Swept wing
A swept wing is a wing angled either backward or occasionally forward from its root rather than perpendicular to the fuselage.
Swept wings have been flown since the pioneer days of aviation. Wing sweep at high speeds was first investigated in Germany as early as 1935 by Albert Betz and Adolph Busemann, finding application just before the end of the Second World War. It has the effect of delaying the shock waves and accompanying aerodynamic drag rise caused by fluid compressibility near the speed of sound, improving performance. Swept wings are therefore almost always used on jet aircraft designed to fly at these speeds.
The term "swept wing" is normally used to mean "swept back", but variants include forward sweep, variable sweep wings and oblique wings in which one side sweeps forward and the other back. The delta wing is also aerodynamically a form of swept wing.
Reasons for sweep
There are three main reasons for sweeping a wing:1. to arrange the center of gravity of the aircraft and the aerodynamic center of the wing to coincide more closely for longitudinal balance, e.g. Messerschmitt Me 163 Komet and Messerschmitt Me 262. Although not a swept wing the wing panels on the Douglas DC-1 outboard of the nacelles also had slight sweepback for similar reasons.
2. to provide longitudinal stability for tailless aircraft, e.g. Messerschmitt Me 163 Komet.
3. most commonly to increase Mach-number capability by delaying to a higher speed the effects of compressibility, e.g. combat aircraft, airliners and business jets.
Other reasons include:
1. enabling a wing carry-through box position to achieve a desired cabin size, e.g. HFB 320 Hansa Jet.
2. providing static aeroelastic relief which reduces bending moments under high g-loadings and may allow a lighter wing structure.
Structural design
For a wing of given span, sweeping it increases the length of the spars running along it from root to tip. This tends to increase weight and reduce stiffness. If the fore-aft chord of the wing also remains the same, the distance between leading and trailing edges reduces, reducing its ability to resist twisting forces. A swept wing of given span and chord must therefore be strengthened and will be heavier than the equivalent unswept wing.A swept wing typically angles backward from its root rather than forwards. Because wings are made as light as possible, they tend to flex under load. This aeroelasticity under aerodynamic load causes the tips to bend upwards in normal flight. Backwards sweep causes the tips to reduce their angle of attack as they bend, reducing their lift and limiting the effect. Forward sweep causes the tips to increase their angle of attack as they bend. This increases their lift causing further bending and hence yet more lift in a cycle which can cause a runaway structural failure. For this reason forward sweep is rare and the wing must be unusually rigid.
There are two sweep angles of importance, one at the leading edge for supersonic aircraft and the other 25% of the way back from the leading edge for subsonic and transonic aircraft. Leading edge sweep is important because the leading edge has to be behind the mach cone to reduce wave drag. The quarter chord line is used because subsonic lift due to angle of attack acts there and, up until the introduction of supercritical sections, the crest was usually close to the quarter chord.
Typical sweep angles vary from 0 for a straight-wing aircraft, to 45 degrees or more for fighters and other high-speed designs.
Aerodynamic design
Subsonic and transonic flight
Shock waves can form on some parts of an aircraft moving at less than the speed of sound. Low-pressure regions around an aircraft cause the flow to accelerate, and at transonic speeds this local acceleration can exceed Mach 1. Localized supersonic flow must return to the freestream conditions around the rest of the aircraft, and as the flow enters an adverse pressure gradient in the aft section of the wing, a discontinuity emerges in the form of a shock wave as the air is forced to rapidly slow and return to ambient pressure.At the point where the density drops, the local speed of sound correspondingly drops and a shock wave can form. This is why in conventional wings, shock waves form first after the maximum Thickness/Chord and why all airliners designed for cruising in the transonic range have supercritical wings that are flatter on top, resulting in minimized angular change of flow to upper surface air. The angular change to the air that is normally part of lift generation is decreased and this lift reduction is compensated for by deeper curved lower surfaces accompanied by a reflex curve at the trailing edge. This results in a much weaker shock wave towards the rear of the upper wing surface and a corresponding increase in critical mach number.
Shock waves require energy to form. This energy is taken out of the aircraft, which has to supply extra thrust to make up for this energy loss. Thus the shocks are seen as a form of drag. Since the shocks form when the local air velocity reaches supersonic speeds, there is a certain "critical mach" speed where sonic flow first appears on the wing. There is a following point called the drag divergence mach number where the effect of the drag from the shocks becomes noticeable. This is normally when the shocks start generating over the wing, which on most aircraft is the largest continually curved surface, and therefore the largest contributor to this effect.
