Boeing 787 Dreamliner - Mark Wagner [12]
In aeronautical engineering, a Sears-Haack body—which traditionally is cigar-shaped—is the best shape for reducing wave drag. This forms when air passing over the aircraft accelerates to supersonic speeds, even if the aircraft itself is going subsonically. The supersonic flow creates shock waves, which produce wave drag. To minimize this drag, Sonic Cruiser designers tailored the cross-sectional area distribution of the airliner to closely resemble the Sears-Haack shape.
Although high-speed aerodynamics had played a pivotal role in helping Boeing achieve high subsonic cruise speeds with the 707 and the 747 in particular, the most obvious deliberate shaping in U.S. airliner design for higher speeds had been the trailing-edge shock bodies on the Convair CV-990. The aircraft’s Mach 0.91 cruise speed, which at the time made it the world’s fastest airliner, was largely due to large, canoelike fairings on the upper trailing edges of the wings. The devices cut wave drag and were nicknamed “Küchemann Carrots” after their designer, Dr. Dietrich Küchemann.
Boeing’s design breakthrough on the Sonic Cruiser was to shape the aft fuselage, delta wing, and semirecessed engine nacelles so it achieved the much-vaunted Sears-Haack area distribution.
The eye-catching shape genuinely looked like it wanted to go faster, and indeed Boeing aerodynamicists believed the Sonic Cruiser was easily capable of low supersonic speed. The difference with the Sonic Cruiser approach was that it was more economically feasible to produce a high-speed airliner from a transonic design than to work the drag reduction problems of speeding up a conventional subsonic design. Mike Bair said, “Instead of making a slow aircraft go faster, we have a design for a fast aircraft that goes a bit slower.”
All the talk of “sonic” performance inevitably raised questions about Boeing’s longer-term ambitions. Was this a stealthy gambit to gain the once “holy grail” of air transport ambitions: a supersonic airliner? “No,” was Gillette’s short answer. Although the aircraft would need to be tested at speeds slightly in excess of Mach 1 as part of certification requirements for its intended cruise speed of Mach 0.98, Gillette insisted the Sonic Cruiser was not a would-be Concorde in disguise. “We are looking at Mach 1 and not any faster, but we think in ten to twelve years the time will come when we believe the technology will be there for a variable supersonic transport [SST]. Frankly, the maturity of the technology is not yet there for a modern Mach 1.4 to Mach 2 SST,” he said.
Project Glacier pinpointed Mach 0.98 and Mach 1.02 as the apparent “sweet spots” for a transonic design. “The baseline focus is near the speed of sound, and that was selected based on environmental requirements, particularly sonic boom and other criteria. We did look at faster aircraft, and the data indicates so far that a Mach 1.2 or 1.6 aircraft would have a much higher fuel burn. With a twin-engined aircraft you just can’t get the nonstop design range we’re looking for on this project,” added Roundhill.
Besides, Boeing said going just a little bit faster did not make that much difference from the travel time perspective. The Mach 1.02 option “doesn’t make a big difference in terms of time. The aircraft is clearly stable at that higher speed, but it would require a bigger engine because the increase in drag is dramatic,” he said.
Tests at the University of Washington’s low-speed tunnel and Boeing’s transonic tunnel confirmed good stability and performance characteristics to Mach 1.08. “It turns out to be longitudinally statically stable. The canard-configured aircraft showed no indication of ‘Mach tuck,’ and pitch characteristics were ‘steady and level.’ The strain gauges on the wing showed no buffet onset,” said Gillette. As a result of the tests, the canard was moved aft slightly in later configurations, and the vertical tails moved outboard and forward. “We looked at the control needs and the interference