Science Fair Project Encyclopedia
Landing is the last part of a flight, where a flying animal or aircraft returns to the ground. Hitting the ground too hard is prevented by wings (including rotor wings), a parachute or rockets or a vertically directed jet engine; in the case of a balloon the buoyancy is slightly decreased for a soft landing. Aircraft usually land at an airport on a runway or helicopter landing pad.
For aircraft or birds, landing is generally accomplished by trading airspeed for lift. The first phase is the flare, where the rate of descent will be reduced by transitioning to a stall attitude. After slowing down, the plane changes pitch into the landing attitude shortly before touching down.
In a perfect touchdown, assuming there is no crosswind, contact with the ground is made just as the forward speed is reduced to the point where there is no longer sufficient lift to remain aloft. If there is a crosswind, techniques such as a crab landing or a slip landing are used to land the plane safely.
During landing, the ground effect becomes significant for aircraft. This tends to make the aircraft "keep flying" when it ordinarly would not (at higher altitudes) and therefore to extend the distance required to land.
Large jet transport aircraft land differently than described above. If the pilot waited for the aircraft to stall too much runway length would be used so the flare just reduces the rate of descent at touchdown and the aircraft is flown onto the runway. Usually lift dumpers are immediately deployed to dramatically reduce the lift and transfer the aircraft's weight to its wheels, where mechanical braking can take effect.
To land on an aircraft carrier, an aircraft (moving at, perhaps, 150 mph (240 km/h)) is equipped with tailhooks to engage one of up to four arresting cables stretched across the deck, stopping the aircraft within 320 feet (100 m) after engaging one of the cables. To assist safe landings, the carrier will usually steam directly into wind at full speed, thus reducing aircraft's speed relative to the carrier deck, and eliminating any crosswind.
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