Busemann biplane
Theoretical biplane that cancels shock waves but originally produces no lift.
The Busemann biplane is a theoretical aircraft design by Adolf Busemann that prevents N-type shock waves, so it produces no sonic boom or wave drag. However, the original version also generates no lift. A modern variant of the Busemann biplane, which does create sufficient lift and can reduce—but not eliminate—wave intensity and drag, has been explored for a "boomless" supersonic transport.
Busemann’s original biplane has two triangular cross-section plates spaced apart, with their flat sides aligned parallel to the airflow. The gap is large enough to avoid choking and maintain supersonic flow between them. In conventional supersonic wings, compressive shock waves form at the leading and trailing edges, with an expansion wave in between, causing pressure changes that create wave drag. In the Busemann biplane, the forward high-pressure shock wave forms internally and reflects symmetrically between the double-wedge inner surfaces. These reflections cancel each other and subsequent shock waves, so no external wave propagates outward, eliminating wave drag. The flat upper and lower surfaces produce no shock waves because the flow remains parallel. This internal alignment minimizes wave drag, but the flat external surfaces and internal symmetry mean the design produces no lift at its optimal design point for shock and drag reduction.
Operating away from the design cruise speed or angle of attack disrupts the constructive interference, leading to shock choking and flow hysteresis, which greatly increase drag. In shock choking, the shockwaves reduce their backward angle with each reflection off the tapered wing surfaces until they form a shock wall across the gap. This builds pressure and slows airflow, causing hysteresis: the choking persists through and beyond the design point until the aircraft reaches a higher speed to clear it.
To generate lift, Newton’s laws require deflecting air downward, which at supersonic speeds creates at least one shock wave. Giving the Busemann biplane a small positive angle of attack produces lift but also generates external shockwaves. The configuration can still minimize the energy of these shock waves and associated drag. Wave drag has two sources: one from the aircraft’s form and one from lift. The Busemann concept can eliminate form shock drag but not lift-related drag.
- Inventor
- Adolf Busemann
- Original form
- two triangular cross-section plates with flat sides parallel to fluid flow
- Key property
- avoids N-type shock waves, eliminates sonic boom and wave drag
- Lift at design point
- none
- Off-design issues
- shock choking and flow hysteresis
- Modern concept
- provides lift while reducing form drag, studied for boomless supersonic transport
Lore & Background
Busemann's original biplane consists of two triangular cross-section plates spaced apart such that supersonic flow is maintained between them. In conventional supersonic wings, compressive shock waves form at leading and trailing edges, creating wave drag. In the Busemann biplane, the forward high-pressure shock wave is created internally and reflects symmetrically between double-wedge inner surfaces, interfering to cancel themselves and following shock waves, leaving no external wave to propagate to infinity. The flat upper and lower surfaces generate no shock waves because the flow is parallel. This internal alignment means the design produces minimum wave drag, but the flat external surfaces and internal symmetry also mean it produces no lift at the design point for optimal shock and drag reduction.
Operation away from the design cruise speed or angle of attack destroys the constructive interference, resulting in shock choking and flow hysteresis effects that greatly increase drag. In shock choking, shockwaves reduce their backwards angle with each reflection until they form a shock wall across the gap, causing pressure buildup and flow slowdown. Flow hysteresis causes the choking to persist through and beyond the design point before clearing at a higher aircraft speed.
To obtain upward lift, the air passing over the wings must be deflected downwards, which at supersonic speeds creates at least one shock wave. The Busemann biplane can be given a small positive angle of attack to generate lift, but it will then generate external shockwaves. The configuration can still minimize the energy of these shock waves and associated drag. Wave drag has two causes: form drag and lift-induced drag. The Busemann concept can eliminate form shock drag but not that due to lift. Modern Busemann-type designs can create lift while still eliminating much or all of the form drag, achieving considerable efficiency improvements.
Reader's Guide
The Busemann biplane is significant because it demonstrates a theoretical method to eliminate sonic boom and wave drag entirely, though at the cost of lift. Its legacy lies in inspiring modern concepts that trade complete drag elimination for practical lift generation, offering a path toward 'boomless' supersonic transport. The article notes that off-design problems such as shock choking and hysteresis can be resolved using variable-geometry devices like flaps and slats, or by modifying airfoil geometry to accept some form drag even at the optimum design point. The concept has been studied specifically for a boomless supersonic transport, indicating its potential to reduce the environmental impact of supersonic flight. The article also draws a parallel to the Pratt & Whitney J58 engine's mixed compression inlet, which is sensitive to 'unstarts'—a related phenomenon of shock expulsion. Additionally, it compares the Busemann biplane to catamaran boats that reduce wave-making resistance via a split hull configuration, highlighting the broader principle of wave cancellation through geometry.
Did You Know?
- The original Busemann biplane produces no lift at its design point for optimal shock and drag reduction.
- Off-design operation causes shock choking, where shockwaves form a shock wall across the gap between the plates.
- Modern Busemann-type designs can eliminate form drag but not the drag due to lift.
- The concept has been studied for a 'boomless' supersonic transport.
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