Information about Wing Loading

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.

Typical 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. The critical limit for bird flight is about 5 lb/ft² (25 kg/m²) (Meunier, 1951).

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 is also a useful measure of the general maneuvering performance of an aircraft. To turn, an aircraft must roll in the direction of the turn (i.e., in a right turn the pilot rolls to right wing low, left wing high), increasing the aircraft's bank angle. Banks lower the wing's lift component against gravity and hence cause a descent. To compensate the total aerodynamic force must be increased by increasing the angle of attack by use of up elevator deflection which increases drag. Also, turning is '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 harder the turn attempted, the more drag. All 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. 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 the bank angle or angle of attack increases. The immediate turn position an aircraft can get into before drag seriously bleeds off speed is its 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. 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. A blended wing-fuselage design often helps to reduce wing loading; in such a design (such as that found on the F-16 Fighting Falcon or MiG-29 Fulcrum), the shape of the fuselage generates some aerodynamic lift itself, improving wing loading while maintaining high performance.



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 preferable: aircraft with 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.

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 of the Messerschmitt design 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.

References

• Meunier, K. (1951): Korrelation und Umkonstruktionen in den Größenbeziehungen zwischen Vogelflügel und Vogelkörper. Biologia Generalis 19: 403-443. [Article in German]