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Variable-sweep wings

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CONTENTS

Abstract

Introduction

Motivation for unswept wings

Motivation for swept wings

Definition

History

Mach sweep programmer

Wing plan form vs. Range/Endurance

Drawbacks

Current usage

Future of variable sweep

Conclusion

References

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ABSTRACT

VARIABLE-SWEEP WING

A variable-sweep wing is a wing configuration that allows to alter planform for various

flight conditions. A variable-sweep wing can be moved during flight-usually between a

swept-back position and a straight position. This allows it to take advantage of the

aerodynamics of a swept wing at high speeds while avoiding the drawbacks of such a

design at lower speeds. It is successful in this respect, but the added mass and complexity

required counter the benefits and stand in the way of widespread adoption. The term

variable-geometry is often used synonymously with swing-wing, though strictly speaking

swing-wing is a specific type of variable geometry.

Approximate wing-planform shape of Grumman F-14A variable-sweep

jet fighter.

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INTRODUCTION

A wing is a surface used to produce lift for flight through the air or gas or fluid medium.

The wing shape is usually an airfoil. The word originally referred only to the foremost

limbs of birds, but has been extended to include the wings of insects (see insect wing),

bats, pterosaurs, and aircraft.

A wing's aerodynamic quality is expressed as a Lift-to-drag ratio. The lift generated by a

wing at a given speed and angle of attack can be 1-2 orders of magnitude greater than the

drag. This means that a significantly smaller thrust force can be applied to propel the

wing through the air in order to obtain a specified lift.

A common misconception is that it is the shape of the wing that is essential to generate

lift by having a longer path on the top rather than the underside. This is not the case, thin

flat wings can produce lift efficiently and aircraft with cambered wings can fly inverted

as long as the nose of the aircraft is pointed high enough so as to present the wing at a

positive angle of attack to the airflow.

The common aerofoil shape of wings is due to a large number of factors many of them

not at all related to aerodynamic issues, for example wings need strength and thus need to

be thick enough to contain structural members. They also need room to contain items

such as fuel, control mechanisms and retracted undercarriage. The primary aerodynamic

input to the wing’s cross sectional shape is the need to keep the air flowing smoothly over

the entire surface for the most efficient operation. In particular, there is a requirement to

prevent the low-pressure gradient that accelerates the air down the back of the wing

becoming too great and effectively “sucking” the air off the surface of the wing. If this

happens the wing surface from that point backwards becomes substantially

ineffective.The shape chosen by the designer is a compromise dependent upon the

intended operational ranges of airspeed, angles of attack and wing loadings. Usually

aircraft wings have devices, such as flaps, which allow the pilot to modify shape and

surface area of the wing to be able to change its operating characteristics in flight.

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Aircraft wings may feature some of the following:

A rounded leading edge cross-section

A sharp trailing edge cross-section

Leading-edge devices such as slats, slots, or extensions

Trailing-edge devices such as flaps or flaperons (combination of flaps and

ailerons)

Ailerons (usually near the wingtips) to provide roll control

Spoilers on the upper surface to disrupt lift and provide additional roll control

Vortex generators to help prevent flow separation in transonic flow

Wing fences to keep flow attached to the wing by stopping boundary layer

separation from spreading

Winglets to keep wingtip vortices from increasing drag and decreasing lift

Dihedral or a positive wing angle to the horizontal. This gives inherent

stability in roll. Anhedral, or a negative wing angle to the horizontal, has a

destabilising effect

Folding wings allow more aircraft to be carried in the confined space of the

hangar of an aircraft carrier.

Variable-sweep wing or 'swing wing' to allow outstretched wings for slow

speed (i.e. take-off and landing) and swept back wings for high speed (usually

supersonic) flight, such as the FB-111 and the F-14.

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MOTIVATION FOR UNSWEPT WINGS

UNSWEPT WINGS: BENEFITS

Unswept wings are efficient at low speeds, providing a great amount of lift

compared to the amount of induced drag exerted on the plane. Induced drag is essentially

another component of the force that allows the plane to fly. As air flows around the

wings, the resultant turning of the airflow deflects the plane up, counteracting gravity

pulling the plane down toward the ground. However, some of that force also resists the

plane's forward movement, resulting in drag. As speed increases, this drag becomes even

more problematic.

Take off becomes pretty easy and quick for the aircraft even on a shorter runway.

The same helps in landing as well where low speeds are safer and don’t require longer

runways. With unswept wings, maneuverability becomes easy as the flaps can be

adjusted as per need. This is useful in dog fights in mid air as well as targeting the enemy

on air as well as on the ground.

