Wing loading

In aerodynamics, wing loading is the loaded weight of the aircraft divided by the area of the wing. [Thom, 1988. p. 6.] It is broadly reflective of the aircraft's lift-to-mass ratio, which affects its rate of climb, load-carrying ability, and turn performance.

Range of wing loadings

Typical aircraft wing loadings range from 20 lb/ft² (100 kg/m²) for general aviation aircraft, to 80 to 120 lb/ft² (390 to 585 kg/m²) for high-speed designs like modern fighter aircraft. [Spick, 1986. p.95.] The critical limit for bird flight is about 5 lb/ft² (25 kg/m²) [Meunier, 1951]

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).

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 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.

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 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 creates even more drag. The tighter the turn radius 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 applied to the aircraft, this has the effect of increasing the wing loading and also the stalling speed. This effect is also experienced during level pitching manouevres. [Spick, 1986. p.24.] 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, which has a very small wing and high wing loading. At the opposite end of the spectrum was the gigantic Convair B-36. 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 at high altitude.

All else being equal, a larger wing generates more drag than a small one. The construction of a large wing also tends to be thicker, which further increases drag. This drag reduces the aircraft's acceleration, particularly at supersonic speeds. A smaller, thinner wing will produce less drag, making it more suitable for high-speed flight (albeit at the cost of higher take-off speeds and reduced turning performance). [Spick, 1986. p.97.]

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 has been criticized for its ride quality, as have most delta wing aircraft (such as the Dassault Mirage III), which tend to have large wings and low 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 heavy wing loading as new equipment is added. Although engines can be replaced or upgraded for additional thrust, the effects on turning and takeoff performance resulting from higher wing loading are not so easily reconciled.

Water ballast use in gliders

Modern gliders often use water ballast carried in the wings to increase wing loading when soaring conditions are strong. By increasing the wing loading the lift-to-drag ratio is increased at higher airspeeds. The ballast can be dumped overboard when conditions weaken. [ [http://en.wikipedia.org/wiki/Gliding#Maximizing_speed Maximizing glider cross-country speed] ]

Design considerations

Fuselage lift

A blended wing-fuselage design such as that found on the F-16 Fighting Falcon or MiG-29 Fulcrum 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 and the Panavia Tornado 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 swept forward position takeoff and landing performance is greatly improved. [Spick, 1986. p.84-87. ]

Fowler flaps

The use of Fowler flaps increases the wing area, decreasing the wing loading which allows slower landing approach speeds.

ee also

*Lift coefficient

References

Notes

Bibliography

* Meunier, K. "Korrelation und Umkonstruktionen in den Größenbeziehungen zwischen Vogelflügel und Vogelkörper-Biologia Generalis" 1951: p403-443. [Article in German]
* Thom, Trevor. "The Air Pilot's Manual 4-The Aeroplane-Technical". 1988. Shrewsbury, Shropshire, England. Airlife Publishing Ltd. ISBN 1-85310-017-X
* Spick, Mike. "Jet Fighter Performance-Korea to Vietnam". 1986. Osceola, Wisconsin. Motorbooks International. ISBN 0-7110-1582-1

External links

* [http://www.hq.nasa.gov/pao/History/SP-468/ch7-3.htm NASA article on wing loading] Retrieved 8 February 2008


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