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| {{About|aviation|car handling|slip angle}}
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| A '''slip''' is an [[aerodynamic]] state where an [[aircraft]] is moving ''somewhat'' sideways as well as forward relative to the oncoming airflow or [[relative wind]]. In other words, for a conventional aircraft, the nose will be pointing in the opposite direction to the bank of the wing(s). The aircraft is not in [[coordinated flight]] and therefore is flying inefficiently.
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| ==Background==
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| Flying in a slip is aerodynamically inefficient, the [[Lift-to-drag ratio|L/D]] ratio is reduced. More drag is at play consuming energy but not producing lift. Inexperienced or [[absent-mindedness|inattentive]] pilots will often enter slips unintentionally during turns by failing to coordinate the aircraft with the [[rudder]]. Airplanes can readily enter into a slip climbing out from take-off on a windy day. If left unchecked, climb performance will suffer. This is especially dangerous if there are nearby obstructions under the climb path and the aircraft is underpowered or heavily loaded.
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| A slip can also be a ''piloting maneuver'' where the pilot deliberately enters one type of slip or another. Slips are particularly useful in performing a short field landing over an obstacle (such as trees, or power lines), or to avoid an obstacle (such as a single tree on the extended centerline of the runway), and may be practiced as part of emergency landing procedures. These methods are also commonly employed when flying into farmstead or rough country airstrips where the landing strip is short. Pilots need to touch down with ample runway remaining to slow down and stop.
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| There are common situations where a [[aviator|pilot]] may deliberately enter a slip by using opposite rudder and [[aileron]] inputs, most commonly in a landing approach at low power.<ref name="SHIF">John S. Denker, ''See How It Flies''. http://www.av8n.com/how/htm/snaps.html#sec-intentional-slip</ref>
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| Without [[Flap (aircraft)|flaps]] or [[Spoiler (aeronautics)|spoilers]] it is difficult to increase the steepness of the glide without adding significant speed. This excess speed can cause the aircraft to fly in [[Ground effect in aircraft|ground effect]] for an extended period, perhaps running out of runway. In a forward slip much more drag is created, allowing the pilot to dissipate altitude without increasing airspeed, increasing the angle of descent (glide slope). Forward slips are especially useful when operating pre-1950s training aircraft, aerobatic aircraft such as the [[Pitts Special]] or any aircraft with inoperative flaps or spoilers.
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| Often, if an airplane in a slip is made to stall, it displays very little of the yawing tendency that causes a [[Skid (aerodynamic)|skidding]] stall to develop into a [[Spin (flight)|spin]]. A stalling airplane in a slip may do little more than tend to roll into a wings level attitude. In fact, in some airplanes stall characteristics may even be improved.<ref>http://www.faa.gov/library/manuals/aircraft/airplane_handbook/media/faa-h-8083-3a-4of7.pdf</ref>
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| ==Forward-slip vs. Sideslip==
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| Aerodynamically these are identical once established, but they are entered for different reasons and will create different ground tracks and headings relative to those prior to entry. '''Forward-slip '''is used to steepen an approach (reduce height) without gaining much airspeed,<ref name="The Forward Slip to Steepen an Approach">{{cite book|last=Thom|first=Trevor|title=The Flying Training Manual|year=1993|publisher=Aviation Theory Centre P/L|location=Vic. Australia|isbn=1-875537-18-x|page=8/19}}</ref> benefiting from the increased drag. The '''Sideslip '''moves the aircraft sideways (often, only in relation to the wind) where executing a turn would be inadvisable, drag is considered a byproduct. Most pilots like to enter sideslip just before flaring or touching down during a crosswind landing.<ref>{{cite book|last=Thom|first=Trevor|title=The Flying Training Manual|year=1993|publisher=Aviation Theory Centre P/L|location=Vic. Australia|isbn=1-875537-18-x|page=8/21}}</ref>
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| === Forward-slip ===
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| The forward slip changes the [[Aircraft heading|heading]] of the aircraft away from the down wing, while retaining the original ''track'' (flight path over the ground) of the aircraft.
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| To execute a forward slip, the pilot [[Aileron|banks]] into the wind and applies opposing rudder (e.g. Right aileron + Left rudder) in order to keep moving towards the target. If you were the target you would see the plane's nose off to one side, a wing off to the other side and tilted down toward you. The pilot must make sure that the plane's nose is low enough to keep airspeed up.<ref>http://www.wikihow.com/Perform-a-Forward-Slip-in-a-Cessna-152-to-Descend-Rapidly</ref> However, Airframe speed limits such as V<sub>A</sub> and V<sub>FE</sub> must be observed.<ref>[[V speeds#Regulatory V-speeds]]</ref>
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| A forward-slip is useful when a pilot has set up for a landing approach with excessive height or must descend steeply beyond a tree line to touchdown near the runway threshold. Assuming that the plane is properly lined up for the runway, the forward slip will allow the aircraft ''track'' to be maintained while steepening the descent without adding excessive airspeed. Since the heading is not aligned with the runway, forward-slip must be removed before touchdown to avoid excessive side loading on the landing gear, and if a cross wind is present an appropriate '''sideslip''' may be necessary at touchdown as described below.