Low landing speeds.

Good subsonic maneuvering.

Improved take-off characteristics.

Precision targeting.

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MOTIVATION FOR SWEPT WINGS

SWEPT WINGS: BENEFITS

A swept wing is a wing planform with a wing root to wingtip direction angled beyond

(usually aft ward) the span wise axis, generally used to delay the drag rise caused by fluid

compressibility. Swept wings provide lateral stability and it was for this reason that the

concept was first employed in the designs

Unswept wings are very bad at dealing with wave drag. Swept wings cut down on drag

caused by turbulence at the wingtips. But the real advantage of swept wings comes in

supersonic flight -- the configuration cuts down on wave drag by redistributing the shock

waves along the plane's aerodynamic profile. They are ideal for these high-speed

conditions.

Also at high mach speeds, the wings should be able to withstand stress. Hence they

should be made strong enough and usage of heavy strong materials increases the weight

in turn. In swept mode, the amount of stress is considerably reduced on the wings. The

weight of the aircraft can thus, be reduced up to a great extent and makes it capable of

flying at higher mach speeds.

Capable of near-sonic or supersonic speeds.

Increases critical mach no.

Save weight for high mach.

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DEFINITION

A variable-sweep wing is an aero plane wing that may be swept back and then

returned to its original position during flight.

Typically, a swept wing is more suitable for high speeds, while an unswept wing

is suitable for lower speeds (such as while taking off or landing).

A variable-sweep wing slows the pilot to select the correct wing configuration for

the planes intended speed.

The variable-sweep wing is most useful for those aircrafts that are expected to

function at both low and high speeds.

It is primarily used in military aircrafts.

F-14 TOMCAT is a typical example of a variable-geometry aircraft.

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HISTORY: BELL X-5

The Bell X-5 was the first aircraft capable of changing the sweep of its wings in flight. It

was inspired by the untested wartime P.1101 design of the German Messerschmitt

company. In contrast with the German design which could only be adjusted on the

ground, the Bell engineers devised a system of electric motors to adjust the sweep in

flight.

It was developed by the USAF and NASA in 1935. It was small in size and was designed

strictly for research purposes. Unlike other X Planes, the X-5 could takeoff and land on

its own and vary its wing sweep several times in flight if needed.

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The X-5 had a complex design with three sweep positions: 20°, 40°, and 60°, creating an

in-flight "variable-geometry" platform. Gearboxes and jackscrews were used to change

the wing sweep. A jackscrew assembly moved the wing's hinge along a set of short

horizontal rails, using disc brakes to lock the wing into its in-flight positions. Moving

from full extension to full sweep took less than 30 seconds. The articulation of the hinge

and pivots partly compensated for the shifts in centre of gravity and centre of pressure as

the wings moved.

Even so, the X-5 had vicious spin characteristics arising from the aircraft's flawed

aerodynamic layout, particularly a poorly positioned tail and vertical stabilizer, which in

some wing positions, could lead to an irrecoverable spin. This violent stall-spin instability

eventually caused the destruction of the second aircraft and the death of its Air Force test

pilot in 1953.

The unfavorable spin characteristics also led to the cancellation of tentative plans by the

US Air Force to modify the X-5's design into a low-cost tactical fighter for NATO and

other foreign countries.

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MACH SWEEP PROGRAMMER (MSP)

The wing sweep is controlled by a "Mach sweep programmer" that automatically moves

the wings through the range of 20 degrees to 68 degrees sweep, as dictated by flight

requirements. The pilot can also set the sweep manually, and can select a special 55-

degree mode for ground attack. The wings can be set back 75 degrees to an "over sweep"

position, overlapping the horizontal tail plane, for carrier-deck storage.

Automatic - Used Computer Controls Sweep

Manual - Pilot Can Override Computer.

Emergency - Pilot Has Complete Control

Ground attack - Locked Sweep

The wing has an automatic variable-sweep from 20° to 68°. The Mach-sweep

programmer provides the fully automatic wing-sweep as a function of Mach number and

of altitude. Thus the pilot can obtain the maximum performance/ Buffet advantages. The

pilot is provided with manual, automatic and emergency sweep-control, but his minimum

sweep-angle is automatically limited as a function of Mach number and of altitude. The

Mach-sweep programmer provides the pilot with improved combat maneuverability since

he does not have to select manually the optimum sweep at each stage of combat.

Maneuver flap actuation is co-ordinate with wing sweep for maximum delay of buffet

onset when wing lift is increased.

