Slip (aerodynamic)
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A slip is an aerodynamic state where an aircraft is moving sideways as well as forward relative to the oncoming wind. In other words, for a conventional aircraft, the nose will not be pointing directly into the wind. Flying in a slip is aerodynamically inefficient and can also cause motion sickness in passengers. Inexperienced pilots will often enter slips unintentionally during turns by failing to coordinate the aircraft using the rudder; however there are two common situations where a pilot may enter a slip deliberately by using opposite rudder and aileron inputs: the sideslip, and the forward slip.
Sideslip
The sideslip is an aircraft maneuver that uses opposite aileron and rudder. This maneuver is often used when landing in a crosswind, as an alternative to a crab landing — if the aircraft can be made to slip sideways at exactly the same speed as the crosswind, the aircraft will remain lined up with the runway, and will appear to be moving straight forward (from a ground perspective).
For example, when approaching a runway that has a crosswind from the left, the pilot may apply left aileron to bank the aircraft slightly and also apply right rudder to keep the aircraft pointing toward the runway. By controlling the angle of bank, the pilot can sideslip the aircraft to exactly counter the crosswind. With a sideslip, the aircraft will land the upwind wing low, and the upwind wheel(s), float, or ski will touch the ground first.
Forward slip
Aerodynamically, the forward slip is identical to the sideslip; however, in this case, the pilot slips the aircraft not to stay lined up with a runway, but to lose altitude quickly — because a slip is inefficient, an aircraft will descend much more quickly in a slip without picking up excessive speed, as it would if the pilot simply pushed the nose down. In a forward slip, the nose will be in a noticeable crab angle — from the ground, it will appear that the aircraft is pointing away from the runway.
Some simple powered aircraft (such as the Piper J-3 Cub) and old gliders do not have spoilers, so the forward slip is the primary way for a pilot to lose excess altitude on approach. For aircraft with spoilers, combining spoilers and a slip can allow a very steep descent without excess ground speed. Forward slips are rarely used for medium and heavy aircraft, but they are possible — in one famous example, the captain of the Gimli Glider used a forward slip to lose altitude while landing a Boeing 767 after running out of fuel over Manitoba. However it is probable that if a large aircraft did attempt a forward slip, it would likely extinguish its engines due to a lower air intake.
Other uses
There are other, specialized circumstances where slips can be useful in aviation. For example, during aerial photography, a slip can lower one side of the aircraft to allow ground photos to be taken through a side window. Pilots will also use a slip to land in icing conditions if the front windshield has been entirely iced over — by landing slightly sideways, the pilot is able to see the runway through the aircraft's side window. Slips also play a role in aerobatics and aerial combat.
See also
External link
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