User Contributed Dictionary

English

Pronunciation

swēp'bǎk'

Noun

Sweepback

the rearward angle of the leading edge of a wing, or airfoil.

Extensive Definition

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. This is a useful drag reducing measure for aircraft flying just below the speed of sound. Forward sweep is also used on some aircraft, to the same end. Swept wings were initially used only on fighter aircraft, but have since become almost universal on jets, including airliners and business jets.

In the transonic

As an aircraft approaches the speed of sound, an effect known as wave drag starts to appear. Due to Bernoulli's principle, airflow accelerates around curved surfaces, and near the speed of sound the acceleration can cause the airflow to reach supersonic speeds. When this occurs, an oblique shock wave is generated at the point where the flow goes supersonic. Since this occurs on curved areas, they are normally associated with the upper surfaces of the wing, the cockpit canopy, and the nose cone of the aircraft, areas with the highest local curvature.

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 over various features of the aircraft, there is a certain "critical mach" speed (or drag divergence mach number) where this effect 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.

Since these shock waves are generated at areas of curvature, the obvious way to reduce their effect is to reduce the curvature. In the case of the fuselage, this suggests long, thin designs that are pointed at the ends. Such designs are common on high speed aircraft, the Concorde being one example, and are referred to as having a high "fineness ratio".

This applies to the wing as well, which suggests that wings should have as little curvature as possible, be as thin as possible, and have a long chord. Examples of this sort of wing planform 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 a very low lift coefficient, and poor performance at slow speeds. The Starfighter has had a large number of landing accidents caused by its very high landing speed that was needed to keep the wing generating enough lift to fly.

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, but no one measures "head on", all ribs are stream-wise. At low angles of attack 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.Early experiments demonstrated that the peak drag was lowered by as much as four times compared to a straight wing.

Swept wings use an aerofoil with the maximum thickness close to the leading edge. It acts like a de Laval nozzle and accelerates the air-flow to supersonic speeds. The middle part of the aerofoil is basically flat, thus the air flow is undisturbed by the aerofoil. Within this supersonic flow an oblique shock can travel span-wise, shocking the flow to subsonic speeds with much higher efficiency than a normal shock in an unswept wing. After the shock the aerofoil is curved downwards to meet the underside of the wing.

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.

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.. For instance, at Mach 1.3 the angle of the Mach cone formed off the body of the aircraft will be at about sinμ = 1/M (μ is the sweep angle of the Mach cone)

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, as on the F-14 Tomcat, 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 high-speed 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 almost straight from front to back. At lower speeds the air does have time to react, and is pushed sidewise by the angled leading edge, pushing some of the air sideways 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 sidewise not only by the leading edge, 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 relative 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. He noted that the airspeed over the wing was dominated by the normal component of the airflow, not the freestream velocity, so by setting the wing at an angle the forward velocity at which the shock waves would form would be higher (the same had been noted by Max Munk in 1924, although not in the context of high-speed flight). Albert Betz immediately suggested the same effect would be equally useful in the transonic. After the presentation the host of the meeting, Arturo Crocco, jokingly sketched "Busemann's airplane of the future" on the back of a menu while they all dined. Crocco's sketched showed a classic 1950's fighter design, with swept wings and tail surfaces, although he also sketched a swept propeller powering it.

At the time, however, 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. 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.

At the time the presentation was largely of academic interest, and soon forgotten. Even notable attendees including Theodore von Kármán and Eastman Jacobs did not recall the presentation ten years later when it was re-introduced to them. Nevertheless, Buseman was in charge of aerodynamics research at Braunschweig, and began a research program studying the concept. By 1939 wind tunnel testing had demonstrated the effect was real, and practical.

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.

von Kármán travelled to Germany near the end of the war as part of Operation Paperclip, and reached Braunschweig on May 7, discovering a number of swept wing models and a mass of technical data from the wind tunnels. One member of the US team was George Schairer, who was at that time working at the Boeing company. He immediately forwarded a letter to Ben Cohn at Boeing stating that they needed to investigate the concept. He also told Cohn to distribute the letter to other companies as well, although only Boeing and North American made immediate use of it.

In February 1945 NACA engineer Robert T. Jones started looking at highly-swept delta wings and V shapes, and discovered the same effects as Busemann. He finished a detailed report on the concept in April, but found his work was heavily criticised by other members of NACA Langley, notably Theodore Theodorsen, who referred to it as "hocus-pocus" and demanded some "real mathematics". However, Jones had already secured some time for free-flight models under the direction of Robert Gilruth, whose reports were presented at the end of May and showed a four-fold decrease in drag at high speeds. All of this was compiled into a report published on 21 June 1945, which was sent out to the industry three weeks later. Ironically, by this point Busemann's work had already been passed around.

The Soviet Union was also intrigued about the idea of swept wings on aircraft at the end of World War II in Europe, when their "captured aviation technology" counterparts to the western Allies spread out across the defeated Third Reich. Artem Mikoyan was asked by the Soviet government, principally by the government's TsAGI aviation research department, to develop a test-bed aircraft to research the swept wing idea-the result was the late 1945-flown, unusual MiG-8 Utka pusher canard layout aircraft, with its rearwards-located wings being swept back for this type of research.

Boeing was in the midst of designing the B-47 Stratojet, which at that time was a straight-wing design similar to the B-45, B-46 and B-48 it competed with. A recent design overhaul completed in June produced a four-engine design with the engines buried in the fuselage to reduce drag, and long-span wings that gave it an almost glider-like appearance. By September the Braunschweig data had been worked into the design, which re-emerged as a larger six-engine design with more robust wings swept at about 35 degrees. Another re-work in November moved the engines to pods on the wings, as the Army was concerned about engine fires potentially destroying the aircraft. The resulting design was almost identical to the final version, and its performance simply trounced the competition.

North American Aviation, meanwhile, was in the midst of working on a jet powered fighter then known as the FJ-1. They also subitted it to the Air Force as the F-86, where it competed against similar straight-wing designs, the P-80 Shooting Star and P-84. Larry Green, who could read German, studied the Busemann reports and convinced management to allow a redesign starting in August 1945. A battery of wind tunnel tests followed, and although little else of the design was changed, including the wing profile (NACA 0009), the performance of the aircraft was dramatically improved and went on to become one of the classic fighters of the 1950s. The P-80 was relegated to training roles, while the P-84 was forced to introduce a swept wing version.

The British also received the German data, and decided that future high-speed designs would have to use it. A particularly interesting victim of this process 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. The Air Ministry introduced a program of experimental aircraft to examine the effects of swept wings (as well as delta wings) and introduced their first combat designs as the Hawker Hunter and Supermarine Swift.

The German research was also "leaked" to SAAB from a source in Switzerland in late 1945. They were in the process of developing the SAAB Tunnan, and quickly adapted the existing straight-wing layout to incorporate a 35 degree sweep. Although not well known outside Sweden, the Tunnan was a very competitive design, remaining in service until 1972 in some roles.

The introduction of the German swept-wing research to aeronautics caused a minor revolution, especially after the dramatic successes of the B-47 and P-86. Eventually almost all design efforts immediately underwent modifications in order to incorporate a swept-wing. By the 1950s nearly every fighter used a swept wing, and by the 1960s, civilian jets as well.

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