Wing loading
Overview
 
In aerodynamics
Aerodynamics
Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a subfield of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used synonymously with gas dynamics, with...

, wing loading is the loaded weight of the aircraft divided by the area of the wing. The faster an aircraft flies, the more lift
Lift (force)
A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...

 is produced by each unit area of wing, so a smaller wing can carry the same weight in level flight, operating at a higher wing loading. Correspondingly, the landing and take-off speeds will be higher. The high wing loading also decreases maneuverability. The same constraints apply to winged biological organisms.
Wing loadings are usually given in either lb
Pound (mass)
The pound or pound-mass is a unit of mass used in the Imperial, United States customary and other systems of measurement...

/ft2 or kg
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...

/m2, and occasionally in N/m2.
Encyclopedia
In aerodynamics
Aerodynamics
Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a subfield of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used synonymously with gas dynamics, with...

, wing loading is the loaded weight of the aircraft divided by the area of the wing. The faster an aircraft flies, the more lift
Lift (force)
A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...

 is produced by each unit area of wing, so a smaller wing can carry the same weight in level flight, operating at a higher wing loading. Correspondingly, the landing and take-off speeds will be higher. The high wing loading also decreases maneuverability. The same constraints apply to winged biological organisms.

Units

Wing loadings are usually given in either lb
Pound (mass)
The pound or pound-mass is a unit of mass used in the Imperial, United States customary and other systems of measurement...

/ft2 or kg
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...

/m2, and occasionally in N/m2. Wing loadings of 10 lb/ft2, 48.8 kg/m2 and 479 N/m2 are the same.

Range of wing loadings

Aircraft Buzz Z3 Fun 160 ASK 21 Nieuport 17
Nieuport 17
|-Specifications :-See also:-Bibliography:* Bruce, Jack. "Those Classic Nieuports". Air Enthusiast Quarterly. Number Two, 1976. Bromley, UK:Pilot Press. pp. 137–153....

Ikarus C42
Ikarus C42
-See also:-External links:* * *...

Cessna 152
Cessna 152
The Cessna 152 is an American two-seat, fixed tricycle gear, general aviation airplane, used primarily for flight training and personal use.-Development:...

Vans RV-4 DC-3 Spitfire
Supermarine Spitfire
The Supermarine Spitfire is a British single-seat fighter aircraft that was used by the Royal Air Force and many other Allied countries throughout the Second World War. The Spitfire continued to be used as a front line fighter and in secondary roles into the 1950s...

Messerschmitt Bf-109 B-17 B-36 Eurofighter F-104 A380
Airbus A380
The Airbus A380 is a double-deck, wide-body, four-engine jet airliner manufactured by the European corporation Airbus, a subsidiary of EADS. It is the largest passenger airliner in the world. Due to its size, many airports had to modify and improve facilities to accommodate it...

B747
Boeing 747
The Boeing 747 is a wide-body commercial airliner and cargo transport, often referred to by its original nickname, Jumbo Jet, or Queen of the Skies. It is among the world's most recognizable aircraft, and was the first wide-body ever produced...

MD-11F
McDonnell Douglas MD-11
The McDonnell Douglas MD-11 is a three-engine medium- to long-range widebody jet airliner, manufactured by McDonnell Douglas and, later, by Boeing Commercial Airplanes. Based on the DC-10, it features a stretched fuselage, increased wingspan with winglets, refined airfoils on the wing and smaller...

Wing Loading
(kg/m2)
3.9 6.3 33 38 38 51 67 123 158 173 190 272 311 514 663 740 844
Role paraglider hang glider glider fighter microlight trainer sports airliner fighter fighter bomber bomber fighter fighter-bomber airliner airliner airliner
Year 2010 2007 1979 1916 1997 1978 1980 1936 1942 1941 1938 1949 1998 1958 2007 1970 1990

The table, which shows wing loadings, is intended to give an idea of the range of wing loadings used by human-built aircraft. Maximum weights have been used. There will be variations amongst variants of any particular type. The dates are approximate, indicating period of introduction.

