slip (aerodynamics)

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A slip is an
aerodynamic study at Wallops Flight Facility, Wallops Island in 1990. A vortex is created by passage of an aircraft wing, revealed by smoke. Vortices are one of the many phenomena associated with the study of aerodynamics. Aerodynamics, from Greek language, ...
state where an
aircraft An aircraft is a vehicle or machine that is able to fly Flies are insect Insects or Insecta (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Lat ...

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 An aircraft is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravi ...
. 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 flightImage:Glider Instrument Panel.png, 250px, Pilot's view of a Schempp-Hirth Janus-C Glider (sailplane), glider. The yaw string (red wool) on the canopy and the turn and bank indicator (top center of the instrument panel) both show the glider is not in ...
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 amount of Lift (force), lift generated by a wing or vehicle, divided by the aerodynamic drag it creates by moving through air. A greater or more favorable L/D ratio is typically one of th ...
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 A ship is a large watercraft that travels the world's oceans and other sufficiently deep Sea lane, waterways, carrying goods or passengers, or in support of specialized missions, su ...

. 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 Aircraft flight control system, directional flight controls. Some other aircrew, aircrew members, such as navigators or flight engineers, are a ...
may deliberately enter a slip by using opposite rudder and
aileron An aileron (French for "little wing" or "fin") is a hinged flight control surface Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude. Development of an effective se ...

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 Ground effect may refer to: * Ground effect (aerodynamics), the increased lift and decreased aerodynamic drag of a wing close to a fixed surface * Ground effect (cars), an effect that creates downforce, primarily in racing cars * Ground effect vehi ...
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. Often, if an airplane in a slip is made to stall, it displays very little of the yawing tendency that causes a Skid (aerodynamics), 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.

# 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 Aircraft heading, heading 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 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. 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 Elevator (aircraft), pitch or Throttle, power. 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 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. The sideslip method for crosswind 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. 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 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 aeronautics, the relative wind is the direction of movement of the atmosphere relative to an aircraft An aircraft is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravi ...
. In flight dynamics 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 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.

# Uses of the slip

## 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 warfare, aerial combat.

## Notable employment of the slip

* Gimli Glider – Robert Pearson, captain of Air Canada Flight 143, a Boeing 767, made a successful engine-out landing in Gimli, Manitoba despite excessive approach altitude by employing a forward slip. He had learned the maneuver as a glider pilot. * TACA Flight 110 – Pilot Carlos Dardano of El Salvador performed a side slip in order to correct the course of a Boeing 737-300 airliner. He landed on a narrow grass levee in New Orleans, bringing the airplane to a safe stop.

# 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.