Sweeping the wing has the effect of reducing the curvature of the body as seen from the airflow, by the cosine of the angle of sweep. For instance, a wing with a 45 degree sweep will see a reduction in effective curvature to about 70% of its straight-wing value. This has the effect of increasing the critical Mach by 30%. When applied to large areas of the aircraft, like the wings and empennage, this allows the aircraft to reach speeds closer to Mach 1.
One limiting factor in swept wing design is the so-called "middle effect". If a swept wing is continuous - an oblique swept wing - the pressure isobars will be swept at a continuous angle from tip to tip. However, if the left and right halves are swept back equally, as is common practice, the pressure isobars on the left wing in theory will meet the pressure isobars of the right wing on the centerline at a large angle. As the isobars cannot meet in such a fashion, they will tend to curve on each side as they near the centerline, so that the isobars cross the centerline at right angles to the centerline. This causes an "unsweeping" of the isobars in the wing root region. To combat this unsweeping, German aerodynamicist Dietrich Küchemann proposed and had tested a local indentation of the fuselage above and below the wing root. This proved to not be very effective. During the development of the Douglas DC-8 airliner, uncambered airfoils were used in the wing root area to combat the unsweeping.
Supersonic flight
Swept wings on supersonic aircraft usually lie within the cone-shaped shock wave produced at the nose of the aircraft so they will "see" subsonic airflow and work as subsonic wings. The angle needed to lie behind the cone increases with increasing speed, at Mach 1.3 the angle is about 45 degrees, at Mach 2.0 it is 60 degrees. The angle of the Mach cone formed off the body of the aircraft will be at about sin μ = 1/M.Disadvantages
When a swept wing travels at high speed, the airflow has little time to react and simply flows over the wing almost straight from front to back. At lower speeds the air does have time to react, and is pushed spanwise by the angled leading edge, towards the wing tip. At the wing root, by the fuselage, this has little noticeable effect, but as one moves towards the wingtip the airflow is pushed spanwise not only by the leading edge, but the spanwise moving air beside it. At the tip the airflow is moving along the wing instead of over it, a problem known as spanwise flow.The lift from a wing is generated by the airflow over it from front to rear. With increasing span-wise flow the boundary layers on the surface of the wing have longer to travel, and so are thicker and more susceptible to transition to turbulence or flow separation, also the effective aspect ratio of the wing is less and so air "leaks" around the wing tips reducing their effectiveness. The spanwise flow on swept wings produces airflow that moves the stagnation point on the leading edge of any individual wing segment further beneath the leading edge, increasing effective angle of attack of wing segments relative to its neighbouring forward segment. The result is that wing segments farther towards the rear operate at increasingly higher angles of attack promoting early stall of those segments. This promotes tip stall on back-swept wings, as the tips are most rearward, while delaying tip stall for forward-swept wings, where the tips are forward. With both forward and back-swept wings, the rear of the wing will stall first creating a nose-up moment on the aircraft. If not corrected by the pilot the plane will pitch up, leading to more of the wing stalling and more pitch up in a divergent manner. This uncontrollable instability came to be known as the Sabre dance in reference to the number of North American F-100 Super Sabres that crashed on landing as a result.
Reducing pitch-up to an acceptable level has been done in different ways such as the addition of a fin known as a wing fence on the upper surface of the wing to redirect the flow to a streamwise direction. The MiG-15 was one example of an aircraft fitted with wing fences. Another closely related design was the addition of a dogtooth notch to the leading edge, used on the Avro Arrow interceptor. Other designs took a more radical approach, including the Republic XF-91 Thunderceptor's wing that grew wider towards the tip to provide more lift at the tip. The Handley Page Victor was equipped with a crescent wing, with three values of sweep, about 48 degrees near the wing root where the wing was thickest, a 38 degree transition length and 27 degrees for the remainder to the tip.
Modern solutions to the problem no longer require "custom" designs such as these. The addition of leading-edge slats and large compound flaps to the wings has largely resolved the issue. On fighter designs, the addition of leading-edge extensions, which are typically included to achieve a high level of maneuverability, also serve to add lift during landing and reduce the problem.
In addition to pitch-up there are other complications inherent in a swept-wing configuration. For any given length of wing, the actual span from tip-to-tip is shorter than the same wing that is not swept. There is a strong correlation between low-speed drag and aspect ratio, the span compared to chord, so a swept wing always has more drag at lower speeds. In addition, there is extra torque applied by the wing to the fuselage which has to be allowed for when establishing the transfer of wing-box loads to the fuselage. This results from the significant part of the wing lift which lies behind the attachment length where the wing meets the fuselage.