The critical limit for bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

 flight is about 5 lb/ft2 (25 kg/m2). An analysis of bird flight which looked at 138 species ranging in mass from 10 g to 10 kg, from small passerines to swan
Swan
Swans, genus Cygnus, are birds of the family Anatidae, which also includes geese and ducks. Swans are grouped with the closely related geese in the subfamily Anserinae where they form the tribe Cygnini. Sometimes, they are considered a distinct subfamily, Cygninae...

s and cranes
Crane (bird)
Cranes are a family, Gruidae, of large, long-legged and long-necked birds in the order Gruiformes. There are fifteen species of crane in four genera. Unlike the similar-looking but unrelated herons, cranes fly with necks outstretched, not pulled back...

 found wing loadings from about 1 to 20 kg/m2. The wing loadings of some of the lightest aircraft fall comfortably within this range. One typical hang-glider (see table) has a maximum wing loading of 6.3 kg/m2, and an ultralight rigid glider 8.3 kg/m2.

Effect on performance

Wing loading is a useful measure of the general maneuvering performance of an aircraft. Wings generate lift owing to the motion of air over the wing surface. Larger wings move more air, so an aircraft with a large wing area relative to its mass (i.e., low wing loading) will have more lift at any given speed. Therefore, an aircraft with lower wing loading will be able to take-off and land at a lower speed (or be able to take off with a greater load). It will also be able to turn faster.

Effect on take-off and landing speeds

Quantitatively, the lift force L on a wing of area A, travelling at speed v is given by


,


where ρ is the density of air and CL is the lift coefficient
Lift coefficient
The lift coefficient is a dimensionless coefficient that relates the lift generated by a lifting body, the dynamic pressure of the fluid flow around the body, and a reference area associated with the body...

. The latter is a dimensionless number of order unity which depends on the wing cross-sectional profile and the angle of attack
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...

. At take-off or in steady flight, neither climbing or diving, the lift force and the weight are equal. With L/A = Mg/A =WSg , where M is the aircraft mass, WS = M/A the wing loading (in mass/area units, i.e. lb/ft2 or kg/m2, not force/area) and g the acceleration due to gravity, that equation gives the speed v through

.

As a consequence, aircraft with the same CL at take-off under the same atmospheric conditions will have take off speeds proportional to . So if an aircraft's wing area is increased by 10% and nothing else changed, the take-off speed will fall by about 5%. Likewise, if an aircraft designed to take off at 150 mph grows in weight during development by 40%, its take-off speed increases to mph.

Some flyers rely on their muscle power to gain speed for take-off over land or water. Ground nesting and water birds have to be able to run or paddle at their take-off speed and the same is so for a hang glider pilot, though he or she may get an assist from a downhill run. For all these a low WS is critical, whereas passerines and cliff dwelling birds can get airborne with higher wing loadings.

Effect on climb rate and cruise performance

Wing loading has an effect on an aircraft's climb rate. A lighter loaded wing will have a superior rate of climb compared to a heavier loaded wing as less airspeed
Airspeed
Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....

 is required to generate the additional lift to increase altitude. A lightly loaded wing has a more efficient cruising performance because less thrust is required to maintain lift for level flight. However, a heavily loaded wing is more suited for higher speed flight because smaller wings offer less drag.

The second equation given above applies again to the cruise in level flight, though and particularly CL will be smaller than at take-off, CL because of a lower angle of incidence and the retraction of flaps or slats; the speed needed for level flight is lower for smaller WS.

The wing loading is important in determining how rapidly the climb is established. If the pilot increases the speed to vc the aircraft will begin to rise with vertical acceleration ac because the lift force is now greater than the weight. Newton's second law tells us this acceleration is given by

or
so the initial upward acceleration is inversely proportional to WS. Once the climb is established the acceleration falls to zero as the sum of the upward components of lift plus engine thrust minus drag becomes numerically equal to the weight.

Effect on turning performance

To turn, an aircraft must roll in the direction of the turn, increasing the aircraft's bank angle. Turning flight lowers the wing's lift component against gravity and hence causes a descent. To compensate, the lift force must be increased by increasing the angle of attack by use of up elevator
Elevator (aircraft)
Elevators are flight control surfaces, usually at the rear of an aircraft, which control the aircraft's orientation by changing the pitch of the aircraft, and so also the angle of attack of the wing. In simplified terms, they make the aircraft nose-up or nose-down...

 deflection which increases drag. Turning can be described as 'climbing around a circle' (wing lift is diverted to turning the aircraft) so the increase in wing angle of attack
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...

 creates even more drag. The tighter the turn radius
Radius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...

 attempted, the more drag induced, this requires that power (thrust) be added to overcome the drag. The maximum rate of turn possible for a given aircraft design is limited by its wing size and available engine power: the maximum turn the aircraft can achieve and hold is its sustained turn performance. As the bank angle increases so does the g-force
G-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...

 applied to the aircraft, this has the effect of increasing the wing loading and also the stalling speed
Stall (flight)
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...

