Delta wing
A triangular wing form optimized for high-speed flight.
A delta wing is a triangular wing, named after the Greek letter delta (Δ). While studied for a long time, it wasn't used much in practice until the Jet Age, when it became ideal for fast subsonic and supersonic flight. At the other end of the speed range, the flexible Rogallo wing proved useful for hang gliders and other ultralight aircraft. The delta shape offers unique aerodynamics and structural benefits, and many design variations have appeared over time, some with extra stabilizing surfaces.
Structurally, the delta wing's long root chord and small outer area make it efficient. It can be built stronger, stiffer, and lighter than a swept wing with the same lift and aspect ratio, and it is also cheap and easy to build—a key reason for the success of the MiG-21 and Mirage. The long root chord allows a thicker wing structure for a given airfoil, saving weight and providing more internal space for fuel and other items without much drag increase. On supersonic designs, however, a thinner airfoil is often used instead to cut drag.
In low-speed flight, a delta wing needs a high angle of attack to generate lift. At a high enough angle, the flow separates, causing high drag. Normally this leads to a stall, but on a sharply swept delta, air spilling up around the leading edge flows inward, creating a vortex over the upper surface. The lower part of this vortex stays attached and speeds up the airflow, maintaining lift. For moderate sweep angles, a retractable "moustache" or fixed leading-edge root extension (LERX) can be added to encourage and stabilize the vortex. The ogee or "wineglass" double curve, seen on Concorde, builds this forward extension into the wing shape. In this condition, the center of lift is roughly at the center of the area covered by the vortex.
In subsonic flight, a delta wing behaves much like a swept wing, with a sideways airflow developing. Lift is greatest along the leading edge, where the air turns most sharply. For a slender delta, the center of lift is about halfway back along the leading edge. The sideways flow reduces overall lift and can increase drag, which can be countered with leading-edge slots, wing fences, or similar devices.
At transonic to low supersonic speeds, if the wing has enough rearward sweep, its leading edge stays behind the shock wave or shock cone from the root.
- Shape
- Triangular, resembling Greek uppercase delta (Δ)
- Structural efficiency
- Can be built stronger, stiffer, and lighter than a swept wing of equivalent aspect ratio
- Low speed behavior
- Requires high angle of attack; generates vortex lift at sufficiently high angles
- Subsonic center of lift
- Approximates halfway back along the leading edge for a slender delta
- Supersonic waveriding example
- North American XB-70 Valkyrie
Lore & Background
The delta wing form has unique aerodynamic characteristics and structural advantages. Its long root chord and minimal area outboard make it structurally efficient, allowing it to be built stronger, stiffer, and lighter than a swept wing of equivalent aspect ratio. This also provides greater internal volume for fuel and other items without significant drag increase, though supersonic designs often use a thinner aerofoil to reduce drag. Many design variations have evolved, including cropped, compound, ogee, tailed, and canard deltas.
At low speeds, a delta wing requires a high angle of attack to maintain lift. For sharply-swept deltas, flow separation generates a characteristic vortex pattern over the upper surface that maintains lift. In subsonic flight, its behavior is similar to a swept wing, with a sideways airflow element that can reduce lift and increase drag. At transonic to low supersonic speeds, the wing's leading edge remains behind the shock cone, allowing sideways flow and maintaining subsonic lifting characteristics.
At high supersonic speeds, the shock cone angles back along the wing surface, eliminating sideways flow and enabling waverider designs like the North American XB-70 Valkyrie. The tailless delta offers structural simplicity and low drag, while the tailed delta allows a smaller, more highly loaded wing. The canard delta configuration, used on many modern fighters, can improve maneuverability and reduce stalling speeds through vortex coupling.
Reader's Guide
The delta wing's significance lies in its structural efficiency and aerodynamic suitability for high-speed flight. Its long root chord allows a thicker wing structure for a given aerofoil section, enhancing weight savings and providing internal volume for fuel. This made it easy and relatively inexpensive to build, contributing to the success of the MiG-21 and Mirage aircraft series. The tailless delta's structural simplicity and low drag helped make the Dassault Mirage III one of the most widely manufactured supersonic fighters.
Design variations address specific challenges. The cropped delta maintains lift outboard and reduces wingtip stalling. The compound delta creates a controlled high-lift vortex without a foreplane, as seen on the Saab Draken. The tailed delta, common on Soviet types like the MiG-21, allows the main wing to be optimized for lift but can cause deep stall issues, as with the Gloster Javelin. The canard delta, used on the JAS 39 Gripen, Eurofighter Typhoon, and Dassault Rafale, offers a smaller shift in center of lift with increasing Mach number and can improve maneuverability through vortex coupling.
The Rogallo flexible wing, a delta variant, proved practical for hang gliders and ultralight aircraft. The ogee delta, used on Concorde, merges two sections into a smooth curve. At high supersonic speeds, the waverider design becomes practicable, as on the XB-70 Valkyrie, where a shock body creates attached shockwaves providing lift without increasing drag.
Did You Know?
- The delta wing is named for its similarity in shape to the Greek uppercase letter delta (Δ).
- The long root chord of the delta wing allows a thicker wing structure for a given aerofoil section, providing greater internal volume for fuel.
- The ogee or 'wineglass' double-curve delta was used on Concorde.
- The tailless delta wing is not suited to high wing loadings and requires a large wing area for a given aircraft weight.
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