Forward-swept wing

Last updated
Forward-swept wing of the Sukhoi Su-47 Sukhoi Su-47 Berkut (S-37) in 2001.jpg
Forward-swept wing of the Sukhoi Su-47

A forward-swept wing or reverse-swept wing is an aircraft wing configuration in which the quarter-chord line of the wing has a forward sweep. Typically, the leading edge also sweeps forward.

Contents

Characteristics

The forward-swept configuration has a number of characteristics which increase as the angle of sweep increases.

Main spar location

The aft location of the main wing spar would lead to a more efficient interior arrangement with more usable space.

Inward spanwise flow

Spanwise airflow over a forward-swept wing is the reverse of flow over a conventional swept wing. Airflow forward and backward swept aircraft.svg
Spanwise airflow over a forward-swept wing is the reverse of flow over a conventional swept wing.

Air flowing over any swept wing tends to move spanwise towards the aftmost end of the wing. On a rearward-swept wing this is outwards towards the tip, while on a forward-swept wing it is inwards towards the root. As a result, the dangerous tip stall condition of a rearward-swept design becomes a safer and more controllable root stall on a forward-swept design. This allows full aileron control despite loss of lift, and also means that drag-inducing leading edge slots or other devices are not required. At transonic speeds, shockwaves build up first at the root rather than the tip, again helping ensure effective aileron control.

With the air flowing inwards, wingtip vortices and the accompanying drag are reduced. Instead, the fuselage acts as a very large wing fence and, since wings are generally larger at the root, this raises the maximum lift coefficient allowing a smaller wing. As a result, maneuverability is improved, especially at high angles of attack.

Yaw instability

One problem with the forward-swept design is that when a swept wing yaws sideways (moves about its vertical axis), one wing retreats while the other advances. On a forward-swept design, this reduces the sweep of the rearward wing, increasing its drag and pushing it further back, increasing the amount of yaw and leading to directional instability. This can lead to a Dutch roll in reverse. [1]

Aeroelasticity

One of the drawbacks of forward swept wings is the increased chance of divergence, an aeroelastic consequence of the lift force on forward swept wings twisting the tip upwards under increased lift. On a forward-swept design, this causes a positive feedback loop that increases the angle of incidence at the tip, increasing lift and inducing further deflection, resulting in yet more lift and additional changes in wing shape. The effect of divergence increases with speed. The maximum safe speed below which this does not happen is the divergence speed of the aircraft.

Such an increase in tip lift under load causes the wing to tighten into turns and may result in a spiral dive from which recovery is not possible. In the worst case, the wing structure can be stressed to the point of failure.

At large angles of sweep and high speeds, in order to build a structure stiff enough to resist deforming yet light enough to be practicable, advanced materials such as carbon fiber composites are required. Composites also allow aeroelastic tailoring by aligning fibers to influence the nature of deformation to a more favorable shape, impacting stall and other characteristics.

Stall characteristics

Any swept wing tends to be unstable in the stall, since the wing tips stalls first causing a pitch-up force worsening the stall and making recovery difficult. This effect is less significant with forward sweep because the rearward end carries greater lift and provides stability.

However, if the aeroelastic bending is sufficient, it can counteract this tendency by increasing the angle of attack at the wing tips to such an extent that the tips stall first and one of the main characteristics of the design is lost, on a conventional wing the tips always stall first. Such a tip stall can be unpredictable, especially where one tip stalls before the other.

Composite materials allow aeroelastic tailoring, so that as the wing approaches the stall it twists as it bends, so as to reduce the angle of attack at the tips. This ensures that the stall occurs at the wing root, making it more predictable and allowing the ailerons to retain full control.

History

Pre-WWII studies

Viktor Belyaev tested forward-swept wing gliders BP-2 and BP-3 in 1934 and 1935. [2] [3] Other prewar design studies included the Polish PWS Z-17, Z-18 and Z-47 "Sęp" series.

World War II and aftermath

Forward-swept wings designs, some whose design had begun during the prewar period, were developed during World War II, independently in Germany, the Soviet Union, Japan, and the United States. An early example to fly, in 1940, was the Soviet Belyayev DB-LK, a twin-boom design with forward-swept outer wing sections and backwards-swept tips. It reportedly flew well. Belyayev's proposed Babochka research aircraft was cancelled following the German invasion.

