YC-14 Advanced Medium STOL Transport (AMST)

Boeing's YC-14 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. In the upper surface blowing (USB), concept, the engines were mounted so that the exhaust spread over the upper surface of the wing for enhanced circulation and lift augmentation in STOL operations. NASA and industry studied the USB concept extensively from the middle 1950s to the 1980s using an extensive variety of wind-tunnel investigations, static engine tests, and piloted simulator studies that culminated with the Boeing YC-14 prototype military transport.

In the 1950s, researchers explored employing the efflux of engines to augment wing lift using the jetflap concept to remove the limitations of conventional high-lift devices. The magnitude of maximum lift obtained in this approach can be dramatically increased - by factors of three to four times as large as those exhibited by conventional configurations - permitting vast reductions in field length requirements and approach speeds. This revolutionary breakthrough to providing high lift led to remarkable research and development efforts.

One of the most promising powered-lift concepts is the upper surface blown (USB) flap. In this approach, the jet engine efflux becomes attached to the wing upper surface and is turned downward over a trailing-edge flap (Coanda effect), there by increasing lift. This mode of operation produces aerodynamic and acoustic loads on the airplane that are significantly higher than those experienced by conventional airplanes. These higher loads indicate a need for special design efforts to prevent fatigue failures and to obtain acceptable cabin-interior noise levels. In July 1969, the Defense Science Board produced a report urging the use of prototyping by DoD to yield better, less costly, more competitive weapon systems. Deputy Secretary of Defense David Packard was a strong advocate of the prototyping approach and, in 1971, an Air Force committee recommended six systems as candidates, including a lightweight fighter (which subsequently evolved into the F-16) and the AMST.

Later that year Boeing's preparations for a response to an anticipated Air Force request for proposal (RFP) to design, build, and flight test an AMST Technology Demonstrator rapidly crystallized as the company began to develop its candidate for the competition. John K. (Jack) Wimpress received the AIAA Design Award in 1978 for the YC-14's conception, design, and development, and he was the only Boeing person to be with the YC-14 Program from its inception to its end.

Boeing had accumulated considerable expertise in powered-lift concepts, having proposed the EBF concept for its unsuccessful C-5 competitor as previously discussed and having conducted flight research with NASA using the Boeing 707 prototype (known as the 367-80) modified with sophisticated leading-edge devices and BLC on both leading- and trailing-edge flaps. Along with most of the aeronautical community, Boeing had maintained an awareness of NASA's development of various powered-lift concepts. In its RFP preparations, Boeing examined several powered-lift concepts, including boundary-layer control and, of course, the EBF. Early on, the company was convinced that a twin-engine design offered considerable advantages for the AMST from the perspectives of cost and safety. BLC would not provide the level of lift required via engine bleed air, and the use of an underwing, pod-mounted twin-engine layout for an EBF configuration would require the engines to be located very close to the fuselage to minimize rolling and yawing moments if an engine became inoperative. Boeing was concerned that large aerodynamic interference effects would occur with such an arrangement, particularly at cruise conditions. Thus, Boeing was searching for a new concept that would permit the deflection of jet flow behind a twin-engine arrangement.

Boeing had analyzed the previously discussed exploratory upper-surface blowing tests published a decade earlier and was interested if NASA had since conducted additional research on the concept. Semispan USB research had been conducted in the NASA Langley 12-ft tunnel. An examination of the preliminary results revealed that the magnitude of lift generated was as high as had ever been seen for any powered-lift system. The Langley data were the key enablers for a twin-engine STOL configuration layout. In particular, with the engines on top of the wing, they could be placed close to the centerline of the airplane without causing large aerodynamic inference with the fuselage. Boeing immediately started to build wind-tunnel models to verify the NASA data with geometric and engine parameters more closely representing configurations that Boeing was actually considering. By the end of 1971, Boeing was hard at work in several wind tunnels assessing and refining the twin-engine configuration.

When the Air Force RFP for the AMST prototypes was released in January 1972, it called for the very impressive capability of operations into and out of a 2,000-ft semiprepared field at the midpoint of a 500 nmi mission while carrying a 27,000-lb payload both ways. By comparison, the C-130 series in operation at that time required field lengths almost twice as long to lift a 27,000 lb payload. Following the submittal of its proposal in March 1972, Boeing conducted many wind-tunnel and engine test-stand investigations to refine its proposed design and to identify and solve potential problems. In November, Wimpress again visited Langley for an update on NASA's USB research activities. Joe Johnson and Dudley Hammond both reported on testing being conducted in their organizations and showed Wimpress experimental data that verified the high-lift performance that Boeing had submitted in its proposal.

