Wing loading is defined as the total weight, including the aircraft itself, that an aircraft is carrying divided by the area of lift producing surface (wing(s)).

Aircraft are usually designed for optimum characteristics within a certain wing loading range. When operated at or near maximum wing loading an aircraft may behave sluggishly, require excessive fuel, and have a high descent rate(= reduced climb rate). Flex wing aircraft such as hang gliders and paragliders can be difficult to control when flown at less than minimum wing loading, since the wing or airframe needs a certain load to flex into the proper shape. Certain cardinal speeds of gliding aircraft depend on wing loading, such as stall speed (Vs), best glide, etc. Pilots can carry ballast to set the wing loading for desired characteristics. Water is commonly used as ballast because it may be safely discarded during flight to adjust for varying conditions. Sailplanes often have tanks for water ballast within their wings, where a powered plane would carry fuel. Hang glider pilots carry ballast in their harnesses, so water is of less use; lead, either shot or scuba diving weights (or rocks!) are often used. Hopefully, these are not dropped on purpose.

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