Throughout World War II, numerous fighter, bomber, and other military aircraft can be described as having forward-swept wings, due to the average chord of their wings being forward-sweeping. However, these designs almost always utilized a rearward-swept leading edge, which would technically render them as high aspect ratio trapezoidal wings.

The American Cornelius Mallard flew on 18 August 1943. The Mallard was powered by a single engine, but it was followed by the Cornelius XFG-1 prototypes, which were flying fuel tanks, unpowered and designed for towing by larger aircraft. These Cornelius designs were unusual for being not only forward swept but also tailless.

A model of the Ju 287 V1 Modellphoto Ju287V1 1.png
A model of the Ju 287 V1

Meanwhile in Germany, Hans Wocke was studying the problems of swept wings at the near-sonic speeds of which the new jet engines were capable. He recognised many of the advantages that forward sweep offered over the backwards-swept designs then being developed, and also understood the implications of aeroelastic bending and yaw instability. His first such design to fly was the Junkers Ju 287, on 16 August 1944. Flight tests on this and later variants confirmed the low-speed advantages but also soon revealed the expected problems, preventing high-speed trials.

Wocke and the incomplete Ju 287 V3 prototype were captured and, in 1946, taken to Moscow where the aircraft was completed and flown the next year as the OKB-1 EF 131. The later OKB-1 EF 140 was essentially the same airframe re-engined with a pair of Mikulin-design Soviet jet engines of greater thrust. In 1948, the Soviet Union created the Tsybin LL-3. [4] The prototype would subsequently have a great impact on the Sukhoi SYB-A, which was completed in 1982.

When the German research reached the United States after the war, a number of proposals were put forward. These included the Convair XB-53 supersonic bomber and forward-swept variants of the North American P-51 Mustang, Bell X-1 rocket plane and Douglas D-558-I. The Bell proposal reached the wind tunnel testing stage, where the problems of aeroelasticity were confirmed. The structural problems confirmed by the Ju 287 series and the Bell X-1 studies proved so severe that the materials available at the time could not make a wing strong and stiff enough without also making it too heavy to be practical. As a result, forward sweep for high-speed designs was abandoned, until many years later when new structural materials would become available.

Post-WWII general aviation

LET L-13 two-seat glider PSU Blanik.JPG
LET L-13 two-seat glider
ARV Super2 ARV prototype.jpg
ARV Super2

Small amounts of sweep do not cause serious problems and even moderate forward sweep allows a significant aft movement of the main spar attachment point and carry-through structure.

In 1954, Wocke returned to the German Democratic Republic, moving to West Germany shortly afterwards and joining Hamburger Flugzeugbau (HFB) as their chief designer. [1] In Hamburg, Wocke completed work on the HFB 320 Hansa Jet business jet which flew in 1964. The forward sweep enabled the main spar to be moved aft behind the cabin so that the spar did not need to project into the cabin.

Moderate forward sweep has been used for similar reasons in many designs, mainly sailplanes and light aircraft. Many high-wing training gliders with two seats in tandem have slightly forward-swept wings in order to enable the wing root to be located further aft to prevent the wing from obscuring the rear occupant's lateral visibility. Typical examples are the Schleicher ASK 13 and the Let Kunovice LET L-13 Blaník.

Other examples include:

Fast jet

Grumman X-29 displaying forward-swept wing configuration X-29 in Banked Flight.jpg
Grumman X-29 displaying forward-swept wing configuration
KB SAT SR-10 trainer Russian Air Force, EX-88004, SAT SR-10 (36976765590).jpg
KB SAT SR-10 trainer

The large angles of sweep necessary for high-speed flight remained impractical for many years.

In the late 1970s, DARPA began investigating the use of newer composite materials to avoid the problem of reduced divergence speed through aeroelastic tailoring. Fly-by-wire technology allowed for the design to be dynamically unstable and improved maneuverability. Grumman built two X-29 technology demonstrators, first flying in 1984, with forward swept wings and canards. Maneuverable at high angles of attack, the X-29 remained controllable at a 67° angle of attack. [6]

Advances in thrust vectoring technology and a shift in air combat tactics toward medium range missile engagements decreased the relevance of a highly agile fighter aircraft.

