Aerodynamic sideforce during slips and skids

Aerodynamic sideforce during slips and skids

Steve Seibel
www.aeroexperiments.org

This page is still under construction!
This page was last modified on August 26, 2006

 

In "What is a turn?", we emphasized that a turn is a curvature in the flight path, not a change in heading. We also emphasized that in the most fundamental sense, a turn is caused by a steady sideways "centripetal" force acting at the CG of a moving body, rather than by a yaw torque. We noted that during a steady, constant-bank, constant-rate turn, the net yaw torque created by an aircraft is actually zero. And we noted that banking the wing to create a horizontal component in the wing's lift vector is a very efficient way to create the "centripetal" force that causes a curvature in the flight path.

However, in some of these tutorial pages (e.g. "More detailed definitions of 'slips' and 'skids'", we've also suggested that allowing or forcing the nose to yaw to point in a different direction than the aircraft is travelling through the airmass, i.e. in a different direction than the relative wind is blowing from at any given moment, will affect the aircraft's turn rate. For any given fixed bank angle, if the nose is yawed to point toward the "inside" or "low" side of the turn (this is a skid), the turn rate will increase, and if the nose is yawed to point toward the "outside" or "high side" of the turn (this is a slip), the turn rate will decrease or even stop entirely. In "More detailed definitions of 'slips' and 'skids'", we also noted that an aircraft can be (inefficiently) forced to turn even when the wings are forced to stay level, by using the rudder to yaw the nose to point in a different direction than the aircraft is actually moving through the airmass. (Note that this is quite different than using the rudder as a roll control, as we discussed in "The rudder as a roll control: aircraft with dihedral".)

If we force the aircraft to remain wings-level, why can we create a turn by yawing the nose to point in a different direction than the aircraft is actually moving through the airmass? If we force the bank angle to remain constant, why does the turn rate increase when the nose is yawed to point toward the "inside" or "low side" of a banked turn, and why does the turn rate decrease when the nose is yawed to point toward the "outside" or "high side" of a banked turn?

To answer these questions, we need to recognize one of the most important (and most widely overlooked) aspects of the dynamics of slips and skids. When the nose of an aircraft is yawed to point toward the left (for example) of the actual direction of the flight path through the airmass and the relative wind, this causes the airflow to impact against the right side of the fuselage and other surfaces of the aircraft (diagram to be inserted.) This causes a real, tangible aerodynamic force toward the left. We call this force the "aerodynamic sideforce". The aerodynamic sideforce is really a form of lift: by yawing the fuselage so that it meets the airflow at a non-zero "angle-of-attack", we create a sideways lift force toward the left. More precisely, the aerodynamic sideforce is defined to act perpendicularly to the flight path and relative wind, and parallel to the wingspan. At shallow bank angles, the aerodynamic sideforce created by yawing the nose away from the actual direction of the flight path and relative wind acts mainly in the horizontal direction. At steeper bank angles, the aerodynamic sideforce created by yawing the nose away from the actual direction of the flight path and relative wind has a significant vertical component. (Diagrams to be inserted.) In the extreme case of sustained, vertically-banked knife-edge flight, the aerodynamic sideforce acts purely in the vertical direction--for more, see "Knife-edge flight" (link to be inserted).

When the pilot tends to "fall" to one side during a turn, and when the slip-skid ball drifts to one side during a turn, this is a direct reflection of the "aerodynamic sideforce" created by the real, tangible impact of the airflow against the side of the fuselage and other surfaces of the aircraft. Far too many "explanations" of the movement of the pilot's body during slips and skids, and of the movement of the slip-skid ball during slips and skids, invoke a "lack of centrifugal force due to an inadequate turn rate in relation to the bank angle", or other similar ideas. This sort of "explanation" completely "puts the cart before the horse." Centrifugal force, as it is usually invoked, is a "pseudoforce" that does not really exist. The sideways force "felt" by the pilot, and by the slip-skid ball, during a slip or skid is nothing other than the real, tangible, aerodynamic sideforce created by the impact of the airflow against the fuselage and other surfaces of the aircraft. This real, tangible, aerodynamic sideforce is the cause, not the result, of the decreased turn rate in a slip and the increased turn rate in a skid. If there is no airflow impacting the side of the fuselage, the aerodynamic sideforce will be zero, and the pilot and slip-skid ball will feel no sideways force of any kind in their own reference frame, regardless of whether not the wing's lift vector happens to be "correctly" sized in relation to the bank angle so that all the vertical force components acting on the aircraft add up to zero, and regardless of whether not the turn rate is "correct" for the bank angle. As long as the wing's lift vector (and drag and thrust) are the only significant aerodynamic forces at play, there will be no sideways forces in the reference frame of the aircraft and pilot, regardless of whether or not the magnitude of the lift vector is "appropriate" for the bank angle. For example, when the wing is "unloaded" to the zero-lift angle-of-attack, the wing's lift vector vanishes and the turn rate is no longer "correct" for the bank angle and the flight path will curve downward due to the pull of gravity and the complete absence of any vertical lift component, but as long as the nose of the aircraft is kept pointing directly into the airflow, the airflow will not impact the side of the fuselage and the aerodynamic sideforce will be zero and the pilot's body and slip-skid ball will not tend to either fall toward the low side of the fuselage or drift toward the high side of the fuselage. (For more on this, see the related article in the Aeroexperiments website entitled "You can't feel gravity".)

It's worth repeating this one more time: in a "coordinated" turn, meaning a turn where the nose of the aircraft is kept aligned with the actual direction of the flight path and relative wind at any given moment, the airflow does not impact either the "high side" or the "low side" of the aircraft, and no sideways aerodynamic forces are generated in the reference frame of the aircraft and pilot, and there will be no tendency for the pilot and other contents of the aircraft to shift toward the "high side" or the "low side" of the turn, regardless of whether the aircraft is in a steady-state condition where the size of the wing's lift vector is correctly "matched" to the bank angle and all vertical forces are in balance and the airspeed is remaining constant, or not.

