A throwback to early 20th Century aviation may hold the key to eliminating the sonic boom - at least according to researchers at MIT and Stanford University. Strongly reminiscent of biplanes still in use today, the researcher's concept supersonic aircraft introduces a second wing which it is claimed cancels the shockwaves generated by objects near or beyond the sound barrier.
In fact the idea is not a new one. The idea of a biplane to negate the sonic boom was proposed in the 1930s by aviation pioneer Adolf Busemann, also responsible for the idea of swept-wing aircraft.
Aircraft traveling at supersonic speeds cause shockwaves in the air around them. A first boom is caused by the rapid compression of air at the front of the plane, literally pushed together by the aircraft. A second is caused by the negative pressure left in the plane's wake - or rather, the rapid return to normal pressure that follows soon after. Though the two booms separate phenomena, they occur so close together that they they are usually perceived as a single sound. An aircraft in supersonic flight creates a continual boom as it goes.
And that's a problem. Sonic booms might be all part of the drama at an air show, but if you were to live under a supersonic flightpath in regular use, the novelty would likely quickly wear off. There are also the concerns of the effect of supersonic flight upon wildlife which might include shock or injury in the short-term, and wholesale habitat abandonment over time. It's no surprise, then, that a return of commercial supersonic flight is considered a doomed enterprise in certain quarters.
Adolf Busemann's design, known as Busemann's Biplane, features two wings with a triangular length-wise cross section. The points of the wings point towards each other so that the outer face of the wings is completely flat, parallel to the air passing over and under. The wings must be sufficiently far apart that the passage of air between isn't stifled. With this design, the first positive shock wave is created and reflected between the wings, filling the space created in the plane's wake, canceling the negative shockwaves and negating the sonic boom.
What's the catch? At sub-supersonic speeds, a Busemann Biplane doesn't produce sufficient lift under acceleration, undergoing considerable drag. The design is said to work perfectly at supersonic speeds - it's getting to them that is the trouble. So though there may be no sonic boom, there's no flight either.
But the joint MIT/Stanford research at least appears to confirm that Busemann's noise-canceling concept was sound. The study, which used computer simulation, demonstrated that the biplane concept exhibited "significantly less drag." Additional research at Tohoku University in Japan claims Busemann's concept is effective in reducing the intensity of shock waves at ground level by 85 percent.
Better yet, the MIT/Stanford team think they might have cracked the problem of lift at sub-supersonic speeds. Through an iterative processes, modeling differing design variations, the team has discovered that smoothing the wing's inner surface eases the passage of air between the wings. By additionally "bumping out" the outer edges of the two wings, the team has come up with a design it claims will fly below the speed of sound, and with half the drag of Concorde.
"If you think about it, when you take off, not only do you have to carry the passengers, but also the fuel, and if you can reduce the fuel burn, you can reduce how much fuel you need to carry, which in turn reduces the size of the structure you need to carry the fuel," said Qiqi Wang, assistant professor of aeronautics and astronautics at MIT. "It's kind of a chain reaction."
The MIT/Stanford team is now pursuing a 3D model to address other practicalities in flight, with the hopes of approaching a static, optimized design. In contrast, the Japanese concept, identified by Wang, would change shape during flight to adapt to supersonic speeds.