Design and developmentEdit
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, and 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 they modified their losing entry into the Boeing 747. They 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 an 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 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 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 — 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 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°. 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.