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| In the [[United States]], student pilots are required to know how to do forward slips before embarking on their first solo flight. The logic is that in the event of an engine failure, the pilot will have to land on the first attempt and will not have a chance to go around if the aircraft is too high or too fast.
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| ===Sideslip===
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| The '''sideslip''' also uses opposite aileron and rudder. In this case it is entered by lowering a wing and applying exactly enough opposite rudder so the airplane does not turn (maintaining the same ''heading''), while maintaining safe airspeed with [[Elevator (aircraft)|pitch]] or [[Throttle|power]].
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| In the sideslip condition, the airplane's longitudinal axis remains parallel to the original flightpath, but the airplane no longer flies straight along its original track. Now, the horizontal component of lift forces the airplane to move sideways toward the low wing.
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| A sideslip may be used exclusively to remain lined up with a runway centerline while on approach in a [[crosswind]] or be employed in the final moments of a crosswind landing. To commence sideslipping, the pilot [[Aileron|rolls]] the airplane toward the wind to maintain runway centerline position while maintaining heading on the centerline with the rudder. Sideslip causes one main landing gear to touch down first, followed by the second main gear. This allows the wheels to be constantly aligned with the track, thus avoiding any side load at touchdown.
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| The sideslip method for cross-wind landings is not suitable for long-winged and low-sitting aircraft such as [[Glider (sailplane)|gliders]], where instead a crab angle (heading into the wind) is maintained until a moment before touchdown.
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| ====Sideslip angle====
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| The sideslip angle, also called angle of sideslip (AOS, AoS, <math>\beta</math>, [[Greek alphabet|Greek letter]] [[beta (letter)|Beta]]), is a term used in [[fluid dynamics]] and [[aerodynamics]] and [[aviation]]. It relates to the rotation of the aircraft centerline from the [[relative wind]]. In [[flight dynamics]] it is given the shorthand notation <math>\beta</math> (beta) and is usually assigned to be "positive" when the relative wind is coming from the right of the nose of the airplane. The sideslip angle <math>\beta</math> is essentially the directional [[angle of attack]] of the airplane. It is the primary parameter in [[stability]]{{disambiguation needed|date=June 2013}} considerations.<ref>{{cite book
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| | last = Hurt
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| | first = H. H., Jr.
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| | origyear = 1960
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| | title = Aerodynamics for Naval Aviators
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| |date=January 1965
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| | publisher = U.S. Navy, Aviation Training Division
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| | location = U.S. Government Printing Office, Washington D.C.
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| | id = NAVWEPS 00-80T-80
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| | pages = 284–5
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| }}</ref>
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| In Vehicle Dynamics, side slip angle is defined as the angle made by the velocity vector to longitudinal axis of the vehicle at the center of gravity in an instantaneous frame. As the lateral acceleration increases during cornering, the side slip angle decreases. Thus at very high speed turns and small turning radius, there is a high lateral acceleration and <math>\beta</math> could be a negative value.
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| ==Uses of the slip==
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| ===Other uses===
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| 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 warfare|aerial combat]].
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| === Notable employment of the slip ===
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| *[[Gimli Glider]] – a 767 captain who happened to have glider experience made a successful engine-out landing despite excessive approach altitude by employing a forward slip.<ref>http://www.damninteresting.com/the-gimli-glider/</ref>
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| *[[TACA Flight 110]] – Pilot Carlos Dardano of El Salvador had performed a side slip<ref>http://www.youtube.com/watch?v=jnX1u5vp1J4</ref> in order to correct the course of the Boeing 737-3T0 airliner and then landed safely on a narrow grass levee in New Orleans, bringing the airplane to a safe stop.
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| ==How a slip affects flight==
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| When an aircraft is put into a side slip with no other changes to the throttle or elevator, the pilot will notice an increased rate of descent (or reduced rate of ''ascent''). This is usually mostly due to increased drag on the fuselage. The airflow over the fuselage is at a sideways angle, increasing the relative frontal area, which increases drag.
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| ==See also==
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| *[[Coordinated flight]]
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| *[[Crosswind landing]]
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| *[[Sideslip angle]]
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| *[[Skid (aerodynamic)]]
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| *[[Slip-turn]]
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| ==External links==
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| *[http://www.bowersflybaby.com/slip.mpg Video of a slip from camera mounted on landing gear ]
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| *[http://www.faa.gov/library/manuals/aircraft/airplane_handbook/media/faa-h-8083-3a-4of7.pdf FAA Airplane Flying Handbook Chapters 7–9]
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| ==References==
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| {{Reflist}}
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| {{Aerobatics}}
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| [[Category:Aerial maneuvers]]
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| [[Category:Aerodynamics]]
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Adrianne Le is the name my parents gave others but you can get a hold of me anything you really like. My house is now inside of South Carolina. Filing is generally my day job so now but soon I'll be on my own. What me and my family appreciation is acting but I've can't make it all of my profession really. See what's new on my very website here: http://circuspartypanama.com
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