The Vietnam War, with its relentless brutality, exposed a weakness in America’s military machine that could no longer be ignored. The venerable C-130 Hercules, once the backbone of U.S. tactical airlift, was faltering under the demands of remote, unforgiving combat zones where short, rough airstrips turned every landing into a gamble.
The need for a new breed of airlifter—one that could conquer the harshest conditions, land in spaces no larger than a football field, and bear the heaviest military hardware—became undeniable. Out of this dire necessity, the blueprints for the Boeing YC-14 were forged, an aircraft so revolutionary it seemed to defy the laws of physics.
While the C-130 needed over 4,000 feet of runway to take off, the YC-14 could soar over a 50-foot obstacle with just 2,000 feet and a jaw-dropping 27,000-pound payload. It harnessed the Coandă effect, manipulating exhaust flow to generate extraordinary lift, allowing it to land in just 387 feet at a mere 99 miles per hour—an achievement that no other airlifter had ever approached.
But the YC-14 was more than just a short-field marvel. It could transport a 109,000-pound M60A2 main battle tank across continents, and with a climb rate of 6,000 feet per minute, it ascended three times faster than the C-130, promising to redefine battlefield logistics and survivability. The YC-14 was poised to become the ultimate military logistics machine, meeting and exceeding nearly every demand the Air Force had set. Its potential to reshape the face of warfare was undeniable—so undeniable, in fact, that even the Soviets took notice.
Rumors swirled that Soviet agents had stolen the YC-14’s design, a bold move to create their own weapon to challenge American air supremacy. Yet, just as the YC-14 was acing every test and primed to take over Military Airlift Command, it vanished…
The Boeing YC-14 is a twinjet short take-off and landing (STOL) tactical military transport aircraft. It was Boeing’s entrant into the United States Air Force’s Advanced Medium STOL Transport (AMST) competition, which aimed to replace the Lockheed C-130 Hercules as the USAF’s standard STOL tactical transport. Although both the YC-14 and the competing McDonnell Douglas YC-15 were successful, neither aircraft entered production. The AMST project was ended in 1979 and replaced by the C-X program.
Design and Development
In mid-1970, the USAF began a paper study, the Tactical Aircraft Investigation (TAI), with Boeing, McDonnell Douglas, and other companies to look at possible tactical transport aircraft designs. This study was a precursor to what became the Advanced Medium STOL Transport program. As a part of this program, Boeing began to look at various high-lift aircraft configurations. Boeing had earlier proposed an underwing externally blown flap solution for their competitor for the Lockheed C-5 Galaxy, and had put this to good use when it modified its losing entry into the Boeing 747. The company had also done studies with the original Boeing 707 prototype, the Boeing 367-80, adding extensive leading and trailing edge devices using blown flaps. For the TAI studies, Boeing again looked at those mechanisms, as well as new mechanisms like boundary layer control. However, none of these studied designs were particularly appealing to Boeing.
The Boeing engineers were aware that NASA had carried out a series of “powered lift” studies some time earlier, including both externally blown flaps, as well as upper-surface blowing (USB), an unusual variation. In the USB system, the engine is arranged over the top surface of the wing, blowing over the flaps. When the flaps are lowered, the Coandă effect makes the jet exhaust “stick” to the flaps and bend down toward the ground. They searched for additional research on the concept and found that half-span upper-surface blowing research had been conducted in the NASA Langley 12-foot (3.7 m) tunnel. An examination of the preliminary results suggested that the system was as effective as any of the other concepts previously studied. Boeing immediately started to build wind-tunnel models to verify the NASA data with layouts more closely matching their own designs. By the end of 1971, several models were being actively studied.
Another NASA project the engineers were interested in was the supercritical airfoil, designed by Richard Whitcomb. The supercritical design promised to lower transonic drag greatly, as much as a swept wing in some situations. This allowed an aircraft with such a wing to have low drag in cruise while also having a wing planform more suitable for lower-speed flight—swept wings have several undesirable characteristics at low speed. Additionally, the design has a larger leading edge radius that makes it particularly suitable for low-speed high-lift applications like a transport. Boeing incorporated the concept into their design, the first non-experimental aircraft to do so.
The request for proposal (RFP) was issued in January 1972, asking for operations into a 2,000-foot (610 m) semi-prepared field at 500 nautical miles (930 km) with a 27,000 lb (12,000 kg) payload in both directions with no refueling. For comparison, the C-130 of that era required about 4,000 ft (1,200 m) for this load. Five companies submitted designs at this stage of the competition, Boeing with their Model 953 in March 1972. On 10 November 1972 the downselect was carried out, and Boeing and McDonnell Douglas won development contracts for two prototypes each.
Wind tunnel tests continued through this period. In November, John K. Wimpress again visited Langley looking for an update on NASA’s own USB program. Joe Johnson and Dudley Hammond both reported on testing and showed Wimpress data that verified the high-lift performance that Boeing had quoted in its proposal. By December 1975, Boeing and NASA Langley had arranged a contract for a full-scale USB testbed, which Boeing built at their Tulalip test facility consisting of a 1/4-scale wing with one JT-15D engine and a partial fuselage. Langley was particularly interested in the effectiveness of the D-shaped nozzle that directed the jet flow over the upper surface of the wing, as well as the resulting sound levels, at that time a major focus of NASA’s civilian aerodynamics research.
Two major problems were found and corrected during testing. The first was a problem with air circulating around the wing when operating at low speeds close to the ground, which had a serious effect on the spreading of the jet flow through the nozzle. This led to flow separation near the flap, and a decrease in the effectiveness of the USB system. In response, Boeing added a series of vortex generators on the upper surface of the wing, which retracted when the flap was raised above 30°. Additionally, the tail surfaces were initially placed well aft in order to maximize control effectiveness. This positioning turned out to interfere with the airflow over the wings during USB operations, and a new tail with a more vertical profile was introduced to move the elevator forward.
Sources: Wikipedia; YouTube