9th of August 1976
Today in 1976, the Boeing YC-14 Short Take-Off and Landing (STOL) tactical military transport aircraft made its first flight. It was one of the competitors in the USAF Advanced Medium STOL Transport competition, another competitor was the McDonnell Douglas YC-15.
The Boeing YC-14
After several ideas were looked at and tested by Boeing (involving aircraft like the Lockheed C-5 Galaxy, the Boeing 747 and the Boeing 707 prototype), they turned their attention to NASA research.
NASA had been doing tests of several “power lift” studies, involving ‘blown flaps’ and ‘upper surface blowing’’. Blown flaps and upper surface blowing both use the exhaust gasses of a jet engine (or the prop wash to increase the lift generated by the flaps.
In January of 1972, both Boeing and McDonnel Douglas were awarded development contracts for an aircraft that was capable of operating into a 2000 ft (610 meters) runway while carrying 27.000 lbs (12.000kg) and returning to its departure airport 500 nm (930 km) with the same payload without refuelling
Together with NASA Boeing went on to do wind tunnel testing on a ¼ scale wing of their proposed aircraft. Several problems with regard to airflow and controllability were identified. These problems were fixed by changing the initial tail design of the aircraft and by adding retractable vortex generators which would be retracted automatically when the flaps were extended <30°. Almost a year after the first flight of the McDonnell Douglas YC-15 the first flight of the Boeing YC-14 took place on this day in aviation history. During the test program, the YC-14 was flown at speeds as low as 59 knots (68 mph / 109 kph) but also at M 0.78 at 38.000 ft. It was found that the drag the aircraft encountered was 11% higher than envisaged during the development. With the aid of wind tunnel testing, extensive modifications were made, they ranged from additional vortex generators to removal of fairings and from re-designed gear pods to strakes on the aft fuselage. All the improvements reduced the drag, but still, it was 7% more than initially envisaged. Testing continued and included a demonstration flight carrying an M60 Patton battle tank, weighing 109.200 lbs (49.500 kg), a feat that the competing YC-15 did not demonstrate.
By the end of the summer in 1977 testing was completed, both aircraft are “preserved”, one on display at the Pima Air & Space Museum and the other airframe is stored at the 309th AMARG (Aerospace Maintenance and Regeneration Group) at the Davis Monthan USAF base.
By this time the emphasis for air transport changed from tactical to strategic airlift capabilities, and in 1979 the AMST program was cancelled and a new Task Force was formed, the C-X Task Force. Their task was to look at the possibilities to still combine the strategic and tactical requirements. They selected a proposal that would be developed into the C-17 Globemaster III.
Specifications of the YC-14
Capacity: 150 troops or 69,000 lb (31,400 kg) (STOL: 27,000 lb (12,300 kg))
Length: 131 ft 8 in (40.14 m)
Wingspan: 129 ft 0 in (39.32 m)
Height: 48 ft 4 in (14.74 m)
Wing area: 1,762 sq ft (163.7 m2)
Empty weight: 117,500 lb (53,410 kg)
Max takeoff weight: 251,000 lb (113,850 kg) (conventional landing and takeoff), 170,000 lb (77,270 kg) (STOL)
Powerplant: 2 × General Electric CF6-50D turbofans, 51,000 lbf (230 kN) thrust each
Maximum speed: 504 mph (811 km/h, 438 kn)
Cruise speed: 449 mph (723 km/h, 390 kn)
Ferry range: 3,190 mi (5,136 km, 2,734 nmi)
Service ceiling: 45,000 ft (13,716 m)
Rate of climb: 6,350 ft/min (32.3 m/s)