In 1997, Sukhoi introduced the Su-47 fighter prototype at the Paris Air Show. It did not enter production, although it underwent a series of flight tests and performed at several air shows.

The KB SAT SR-10 is a prototype Russian single-engine jet trainer aircraft, fitted with forward-swept wings. It first flew in 2015.

In biology

Large-headed pterosaurs had forward swept wings in order to better balance in flight. [7]

See also

Related Research Articles

<span class="mw-page-title-main">Stall (fluid dynamics)</span> Abrupt reduction in lift due to flow separation

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack exceeds its critical value. The critical angle of attack is typically about 15°, but it may vary significantly depending on the fluid, foil – including its shape, size, and finish – and Reynolds number.

<span class="mw-page-title-main">Delta wing</span> Triangle shaped aircraft wing configuration

A delta wing is a wing shaped in the form of a triangle. It is named for its similarity in shape to the Greek uppercase letter delta (Δ).

<span class="mw-page-title-main">Republic XF-91 Thunderceptor</span> Experimental interceptor aircraft

The Republic XF-91 Thunderceptor is a mixed-propulsion prototype interceptor aircraft, developed by Republic Aviation. The aircraft would use a jet engine for most flight, and a cluster of four small rocket engines for added thrust during climb and interception. The design was largely obsolete by the time it was completed due to the rapidly increasing performance of contemporary jet engines, and only two prototypes were built. One of these was the first American fighter to exceed Mach 1 in level flight.

<span class="mw-page-title-main">Swept wing</span> Plane wing that angles backwards or forwards

A swept wing is a wing angled either backward or occasionally forward from its root rather than perpendicular to the fuselage.

<span class="mw-page-title-main">Lockheed L-2000</span> Proposed US supersonic airliner design

The Lockheed L-2000 was Lockheed Corporation's entry in a government-funded competition to build the United States' first supersonic airliner in the 1960s. The L-2000 lost the contract to the Boeing 2707, but that competing design was ultimately canceled for political, environmental and economic reasons.

<span class="mw-page-title-main">Variable-sweep wing</span> Airplane wings capable of changing position to alter their geometry

A variable-sweep wing, colloquially known as a "swing wing", is an airplane wing, or set of wings, that may be modified during flight, swept back and then returned to its previous straight position. Because it allows the aircraft's shape to be changed, it is an example of a variable-geometry aircraft.

<span class="mw-page-title-main">Dihedral (aeronautics)</span> Angle between each wing or tail surface within a pair

Dihedral angle is the upward angle from horizontal of the wings or tailplane of a fixed-wing aircraft. "Anhedral angle" is the name given to negative dihedral angle, that is, when there is a downward angle from horizontal of the wings or tailplane of a fixed-wing aircraft.

<span class="mw-page-title-main">Junkers Ju 287</span> Prototype German jet bomber

The Junkers Ju 287 was a multi-engine tactical jet bomber built in Nazi Germany in 1944. It featured a novel forward-swept wing, and the first two prototypes were among the very few jet propelled aircraft ever built with fixed landing gear.

<span class="mw-page-title-main">Grumman X-29</span> 1984 experimental aircraft family by Grumman

The Grumman X-29 is an American experimental aircraft that tested a forward-swept wing, canard control surfaces, and other novel aircraft technologies. Funded by NASA, the United States Air Force and DARPA, the X-29 was developed by Grumman, and the two built were flown by NASA and the United States Air Force. The aerodynamic instability of the X-29's airframe required the use of computerized fly-by-wire control. Composite materials were used to control the aeroelastic divergent twisting experienced by forward-swept wings, and to reduce weight. The aircraft first flew in 1984, and two X-29s were flight tested through 1991.

<span class="mw-page-title-main">Sukhoi Su-47</span> Experimental fighter aircraft

The Sukhoi Su-47 Berkut, also designated S-32 and S-37 during initial development, was a Russian experimental supersonic jet fighter developed by the JSC Sukhoi Company. A distinguishing feature of the aircraft was its forward-swept wing which gave the aircraft excellent agility and maneuverability. While serial production of the type never materialized and the configuration was not further pursued, the sole aircraft produced served as a technology demonstrator prototype for a number of advanced technologies later used in fourth-generation fighter Su-35 and fifth-generation fighter Su-57.