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WING PLAN FORM VS RANGE/ENDURANCE

Unswept:

Plan form area = 521.8 sq.ft, AR=7.3

Swept:

Plan form area = 281.8 sq.ft, AR = 5.0

Under equal flight conditions, the unswept wing will experience a 54% increase in range

and endurance. A plan form or plan view is a vertical orthographic projection of an object

on a horizontal plane, like a map.

In aviation, a planform is the shape and layout of an airplane's wing and fuselage. Of all

the myriad planforms used, they can typically be grouped into those used for low-speed

flight, found on general aviation aircraft, and those used for high-speed flight, found on

many military aircraft and airliners.

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DRAWBACKS

There were several reasons for the move away from this technology, but the primary

reason was that the large metal gearbox needed to move the wings was complicated and

heavy. This increased maintenance requirements and decreased fuel performance. An

aircraft capable of moving its wing forward for fuel-efficient flight could never be as

efficient as an airplane equipped with a straight wing. The same was true for aircraft with

swept-back wings; they would always be more efficient than aircraft with swing-wings.

The B-1B Lancer, for example, has never been able to achieve its original range

requirements and has to refuel in the air more often than planned. It also rarely flew at the

high speeds that sweeping back the wings allowed it to do.

Stability problems arise due to ac shift in supersonic regime.

Mechanically complex wing boxes.

High cost.

Sweepback has been used primarily in the interest of minimizing transonic and

supersonic wave drag. At subsonic Mach numbers, however, the disadvantages are

dominant. They include high induced drag (due to small wing span or low aspect ratio),

high angles of attack for maximum lift, and reduced effectiveness of trailing-edge flaps.

The straight-wing airplane does not have these disadvantages. For an airplane which is

designed to be multimission, for example, subsonic cruise and supersonic cruise, it would

be advantageous to combine a straight wing and swept wing design. This is the logic for

the variable sweep or swing-wing. Figure 2 shows (L/D)max, a measure of aerodynamic

efficiency, plotted against Mach number for an optimum straight-wing and swept-wing

airplane. Although not necessarily equal to the optimum configurations in their respective

speed regimes, it is evident that an airplane with a swing-wing capability can in a multi-

missioned role, over the total speed regime, be better than the other airplanes

individually. One major drawback of the swing-wing airplane is the added weight and

complexity of the sweep mechanisms. But technological advances are solving these

problems also.

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Figure 1

In addition to low-aspect-ratio wings at supersonic speeds, supersonic wave drag

may also be minimized by employing thin wings and using area ruling. Also long,

slender, cambered fuselages minimize drag and also improve the span wise lift

distribution. But this turned out to be expensive.

Ultimately, aircraft designers decided that the flexibility of the variable-sweep wing was

not worth the compromises it demanded.

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CURRENT USAGE

F-14

The Grumman F-14 Tomcat is a supersonic, twin-engine, two-seat, variable-sweep wing

aircraft. The F-14 was the United States Navy’s primary maritime air superiority fighter,

fleet defense interceptor and tactical reconnaissance platform from 1974 to 2006. It later

performed precision strike missions once it was integrated with the Low Altitude

Navigation and Targeting Infrared for Night LANTIRN system. The F-14 was developed

after the collapse of the F-111B project, and was the first of the American teen-series

fighters which were designed incorporating the experience of air combat against MiG’s

during the Vietnam War.

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F-111

The General Dynamics F-111 "Aardvark" is a medium-range interdictor and tactical

strike aircraft that also fills the roles of strategic bomber, reconnaissance, and electronic

warfare in its various versions. Developed in the 1960s and first entering service in 1967,

the United States Air Force (USAF) variants were officially retired by 1998. The Royal

Australian Air Force (RAAF) is the sole remaining operator of the F-111.

The F-111 pioneered several technologies for production military aircraft including

variable-sweep wings, afterburning turbofan engines, and automated terrain following

radar for low-level, high-speed flight. Its design was influential, being reflected in later

Soviet aircraft such as the Sukhoi Su-24, and some of its advanced features have since

become commonplace. During its inception, however, the F-111 suffered a variety of

development problems, and several of its intended roles, such as naval interception, failed

to materialize.

