Ground effect (aerodynamics)
In fixed-wing aircraft, ground effect is the increased lift and decreased drag that an aircraft's wings generate when they are close to a fixed surface. When landing, ground effect can give the pilot the feeling that the aircraft is "floating". When taking off, ground effect may temporarily reduce the stall speed. The pilot can then fly level just above the runway while the aircraft accelerates in ground effect until a safe climb speed is reached. 
Principle of ground effect
When an aircraft is flying at an altitude that is approximately at or below the same distance as the aircraft's wingspan or helicopter's rotor diameter, there is, depending on airfoil and aircraft design, an often noticeable ground effect. This is caused primarily by the ground interrupting the wingtip vortices and downwash behind the wing. When a wing is flown very close to the ground, wingtip vortices are unable to form effectively due to the obstruction of the ground. The result is lower induced drag, which increases the speed and lift of the aircraft.
A wing generates lift, in part, due to the difference in air pressure gradients between the upper and lower wing surfaces. During normal flight, the upper wing surface experiences reduced static air pressure and the lower surface comparatively higher static air pressure. Flying close to a surface increases air pressure on the lower wing surface, known as the "ram" or "cushion" effect, and thereby improves the aircraft lift-to-drag ratio. As the wing gets lower, the ground effect becomes more pronounced. While in the ground effect, the wing will require a lower angle of attack to produce the same amount of lift. If the angle of attack and velocity remain constant, an increase in the lift coefficient will result, which accounts for the "floating" effect. Ground effect will also alter thrust versus velocity, in that reducing induced drag will require less thrust to maintain the same velocity.
Low winged aircraft are more affected by ground effect than high wing aircraft. Due to the change in up-wash, down-wash, and wingtip vortices there may be errors in the airspeed system while in ground effect due to changes in the local pressure at the static source.
Another important issue regarding ground effect is that the makeup of the surface directly affects the intensity; this is to say that a concrete or other smooth hard surface will produce more effect than water or broken ground.
Ground effect aircraft
Many vehicles have a design that makes use of the wing in ground effect. Although all airplanes fly through ground effect at some point, aircraft that do so in a dedicated manner are designed in such a way that their wings are normally unable to take them into flight out of ground effect (free flight). Those that can fly out of ground effect are often only capable of short distance hops into free flight. Because of this, these craft are often licenced as ships rather than as aircraft. These specially designed craft may use delta wings, ekranoplan wings or tandem wings.
Ground effect with airships
Airships can benefit from the wing in ground effect, too. Feasibility studies have been made of specially designed airships taking advantage of the effect.
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