<span class="mw-page-title-main">Messerschmitt P.1101</span> German fighter prototype

The Messerschmitt P.1101 was a single-seat, single-jet fighter project of World War II, developed as part of the 15 July 1944 Emergency Fighter Program which sought a second generation of jet fighters for the Third Reich. A prominent feature of the P.1101 prototype was that the sweep angle of the wings could be changed before flight, a feature further developed in later variable-sweep aircraft such as the Bell X-5 and Grumman XF10F Jaguar.

<span class="mw-page-title-main">Pitch-up</span>

In aerodynamics, pitch-up is an uncommanded nose-upwards rotation of an aircraft. It is an undesirable characteristic that has been observed mostly in experimental swept-wing aircraft at high subsonic Mach numbers or high angle of attack.

Wing twist is an aerodynamic feature added to aircraft wings to adjust lift distribution along the wing.

<span class="mw-page-title-main">Bölkow Bo 46</span> Experimental high-speed helicopter

The Bölkow Bo 46 was a West German experimental helicopter built to test the Derschmidt rotor system that aimed to allow much higher speeds than traditional helicopter designs. Wind tunnel testing showed promise, but the Bo 46 demonstrated a number of problems and added complexity that led to the concept being abandoned. The Bo 46 was one of a number of new designs exploring high-speed helicopter flight that were built in the early 1960s.

<span class="mw-page-title-main">Washout (aeronautics)</span> Characteristic of aircraft wing design

Washout is a characteristic of aircraft wing design which deliberately reduces the lift distribution across the span of an aircraft’s wing. The wing is designed so that the angle of incidence is greater at the wing roots and decreases across the span, becoming lowest at the wing tip. This is usually to ensure that at stall speed the wing root stalls before the wing tips, providing the aircraft with continued aileron control and some resistance to spinning. Washout may also be used to modify the spanwise lift distribution to reduce lift-induced drag.

<span class="mw-page-title-main">BERP rotor</span>

The BERP rotor blade design was developed under the British Experimental Rotor Programme. The initial BERP rotor blades were developed in the late 1970s to mid-1980s as a joint venture programme between Westland Helicopters and the Royal Aircraft Establishment (RAE), with Professor Martin Lowson as a co-patentee. The goal was to increase the helicopters lifting-capability and maximum speed using new designs and materials.

<span class="mw-page-title-main">Wing configuration</span> Describes the general shape and layout of an aircraft wing

The wing configuration or planform of a fixed-wing aircraft is its arrangement of lifting and related surfaces.

<span class="mw-page-title-main">Trapezoidal wing</span> Aircraft wing shape

In aeronautics, a trapezoidal wing is a straight-edged and tapered wing planform. It may have any aspect ratio and may or may not be swept.

<span class="mw-page-title-main">Crescent wing</span> Aircraft wing configuration

The crescent wing is a fixed-wing aircraft configuration in which a swept wing has a greater sweep angle on the inboard section than the outboard, giving the wing a crescent shape.

<span class="mw-page-title-main">Blohm & Voss P 215</span> Type of aircraft

The Blohm & Voss P215 was an advanced jet night fighter project by Blohm & Voss during the Second World War. With a crew of three and twin jet engines, it featured a tailless swept-wing layout and heavy armament. An order for three prototypes was received just weeks before the war ended.

References

Inline citations

  1. 1 2 Miller, J.; The X-Planes, Speciality Press, Second Printing (1985), pp. 175–177.
  2. "Беляев БП-2(ЦАГИ-2)". www.airwar.ru.
  3. "Механические птицы профессора Беляева / Авиация и время 2008 04". www.k2x2.info.
  4. Russian Aviation Page: Sukhoi S-37 Berkut (S-32) Archived 2006-02-13 at the Wayback Machine
  5. "airplane.cz". www.airplane.cz.
  6. NASA. "Dryden Fact Sheet - X-29". Retrieved 22 August 2005.
  7. https://qmro.qmul.ac.uk/xmlui/bitstream/handle/123456789/10947/Hone%20The%20wingtips%20of%20the%20pterosaurs%202015%20Accepted.pdf?sequence=1&isAllowed=y [ bare URL PDF ]

General references