Qiqi Wang (MIT), Rui Hu (MIT) and Antony Jameson (Stanford) have shown through a computer model that a modified biplane can, in fact, produce significantly less drag than a conventional single-wing aircraft at supersonic cruise speeds. The group will publish their results in the Journal of Aircraft.
The design changes could cut the amount of fuel needed for a supersonic aircraft by half. That could also help hypersonic military weapons or hypersonic civilian jetliners that travel at more than five times the speed of sound. The two wing can be designed to cancel out each sonic boom.
With Wang’s design, a jet with two wings — one positioned above the other — would cancel out the shock waves produced from either wing alone. Wang credits German engineer Adolf Busemann for the original concept. In the 1950s, Busemann came up with a biplane design that essentially eliminates shock waves at supersonic speeds.
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.
Japanese also working on supersonic biplane
A new theory that significantly reduces shock waves for supersonic transport (SST) has been established by Prof. Kusunose’s group at Tohoku University under the 21st Century COE Program. The theory introduces a second wing nearly parallel to the conventional wing. Kusunose team verified that the biplane configuration reduces shock wave effects felt on the ground by 85%.
Shock waves created by airfoils during supersonic flight. single airfoil (diamond airfoil) shock canceling biplane airfoils
A supersonic biplane concept created by Kazuhiro Kusunose and colleagues at Tohoku University in Japan.
Conceptual drawing of silent supersonic Japanese aircraft in flight
MIT Supersonic Biplane
This paper addresses the aerodynamic performance of Busemann type supersonic biplane at off-design conditions. An adjoint based optimization technique is used to optimize the aerodynamic shape of the biplane to reduce the wave drag at a series of Mach numbers ranging from 1.1 to 1.7, at both acceleration and deceleration conditions. The optimized biplane airfoils dramatically reduces the effects of the choked flow and flow-hysteresis phenomena, while maintaining a certain degree of favorable shockwave interaction effects at the design Mach number. Compared to a diamond shaped single airfoil of the same total thickness, the wave drag of our optimized biplane is lower at almost all Mach numbers, and is significantly lower at the design Mach number.
In this paper, the favorable shock wave interaction of the supersonic biplane airfoil is studied. Two dimensional numerical simulation results show that the Busemann biplane airfoil produces very low wave drag at design condition due to the perfect shock-expansion wave cancellation. But for off-design conditions, the Busemann biplane airfoil performance is poor. To overcome the choked-flow and flow-hysteresis problems of the Busemann biplane at off-design conditions, the inviscid compressible flow (Euler) optimization techniques based on control theory have been applied.
In order to obtain an optimized supersonic airfoil with lower wave drag within the given optimization Mach number range, a multiple design point strategy is employed. The optimized biplane airfoil shows good performance at both design and off-design conditions. The flow-hysteresis phenomenon of the optimized airfoil still exists but the area is greatly reduced compared to that of the baseline Busemann biplane and the wave drag caused by choked flow is also much lower. For inviscid flow, the wave drag of the optimized biplane airfoil is lower than that of the diamond airfoil with the same total thickness throughout the optimization range. The two sensitivity studies show that the optimized design is robust and not very sensitive to the change of the angle of attack or the separation distance.
MIT Supersonic Biplane Giving lift to a grounded theory
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,” Wang says. “It’s kind of a chain reaction.”
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 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.
“Now people are having more ideas on how to improve [Busemann’s] design,” Wang says. “This may lead to a dramatic improvement, and there may be a boom in the field in the coming years.”