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MIG-23

The Mikoyan-Gurevich MiG-23 (Russian: Микоян и Гуревич МиГ-23; NATO

reporting name: Flogger) is a swing-wing fighter aircraft, designed by the Mikoyan-

Gurevich bureau in the Soviet Union. It is considered to belong to the Soviet "Third

Generation" aircraft category along with similar-aged Russian-produced fighters like

the MiG-25 "Foxbat". It was the first Soviet fighter with look-down/shoot-down radar

and beyond visual range missiles, and the first MiG production fighter plane to have

intakes at the sides of the fuselage. Production started in 1970 and reached large

numbers with over 5,000 aircraft built. Today the MiG-23 remains in limited service

with various export customers

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MIG-27

The Mikoyan MiG-27 (Russian: Микоян и Гуревич МиГ-27) (NATO reporting

name "Flogger-D/J") is a ground-attack aircraft, originally built by the Mikoyan

design bureau in the Soviet Union and later license-produced in India by Hindustan

Aeronautics as the Bahadur ("Valiant"). It is based on the Mikoyan-Gurevich MiG-23

fighter aircraft, but optimized for the air-to-ground role. However unlike the MiG-23,

the MiG-27 did not see widespread use outside Russia, as most countries opted for

the MiG-23BN and Sukhoi Su-25 instead. It currently remains only in service with

the Indian and Sri Lankan Air Forces in the ground attack role. All Russian and

Ukrainian examples have now been retired.

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OTHER AIR CRAFTS WITH VARIABLE GEOMETRY WINGS:

B-1 Lancer

Bell X-5

Sukhoi Su-17

Sukhoi Su-24

Dassault Mirage G

NASA AD-1

Northrop Grumman Switchblade

Northrop Switchblade

Tupolev Tu-160

Tupolev Tu-22M

EF-111A Raven

Panavia Tornado

Panavia Tornado ADV

XF10F Jaguar

General Dynamics F-111

General Dynamics/Grumman F-111B

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FUTURE OF VARIABLE SWEEP

Morphing Wing

One primary advantage would be the increased cost effectiveness of aircraft through

eliminating the need for multiple, expensive, mission specific aircraft. Morphing wings

can also be useful for military defense and homeland security when applied to unmanned

surveillance planes that need to fly quickly to a distant point, loiter at slow speed for a

period of time and then return, Lesieutre explains. Flying efficiently at high speed

requires small, perhaps, swept wings. Flying at slow speed for long periods requires long

narrow wings. The morphing wings designed by the Penn State team can change both

wing area and cross section shape to accommodate both slow and fast flight

requirements.

There are typically four applications of morphing:

Improve aircraft performance to expand its flight envelope;

Replace conventional control surfaces for flight control to improve performance

and stealth

Reduce drag to improve range and

Reduce vibration or control flutter.

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Micro Air Vehicle

The term micro air vehicle (MAV) or micro aerial vehicle refers to a type of unmanned

air vehicle (UAV) that is remotely controlled. Today's MAVs are significantly smaller

than those previously developed, with target dimensions reaching a maximum of

approximately 15 centimeters (six inches). Development of insect-size aircraft is

reportedly expected in the near future. Potential military use is one of the driving factors

of development, although MAVs are also being used commercially and in scientific,

police and mapping applications. Another promising area is remote observation of

hazardous environments that are inaccessible to ground vehicles. Because these aircraft

are often in the same size range as radio-controlled models, they are increasingly within

the reach of amateurs, who are making their own MAVs for aerial robotics contests and

aerial photography.

Deployed into unsafe or toxic conditions for humans.

Launched from a tube.

High speed flight to designated area.

Low speed loiters to collect necessary data.

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CONCLUSION

This would be yet another technology that can be used on aircrafts and has got

its own set of advantages and disadvantages. It cannot be taken as a

replacement to the usual non variable winged aircrafts as for now but the

designers are trying to develop a much more efficient design for such

aircrafts. It holds a promising future.

Other similar technologies include the variable incidence wings, variable

camber wings etc.A variable-sweep wing was also selected as the winning

design used by Boeing’s entry in the FAA's study for a supersonic transport,

the 2707. However it evolved through several configurations during the

design stage, finally adding a canard and it eventually became clear that the

design would be so heavy that it would be lacking sufficient payload for the

fuel needed. The design was later abandoned in favour of a more conventional

tailed delta wing.

While variable-sweep provides many advantages, particularly in takeoff

distance, load-carrying ability, and the fast, low-level penetration role,

variable-sweep wing impose a considerable penalty in weight and complexity.

The advent of relaxed stability flight control systems in the 1970s negated

many of the disadvantages of a fixed platform. No new variable-sweep wing

aircraft have been built since the Tu-160, though it has been noted that the

F-14's replacement - the F/A-18E - has a reduced payload/range capability

largely because of its small fixed wings.

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REFERENCES