# Slip (aerodynamic)

(Redirected from Side slip)

A slip is an aerodynamic state where an aircraft is moving somewhat sideways as well as forward relative to the oncoming airflow. In other words, for a conventional aircraft, the nose will not be pointing directly into the relative wind (in the side-to-side sense). A slip is also a piloting maneuver where the pilot deliberately puts the aircraft into a slip.

## Background

Flying in a slip is aerodynamically inefficient. Inexperienced pilots will often enter slips unintentionally during turns by failing to coordinate the aircraft using the rudder; however there are common situations where a pilot may enter a slip deliberately by using opposite rudder and aileron inputs, most commonly in a landing approach at low power.[1] Without a slip 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 slip much more drag is created, allowing the pilot to dissipate altitude without increasing airspeed, increasing the angle of descent (glide slope). Additional airspeed will further increase the steepness of descent.

Slips are especially useful when operating aircraft that have neither high-drag flaps nor spoilers (typically pre-1950s training aircraft, or in aerobatic aircraft such as the Pitts Special), or in any aircraft in which the flaps cannot be extended due to a system failure, or when finer control is needed.

As with any low altitude maneuver it is important to maintain correct airspeed in order to prevent a stall. However, 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. The 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.[2]

## Forward-slip vs. sideslip

Two forms are employed, the forward-slip and the sideslip. Aerodynamically these are identical once established, but they are entered in different manners and will create different ground tracks and headings relative to those prior to entry. 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 in flying into farmstead or rough country airstrips where approach hazards are present.

### Forward-slip

The forward slip will change the heading of the aircraft away from the down wing, while retaining the original track (flight path over the ground) of the aircraft.

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 start of a short runway. Assuming that the runway is properly lined up, 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, the slip must be removed before touchdown to avoid excessive side loading on the landing gear, and if a cross wind is present an appropriate side slip may be necessary at touchdown as described below.

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 and/or too fast.

### Sideslip

The sideslip also uses opposite aileron and rudder. In this case it is entered by lowering a wing and exactly enough opposite rudder so the airplane does not turn (maintaining the same heading), while adding airspeed as required.

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.

A sideslip is also one of the methods used by pilots to execute a crosswind landing. In order to land crosswind using the sideslip method, the pilot puts the airplane into a sideslip toward the wind to maintain runway centerline position while maintaining heading on the centerline with the rudder, touching one main landing gear, followed by the second main gear, and finally the nose gear (or tail gear if employed). This allows the wheels to be constantly aligned with the track, thus avoiding any side load at touchdown.

The sideslip method for cross-wind landings 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.

#### Sideslip angle

The sideslip angle, also called angle of sideslip (AOS, AoS, $\beta$, Greek 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 $\beta$ (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 [[ l stability]] considerations.[3]

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 combat.

### Notable employment of the slip

• 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.[4]

## How a slip affects flight

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.