A swept wing is a wing planform favored for high subsonic jet speeds first investigated by Germany during the Second World War. Since the introduction of the MiG-15 and North American F-86 which demonstrated a decisive superiority over the slower first generation of straight-wing jet fighters during the Korean War, swept wings have become almost universal on all but the slowest jets (such as the A-10). Compared with straight wings common to propeller-powered aircraft, they have a "swept" wing root to wingtip direction angled beyond (usually aftward) the spanwise axis. This has the effect of delaying the drag rise caused by fluid compressibility near the speed of sound as swept wing fighters such as the F-86 were among the first to be able to exceed the speed of sound in a slight dive, and later in level flight.

Unusual variants of this design feature are forward sweep, variable sweep wings and pivoting wings. Swept wings as a means of reducing wave drag were first used on jet fighter aircraft. The four-engine propeller-driven Tu-95 aircraft also has swept wings.

The ''angle of sweep'' which characterizes a swept wing is conventionally measured along the 25% chord line. If the 25% chord line varies in sweep angle, the leading edge is used; if that varies, the sweep is expressed in sections (e.g., 25 degrees from 0 to 50% span, 15 degrees from 50% to wingtip).

Subsonic and transonic behavior

As an aircraft enters the transonic speeds just below the speed of sound, an effect known as wave drag starts to appear. Using conservation of momentum principles in the direction normal to surface curvature, 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 slows down back to subsonic speed. 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 Lockheed 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 for the transonic range

Supersonic behavior

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 Convair 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 Grumman 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 (that is, the cone becomes narrower), 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 lift vector 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 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. This creates a nose-up pressure on the aircraft. If this is not corrected by the pilot it 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 the ''Sabre dance'' in reference to the number of North American F-100 Super 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 addition of a dogtooth notch to the leading edge (Avro Arrow). 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 had a planform based on a crescent ''compound sweep'' or ''scimitar wing'' that had substantial sweep-back near the wing root where the wing was thickest, and progressively reducing sweep along the span as the wing thickness reduced towards 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, 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 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. The rearmost section of the wing will stall first causing a pitch-up moment pushing the aircraft further into stall similar to a swept back wing design. Thus swept-forward wings are unstable in a fashion similar to the low-speed problems of a conventional swept wing. However unlike swept back wings, the tips on a forward swept design wil stall last maintaining roll control.

Forward-swept wings can also experience dangerous flexing effects compared to aft-swept wings that can negate the tip stall advantage if the wing is not sufficiently stiff. In aft-swept designs, when the airplane maneuvers at high load factor the wing loading and geometry twists the wing in such a way as to create washout (tip twists leading edge down). This reduces the angle of attack at the tip, thus reducing the bending moment on the wing, as well as somewhat reducing the chance of tip stall. However, the same effect on forward-swept wings produces a wash-in effect which increases the angle of attack promoting tip stall.

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

The first aircraft with swept wings were those designed by the British designer J.W.Dunne in the first decade of the 20th century. Dunne successfully employed severely swept wings in his tailless aircraft as a means of creating positive longitudinal static stability. Historically, many low-speed aircraft have had swept wings in order to avoid problems with their center of gravity, to move the wing spar into a more convenient location, or to improve the sideways view from the pilot's position. For instance, the Douglas DC-3 had a slight sweep to the leading edge of its wing. The wing sweep in low-speed aircraft was not intended to help with transonic performance, and although most have a small amount of ''wing sweep'' they are rarely described as ''swept wing'' aircraft. The Curtiss XP-55 was the first American swept wing airplane, although it was not considered successful.

Introduction

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.

Hubert Ludewieg of the High-Speed Aerodynamics Branch at the AVA Göttingen in 1939 conducted the first wind tunnel tests to investigate Busemann's theory. Two wings, one with no sweep, and one with 45 degrees of sweep were tested at Mach numbers of 0.7 and 0.9 in the 11 x 13 cm wind tunnel. The results of these tests confirmed the drag reduction offered by swept wings at transonic speeds. The results of the tests were communicated to Albert Betz who then passed them on to Willy Messerschmitt in December 1939. The tests were expanded in 1940 to include wings with 15, 30 and -45 degrees of sweep and Mach numbers as high as 1.21.

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 . 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 10 years later when it was re-introduced to them. Buseman was in charge of aerodynamics research at Braunschweig, and in spite of the limited interest he 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. This resulted in a crash program to introduce new swept wing designs, both for fighters as well as bombers. The Focke-Wulf Ta 183 was a swept wing fighter design with a layout very similar to that later used on the MiG-15 that was not produced before war's end.

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 fighter or bomber 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 last jet fighter designed by Willy Messerschmitt the HA-300 had swept wings, Delta Wing in this case.

Technology impact

The Soviet Union was 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. When applied to the jet-powered Mig-15, its maximum speed of outclassed the straight-winged American jets and piston-engined fighters first deployed to Korea.