. This effect is also experienced during level pitching maneuvers.

Aircraft with low wing loadings tend to have superior sustained turn performance because they can generate more lift for a given quantity of engine thrust. The immediate bank angle an aircraft can achieve before drag seriously bleeds off airspeed is known as its instantaneous turn performance. An aircraft with a small, highly loaded wing may have superior instantaneous turn performance, but poor sustained turn performance: it reacts quickly to control input, but its ability to sustain a tight turn is limited. A classic example is the F-104 Starfighter
F-104 Starfighter
The Lockheed F-104 Starfighter is a single-engine, high-performance, supersonic interceptor aircraft originally developed for the United States Air Force by Lockheed. One of the Century Series of aircraft, it served with the USAF from 1958 until 1969, and continued with Air National Guard units...

, which has a very small wing and high wing loading. At the opposite end of the spectrum was the gigantic Convair B-36
Convair B-36
The Convair B-36 "Peacemaker" was a strategic bomber built by Convair and operated solely by the United States Air Force from 1949 to 1959. The B-36 was the largest mass-produced piston engine aircraft ever made. It had the longest wingspan of any combat aircraft ever built , although there have...

. Its large wings resulted in a low wing loading, and there are disputed claims that this made the bomber more agile than contemporary jet fighters (the slightly later Hawker Hunter
Hawker Hunter
The Hawker Hunter is a subsonic British jet aircraft developed in the 1950s. The single-seat Hunter entered service as a manoeuvrable fighter aircraft, and later operated in fighter-bomber and reconnaissance roles in numerous conflicts. Two-seat variants remained in use for training and secondary...

 had a similar wing loading of 250 kg/m2) at high altitude. Whatever the truth of that, the delta winged Avro Vulcan
Avro Vulcan
The Avro Vulcan, sometimes referred to as the Hawker Siddeley Vulcan, was a jet-powered delta wing strategic bomber, operated by the Royal Air Force from 1956 until 1984. Aircraft manufacturer A V Roe & Co designed the Vulcan in response to Specification B.35/46. Of the three V bombers produced,...

 bomber, with a wing loading of 260 kg/m2 could certainly be rolled at low altitudes.

Like any body in circular motion
Circular motion
In physics, circular motion is rotation along a circular path or a circular orbit. It can be uniform, that is, with constant angular rate of rotation , or non-uniform, that is, with a changing rate of rotation. The rotation around a fixed axis of a three-dimensional body involves circular motion of...

, an aircraft that is fast and strong enough to maintain level flight at speed v in a circle of radius R accelerates towards the centre at . That acceleration is caused by the inward horizontal component of the lift, , where is the banking angle. Then from Newton's second law ,
Tidying up gives
The smaller the wing loading, the tighter the turn.

Gliders designed to exploit thermals need a small turning circle in order to stay within the rising air column, and the same is true for soaring birds. Other birds, for example those that catch insects on the wing also need high maneuverability. All need low wing loadings.

Effect on stability

Wing loading also affects gust response, the degree to which the aircraft is affected by turbulence and variations in air density. A small wing has less area on which a gust can act, both of which serve to smooth the ride. For high-speed, low-level flight (such as a fast low-level bombing run in an attack aircraft), a small, thin, highly loaded wing is preferable: aircraft with a low wing loading are often subject to a rough, punishing ride in this flight regime.
The F-15E Strike Eagle
F-15E Strike Eagle
The McDonnell Douglas F-15E Strike Eagle is an all-weather multirole fighter, derived from the McDonnell Douglas F-15 Eagle. The F-15E was designed in the 1980s for long-range, high speed interdiction without relying on escort or electronic warfare aircraft. United States Air Force F-15E Strike...

 has a wing loading of 650 kg/m2 (excluding fuselage contributions to the effective area), as have most delta wing
Delta wing
The delta wing is a wing planform in the form of a triangle. It is named for its similarity in shape to the Greek uppercase letter delta .-Delta-shaped stabilizers:...

 aircraft (such as the Dassault Mirage III
Dassault Mirage III
The Mirage III is a supersonic fighter aircraft designed by Dassault Aviation during the late 1950s, and manufactured both in France and a number of other countries. It was a successful fighter aircraft, being sold to many air forces around the world and remaining in production for over a decade...

