Forward-swept wing

A forward-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.

Characteristics

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

Main spar location

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

Inward spanwise flow

Air flowing over any swept wing tends to move spanwise towards the rearmost 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 backwards-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.

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 improves lift allowing a smaller wing.

As a result manoeuvrability is improved, especially at high angles of attack.

At transonic speeds, shockwaves build up first at the root rather than the tip, again helping to ensure effective aileron control.

Yaw instability

One problem with the forward-swept design is that when a swept wing yaws sideways, one wing moves rearwards. 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

The other problem is a structural one. Due to aeroelasticity, the lift force on any wing tends to bend it upwards at the tip. On a forward-swept design, this increases the angle of incidence at the tip, increasing lift and causing further bending which causes further lift and so on. In the worst case, the tip structure can bend so far it fails. This divergence speed sets a maximum safe speed for the aircraft.

Another dangerous consequence of this increase in tip lift under load is a tendency to tighten into a turn, even though the pilot is not trying to.

At large angles of sweep and high speeds, in order to build a structure stiff enough to resist bending and light enough to be practicable, advanced materials such as carbon fibre composite are required. Composites also allow aeroelastic tailoring, by running the fibres such that the wing bends under load in a desirable way rather than an undesirable way, for example to ensure safe stall characteristics.

Stall characteristics

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

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. 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

Prewar studies

Sweeping the wings forward may first have been proposed in 1936 by German aircraft designers.^[2]

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 were first developed before and during the Second World War, independently in Germany, Russia and the USA.

An early example to fly, in 1940, was the 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.

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

Meanwhile in Germany, Dipl. Ing. 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 Soviet type.

In 1948 the Soviet Union created the Tsybin's LL-3.^[3] The prototype would subsequently have a great impact on the Sukhoi's 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 practicable. As a result, forward sweep for high-speed designs was abandoned, until many years later new structural materials would become available.

Postwar general aviation

LET L-13 two-seat glider showing forward swept wing

Meanwhile, small amounts of sweep do not cause serious problems and the ability of even moderate forward sweep to adjust the position of the main spar has proved to be a useful feature.

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.

Schleicher ASK 13 glider showing forward-swept wing

Since then, moderate forward sweep has been used for similar reasons in many designs. These include:

• The CZAW Parrot^[4]
• The 1964 HFB-320 business jet, of which 50 were built,
• The Saab Safari, Bölkow Junior & ARV Super2 all have shoulder wings for increased visibility, necessitating forward-swept wings to maintain correct CofG.
• The Scaled Composites Boomerang, a prototype piston twin design which would allow for safe handling in the event of a single engine failure.
• The Cessna NGP, a prototype single-engine aircraft intended to eventually replace the Cessna 172 and Cessna 182.
• The SZD-9 Bocian and PZL Bielsko SZD-50 Puchacz, multi-purpose two-seat sailplanes designed and built in Poland.

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.

Return of the fast jet

Grumman X-29 displaying forward-swept wing configuration

Sukhoi Su-47

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 67° angle of attack.^[5]

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.

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.

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

See also

• Swept wing
• Sweep theory
• Trapezoidal wing
• Variable-sweep wing

References

Inline citations

General references

• Miller, J.; The X-planes, X-1 to X-29 (UK Edition), MCP, 1983, Pages 175-179.