Wing loading


In aerodynamics, wing loading is the total weight of an aircraft divided by the area of its wing. The stalling speed of an aircraft in straight, level flight is partly determined by its wing loading. An aircraft with a low wing loading has a larger wing area relative to its mass, as compared to an aircraft with a high wing loading.
The faster an aircraft flies, the more lift can be produced by each unit of wing area, so a smaller wing can carry the same mass in level flight. Consequently, faster aircraft generally have higher wing loadings than slower aircraft. This increased wing loading also increases takeoff and landing distances. A higher wing loading also decreases maneuverability. The same constraints apply to winged biological organisms.

Range of wing loadings

AircraftTypeIntroductionMTOWWing areakg/m2lb/sqft
Monarch ButterflyAnimalCenozoic-
birdsAnimalCretaceous-
bird flight upper critical limitAnimal-
Ozone Buzz Z3 MSParaglider2010-
Wills Wing Sport 2 155Hang glider2004-
upper limitMicrolift glider2008 max. min.-
CAA regulationsmicrolight wing loading limit2008 max. min.-
Schleicher ASW 22Glider1981-
Piper WarriorGeneral aviation1960-
Beechcraft BaronGeneral aviation twin-engine1960-
Supermarine SpitfireFighter 1938-
Beechcraft AirlinerAirliner 1968-
Learjet 31Business jet1990-
Mikoyan MiG-23Fighter 1970-
General Dynamics F-16Fighter 1978-
Fokker F27Airliner 1958-
McDonnell Douglas F-15 EagleFighter 1976-
Fokker F28 FellowshipAirliner 1969-
Boeing 737-300Airliner 1984-
Boeing 737-900Airliner 2001-
Boeing 767Airliner 1982-
ConcordeAirliner 1976-
Rockwell B-1B LancerBomber 1983-
Boeing 777Airliner 1995-
Boeing 747Airliner 1977-
Airbus A380Airliner 2007-

Effect on performance

Wing loading is a useful measure of the stalling speed of an aircraft. Wings generate lift owing to the motion of air around the wing. Larger wings move more air, so an aircraft with a large wing area relative to its mass will have a lower stalling speed. Therefore, an aircraft with lower wing loading will be able to take off and land at a lower speed. It will also be able to turn at a greater rate.

Effect on takeoff and landing speeds

The lift force L on a wing of area A, traveling at true airspeed v is given by

,

where ρ is the density of air and CL is the lift coefficient. The lift coefficient is a dimensionless number which depends on the wing cross-sectional profile and the angle of attack. At take-off or in steady flight, neither climbing nor 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 and g the acceleration due to gravity, that equation gives the speed v through
.

As a consequence, aircraft with the same CL at takeoff under the same atmospheric conditions will have takeoff speeds proportional to. So if an aircraft's wing area is increased by 10% and nothing else is changed, the takeoff 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 takeoff speed increases to = 177 mph.
Some flyers rely on their muscle power to gain speed for takeoff over land or water. Ground nesting and water birds have to be able to run or paddle at their takeoff speed before they can take off. The same is true for a hang glider pilot, though they may get assistance 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 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' 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 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 having the effect of increasing the wing loading and also the stalling speed. This effect is also experienced during level pitching maneuvers.
As stalling is due to wing loading and maximum lift coefficient at a given altitude and speed, this limits the turning radius due to maximum load factor.
At Mach 0.85 and 0.7 lift coefficient, a wing loading of can reach a structural limit of 7.33 g up to and then decreases to 2.3 g at. With a wing loading of the load factor is twice smaller and barely reaches 1g at 40,000 feet.
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 large Convair B-36: its large wings resulted in a low wing loading that could make it sustain tighter turns at high altitude than contemporary jet fighters, while the slightly later Hawker Hunter had a similar wing loading of. The Boeing 367-80 airliner prototype could be rolled at low altitudes with a wing loading of at maximum weight.
Like any body in circular motion, an aircraft that is fast and strong enough to maintain level flight at speed v in a circle of radius R accelerates towards the center at. That acceleration is caused by the inward horizontal component of the lift,, where is the banking angle. Then from Newton's second law,
Solving for R 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, 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 a wing loading of , whereas most delta wing aircraft tend to have large wings and low wing loadings.
Quantitatively, if a gust produces an upward pressure of G 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. 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 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 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 is achieved at a higher airspeed than with a lower wing loading, and this allows a faster average speed across country. The ballast can be ejected overboard when conditions weaken to maximize glider cross-country speed in gliding competitions.

Design considerations

Fuselage lift

A blended wing-fuselage design such as that found on the General Dynamics F-16 Fighting Falcon or Mikoyan 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 Grumman F-14 Tomcat and the Panavia Tornado employ variable-sweep wings. As their wing area varies in flight so does the wing loading. When the wing is in the forward position takeoff and landing performance is greatly improved.

Fowler flaps

Like all aircraft flaps, Fowler flaps increase the camber and hence CL, lowering the landing speed. They also increase wing area, decreasing the wing loading, which further lowers the landing speed.