In the early 1970s the U.S. Air Force issued its Advanced Medium STOL Transport ( AMST ) requirement for a tactical airlifter with exceptional field performance. Five American companies submitted proposals, and in 1972 two of these 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 each utilized off-the-shelf engines to achieve the "Coanda effect" (air turning on the convex side of an aerodynamic surface) to maximize lifting capability. The Boeing YC-14, first flown on 9 August 1976, used two GE F103/F1A engines mounted forward and above the wing, their exhaust blown across the upper surface of the wing and flap system in order to create powered lift. This location also gave the airplane a quieter noise footprint. Two aircraft were built, tail numbers 01873 and 01874.
The McDonnell Douglas YC-15, first flown on 26 August 1975, 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. Also, employing "under-surface blowing" to achieve STOL (Short-Takeoff and Landing) capability, its 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." The two YC-15s built, tail numbers 01875 and 01876, flew with two different size wingspans, 132 feet and 110 feet, respectively. Both aircraft are 124 feet in length.
Originally conceived as a potential replacement for the venerable C-130 Hercules, 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 C-17 transport.
A New Breed
The C-17 Globemaster III (#87-0025) made its maiden flight on 15 September 1991. The aircraft is operated by the Air Mobility Command with initial operations at Charleston AFB, SC. As of this posting, the C-17 currently owns over 20 world-class airlift records, including payload to altitude time-to-climb, and short takeoff and landing mark in which the C-17 took off in less than 1,400 feet, carried a payload of 44,000 pounds to altitude, and landed in less than 1,400 feet.
The C-17 incorporates many of the military transport standards, as well as numerous advanced technological features. Able to accommodate nearly 100 percent more cargo volume than the C-141B Starlifter, this new, four-engined high-wing T-tailed military jet transport incorporates a rear ramp/door arrangement, heavy-duty retractable landing gear with fuselage blister fairings, the blown flap system developed for the YC-15, a "glass cockpit," and a GEC fly-by-wire control system.
Cargo Compartment - Capacity for: 18 cargo pallets; 102 troops/paratroops; 48 litter and 54 ambulatory patients and attendants.
Cargo is loaded onto the C-17 through a large aft ramp/door assembly that accommodates military vehicles and palletized cargo. The C-17 can carry virtually all of the Army's air-transportable, outsized combat equipment. It is also able to airdrop paratroopers and cargo.
Powered Lift - A key element of the aircraft is a flap system developed by a team of researchers at NASA-Langley Research Center in the mid-1950s. The "externally blown flap" or "powered-lift system" enables the airplane to make slow, steep approaches with heavy cargo loads. This is accomplished by diverting engine exhaust downward, giving the wing more lift. In the flap system, the engine exhaust from pod-mounted engines impinges directly on conventional slotted flaps and is deflected downward to augment the wing lift. This allows aircraft with "blown flaps" to operate at roughly twice the lift coefficient of that of conventional jet transport aircraft.
The C-17 Globemaster III can operate on small, austere airfields. It can take off and land on runways as short as 3,000 feet and as narrow as 90 feet wide. Even on such narrow runways, the aircraft can turn around by using its backing capability while performing a three-point star turn.
Supercritical Wing - Like other military transports, the C-17 uses a "supercritical" wing. These are advanced airfoil designs that enhance the range, cruising speed, and fuel efficiency of jet aircraft by producing weaker shock waves that create less drag and permit high efficiency.
The Winglet - In the mid-1970s, NASA-Langley developed the winglet concept through wind tunnel research. Winglets are small, winglike vertical surfaces at each wingtip of an aircraft that enable the airplane to fly with greater efficiency. They curve flow at the wingtip to produce a forward force on the airplane, similar to the sail on a sail boat.
Fly-by-Wire - Another NASA contribution to the C-17 was fly-by-wire flight-control technology, a lower volume, lightweight replacement for hydraulic control systems.
Composite Materials - Sixteen-thousand pounds of composite materials have been applied to the C-17. Several of the major control surface and secondary structural components of the aircraft are made of composites. The most direct contribution to C-17 applications was the development of the DC-10 graphite-epoxy upper aft rudders. These rudders have accumulated more than 500,000 flight hours since they were introduced into regular airline service in 1976. The high-time rudder alone has flown for 75,000 hours. The control surfaces of the C-17 follow the same multi-rib configuration as the DC-10 rudders.