"Skidding" turn

Definition of a "skidding" turn

Steve Seibel
www.aeroexperiments.org

This page is still under construction!
This page was last modified on August 3, 2006

 

An aircraft is engaged in a "skidding" turn whenever the nose of the aircraft has been allowed or forced to yaw to point to the "low side" or "inside" of the turn in relation to the aircraft's actual direction of travel through the airmass at any given moment. In other words, an aircraft is "skidding" whenever the nose of the aircraft has been allowed or forced to yaw to point to the "low side" or "inside" of the turn in relation to the actual direction from which the relative wind is blowing at any given moment. (Diagram to be added.) In a left turn, an aircraft is skidding if the aircraft's nose is pointing to the left of the actual direction of the flight path through the airmass at any given moment, i.e. to the left of the direction from which the relative wind is blowing at any given moment. (Diagram to be added.)

When an aircraft is "skidding", there is a sideways component in the relative wind or airflow over the aircraft. This sideways component blows from the "leading" wing to the "following" wing, i.e. from the "outside" wingtip to the "inside" wingtip, i.e. from the high wingtip to the low wingtip. (Diagram to be added.) For example, in a left turn, if the aircraft is "skidding", i.e. if the nose is pointing too far toward the left in relation to the actual direction of the flight path and relative wind at any given moment, there will be a right-to-left component in the airflow over the aircraft. (Diagram to be added.)

In "'Conventional' use of the rudder", we noted that sailplane pilots use a "yaw string" (photo to be added) to show the precise direction of the flight path and relative wind: if the yaw string blows to one side, the nose of the aircraft is not pointing directly into the relative wind. (Photo to be added). In a "skidding" left turn, the yaw string blows toward the left. (Photo to be added.) If the aircraft has a rudder, the skid can be cured by "stepping opposite the yaw string", i.e. by applying right rudder, so that the nose yaws to the right and becomes aligned with the actual direction of the flight path and relative wind at any given moment.

In "'Conventional' use of the rudder", we noted that airplane pilots typically use a slip-skid ball (or bubble) (photo to be added) as a guide to rudder usage: if the nose of the aircraft is not pointing directly into the airflow, the slip-skid ball (or bubble) drifts to one side. (Photo to be added.) In a "skidding" left turn, the slip-skid ball drifts toward the right. (Photo to be added.) If the aircraft has a rudder, the skid can be cured by "stepping on the ball", i.e. by applying right rudder, so that the nose yaws to the right and becomes aligned with the actual direction of the flight path and relative wind at any given moment. A slip-skid bubble displaces in the same direction as the yaw string, and in the opposite direction as the slip-skid ball--a slip-skid bubble displaces to the right in a "skidding" left turn. (Photo to be added.)

In "What is a turn?", we noted that "adverse yaw" is one of the main reasons that a turn might tend to become "uncoordinated". In particular, adverse yaw tends to cause a skid as an aircraft rolls from a bank back to wings-level. For example, as an aircraft rolls from a left bank back to wings-level, adverse yaw tends to swing the nose to the left, both in an absolute sense, and in relation to the actual direction of the flight path and relative wind at any given moment. The yaw string (if present) will blow toward the left and the slip-skid ball (if present) will drift toward the right. (Photo to be added.) By applying the proper amount of right rudder as he rolls the aircraft from the left bank back toward wings-level, a pilot can overcome adverse yaw and keep the turn "coordinated", so that the nose of the aircraft remains pointing directly into the relative wind.

In an aircraft without a rudder, the pilot has no way to keep the aircraft perfectly "coordinated." The nose of the aircraft will often point in slightly different direction than the aircraft is actually travelling through the airmass. In particular, "adverse yaw" will tend to cause the aircraft to skid as it rolls from a bank back to wings-level. (Photo to be added.) One way to minimize or eliminate this skidding tendency is to use spoilerons rather than ailerons or weight-shift for roll control.

In "The rudder as a roll control--aircraft with dihedral", we noted that some aircraft use the rudder as the sole means of roll control. In such an aircraft, since the rudder cannot be used as a yaw control without also affecting roll, the pilot again has no way to keep the aircraft perfectly "coordinated." The nose of the aircraft will often point in slightly different direction than the aircraft is actually travelling through the airmass. All rolling motions will be initiated by first yawing the nose to point in the desired direction of roll, in relation to the actual direction of the flight path and relative wind at that moment. This means that the aircraft will skid whenever the pilot is rolling the aircraft into a turn, and the aircraft will slip whenever the pilot is rolling the aircraft back toward wings-level.

Fundamentally, a skid is a yaw-axis phenomenon. In a skid, the nose of the aircraft has been forced or allowed to yaw to point in a different direction than the aircraft is actually travelling through airmass at a given moment. In other words, the nose of the aircraft has been forced or allowed to yaw to point in a different direction than the relative wind is actually blowing from at a given moment. To put it yet another way, there is a sideways component in the relative wind or airflow over the aircraft. A skid is not a pitch-axis phenomenon--it is not caused by a "excess of lift" in relation to the bank angle. For example, if the vertical components of the aerodynamic forces generated by the aircraft add up to greater than 1 "G", the flight path will curve upward and the aircraft will decelerate, but this generally does not cause the aircraft to "skid" sideways through the air, even if the aircraft is steeply banked.

 

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