On November 10, 1972, the Air Force selected Boeing and McDonnell Douglas as contractors to work on the AMST prototypes. Following the contract award, Boeing launched an aggressive development program to actually design the airplane. Considerable efforts were required for the development of an acceptable USB nozzle, and a major technical surprise occurred when Boeing discovered that the forward flow over the airplane during lowspeed operations had a degrading effect on the USB flap, reducing the jet spreading and causing separation ahead of the flap trailing edge. This phenomenon had not been noted in earlier NASA or Boeing wind-tunnel testing. Results from those earlier tests had led to the conclusion that forward speed effects would not significantly impact the flow-turning capability of the nozzle. Boeing added vortex generators to the YC-14 configuration to re-energize the flow and promote attachment on the USB flap during STOL operations. The vortex generators were extended only when the USB flap was deployed beyond 30° and were retracted against the wing surface during cruise. Boeing adopted a supercritical airfoil for the wing of the YC-14 based on internal aerodynamic research following the 747's design. Initially, senior aerodynamicists at the company were reluctant to accept such a radical airfoil shape.

After reviewing ongoing supercritical wing research at Langley led by Richard T. Whitcomb, they were impressed by the performance of a supercritical airfoil applied to a Navy T-2C aircraft in a research program by Langley. Confidence in the design methodology for the new family of airfoils was provided by close correlation of wind-tunnel predictions and actual flight results obtained with the T-2C. With the NASA data in hand, Boeing proceeded to implement the supercritical technology for the YC-14 and for its subsequent civil commercial transports, including the 777.

Applications of supercritical wing technology to larger transport aircraft in the United States began with prototype military transports in 1976-Boeing's application to the YC-14 and McDonnell Douglas' application to the YC-15. The supercritical airfoil, developed at the Langley Research Center, uses a unique geometric shape to control the characteristics of the supersonic flow in a manner to minimize drag and enhance the cruise efficiency of the transport. The curvature of the middle region of the upper surface of the supercritical airfoil is significantly reduced and carefully tailored to result in a more rearward location and substantial decrease in the strength of the shock wave, and drag for a given lift coefficient is reduced.

During the development process, Boeing was faced with determining the size of the horizontal tail and its placement on the configuration. The initial proposal airplane had a horizontal tail mounted on the end of a long extended body atop a vertical tail with relatively high sweep. However, as the design evolved it became apparent that the proposal configuration would not adequately accommodate the large nose-down pitching moments of the powered-lift system or ground effects. Boeing examined the parametric design information on longitudinal stability and trim that Langley tests had produced in the Full-Scale Tunnel and the V/STOL Tunnel, indicating that it was very desirable to place the horizontal tail in a position that was more forward and higher than the position that Boeing had used for the proposal configuration. These Langley data provided critical guidelines in the tail configuration's revision for the YC-14's final version.

By December 1975, Langley had negotiated with Boeing to obtain full-scale data on a USB high-lift system. Boeing conducted full-scale powered ground tests of a complete YC-14 wing-flap-fuselage segment at its Tulalip test facility to evaluate the effectiveness and noise levels of its powered system. During the tests, sound levels and pressure distributions were measured by Boeing over the USB flap and the fuselage next to the flap. These data were made available to Langley under the special research contract. Langley's interest was stimulated in part by the fact that the engine nozzle of the YC-14 design incorporated a D-nozzle (a semielliptical exit shape), which differed from the high aspect-ratio rectangular nozzles that had been used at Langley in the full-scale Aero Commander tests previously discussed. With the full-scale YC-14 data in hand, Langley proceeded with a test program to determine the adequacy of subscale models to predict such information, including the development of scaling relationships required for the various technologies involved.

A 0.25-scale model static ground tests of the Boeing YC-14 powered lift system were conducted at the outdoor test site near the Full-Scale Tunnel for correlation with full-scale test results. The model used a JT-15D turbofan engine to represent the CF6-50D engine used on the YC-14. The tests included evaluations of static turning performance, static surface pressure and temperature distributions, fluctuating loads, and physical accelerations of portions of the wing, flaps, and fuselage. Results were obtained for the landing flap configuration over a range of fan pressure ratio for various ground heights and vortex generator modifications.

