The Busemann biplane
In 1935, Adolf Busemann proposed a biplane concept that divided a diamond airfoil into two components and placed the triangular surfaces facing each other—i.e., the flat sides are parallel to the fluid flow.
The Busemann airfoil reduces the wave drag by the interference of the shock and expansion waves which generate between the biplane.
Professor Kusunose’s group at Tohoku University in Japan verified that the Busemann biplane configuration reduces shock wave effects felt on the ground by 85%.

For decades, the speed of commercial aircraft was constrained by the sound barrier. Even with the most successful Concorde, supersonic flight was only available on a small number routes and for those are willing and able to pay for the expensive airplane tickets. The two major challenges for supersonic flight are high drag due to shock waves and the sonic boom.

The biplane concept proposed by Adolf Busemann can potentially solve both the high drag and the sonic boom problems. At the design condition, the Busemann biplane produces zero wave drag and no sonic boom will escape from the biplane system due to the wave cancellation between two airfoil components.

—Hu et al. (2011)

Much research was performed on the Busemann biplane concept from 1935 to 1960s, and interest has recently ticked up amongst a number of research groups. Although the Busemann airfoil demonstrates very good performance at the design Mach number, Wang and his colleagues note, the drag of the Busemann airfoil at the off-design conditions is much higher due to several phenomena (choked-flow and flow-hysteresis).

Wang and colleagues used multiple point adjoint based aerodynamic design and optimization method to improve the baseline Busemann biplane airfoil’s off-design performance and alleviate the flow hysteresis problem.

Normally, as a conventional jet nears the speed of sound, air starts to compress at the front and back of the jet. As the plane reaches and surpasses the speed of sound, or Mach 1, the sudden increase in air pressure creates two huge shock waves that radiate out at both ends of the plane, producing a sonic boom.

Through calculations, Busemann found that a biplane design could essentially do away with shock waves. Each wing of the design, when seen from the side, is shaped like a flattened triangle, with the top and bottom wings pointing toward each other. The configuration, according to his calculations, cancels out shock waves produced by each wing alone.

However, the design lacks lift: the two wings create a very narrow channel through which only a limited amount of air can flow. When transitioning to supersonic speeds, the channel, Wang says, could essentially “choke,” creating incredible drag. While the design could work beautifully at supersonic speeds, it can’t overcome the drag to reach those speeds.

To address the drag issue, Wang, Hu and Jameson designed a computer model to simulate the performance of Busemann’s biplane at various speeds. At a given speed, the model determined the optimal wing shape to minimize drag. The researchers then aggregated the results from a dozen different speeds and 700 wing configurations to come up with an optimal shape for each wing.

They found that smoothing out the inner surface of each wing slightly created a wider channel through which air could flow. The researchers also found that by bumping out the top edge of the higher wing, and the bottom edge of the lower wing, the conceptual plane was able to fly at supersonic speeds, with half the drag of conventional supersonic jets such as the Concorde. Wang says this kind of performance could potentially cut the amount of fuel required to fly the plane by more than half.

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. It’s kind of a chain reaction.

—Qiqi Wang

The team’s next step is to design a three-dimensional model to account for other factors affecting flight. While the MIT researchers are looking for a single optimal design for supersonic flight, Wang points out that a group at Tohoku University in Japan has made progress in designing a Busemann-like biplane with moving parts—the wings would essentially change shape in mid-flight to attain supersonic speeds.

There are many challenges in designing realistic supersonic aircraft, such as high drag, efficient engines and low sonic-boom signature. Dr. Wang’s paper presents an important first step towards reducing drag, and there is also potential to address structural issues.

—Karthik Duraisamy, assistant professor of aeronautics and astronautics at Stanford University, who was not involved in the research

Resources

• R. Hu, Q. Wang and A. Jameson. Adjoint based aerodynamic optimization of supersonic biplane airfoils, accepted for publication in Journal of Aircraft.

• Rui Hu, Antony Jameson and Qiqi Wang. Adjoint based aerodynamic optimization of supersonic biplane airfoils. (AIAA 2011-1248)

• Silent Supersonic Transport (Tohoku University)

• Busemann, A. (1935) Aerodynamic Lift at Supersonic Speeds, No. 6, Luftfahrtforschung, 12th ed. pp. 210–220.