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The crescent wing is a fixed-wing aircraft configuration in which a
swept wing A swept wing is a wing that angles either backward or occasionally forward from its root rather than in a straight sideways direction. Swept wings have been flown since the pioneer days of aviation. Wing sweep at high speeds was first investigate ...
has a greater sweep angle on the inboard section than the outboard, giving the wing a crescent shape. The
planform In technical drawing and computer graphics, a multiview projection is a technique of illustration by which a standardized series of orthographic two-dimensional pictures are constructed to represent the form of a three-dimensional object. Up to ...
attempts to reduce several unpleasant side-effects of the swept wing design, notably its tendency to "
pitch-up 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. History ...
", sometimes violently, when it nears a stall.


Basic concept

As an aircraft enters the
transonic Transonic (or transsonic) flow is air flowing around an object at a speed that generates regions of both subsonic and supersonic airflow around that object. The exact range of speeds depends on the object's critical Mach number, but transonic ...
region close to the
speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as w ...
, the acceleration of air over curved areas can cause the flow to go
supersonic Supersonic speed is the speed of an object that exceeds the speed of sound ( Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately . Speeds greater than five times ...
. This generates a
shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a med ...
and creates considerable drag, known as
wave drag In physics, mathematics, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities. Waves can be periodic, in which case those quantities oscillate repeatedly about an equilibrium (res ...
. The increase in drag is so rapid and powerful that it gives rise to the concept of a
sound barrier The sound barrier or sonic barrier is the large increase in aerodynamic drag and other undesirable effects experienced by an aircraft or other object when it approaches the speed of sound. When aircraft first approached the speed of sound, th ...
. The speed at which this effect becomes noticeable, known as ''
critical mach In aerodynamics, the critical Mach number (Mcr or M*) of an aircraft is the lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound, but does not exceed it.Clancy, L.J. ''Aerodynamics'', Section 11.6 At t ...
'', is based on the rate of curvature on the upper and lower surfaces; airfoils with greater curvature will have a lower critical Mach speed and thus suffer more heavily from wave drag. A wing designed for good transonic and supersonic performance should spread the curvature of the wing over a longer distance. This naturally leads to thin, long-chord, low aspect-ratio designs like the wing on the
Lockheed F-104 Starfighter The Lockheed F-104 Starfighter is an American single-engine, supersonic air superiority fighter which was extensively deployed as a fighter-bomber during the Cold War. Created as a day fighter by Lockheed as one of the "Century Series" of fi ...
. Such designs suffer from much greater
induced drag In aerodynamics, lift-induced drag, induced drag, vortex drag, or sometimes drag due to lift, is an aerodynamic drag force that occurs whenever a moving object redirects the airflow coming at it. This drag force occurs in airplanes due to wings or ...
, making them less efficient at slower speeds. They also have practical drawbacks, notably a lack of room for fuel and storage of the
landing gear Landing gear is the undercarriage of an aircraft or spacecraft that is used for takeoff or landing. For aircraft it is generally needed for both. It was also formerly called ''alighting gear'' by some manufacturers, such as the Glenn L. Martin ...
.
Swept wing A swept wing is a wing that angles either backward or occasionally forward from its root rather than in a straight sideways direction. Swept wings have been flown since the pioneer days of aviation. Wing sweep at high speeds was first investigate ...
s are a way to lower the amount of effective curvature of a wing without having a longer physical chord. Instead of meeting the
leading edge The leading edge of an airfoil surface such as a wing is its foremost edge and is therefore the part which first meets the oncoming air.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 305. Aviation Supplies & Academics, ...
curvature directly, the sweep of the wing lengthens the path of the airflow over the wing by the sine of the sweep angle, increasing the effective chord. This allows a thicker wing to have the same critical Mach as a thinner unswept design. Most transonic designs use sweep for this reason, allowing them to use a wing that is thick enough for practical internal storage without incurring a heavy wave drag penalty. In real-world designs, the
wing root The wing root is the part of the wing on a fixed-wing aircraft or winged-spaceship that is closest to the fuselage,Peppler, I.L.: ''From The Ground Up'', page 9. Aviation Publishers Co. Limited, Ottawa Ontario, Twenty Seventh Revised Edition, 1996 ...
, where the wing meets the fuselage, is thicker than the
wing tip A wing tip (or wingtip) is the part of the wing that is most distant from the fuselage of a fixed-wing aircraft. Because the wing tip shape influences the size and drag of the wingtip vortices, tip design has produced a diversity of sha ...
