Boeing YC-14

Boeing YC-14
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One of the YC-14 prototypes conducting flight testing.
Type	Tactical airlifter
Manufacturer	Boeing
Maiden flight	9 August 1976
Status	Retired
Number built	2

The YC-14 was Boeing's entrant into the USAF's Advanced Medium STOL Transport (AMST) competition, which aimed to replace the C-130 Hercules as the USAF's standard STOL tactical transport. Although both the YC-14 and the competing McDonnell Douglas YC-15 were very successful, neither aircraft entered production, and the AMST project was eventually shut down in 1979.

History

In 1971 the USAF started work on a series of prototype proposals, which would lead to both the AMST project and the Light Weight Fighter. In preparation for a formal RFP, Boeing started examining various high-lift solutions. Boeing had earlier proposed an underwing externally-blown flap solution for their competitor for the C-5 Galaxy, and had put this to good use when they modified their losing entry into the Boeing 747. They had also done studies with the original Boeing 707 prototype, the 367-80, adding extensive leading and trailing edge devices using blown flaps. However they had since become wary of all of these solutions due to the highly asymmetric lift that would develop in an engine-out situation. For safety, they considered a twin-engine layout with the engines mounted very close to the fuselage to be the only acceptable solution.

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 a upper-surface blowing (USB), a dramatic 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 towards the ground. They were interested if they had conducted additional research on the concept, and found that half-span upper-surface blowing research had been conducted in the NASA Langley 12 foot tunnel. An examination of the preliminary results suggested 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 the already famous Richard Whitcomb. The supercritical design promised to greatly lower transonic drag, 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 to lower-speed flight. 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 official RFP was issued in January 1972, asking for operations into a 2,000 foot semi-prepared field at 500 nmi with a 27,000 lb payload in both directions with no refueling. For comparison, the C-130 of that era required about 4,000 ft 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. (Jack) 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.

During this testing several problems were noted. One was that air circulating around the wing when operating at low speeds close to the ground had a serious effect on the spreading of the jet flow though the nozzle. This led to flow separation near the flap, and a decrease in 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°. Another problem that appeared was the positioning of the tail surfaces, which were initially placed very far back to increase 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.

Testing

The first Boeing YC-14 (#72-1873) took to the skies on 9 August 1976. Two aircraft were built, the second being tail number #72-1874. The competing YC-15 had started flights almost a year earlier. head-to-head flight testing at Edwards Air Force Base started in early November 1976. At the completion of testing in the late summer of 1977, the YC-14 prototypes were returned to Boeing. The prototypes were not scrapped, and one is stored at AMARC, located at Davis-Monthan Air Force Base and the other is on display at the nearby Pima Air & Space Museum.

By this point the seeds of the AMST program's demise had already been sown. In March 1976 the Air Force Chief of Staff Gen. David C. Jones asked the Air Force Systems Command to see if it was possible to use a single model of the AMST for both strategic and tactical airlift roles, or alternately, if it was possible to develop non-STOL derivatives of the AMST for the strategic airlift role. This led to a series of studies which basically stated that such a modification was not easy, and would require major changes to either design to produce a much larger aircraft.

Although the YC-14 and YC-15 both met or exceeded the specifications of the contest, the increasing importance of the strategic vs. tactical mission eventually led the Air Force to conclude that they were better off with an updated C-130 in the short term. In the longer term, the much larger aircraft would be developed out of the YC-15 wich was developed into the C-17 Globemaster III.

USB remains a fairly rare concept in use, and has been seen only on a few other aircraft, such as the Antonov An-72 'Coaler'. It has been speculated the Coaler was developed on the YC-14 basis.

References

• Boeing YC-14

External links

• Pima Air & Space Museum: Boeing YC-14
• Edwards AFB: A Pair of Most Unusual Transports (Internet Archive)