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.
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[edit] Forward-slip
The forward-slip is an aircraft maneuver that uses opposite aileron and rudder, that is, the wing on the side toward which the slip is to be made is lowered by the use of ailerons, while simultaneously yawing the aircraft's nose in the opposite direction by applying opposite rudder. In this maneuver, the airplane's direction of motion continues the same as before the slip was begun, however much more drag is created, allowing the pilot to dissipate altitude without increasing airspeed.
[edit] Sideslip
A sideslip is one of the methods used by pilots to execute a crosswind landing. Sideslips are entered by lowering a wing and applying enough opposite rudder so the airplane does not turn. In a sideslip condition, the airplane's longitudinal axis remains parallel to the original flightpath, but the airplane no longer flies straight. Now, the horizontal component of lift forces the airplane to move sideways toward the low wing. In order to land using the sideslip method, the pilot puts the airplane into a sideslip, touching one main landing gear, followed by the second main gear, and finally the nose gear.
[edit] 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.
