In retrospect, the Concorde was doomed from the start. Besides being fuel-hungry, the legendary supersonic jet created sonic booms that were simply too loud for comfort, which prevented the aircraft from flying lucrative overland routes, such as New York to Los Angeles, and kept it an expensive luxury.
More than a decade after Concorde was retired, researchers are investigating new ways to build supersonic aircraft by going back to an old idea: the biplane. New research, most recently from the Massachusetts Institute of Technology, shows promise for solving both major challenges to supersonic transports—cost and noise—by turning to this configuration long abandoned by aircraft designers.
Ahead of Its Time
Back in 1935, German aeronautical engineer Adolf Busemann proposed a radical solution to a problem that did not yet exist. In that era, airplane builders foresaw that biplanes would soon be replaced by single-wing aircraft. But without access to modern computational tools, Busemann attacked the theoretical problem of supersonic flight and showed that biplanes could actually exceed the speed of sound without creating the hefty shock waves (and resulting sonic booms) that jets such as the Concorde would encounter, and with a minimum of drag. In Busemann's biplane design, the noise-producing shock waves would not propagate outward from the front and rear of the craft, but rather from between the two sets of stacked wings, canceling each other out.
There was one major catch (besides the fact that it wasn't actually possible to build the plane with 1935 technology): Busemann's biplane design would work only at a set design speed, say Mach 1.7, and then only if the proposed craft first accelerated to a much higher speed, Mach 2.18, before dropping back to reduced-drag flight at Mach 1.7. Getting up to that much higher speed would be dicey and perhaps impossible, given the extreme drag encountered on the way.
And so, Busemann's biplane concept remained a historical curiosity for 70 years. Then, beginning in 2004, researchers led by Kazuhiro Kusunose at Tohoku University in Japan applied modern computational fluid dynamics to the venerable design. , the group found that by varying the shape of the four sets of wings in flight via moveable flaps and slats, they could get a theoretical airplane to fly with acceptable drag through all phases of flight, from takeoff to landing.
Meanwhile, at MIT...
When Rui Hu was a Ph.D. student at Stanford University, her adviser, Antony Jameson, introduced her to the Tohoku work. Hu saw it as an inelegant solution. "For supersonic flight, if you need to operate these moving parts, it will increase the possibility for instability," she tells PM. Coming to grips with the limitations of Busemann's biplane became the focus of her Ph.D. thesis. She continued her work on the subject as a postdoc at MIT, where she was further aided in her quest for a better supersonic biplane by Qiqi Wang.
The result of the trio's work, to be published in the Journal of Aircraft, is a biplane airfoil design that should enable smooth flight to supersonic cruising speed with minimal drag, and without having to first accelerate beyond the optimal design speed. The team reached its final design by running computer models of about 700 airfoil configurations and a dozen possible supersonic cruise speeds.
"We really want to make Concord-like flight feasible in the future," Wang tells PM. "If the drag problem is reduced, that will reduce the cost of fuel and reduce the ticket price for supersonic transport. If we can reduce the sonic boom of the airplane by a large factor, then there is a possibility that this airplane is going to be allowed to fly supersonically over land."
The optimized Busemann biplane design could overcome both barriers to supersonic passenger flight. But don't start saving up for supersonic tickets just yet. While the Japanese researchers have modeled their designs in three dimensions, the MIT group is still working with easier-to-calculate 2D computer models. "If we can do this in three dimensions, it will become very interesting," Hu says. "[But] how we design a real airfoil and airplane, I have to say, we don't know."