***image1***The Air Force’s Boeing YC-14, pictured above, was a mid-1970s prototype for an Advanced Medium STOL Transport intended to replace the C-130. The unique, signature feature of the YC-14 was its two high-bypass-ratio turbofan engines mounted forward and above the high wing’s leading edge. From this position, their jet exhaust blew directly onto the wing’s upper surface and over large, double-slotted flaps that spanned 75 percent of the wing’s trailing edge. During takeoff and landing, when these large flaps were extended they “pulled” the jet blast downward for additional lift. The jet exhaust was pulled down and clung to the surface of the flaps in the same way that water from a faucet follows the curve of a spoon, what is known as the “Coanda (pronounced “kwanda”) Effect.” The result of the lowered flaps and the Coanda Effect was called upper surface blowing and, it was hoped, would provide considerable additional lift. The flaps were made largely of titanium to resist the intense heat of the jet exhaust, and to avoid excessive vibration on the flaps the engines were very quiet.
***image2***When the system was tested it was found that the extra lift was provided without excessively compromising performance or weight, and the YC-14 and its competitor, the four-engine McDonnell-Douglas YC-15, proved to be excellent aircraft that fulfilled every requirement. However, budgets were constrained and the Department of Defense decided not to replace the C-130s because the new aircraft were deemed too expensive for use solely as a short range tactical transport. Still, the principles were sound and the same day the Air Force cancelled the AMST tactical transport program it took the basic principles and applied them to a proposed strategic transport program, the C-X. This program would ultimately lead to the C-17.
The two YC-14s that were built still survive. One is stored in the “bone yard” at Davis Monthan Air Force Base in Tucson, Ariz., while the other is on display at the nearby Pima Air and Space Museum, also in Tucson.
Not only do the two YC-14s survive, but their ancestor, seemingly a direct copy, occasionally visits Ramstein. It is the only production aircraft to use the USB system, the Russian Antonov AN-72 “Coaler.” The AN-72, pictured below, is much smaller than the YC-14 – about half the size – and carries about a quarter of the payload, but the resemblance in unmistakable. Like the YC-14, the AN-72’s twin turbofans blow the exhaust over the wings and flaps and provides very high maximum lift coeffieicents, and fully loaded the AN-72 can take off or land in about 1,500 feet. This makes it ideal for operating the small, relatively unimproved fields that dot the former Soviet Union.