SWEPT WING

A 'swept-wing' is a wing planform common on high-speed aircraft, with the wing swept back instead of being set at right angles to the fuselage. Forward sweep is also used on some aircraft. They were initially used only on fighter aircraft, but have since become almost universal on jets, including airliners and business jets.

Contents

In the transonic

Supersonic

Disadvantages

Forward sweep

History

References

See also

In the transonic

As an aircraft approaches the speed of sound, an effect known as wave drag starts to appear. The air reaches supersonic speed locally over an area on the top of the wing, and this local supersonic region ends in an oblique shock wave, which becomes nearly at ninety degrees to the wing's upper surface. Shock waves require energy to generate, and the energy losses to the shock wave increase drag. The shock can be made more oblique by having the profile of the aircraft change as slowly as possible (high fineness ratio).
This applies to the wing as well, which suggests that wings should have very low aspect ratios, long chord, and be very thin. Examples of this sort of wing can be found on the F-104 Starfighter for instance, which is highly optimized for high-speed performance. However, these same characteristics make a wing have much higher drag at low speeds, and generally have poor performance. The Starfighter has had a large number of landing accidents caused by its very fast landing speeds.
Swept wings essentially "fool" the airflow at high speeds into thinking the wing has a longer and flatter profile than it has as measured "head on" to the wing. At high speeds airflow over the wing travels almost directly front to back, so a wing swept at 45 degrees would see an effective chord 1.4 times the actual chord. This reduces the effects of wave drag, making transonic flight much more economical.^[1]

Supersonic

Airflow at supersonic speeds generates lift through the formation of shock waves, as opposed to the patterns of airflow over and under the wing. These shock waves, as in the transonic case, generate large amounts of drag. One of these shock waves is created by the leading edge of the wing, but contributes little to the lift. In order to minimize the strength of this shock it needs to remain "attached" to the front of the wing, which demands a very sharp leading edge. To better shape the shocks that will contribute to lift, the rest of an ideal supersonic airfoil is roughly diamond-shaped in cross-section. For low-speed lift these same airfoils are very inefficient, leading to poor handling and very high landing speeds.^[2]
One way to avoid the need for a dedicated supersonic wing is to use a highly swept subsonic design. Airflow behind the shock waves of a moving body are reduced to subsonic speeds. This effect is used within the intakes of engines meant to operate in the supersonic, as jet engines are generally incapable of ingesting supersonic air directly. This can also be used to reduce the speed of the air as seen by the wing, using the shocks generated by the nose of the aircraft. As long as the wing lies behind the cone-shaped shock wave, it will "see" subsonic airflow and work as normal. 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.^[3]
Generally it is not possible to arrange the wing so it will lie entirely outside the supersonic airflow and still have good subsonic performance. Some aircraft, like the English Electric Lightning or F-106 Delta Dart are tuned entirely for high-speed flight and feature highly-swept planforms without regard to the low-speed problems this creates. In other cases the use of variable geometry wings allows an aircraft to move the wing to keep it at the most efficient angle regardless of speed, although the cost in complexity and weight makes this a rare feature.
Most aircraft have a wing that spends at least some of its time in the supersonic airflow. But since the shock cone moves towards the fuselage with increased speed, the portion of the wing in the supersonic flow also changes with speed. Since these wings are swept, as the shock cone moves inward, the center of lift moves forward as the outer, rearward, portions of the wing are generating less lift. This results in powerful pitching moments and their associated required trim changes.

Disadvantages

When a swept-wing travels at high speed, the airflow has little time to react and simply flows over the wing. However at lower speeds some of the air is pushed to the side towards the wing tip. At the wing root, by the fuselage, this has little noticeable effect, but towards the tip the airflow is pushed sidewise not only by the wing, but the sidewise 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 on a wing is generated by the airflow over it from front to rear. As an increasing amount travels spanwise, the amount flowing front to rear is reduced, leading to a loss of lift. Normally this is not much of a problem, but as the plane slows for landing the tips can actually drop below the stall point even at aircraft speeds where stalls should not occur. When this happens the tip stalls first, and since the tip is swept to the rear of the center of lift, the net lift moves forward. This causes the plane to pitch up, leading to more of the wing stalling, leading to more pitch up, and so on. This problem came to be known as ''Sabre dance'' in reference to the number of North American F-86 Sabres that crashed on landing as a result.
The solution to this problem took on many forms. One was the addition of a fin known as a ''wing fence'' on the upper surface of the wing to redirect the flow to the rear (see the MiG-15 as an example), another closely related design was to add a dogtooth notch to the leading edge (Avro Arrow). Other designs took a more radical approach, including the XF-91 Thunderceptor's wing that grew wider towards the tip to provide more lift there, and the British-favoured a crescent ''compound sweep'' or ''scimitar wing'' that reduced the sweep along the span, used on the Handley Page Victor, one of their V bombers.
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, included for high maneuverability, also serve to add lift during landing and reduce the problem.
The swept-wing also has several more problems. One is that for any given length of wing, the actual span from tip-to-tip is shorter than the same wing that is not swept. Low speed drag is strongly correlated with the aspect ratio, the span compared to chord, so a swept wing always has more drag at lower speeds. Another concern is the torque applied by the wing to the fuselage, as much of the wing's lift lies behind the point where the wing root connects to the plane. Finally, while it is fairly easy to run the main spars of the wing right through the fuselage in a straight wing design to use a single continuous piece of metal, this is not possible on the swept wing because the spars will meet at an angle.

