A Shift to Low-Altitude Flight Operations Dictated a Wing Redesign
Aviation Week & Space Technology
03/27/2006, page 59
William B. Scott Los Angeles
Revised USAF requirements forced Northrop’s B-2 design engineers to essentially start over
Printed headline: Wing Redesign
Two years into the B-2 development program, Northrop managers decided that an altered U.S. Air Force requirement to operate at high speeds and low altitudes would force a redesign of the bomber’s wing to handle high gust-induced loading.
The original dual-“V” trailing-edge planform was tailored for high-altitude flight, and would not handle the high gust loads of low-level turbulence (see graphic, p. 57).
Initially, the analytical tools necessary to identify this gust-loading deficiency were not available. A team of experts devoted a year to developing a sophisticated computer model of the B-2, which ultimately showed “we couldn’t get adequate gust-load alleviation with the [original] planform and still meet range-payload requirements. It just wasn’t possible,” recalls Albert F. Myers, Northrop Grumman’s corporate vice president for strategy and technology. “With the [analytical] model, it took about another year to develop a solution. We had to completely change the wing carry-through structure.”
The B-2 is one of aviation history’s few aircraft with a structure designed specifically to meet gust-loading criteria. “Most military aircraft are maneuvering-load designs. The B-2’s a gust-load design,” Myers says.
The original B-2 planform could not develop enough flight control power to compensate for sharp gusts. “For gust-load alleviation, you have to detect the gust [via] the air data system, then weathercock the aircraft into that gust,” he explains. Before a gust shoves the B-2’s nose up, the air data system senses that pending upward motion and sends a signal to the flight control computers, which command trailing-edge elevons to rapidly pitch the nose down. In other words, the flight control system must quickly respond and counteract each gust to ensure a smooth ride for the crew, and to minimize loads.
“Most of the energy for structural loads is in the wing first-bending mode, the lowest-frequency mode,” Myers says. In the original B-2 design, “the only control surfaces that could pitch the aircraft into the gust were almost on top of the bending node line. You can’t put work into something if you don’t have leverage.” Consequently, the wing planform was changed to add an inboard-elevon control surface, giving the planform its current sawtooth, quad-“V” trailing-edge.
“THIS INBOARD ELEVON–a huge control surface, bigger than most fighter-airplane wings–is in an excellent location with respect to the first wing-bending node line, and is able to take energy out of it,” he notes. “The primary load paths of the aircraft–those across the wing carry-through structure–are now designed to benefit from a nearly 40% reduction in [gust loading].”
Although a moveable “beavertail,” the aft-center tip of the aircraft’s fuselage, is called the Gust Load Alleviation Surface (GLAS), it has minimal ability to counter wing loads. “Most people today believe this surface [GLAS] provides gust-load alleviation, because of its name. Not so. It has too much [wing] chord in front of it,” Myers declares. “Frankly, if it hadn’t cost more to take it off the airplane than to leave it–[due] to where we were in the design process–we’d have probably eliminated it.”
The redesigned wing’s new inboard elevon provided most of the gust-load-alleviation power needed, but it had to react very quickly. “It’s like moving an entire fighter wing at 100 deg./sec.,” Myers says. “You have to detect the gust with the air data system . . . and be able to beat the gust’s [effect on the aircraft]. So, this is a very, very phase-sensitive design. It’s so phase-sensitive that we moved the air data sensors from the [wing] to right on the tip of the nose, just to get them a few feet closer to the aircraft’s leading edge.” That relatively short distance had a positive impact on detection-versus-response times, significantly improving the degree of gust-load alleviation achieved.
THE WING REDESIGN also improved B-2 handling qualities. Originally, the design featured a sharp wing leading edge that ran from wingtip to nose. At higher angles-of-attack (AOA), airflow over the wing would suddenly “break, and what was already an unstable configuration would become wildly unstable,” Myers says. “That was a real concern. As it turned out, that break–[the AOA] where the stability level changed–was in the [AOA regime] we needed to operate. It was a single problem that had severe longitudinal- and lateral-directional implications.” Stated another way, when the wing stalled, the B-2 became unstable in all three axes–pitch, roll and yaw.
To resolve high-AOA instability, Northrop reshaped the wing’s leading edge as part of the redesign. Now, the bomber has sharp points at its outer wingtip and nose, but a rounded leading edge in between.
And why does the B-2 have a beak-like nose? “There’s very little on this airplane that doesn’t have low-observability implications,” Myers says obliquely. “Unfortunately, that’s about all I can say about it.”
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