, for which WS = 387 kg/m2) which tend to have large wings and low wing loadings.

Quantitatively, if a gust produces an upward pressure of G (in N/m2, say) on an aircraft of mass M, the upward acceleration a will, by Newton's second law be given by
,
decreasing with wing loading.

Effect of development

A further complication with wing loading is that it is difficult to substantially alter the wing area of an existing aircraft design (although modest improvements are possible). As aircraft are developed they are prone to "weight growth" -- the addition of equipment and features that substantially increase the operating mass of the aircraft. An aircraft whose wing loading is moderate in its original design may end up with very high wing loading as new equipment is added. Although engines can be replaced or upgraded for additional thrust, the effects on turning and take-off performance resulting from higher wing loading are not so easily reconciled.

Water ballast use in gliders

Modern gliders
Glider (sailplane)
A glider or sailplane is a type of glider aircraft used in the sport of gliding. Some gliders, known as motor gliders are used for gliding and soaring as well, but have engines which can, in some cases, be used for take-off or for extending a flight...

 often use water ballast carried in the wings to increase wing loading when soaring
Lift (soaring)
Gliding flight is heavier-than-air flight without the use of thrust. It is employed by gliding animals and by aircraft such as gliders. The most common human application of gliding flight is in sport and recreation using aircraft designed for this purpose...

 conditions are strong. By increasing the wing loading the average speed achieved across country can be increased to take advantage of strong thermals. With a higher wing loading, a given lift-to-drag ratio
Lift-to-drag ratio
In aerodynamics, the lift-to-drag ratio, or L/D ratio, is the amount of lift generated by a wing or vehicle, divided by the drag it creates by moving through the air...

 is achieved at a higher airspeed
Airspeed
Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....

 than with a lower wing loading, and this allows a faster average speed across country. The ballast can be dumped overboard when conditions weaken. (See Gliding competitions
Gliding competitions
Some of the pilots in the sport of gliding take part in gliding competitions. These are usually racing competitions, but there are also aerobatic contests and on-line league tables.-History of competitions:...

)

Fuselage lift

A blended wing-fuselage design such as that found on the F-16 Fighting Falcon
F-16 Fighting Falcon
The General Dynamics F-16 Fighting Falcon is a multirole jet fighter aircraft originally developed by General Dynamics for the United States Air Force . Designed as an air superiority day fighter, it evolved into a successful all-weather multirole aircraft. Over 4,400 aircraft have been built since...

 or MiG-29 Fulcrum
Mikoyan MiG-29
The Mikoyan MiG-29 is a fourth-generation jet fighter aircraft designed in the Soviet Union for an air superiority role. Developed in the 1970s by the Mikoyan design bureau, it entered service with the Soviet Air Force in 1983, and remains in use by the Russian Air Force as well as in many other...

 helps to reduce wing loading; in such a design the fuselage generates aerodynamic lift, thus improving wing loading while maintaining high performance.

Variable-sweep wing

Aircraft like the F-14 Tomcat
F-14 Tomcat
The Grumman F-14 Tomcat is a supersonic, twin-engine, two-seat, variable-sweep wing fighter aircraft. The Tomcat was developed for the United States Navy's Naval Fighter Experimental program following the collapse of the F-111B project...

 and the Panavia Tornado
Panavia Tornado
The Panavia Tornado is a family of twin-engine, variable-sweep wing combat aircraft, which was jointly developed and manufactured by the United Kingdom, West Germany and Italy...

 employ variable-sweep wings. As their wing area varies in flight so does the wing loading (although this is not the only benefit). In the forward position takeoff
Takeoff
Takeoff is the phase of flight in which an aerospace vehicle goes from the ground to flying in the air.For horizontal takeoff aircraft this usually involves starting with a transition from moving along the ground on a runway. For balloons, helicopters and some specialized fixed-wing aircraft , no...

 and landing
Landing
thumb|A [[Mute Swan]] alighting. Note the ruffled feathers on top of the wings indicate that the swan is flying at the [[Stall |stall]]ing speed...

 performance is greatly improved.

Fowler flaps

The use of Fowler flaps
Flap (aircraft)
Flaps are normally hinged surfaces mounted on the trailing edges of the wings of a fixed-wing aircraft to reduce the speed an aircraft can be safely flown at and to increase the angle of descent for landing without increasing air speed. They shorten takeoff and landing distances as well as...

increases the wing area, decreasing the wing loading which allows slower landing approach speeds.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK