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A slip is an
aerodynamic Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...
state where an
aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engine ...
is moving ''somewhat'' sideways as well as forward relative to the oncoming airflow or
relative wind In aeronautics, the relative wind is the direction of movement of the atmosphere relative to an aircraft or an airfoil. It is opposite to the direction of movement of the aircraft or airfoil relative to the atmosphere. Close to any point on the su ...
. 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 In aviation, coordinated flight of an aircraft is flight without sideslip.Clancy, L.J., ''Aerodynamics'', Section 14.6 When an aircraft is flying with zero sideslip a turn and bank indicator installed on the aircraft's instrument panel usuall ...
and therefore is flying inefficiently.


Background

Flying in a slip is aerodynamically inefficient, since the
lift-to-drag ratio In aerodynamics, the lift-to-drag ratio (or L/D ratio) is the lift generated by an aerodynamic body such as an aerofoil or aircraft, divided by the aerodynamic drag caused by moving through air. It describes the aerodynamic efficiency under gi ...
is reduced. More drag is at play consuming energy but not producing lift. Inexperienced or inattentive pilots will often enter slips unintentionally during turns by failing to coordinate the aircraft with the
rudder A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, aircraft, or other vehicle that moves through a fluid medium (generally air or water). On an aircraft the rudder is used primarily to counter adve ...
. 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. 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. There are common situations where a
pilot An aircraft pilot or aviator is a person who controls the flight of an aircraft by operating its directional flight controls. Some other aircrew members, such as navigators or flight engineers, are also considered aviators, because they a ...
may deliberately enter a slip by using opposite rudder and aileron inputs, most commonly in a landing approach at low power.John S. Denker
''See How It Flies''
Without flaps or 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 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 The Pitts Special (company designations S-1 and S-2) is a series of light aerobatic biplanes designed by Curtis Pitts. It has accumulated many competition wins since its first flight in 1944. The Pitts biplanes dominated world ae ...
or any aircraft with inoperative flaps or spoilers. Often, if an airplane in a slip is made to stall, it displays very little of the yawing tendency that causes a skidding stall to develop into a 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.


Forward-slip vs. sideslip

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, 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.


Forward-slip

The forward slip changes the
heading Heading can refer to: * Heading (metalworking), a process which incorporates the extruding and upsetting processes * Headline, text at the top of a newspaper article * Heading (navigation), the direction a person or vehicle is facing, usually s ...
of the aircraft away from the down wing, while retaining the original ''track'' (flight path over the ground) of the aircraft. To execute a forward slip, the pilot
banks A bank is a financial institution that accepts deposits from the public and creates a demand deposit while simultaneously making loans. Lending activities can be directly performed by the bank or indirectly through capital markets. Becaus ...
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. However, airframe speed limits such as VA and VFE must be observed. 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.


Sideslip

The ''sideslip'' also uses aileron and opposite 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 pitch or
power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...
. Compared to Forward-slip, less rudder is used: just enough to stop the change in the heading. In the sideslip condition, the airplane's longitudinal axis remains parallel to the original flightpath, but the airplane no longer flies along that track. The horizontal component of lift is directed toward the low wing, drawing the airplane sideways. This is the still-air, headwind or tailwind scenario. In case of crosswind, the wing is lowered into the wind, so that the airplane flies the original track. This is the sideslip approach technique used by many pilots in crosswind conditions (sideslip without slipping). The other method of maintaining the desired track is the crab technique: the wings are kept level, but the nose is pointed (part way) into the crosswind, and resulting drift keeps the airplane on track. A sideslip may be used exclusively to remain lined up with a runway centerline while on approach in a
crosswind A crosswind is any wind that has a perpendicular component to the line or direction of travel. This affects the aerodynamics of many forms of transport. Moving non-parallel to the wind's direction creates a crosswind component on the object and th ...
or be employed in the final moments of a crosswind landing. To commence sideslipping, the pilot 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. The sideslip method for
crosswind landing In aviation, a crosswind landing is a landing maneuver in which a significant component of the prevailing wind is perpendicular to the runway center line. Significance Aircraft in flight are subject to the direction of the winds in which the a ...
s is not suitable for long-winged and low-sitting aircraft such as gliders, where instead a crab angle (heading into the wind) is maintained until a moment before touchdown. Aircraft manufacturer Airbus recommends sideslip approach only in low crosswind conditions.


Sideslip angle

The sideslip angle, also called angle of sideslip (AOS, AoS, \beta,
Greek letter The Greek alphabet has been used to write the Greek language since the late 9th or early 8th century BCE. It is derived from the earlier Phoenician alphabet, and was the earliest known alphabetic script to have distinct letters for vowels as ...
beta), is a term used in fluid dynamics and
aerodynamics Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dy ...
and
aviation Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air craft such as hot a ...
. It relates to the rotation of the aircraft centerline from the
relative wind In aeronautics, the relative wind is the direction of movement of the atmosphere relative to an aircraft or an airfoil. It is opposite to the direction of movement of the aircraft or airfoil relative to the atmosphere. Close to any point on the su ...
. In
flight dynamics Flight dynamics in aviation and spacecraft, is the study of the performance, stability, and control of vehicles flying through the air or in outer space. It is concerned with how forces acting on the vehicle determine its velocity and attit ...
it is given the shorthand notation \beta (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 \beta is essentially the directional angle of attack of the airplane. It is the primary parameter in
directional stability Directional stability is stability of a moving body or vehicle about an axis which is perpendicular to its direction of motion. Stability of a vehicle concerns itself with the tendency of a vehicle to return to its original direction in relation ...
considerations. 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 \beta could be a negative value.


Other uses of the slip

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 Aerobatics is the practice of flying maneuvers involving aircraft attitudes that are not used in conventional passenger-carrying flights. The term is a portmanteau of "aerial" and "acrobatics". Aerobatics are performed in aeroplanes and glide ...
and aerial combat.


How a slip affects flight

When an aircraft is put into a forward 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.


See also

* Drifting * Rudder roll * Slip-turn


References


External links


Video of a slip from camera mounted on landing gear

FAA Airplane Flying Handbook Chapters 7–9
{{Aerobatics Aerial maneuvers Aerodynamics Gliding technology