In a slip, when the nose of the aircraft is allowed or forced to point toward the "outside" or "high side" of the turn in relation to the actual direction of the flight path and relative wind at any given moment, the airflow impacts against the "low side" or "inside" of the fuselage, and this creates a real, tangible aerodynamic sideforce toward the "high side" or "outside" of the turn. This sideforce includes a horizontal component that is "anti-centripetal" or "centrifugal" in nature. This opposes the horizontal component of the lift from the banked wing. The turn rate is decreased or even brought to zero. (Diagrams to be inserted.)

This real, tangible aerodynamic sideforce toward the "high side" or "outside" of the turn is eventually transmitted through the aircraft structure to all the aircraft contents, including the pilot, the slip-skid ball, and the pencil lying atop the instrument panel. However, as the aircraft's flight path changes in response to the sideforce, the detailed physics are such that the pilot cannot receive the full measure of this sideforce until he has tilted over to lean against the "low side" of the seatbelts or the "low side" of the cockpit wall, and the slip-skid ball cannot receive the full measure of this sideforce until it has drifted toward the "low side" of its curved glass tube, and the pencil atop the instrument panel cannot receive the full measure of this sideforce until it has skittered across the top of the panel and come to rest against the "low side" of the cockpit wall.

If a slipping hang glider were to experience a significant aerodynamic sideforce toward the "high side" or "outside" of the turn, the full measure of this sideforce could not be transmitted through the hang strap to the pilot's body until the pilot's body had swung several inches toward the low side of the turn.

In a skid, when the nose of the aircraft is allowed or forced to point toward the "inside" or "low side" of the turn in relation to the actual direction of the flight path and relative wind at any given moment, the airflow impacts against the "high side" or "outside" of the fuselage, and this creates a real, tangible aerodynamic sideforce toward the "low side" or "inside" of the turn. This sideforce includes a horizontal component that is "centripetal" in nature. This adds to the horizontal component of the lift from the banked wing. The turn rate is increased (diagrams to be inserted.) However, just as in a slip, the sideways airflow over the aircraft also increases drag. It is much more efficient to increase the turn rate by increasing the bank angle, while leaving the nose of the aircraft pointing directly into the relative wind. After all, as we noted in "What is a turn", the wing is a very efficient generator of aerodynamic force.

A skid, unlike a slip, tends to invite a spin. Pilots generally do not intentionally use "skidding turns" to increase drag, though the rudder is sometimes intentionally used to create very gentle skidding turns for small course corrections, as we'll see below.

In a skid, the real, tangible aerodynamic sideforce toward the "low side" or "inside" of the turn is eventually transmitted through the aircraft structure to all the aircraft contents, including the pilot, the slip-skid ball, and the pencil lying atop the instrument panel. However, as the aircraft's flight path changes in response to the sideforce, the detailed physics are such that the pilot cannot receive the full measure of this sideforce until he has tilted over to lean against the "high side" of the seatbelts or the "high side" of the cockpit wall, and the slip-skid ball cannot receive the full measure of this sideforce until it has drifted toward the "high side" of its curved glass tube, and the pencil atop the instrument panel cannot receive the full measure of this sideforce until it has skittered across the top of the panel and come to rest against the "high side" of the cockpit wall.

If a skidding hang glider were to experience a significant aerodynamic sideforce toward the "low side" or "inside" of the turn, the full measure of this sideforce could not be transmitted through the hang strap to the pilot's body until the pilot's body had swung several inches toward the "high side" of the turn.

As a practical matter, we've noted elsewhere in these tutorial pages that many light aircraft can be "steered" with the rudder alone, which is handy if the pilot needs to take both hands off the control yoke or stick for a moment to attend to some task in the cockpit. For a typical light plane with a rather modest amount of dihedral, a gentle, relatively brief rudder input will produce a course change (curvature in the flight path) primarily by yawing the nose to the side in relation to the actual direction of the flight path and airflow, so that the airflow strikes the side of the fuselage and other surfaces of the aircraft and generates an aerodynamic sideforce. Skidding the aircraft in this manner isn't the most efficient way to turn, but it does work, and for small course corrections the resulting increase in drag is rather small. However, if the rudder input is sustained for any length of time with no opposing roll input, the bank angle will change quite significantly, due to the roll torque generated by the interaction between the sideways airflow and the dihedral geometry of the wing, as we explored in "The rudder as a roll control: aircraft with dihedral". In this manner, a typical light plane can be flown through a series of reversing 45-degree banks with the rudder alone, with the pilot's hands off the stick or yoke. Again, this isn't the most efficient way to maneuver an aircraft, because the rudder is not being used for its normal purpose of keeping the nose pointing directly into the actual direction of the relative wind at all times, but it can be done. (A pilot experimenting with this procedure should keep the roll rate modest, or he or she will also receive a dramatic demonstration of how rapid changes in bank angle with no matching pitch inputs can lead to some rather dramatic climbs or dives due to an "imbalance" in the vertical component of lift! Also, in some spin-prone aircraft it might be unsafe to use the rudder in this manner, though most spin accidents are caused by a chain of events that involves the pilot pulling the control stick aft as well as applying too much "inside" rudder.) At any rate the basic point here is that in a typical light airplane, a rudder input that is not matched with an opposing roll command to keep the wings level will create a turn that is caused partly by the aerodynamic sideforce generated by the airflow striking the side of the fuselage and other parts of the aircraft, and partly by banking.

 

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