Boeing 787 Dreamliner - Mark Wagner [13]
As Boeing continued to drive toward finalizing the shape through early 2001, it became increasingly obvious that simple derivatives of 777 engines would not work. “This aircraft has a different relationship of climb thrust to takeoff thrust, and we found the optimal core size was smaller than the 777,” said Roundhill. The findings “argue for a major change, if not a brand-new engine.”
Core size is traditionally set by top-of-climb thrust requirements, while noise limits generally govern fan pressure ratio (PR) and bypass ratio, hence the configuration of the low-pressure spool. Under the standard relationship between these factors, the Sonic Cruiser engine would have had a sea-level thrust of more than one hundred thousand pounds. However, the integrated engine configuration of the Sonic Cruiser, added to a 9.2-foot-diameter size restriction of the “semi S-duct” inlet and Boeing’s exhaust velocity requirements, meant the takeoff thrust target was closer to ninety thousand pounds. As a result, all three enginemakers began studying powerplants using core technology derived from their respective 777 engine families (see chapter 6)—a move that would prove vital to the eventual birth of the 787.
The British Aircraft Corporation/Sud-Aviation–designed Concorde relied on a high finesse ratio, slim fuselage, and a shapely ogee-delta wing platform to smoothly transition the transonic region on its way to Mach 2.2. Boeing’s high-speed concepts, including all the Sonic Cruiser variants, required additional area ruling because they were all wider-fuselage designs. Some area ruling was, however, needed to deal with shock effects on Concorde’s curvaceous belly fairing between the engine nacelles, as can be seen in this view of a British Airways aircraft departing on a spring evening in April 1997. Mark Wagner
Debate meanwhile raged over how big the baseline aircraft should be. Initially sizes ranged from a single-aisle 100-seater to a twin-aisle 300-seater, with cruise speeds between Mach 0.95 and 0.98, and ranges from 6,000 to 9,000 nautical miles. Although the airlines strongly pushed Boeing toward a larger 300-seat family, Boeing was eager to develop the longer-range, point-to-point capability of smaller, midcapacity Sonic Cruisers. The main configuration was a 767-size 250-seater with a 9,000 nautical miles range, giving it the “legs” to fly Singapore to Los Angeles and save three hours over the 747. Increasing range to 10,000 nautical miles would give it capability for London to Sydney nonstop, saving up to five hours—but the extra-range variant would be limited in size to 225 seats.
An innovative design feature of the Sonic Cruiser was its low-drag inlets beneath the wings. Similar in cross section to the ovoid F-16 inlet but skewed outward, they stood proud from the airframe and clear of the boundary layer, ensuring a flow of “clean,” undisturbed air into the shallow S-ducts feeding the buried engines. The configuration, revealed in some detail for the first time at the 2001 Paris Air Show, also posed some challenges, particularly with foreign-object ingestion. Mark Wagner
Early Sonic Cruiser configuration studies included optional aft body configurations with vertical stabilizers canted both inward and outward. They also looked at empennage designs with split elevons and others with a single, wide “beaver tail.” This offered the advantage of increasing the overall length of the inboard wing, thereby reducing the thickness-to-chord ratio (the chord being the distance between the leading and the trailing edges of the wing). This reduced drag and distributed lift over the aft body, thereby reducing loads on the wingbox. A drawback of the beaver tail was reduced cabin length, which made it less suitable to a Y-2 market, but better for a supersonic one-hundred-seater or business-jet design. Further work on this was pursued as the Model 765-071, under NASA’s Fundamental Aeronautics Program. Lia Ravelo
As the Sonic Cruiser concept evolved, the engine moved aft into the nacelle