The USAF YC-14 prototype STOL aircraft, first flight tested in 1976, successfully implemented optical data links to exchange data between the triplex computers. Optical coupling was selected to maintain inter-channel integrity. Each sensor output is coupled t o the other channels so that each computer has data from each of the sensors. Identical algorithms in each computer consolidate the data enabling equalization and fault detection / isolation of the inputs. The computers are synchronized to avoid sampling time differences and to assure all computers are receiving identical data inputs. The optical communication medium was used to eliminate electromagnetic interference effects, electrical grounding loop problems, and the potential propagation of electrical malfunctions between channels.

The behavior of pressure fluctuations measured on the airframe of a prototype high lift jet transport (YC-14) are characterized in terms of a particular jet exhaust flow field idealization, jet mixing noise, and exhaust shock noise. Generalized spectrum shapes and scaling relations for peak level and frequency of peak level were developed, and the frequency is found to depend on jet exhaust velocity and aircraft velocity. Comparisons are made with near-field engine exhaust noise of a conventional jet, and results suggest that the same two exhaust noises are important for both aircraft types. Surface fluctuating pressure data are assessed, and results suggest that the jet mixing and exhaust shock noise source characterizations for the YC-14 have useful applicability to conventionally configured jets.

One quarter scale static ground tests of the Boeing YC-14 powered lift system were conducted for correlation with full scale test results. The 1/4 scale model utilized a JT-15D turbofan engine to represent the CF6-50D engine employed on the YC-14 advanced medium STOL transport prototype aircraft. The tests included evaluation of static turning performance, static surface pressure and temperature distributions, fluctuating loads, and accelerations of portions of the wing, flaps, and fuselage. Results are presented for the landing flap configuration over an appropriate range of fan pressure ratio as affected by several variables including ground height and vortex generator modifications. Static turning angles of the order of 60 deg were obtained. The highest surface pressures and temperatures were concentrated over the upper surface of the flaps in the region immediately aft of the upper surface blown nozzle.

Flow turning parameters, static pressures, surface temperatures, surface fluctuating pressures and acceleration levels were measured in the environment of a full-scale upper surface blowing (USB) propulsive-lift test configuration. The test components included a flightworthy CF6-50D engine, nacelle and USB flap assembly utilized in conjunction with ground verification testing of the USAF YC-14 Advanced Medium STOL Transport propulsion system. Results, based on a preliminary analysis of the data, generally show reasonable agreement with predicted levels based on model data. However, additional detailed analysis is required to confirm the preliminary evaluation, to help delineate certain discrepancies with model data and to establish a basis for future flight test comparisons.

The YC-14 prototype's first flight occurred on August 9, 1976. YC-14 and YC-15 airplane capabilities were evaluated in a flight test program at Edwards Air Force Base in early November 1976. By the end of April 1977, the very successful YC-14 Program had exceeded all its projected goals in terms of flight hours, test conditions accomplished, and data accumulated. The performance goals were met in terms of maneuvering, field length, and touchdown dispersion. Following the flight test program, Boeing demonstrated the YC-14 to U.S. forces in Europe, including an appearance at the Paris Air Show in June. The airplane impressed the crowds at the air show, performing maneuvers formally considered impossible for a medium-sized transport. After the European tour, the YC-14 arrived for a demonstration at Langley Air Force Base on June 18, 1977, where its outstanding STOL capability and crisp maneuvers stunned not only the Air Force observers but many of the NASA-Langley researchers who had participated in USB studies that helped contribute to the design and success of this remarkable airplane.

The YC-14 flight test program ended on August 8, 1977, exactly 1 year after it began. Unfortunately, the anticipated mission of the AMST did not meet with Air Force funding priorities at the end of the flight evaluations (the B-1B bomber was by then the top Air Force priority), and the AMST Program ended. In 1981, the Air Force became interested in another transport, one having less STOL capability but more strategic airlift capability than the AMST YC-14 and YC-15 airplanes. That airplane was ultimately developed to become today's C-17 transport. The two YC-14 prototype aircraft were placed in storage at the Davis Monthan Air Force Base, and one was later moved to the Pima Air Museum in Tucson, Arizona, where it is displayed next to one of the YC-15 aircraft.