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In aerodynamics, wing loading is the loaded weight of the aircraft divided by the area of the wing. It is broadly reflective of the aircraft's lift-to-mass ratio, which affects its rate of climb, load-carrying ability, and turn performance.
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 tend to have a superior rate of climb because less additional forward speed is necessary to generate the additional lift to increase altitude. It may also be capable of more efficient cruising performance because less thrust is required to maintain the lift for sustained flight.
Wing loading also is a useful measure of the general maneuvering performance of an aircraft. To turn, an aircraft must roll in the direction opposite the turn (i.e., a starboard turn rolls the top of the aircraft towards port), increasing the bank angle . This banking, however, reduces the wing's ability to generate vertical lift, which must be offset by an increase in thrust. There is a maximum rate of turn possible for any given aircraft design limited by its wing size and available engine power: this limit is known as sustained turn performance. Aircraft with low wing loading tend to have superior sustained turn performance because they can generate more lift for a given quantity of engine thrust. Conversely, however, a large, lightly loaded wing will tend to have greater mass and inertia and create greater induced drag when bank angle or angle of attack increases. This reduces the aircraft's instantaneous turn performance, its ability to rapidly change direction. 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.)
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 (all else being equal) have less drag, making it more suitable for high-speed flight (albeit at the cost of higher take-off speeds and reduced turning performance).
Wing loading also affects gust response, the degree to which the aircraft is affected by turbulence and variations in air density. A highly loaded wing has more inertia and 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 preferrable: aircraft with low wing loading are often subject to a rough, punishing ride in this flight regimen. 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.
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 gear 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. This was a major reason for the well-known disparity between the World War II-vintage Supermarine Spitfire and Messerschmitt Bf 109. Earlier marks were significantly lighter than later ones as armament, armor, and equipment increased, and while improved engine power maintained the power-to-weight ratio, later models had such heavily loaded wings that their maneuverability suffered badly, eventually tilting the balance in favor of the Spitfire. Pilots often felt that the later marks, with heavier guns and more powerful engines, were more effective, but the earlier, simpler models were far sweeter to fly, thanks mainly to their lighter wing loading.
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