Thursday, March 22, 2012

Supersonic Stringbag? New Look at Busemann's Biplane

From Aviation Week:

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AW&ST On Technology
Biplanes are slow, right? Well researchers in Japan and the US are investigating a biplane configuration to reduce the shockwave drag and sonic boom of supersonic transports.

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Concept: MIT/Tohoku University
The basic idea, first proposed by aerodynamicist Adolf Busemann at the 1935 Volta Congress, is that the shockwaves from the two airfoils cancel each other out, virtually eliminating wave drag.

It's more complicated than that, of course, and researchers atTohoku University in Japan and Massachusetts Institute of Technology and Stanford University in the US are wielding the latest computational fluid dynamics tools in a bid to turn the Busemann Biplane into a practical design.

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Graphics: Tohoku University

A wing generates shockwaves through two different mechanisms: lift and thickness. The Busemann Biplane (left, above) splits the supersonic airfoil into two and divides the lift between them. As wave drag is proportional to the square of lift, this reduces drag. Between the airfoils, compression and expansion waves from the upper and lower surfaces also cancel each other out, almost eliminating wave drag from thickness. Together, these two mechanisms reduce drag dramatically from a conventional supersonic aerofoil.

That works for the design point, but the Busemann Biplane has poor performance "off design", at Mach numbers other than the aircraft's intended supersonic cruise speed. As the aircraft has to accelerate through lower Mach numbers to get to its cruise speed, that's a pretty big disadvantage for the biplane configuration.

The problem is that the biplane acts like a nozzle and flow between the airfoils chokes at transonic speeds, dramatically increasing drag. As the aircraft accelerates, the choking persists to higher Mach numbers - a phenomenon called hysteresis. Only when the choking has cleared does the biplane generate less wave drag than a conventional supersonic airfoil.

Follow the sequence of images below, from M0.6 at the top left to M2.18 at the bottom left, and you can see a strong bow shock form and the biplane remain choked until the shock attaches to the leading edges and is swallowed - the desired low wave-drag condition.

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Tohoku researchers are tackling the choking problem with variable geometry. They looked at three options (left to right, below): morphing to change the throat area between the airfoils; slats and flaps to move the leading and trailing edges; and a Fowler action to extend the wing chord. They selected the slats and flaps, which can also be used as high-lift devices for take-off and landing.

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At MIT and Stanford, profs Qiqu Wang and Antony Jameson and postdoc Rui Hu took a different approach - modeling 700 wing configurations to come up with the optimum fixed shapes for the airfoils. Keeping the distance between the airfoils constant, the leading and trailing edges were moved until choking was minimized. In addition, the airfoils were modified from triangles to diamonds to further improve performance.

The result, says MIT, is an aircraft concept with half the drag of Concorde. According to Hu, compared with a classic Busemann Biplane, the optimized design has a smaller off-design wave-drag penalty from Mach 1.2 to Mach 1.5. Choking has disappeared before the wing reaches its design point at Mach 1.7, where wave drag is dramatically lower than for a conventional supersonic airfoil.
 The next step is to turn the two-dimension airfoil work into a three-dimensional wing model - something Tohoku has already done with its design.

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