Boeing’s YC-14 airlifter was built as the company’s entrant in the Force’s Advanced Medium Short Takeoff Landing [STOL] Transport (AMST) competition for the United States Air Force (USAF). The competition actually began in 1970 when the Air Force and key aerospace contractors began the Tactical Aircraft Investigation (TAI) to look at potential new airlifters. This video, uploaded to YouTube by Jeff Quitney, is a look at the program as it stood during 1977 after the two prototype YC-14s had been built and were in test by the USAF.
The YC-14 was built specifically to take advantage of high-lift aircraft configurations. Blown leading edge slats and trailing edge flaps as well as various boundary layer control systems were all investigated. Boeing decided instead to take advantage of research previously conducted by the National Aeronautics and Space Administration (NASA) into powered lift- specifically upper-surface blowing (USB). NASA had done wind tunnel testing of experimental shapes with USB and Boeing was able to examine the data.
By mounting the turbofan engines high on the wings so that the exhaust was blown over the wing’s upper surface and trailing edge flaps instead of under the lower surfaces of the wing, the exhaust would aerodynamically couple with the trailing edge flaps when they were deployed and the exhaust would be deflected downward, thereby augmenting lift. This phenomenon, known as the Coandă Effect, was responsible for much of the aircraft’s STOL performance but the lift augmentation was minimal when the flaps were retracted. The USB configuration of the engines coupled with a supercritical wing shape combined to make the YC-14 a stellar STOL performer for its size and weight.
And it needed to be! When the USAF’s request for proposal (RFP) went out in early 1972 the expectations for the competing designs was ability to haul a 27,000 pound payload more than 1,000 miles without refueling- all after taking off from a 2,000 foot runway. These seemingly impossible operational requirements made it absolutely necessary to think outside the box. Boeing certainly did. Eventually the competitors were whittled down to Boeing’s YC-14 and McDonnell Douglas’ YC-15 in 1972. Each company was awarded a development contract for two prototypes.
More wind tunnel testing took place at NASA Langley in Virginia. Between the NASA USB program and Boeing’s YC-14 testing much was learned about the viability of USB. During the testing a couple of challenges were met and bested. Boeing added retractable vortex generators behind the engine exhausts to maintain trailing edge flap effectiveness at low speeds and altitudes. They also revised the design of the empennage by moving it forward and reducing the aft rake of the vertical stabilizer. The Soviets more or less copied the design of the YC-14 in the Antonov An-72 Coaler transport.
The YC-14 first flew on August 9th 1976- nearly a year after the YC-15 first took to the skies. The two YC-14 prototypes (serial numbers 72-1873 and 72-1874) were first tested at Edwards Air Force Base (AFB) in California. The YC-14 was flown as slowly as 59 knots and as fast as 520 knots at 38,000 feet. In order to address higher than expected drag numbers the aircraft received several modifications including revised landing gear pods, fuselage strakes, and addition of vortex generators on the engine nacelles. One thing the YC-14 did that the YC-15 could not do was tote a 55 ton M-60 Patton tank. The airlifter’s two General Electric CF6-50D turbofans, each capable of delivering 51,000 pounds of thrust, enabled that feat.
When the Air Force finished testing of the YC-14 prototypes during August of 1977 (right after the film above was produced) they returned the prototypes to Boeing. The YC-14 prototypes both exist today. One is stored at the AMARG boneyard at Davis Monthan AFB near Tucson. The other is on display at the Pima Air and Space Museum. And the YC-15s? The YC-15 didn’t go into production either. Changing priorities and requirements from the Air Force ended both programs. But the McDonnell Douglas YC-15 served as the basis for…the Boeing C-17 Globemaster III. And there is certainly a Boeing family resemblance in the C-17’s empennage.