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 S. 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 fourfold decrease in drag at high speeds. All of this was compiled into a report published on June 21, 1945, which was sent out to the industry three weeks later. Ironically, by this point Busemann's work had already been passed around.

Boeing was in the midst of designing the Boeing B-47 Stratojet, and the initial Model 424 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 the Model 432, another 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 the Model 448, a larger six-engine design with more robust wings swept at about 35 degrees. Another re-work in November moved the engines to pods under the wings, as the Army was concerned about engine fires potentially destroying the aircraft. The resulting design would have performance rivaling the fastest fighters, and trounced the straight-winged competition. The basic layout of engines on pylons under swept wings is still used on most airliners today.

In fighters, North American Aviation was in the midst of working on a straight-wing jet-powered naval fighter then known as the FJ-1. It was submitted it to the Air Force as the F-86. 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 over straight-winged jets. With the appearance of the Mig-15, the F-86 was rushed into combat and straight-wing jets like the Lockheed P-80 Shooting Star and Republic F-84 Thunderjet were soon relegated to ground attack. Some such as the F-84 and Grumman F-9 Cougar were later redesigned with swept wings from straight-winged aircraft. Later planes such as the North American F-100 Super Sabre would be designed with swept wings from the start, though additional innovations such as the afterburner, area-rule and new control surfaces would be necessary to master supersonic flight.

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 jet fighter Saab 29 Tunnan, and quickly adapted the existing straight-wing layout to incorporate a 25 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 F-86. Eventually almost all design efforts immediately underwent modifications in order to incorporate a swept wing. The classic Boeing B-52, designed in the 1950s, would remain in service until into the 21st century as a high subsonic long-range heavy bomber despite the development of the triple-sonic North American B-70 Valkyrie, supersonic swing-wing Rockwell B-1 Lancer, and flying wing designs. While the Soviet never matched the performance of the Boeing B-52 Stratofortress with a jet design, the intercontinental range Tupolev Tu-95 turboprop bomber also remains in service today. With a near-jet class top speed of 920 km/h, it is an unusual in combining swept wings with propeller propulsion and remains the fastest propeller powered production aircraft. By the 1960s, most civilian jets such as the Boeing 707 adopted swept wings as well.

By the early 1950s nearly every new fighter was either rebuilt or designed from scratch with a swept wing. The Douglas A-4 Skyhawk and Douglas F4D Skyray were examples of delta wings which also have swept leading edges with or without a tail. Most early transonic and supersonic designs such as the MiG-19 and F-100 used long, highly swept wings. Swept wings would reach Mach 2 in the arrow-winged BAC Lightning, and stubby winged Republic F-105 Thunderchief, which was found to be wanting in turning ability in Vietnam combat. By the late 1960s, the F-4 Phantom and Mikoyan-Gurevich MiG-21 which both used variants on tailed delta wings came to dominate front line air forces. Variable geometry wings were employed on the American F-111, Grumman F-14 Tomcat and Soviet Mikoyan Mig-27, although the idea would be abandoned for the American SST design. After the 1970s, most newer generation fighters optimized for maneuvering air combat since the USAF F-15 and Soviet Mikoyan MiG-29 have employed relatively short-span fixed wings with relatively large wing area.

References

External links

''Swept Wings and Effective Dihedral''

The development of swept wings

The L-39 and swept wing research

Swept wings and lateral stability

CFD results showing the 3 dimensional supersonic bubble over the wing of an A 320. Another CFD result showing the MDXX and how the shock vanishes close to the fuselage where the aerofoil is more slender

See also

Delta wing

Forward-swept wing

Planform

Sweep theory

Mach number

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

Trapezoidal wing

Category:Wing configurations Category:Wing design Category:Aviation terminology

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Deke Dickerson (June 3, 1968 in St. Louis, Missouri) is an American singer, songwriter, and guitarist.

After playing in several local rockabilly bands, Deke formed The Untamed Youth at age 17 in his hometown of Columbia, Missouri. In 1991 he moved to Los Angeles and joined Dave Stuckey to form the Dave & Deke Combo, a partnership that yielded two albums and a much-touted 2005 reunion at major rockabilly festivals. Joining the Ecco-Fonics in 1998, Deke toured non-stop, signed to HighTone Records and released three albums for the label. His style incorporates country, alternative country, rockabilly, hillbilly, blues, Western swing and rock 'n' roll.

Dickerson writes a regular column in ''Guitar Player'' magazine and feature articles in Vintage Guitar magazine and The Fretboard Journal. He also organizes an annual "Guitar Geek Festival" held in Anaheim, California, every January, during the NAMM Show. Strong attendance in 2011 might lead to expansion in 2012, to a two-day event.

References

External links

Deke Dickerson homepage

Deke's Guitar Geek Festival Website

Deke Dickerson bio

Category:American guitarists Category:1968 births Category:Living people Category:People from Columbia, Missouri