. This is because the
wing spar In a fixed-wing aircraft, the spar is often the main structural member of the wing, running spanwise at right angles (or thereabouts depending on wing sweep) to the fuselage. The spar carries flight loads and the weight of the wings while on t ...
has to support the forces from the entire wing outboard, meaning there is very little force on the spar at the tip, but the lift force of the entire wing at the root. Spars generally get much larger as they approach the root to account for these forces, and streamlining the wing profile around such designs generally requires the wing to be much thicker and be more heavily curved at the root than the tip. If one desires to keep the critical Mach number close to constant on such a design, the thinner outboard sections of the wing should have less sweep than the thicker root. Shaping a wing to incorporate a constant critical Mach along the span naturally gives it the crescent shape. The design has two added advantages, which are related to each other. The combination of these effects allows the crescent wing to have better handling characteristics across a wider range of speeds. When air flows over a swept wing, it encounters a force towards the wing tip. At high speeds, this force is too small to have an effect before the air is past the wing. At lower speeds, this sideways motion becomes more evident, and as the sideways motion pushes on the air outboard of it, this ''spanwise flow'' becomes more and more noticeable towards the wing tips. At very low speeds, the flow can become so sideways that the front-to-back flow, which is what gives rise to lift, is no longer above the
stall speed In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', p. 486. Aviation Supplies & Academics, 1997. This occurs when the ...
of the airfoil, and the wing tips may stall. Because the sweep means the tips are behind the
center of gravity In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weight function, weighted relative position (vector), position of the distributed mass sums to zero. Thi ...
, this loss of lift at the rear of the aircraft causes a nose-up force, which may cause further stalling. A dangerous runaway effect may occur, known as
pitch up 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. History ...
. A crescent wing reduces this problem. Since the sweep angle at the tip is less than at the root, the sideways force is reduced. When considered over the entire width of the wing, this can greatly reduce the spanwise flow, and thereby lower the speeds where the tips stall. Moreover, even when the wing tips do stall, they are located more forward than they would be in the case of a straight swept wing. This means the loss of lift occurs closer to the center of gravity, and thereby reduces the magnitude of the pitching forces. At the opposite end of the speed range, another effect comes into play. As a wing is loaded, it bends upward. In the case of a swept wing, because these loads are rear of the mean chord, this upward force becomes a
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
around the spar, causing the tips to rotate tips-down. This lowers the amount of lift at the tips as they become flatter to the airflow, or "wash out". This causes the same nose-up force as the low-speed case, and at high speeds, the forces involved can be very high and lead to structural problems. Again, as the crescent wing's tips are closer to the center of pressure, these forces are reduced.
Aileron An aileron (French for "little wing" or "fin") is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll (or movement around ...
s, located at the wing tips, also create a large torque force when actuated. This can cause a problem known as ''aileron reversal'', where the twisting motion of the entire wing causes the opposite force to be applied. This problem was well known on the
Supermarine Spitfire The Supermarine Spitfire is a British single-seat fighter aircraft used by the Royal Air Force and other Allied countries before, during, and after World War II. Many variants of the Spitfire were built, from the Mk 1 to the Rolls-Royce Grif ...
and required its wing to be greatly strengthened to counteract this effect. In the case of a crescent wing, this effect is no more or less pronounced than on other designs. However, it does set a minimum torsional strength requirement which may be higher than would otherwise be needed due to the crescent shape's lowering of the maneuvering loads, thus potentially offsetting this advantage. The self-flaring capability, often mentioned in discussions of the
Handley Page Victor The Handley Page Victor is a British jet-powered strategic bomber developed and produced by Handley Page during the Cold War. It was the third and final '' V bomber'' to be operated by the Royal Air Force (RAF), the other two being the Avro ...
, is not inherent to the crescent wing, but may occur on any high-tailed aircraft with some wing sweep. This effect is caused by the fact that the wing enters the ground effect before the tail, which is mounted high in the T-tail case. This creates a brief period of added lift on the wing that is not countered by the tail, causing the nose to rise. This rotation stops as soon as the aircraft descends low enough that the tail begins to enter the ground effect as well.


