More detailed definitions of "slips" and "skids"

More detailed definitions of "slips" and "skids"

Steve Seibel
www.aeroexperiments.org

This page is still under construction!
This page was last modified on August 24, 2006

 

In "Definition of a slipping turn", we stated that "An aircraft is engaged in a 'slipping' turn whenever the nose of the aircraft has been allowed or forced to yaw to point to the 'high side' or 'outside' 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 'slipping' whenever the nose of the aircraft has been allowed or forced to yaw to point to the 'high side" or 'outside' of the turn in relation to the actual direction from which the relative wind is blowing at any given moment."

This definition implies that the aircraft is in fact turning, i.e. the flight path is curving. However, if we yaw the nose so far away from the actual direction of the flight path through the airmass, and the relative wind, that we force the turn to stop entirely, we still call the situation a "slip". We can no longer talk about the direction of the turn, but we can still say that the aircraft's nose has been yawed to point too far toward the "high side of the bank". In other words, if the aircraft is banked to the left, the nose has been yawed to point to the right of the actual direction of the flight path and relative wind at any given moment.

In "Definition of a skidding turn", we stated that "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."

When speak of the "low side of the turn", we imply that the aircraft is banked, which hints that the bank is probably the primary cause of the turn. Normally this is indeed the case. However, in many aircraft it is in fact possible--though extremely inefficient--to make the flight path curve even when the wings are kept level. The pilot just applies a heavy rudder input to yaw the nose away from the actual direction of the flight path and relative wind, while making whatever roll input is needed to prevent the aircraft from banking. (If the aircraft has dihedral, the sideways airflow will interact with the dihedral to create a roll torque, which the pilot must counteract by applying the ailerons in the opposite direction as the rudder.) For reasons that we'll explore in more detail elsewhere in these tutorial pages, yawing the nose to the side in this manner will in fact cause the flight path to curve in most aircraft. Since the wings are level, it no longer makes sense to speak of the "low side" of the turn. But we still call this situation a "skidding turn", because the nose is still yawed to point toward the "inside" of the turn in relation to the actual direction of the flight path and relative wind at any given moment.

There is one situation where it's ambiguous as to whether we should say that the aircraft is "slipping" or "skidding". As an example, we'll start in a banked left turn, and we'll hold excessive left rudder input, so that the nose will point toward the low side or inside of the turn: this is a "skidding left turn". Then we'll relax our excessive left rudder input to allow the nose to swing into alignment with the flight path and relative wind, so that we enter a normal, banked, "coordinated", nonslipping, nonskidding, left turn. Then we'll gradually apply more and more right rudder to yaw the nose to point further and further to the right of the actual direction of the flight path and relative wind at any given moment, while applying whatever roll inputs are required to hold the bank angle constant. The turn rate (to the left) will slow, and then stop completely. At this point we've passed through the "slipping left turn" and arrived at a "non-turning" or "straight-line" slip. If we then apply even more right rudder, the flight path will begin curving to the right--in the opposite direction as the wing is banked. Now the aircraft is banked toward the left, and the nose is yawed to point to the right of the actual direction of the flight path and relative wind at any given moment, and the flight path is curving toward the right. We can argue that this is a "skid" because the flight path is curving in the same direction that the nose is pointing, in relation to the actual direction of the flight path and relative wind at any given moment. In other words, the nose is pointing "too far" toward the "inside" of the turn, in relation to the actual direction of the flight path and relative wind at any given moment. Or we can argue that this is a "slip", because the nose is pointing too far toward the "high side" of the bank, or too far "toward the high wingtip", in a manner of speaking. Or we can recognize that this situation shows that it's a bit superfluous, from a fundamental aerodynamic point of view, to categorize all "un-coordinated" situations into "slips" and "skids"!

Every one of the above situations has one thing in common--regardless of the direction of the bank and regardless of the direction of the curvature in the flight path, the nose of the aircraft has been allowed or forced to yaw to point in a different direction than the aircraft is actually moving through the airmass at any given moment. In other words, the nose is pointing in a different direction than the relative wind is blowing from at any given moment. This creates a sideways airflow over the aircraft. Engineers often use the term "slip angle" or "sideslip angle" as a blanket term for the angular distance that nose has been yawed away from the actual direction of the flight path and relative wind, regardless of the direction of the bank and regardless of the direction of the curvature of the flight path and regardless of whether a pilot might categorize the situation as a "slip" or a "skid". In these tutorial pages, we've been following the same practice when we've spoken of a "positive coupling between yaw (slip) and roll" or a "negative coupling between yaw (slip) and roll"--when we've used these particular phrases, we've made no effort to distinguish whether a pilot might say the aircraft is "slipping" or is "skidding".

 

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