. The four Pratt & Whitney JT8D-17 turbofan engines were mounted forward and very closely under the wing, which had large double-slotted flaps that spanned 75 percent of the wing's trailing edge. During takeoff and landing, the flaps were extended directly into the engine exhaust, deflecting the jet blast downward for additional lift. Part of the blast would flow through gaps between the flap segments and continue downward, increasing lift on the flap upper surfaces a phenomenon called the Coanda effect. The flaps were made largely of titanium to resist the intense heat of the jet exhaust, which was also mixed with ambient air to reduce its temperature and load intensities at the flap surfaces.

The first of two Boeing YC-14s arrived at Edwards on Nov. 9, 1976. The rival transport was a high wing aircraft with two high-bypass-ratio turbofan engines mounted forward and above the wing's leading edge. From this position, the jet exhaust impinged directly onto the wing's upper surface.

When the large, compound flaps were lowered, the jet exhaust would cling to the surface and be deflected downward the Coanda effect again in much the same way that water from a faucet follows the curve of a spoon. The result of this technique, called upper surface blowing (USB) was additional lift. The YC-14 was also equipped with thrust reversers, variable camber leading edge flaps, and an electronic flight control system.

The AFFTC and the contractors flew the four concept demonstrators through two years of development and evaluation testing, and assisted in utility testing conducted by the Air Force Test and Evaluation Center.

The Graham Ranch runway beyond the end of Runway 22 was used for testing on unimproved runways. Takeoffs and landings were made with heavy payloads, and loading tests were made with Army tanks and howitzers. Both the YC-15 and the YC-14 proved themselves in the grueling tests and turned in some remarkable performances. Pilots even claimed that the YC-14, with its advanced flight control system, flew like a fighter.

Unfortunately for all of this inspiration and effort, the AMST program was terminated in 1977.

The YC-15 is pictured taking off from the dry lake bed. Notice the double-slotted flaps.There were no funds for future development and the four concept demonstrators quickly went into storage. However, many of the design innovations that the YC-15 displayed went on to be used in the highly successful C-17 Globemaster III. And after languishing at Davis-Monthan Air Force Base for 18 years, McDonnell Douglas refurbished YC-15 Number 1 and it began a second career as test-bed aircraft for still newer technologies.




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