History

The crescent wing planform was invented by the German aerodynamicist Dipl.-Ing. Rüdiger Kosin and Walther Lehmann, while working for Arado Flugzeugwerke Gmbh during the Second World War. A prototype wing was constructed by April 1945, with the intention of fitting it to the
Arado Ar 234 The Arado Ar 234 ''Blitz'' (English: lightning) is a jet-powered bomber designed and produced by the German aircraft manufacturer Arado. It was the world's first operational turbojet-powered bomber, seeing service during the latter half of the ...
V16 prototype airframe. However, before it could be fitted, the
British Army The British Army is the principal land warfare force of the United Kingdom, a part of the British Armed Forces along with the Royal Navy and the Royal Air Force. , the British Army comprises 79,380 regular full-time personnel, 4,090 Gurk ...
overran the site and the wing was destroyed. Design staff from the British aircraft manufacturer
Handley Page Handley Page Limited was a British aerospace manufacturer. Founded by Frederick Handley Page (later Sir Frederick) in 1909, it was the United Kingdom's first publicly traded aircraft manufacturing company. It went into voluntary liquidation a ...
- amongst whose staff was engineer
Gustav Lachmann Gustav Victor Lachmann (3 February 1896 – 30 May 1966) was a German aeronautical engineer who spent most of his professional life working for the British aircraft company Handley Page. He was, with Frederick Handley Page, the co-inventor of the ...
- were sent to Germany, where they were impressed by the work at Arado. They subsequently incorporated the configuration in their proposal for the HP.80 V-bomber, later to be named the
Victor The name Victor or Viktor may refer to: * Victor (name), including a list of people with the given name, mononym, or surname Arts and entertainment Film * ''Victor'' (1951 film), a French drama film * ''Victor'' (1993 film), a French shor ...
. Handley Page proposed a one-third scale research glider, the HP.87, but soon abandoned it in favour of a powered research aircraft, the HP.88 having a 0.36-scale wing. The HP.88 first flew on 21 June 1951. During its brief career it showed a tendency to pitching oscillations and, on 26 August 1951, this was observed to occur increasingly violently before the aircraft broke up in the air. By then the Victor design was already well advanced, with the first prototype flying on 24 December 1952 and production examples entering service in April 1958. The problem seen on the HP.88 was ultimately traced to a servomechanism on the tail controls, not a problem inherent to the layout of the bomber. Meanwhile in France, Bréguet proposed the Br.978A design for a crescent-winged airliner, which they referred to as the "croissant". The design was not built. The layout was also selected for the Supermarine 545, a supersonic version of the Supermarine Swift, but this was not put into production. Early versions of the
Avro Vulcan The Avro Vulcan (later Hawker Siddeley Vulcan from July 1963) is a jet-powered, tailless, delta-wing, high-altitude, strategic bomber, which was operated by the Royal Air Force (RAF) from 1956 until 1984. Aircraft manufacturer A.V. Roe and ...
had straight leading edges, and these displays problems at high transonic speeds. This included extensions on the leading edge that gave the inner portions less sweep. The result was a revised wing layout that is essentially a delta-version of the crescent wing.


Handley Page Victor

The Victor was the only crescent-wing type to enter production. It served with the Royal Air Force for many years, serving in a variety of roles besides bomber, including as an inflight refuelling tanker during the
Falklands War The Falklands War ( es, link=no, Guerra de las Malvinas) was a ten-week undeclared war between Argentina and the United Kingdom in 1982 over two British dependent territories in the South Atlantic: the Falkland Islands and its territorial de ...
. The profile and shaping of the crescent wing was subject to considerable fine-tuning and alterations throughout the early development stages, particularly to counter unfavourable pitching behavior in flight. During the flight tests of the first prototype, the Victor proved its aerodynamic performance, flying up to Mach 0.98 without handling or buffeting problems; there were next to no aerodynamic changes between prototype and production aircraft. Production aircraft featured an automated nose-flap operation to counteract a tendency for the aircraft to pitch upwards during low-to-moderate Mach numbers. One unusual flight characteristic of the early Victor was its self-landing capability; once lined up with the runway, the aircraft would naturally
flare A flare, also sometimes called a fusée, fusee, or bengala in some Latin-speaking countries, is a type of pyrotechnic that produces a bright light or intense heat without an explosion. Flares are used for distress signaling, illumination, ...
as the wing entered into ground effect while the tail continued to sink, giving a cushioned landing without any command or intervention by the pilot.Gunston, W.; ''Aeroplane Monthly'' February 1981, p. 63.Butler and Buttler 2009, p. 29. The Victor had good handling and excellent performance, along with favourable slow speed flight characteristics and has been described as an agile aircraft, atypical for a large bomber aircraft; in 1958, a Victor had performed several loops and a
barrel roll A barrel roll is an aerial maneuver in which an airplane makes a complete rotation on both its longitudinal and lateral axes, causing it to follow a helical path, approximately maintaining its original direction. It is sometimes described as a ...
during practices for a display flight at
Farnborough Airshow The Farnborough Airshow, officially the Farnborough International Airshow, is a trade exhibition for the aerospace and defence industries, where civilian and military aircraft are demonstrated to potential customers and investors. Since its fir ...
. The Victor was designed for flight at high subsonic speeds, although multiple instances have occurred in which the
sound barrier The sound barrier or sonic barrier is the large increase in aerodynamic drag and other undesirable effects experienced by an aircraft or other object when it approaches the speed of sound. When aircraft first approached the speed of sound, th ...
was broken.Butler and Buttler 2009, pp. 33-34.


References


Notes


Bibliography

*Butler, P.; and Buttler, A.; ''Aerofax: Handley Page Victor''. Midland Publishing, 2009. . *Green, W.; ''Warplanes of the Third Reich'', Macdonald and Jane's, 1970. *Hygate, B. ''British experimental jet aircraft'', Argus, 1990; Pages 106-112. * {{cite magazine , url=https://www.flightglobal.com/FlightPDFArchive/1954/1954%20-%201386.PDF , title=Aerodynamics of the Crescent Wing , first=G.H. , last=Lee , magazine=Flight International , date=14 May 1954 , pages=611–612


External links


"The Crescent Wing"
a video by Handley Page on the Victor's design Wing configurations