Forward sweep

Sweeping a wing forward has the same effect as rearward in terms of drag reduction, but has other advantages in terms of low-speed handling where tip stall problems simply go away. In this case the low-speed air flows in towards the fuselage, which acts as a very large wing fence. Additionally wings are generally larger at the root anyway, which allows them to have better low-speed lift.
However, this arrangement also has serious stability problems. When such a wing is angled up to the effective wind, the tip rotates to a higher effective angle of attack, producing more lift. This is because the tips are in front of the line of rotation of the wing as a whole, so they move upward as well as rotating. This leads to serious flexing problems, with an above normal amount of lift coming from the wing tips, which, being in front of the center of lift, wants to make the wing rotate even higher.
Thus swept-forward wings are unstable in a fashion similar to the low-speed problems of a conventional swept wing. Small amounts of sweep do not cause serious problems, and had been used on a variety of aircraft to move the spar into a convenient location, as on the Junkers Ju 287 or HFB-320 Hansa Jet. But larger sweep suitable for high-speed aircraft like fighters was generally impossible until the introduction of fly by wire systems that could react quickly enough to damp out these instabilities. The Grumman X-29 was an experimental technology demonstration project designed to test the forward swept wing for enhanced maneuverability in 1984. The Su-47 Berkut is another notable example using this technology. However no highly swept-forward design has entered production.

History

Many aircraft have had wings swept in order to fix problems with their center of gravity or to move the wing spar into a more convenient location. For instance, the DC-3 had a slight sweep to its wing. However these designs were not intended to help with transonic performance, and while they have "wing sweep" it is not really proper to refer to them as "swept wing".
The idea of using swept wings to reduce high-speed drag was first developed in Germany in the 1930s. At a Volta Conference meeting in 1935 in Italy, Dr. Adolf Busemann suggested the use of swept wings for supersonic flight. Albert Betz immediately suggested it would be equally useful in the transonic.^[4] A thick wing could be made "effectively thinner" by rotating it at an angle relative to the airflow, sweeping it back.
However, at the time there was no way to power an aircraft to these sorts of speeds, and even the fastest aircraft of the era were only approaching 400 km/h. The presentation was largely of academic interest, and soon forgotten. Large engines at the front of the aircraft made it difficult to obtain a reasonable fineness ratio, and although wings could be made thin and broad, doing so made them considerably less strong. The British Supermarine Spitfire used as thin a wing as possible for lower high-speed drag, but later paid a high price for it in a number of aerodynamic problems such as control reversal. German design instead opted for thicker wings, accepting the drag for greater strength and increased internal space for landing gear, fuel and weapons.
With the introduction of jets in the later half of World War II applying sweep became relevant. The German jet powered Messerschmitt Me 262 and rocket powered Messerschmitt Me 163 suffered from compressibility effects that made them very difficult to control at high speeds. In addition the speeds put them into the wave drag regime, and anything that could reduce this drag would increase the performance of their aircraft, notably the notoriously short flight times measured in minutes. The result was a crash program to introduce new swept wing designs, both for fighters as well as bombers. A prototype test aircraft, the Messerschmitt Me P.1101, was built to research the tradeoffs of the design and develop general rules about what angle of sweep to use. None of the designs were ready for use by the time the war ended, but the P.1101 was captured by US forces and returned to the United States, where two additional copies with US built engines carried on the research as the Bell X-5.
The introduction of the German swept-wing research to aeronautics caused a minor revolution, and almost all design efforts immediately underwent modifications in order to incorporate a swept-wing. A particularly interesting victim was the cancellation of the Miles M-52, a straight-wing design for an attempt on the speed of sound. When the swept-wing design came to light the project was cancelled, as it was thought it would have too much drag to break the sound barrier, but soon after the US nevertheless did just that with the Bell X-1. In 1945, NACA engineer Robert T. Jones developed the mathematical sweep theory that perfected the concept of swept wings and defined its ability to reduce shockwave effects at critical mach numbers. By the 1950s nearly every fighter used a swept wing.

References

1. Wings for all Speeds
2. Supersonic Wing Design
3. The Mach cone becomes increasingly swept back with increasing Mach numbers.
4. Comment by Hans von Ohain during public talks with Frank Whittle, p. 28


★ ''Swept Wings and Effective Dihedral''

★ The development of swept wings

★ The L-39 and swept wing research

★ Swept wings and lateral stability

See also

★ Delta wing

★ Planform

★ Sweep theory

★ Mach number

★ Theodore von Karman, first to recognize the importance of the swept wing.