What causes a slip/skid turn? [duplicate]

Question

Is the rate of turn being too low/high for a given bank angle the cause of slip/skid turn? Or is it yawing too much inside/outside turn causing an imbalance in centripetal/fugal force and thus the slip/skid turn?

According to the FAA handbook: “In a slipping turn, the aircraft is not turning at the rate appropriate to the bank being used, since the aircraft is yawed toward the outside of the turning flight path. The aircraft is banked too much for the ROT, so the horizontal lift component is greater than the centrifugal force.”

Is that statement true?

Answer 1

Is the rate of turn being too low/high for a given bank angle the cause of slip/skid turn?

One could say that the rate of turn being "too low" or "too high" for a given bank angle is a symptom of something else, which is really the more fundamental cause of slip or skid in a turn.

Or is it yawing too much inside/outside turn causing an imbalance in centripetal/fugal force and thus the slip/skid turn?

Yes, if we understand the word "yawing" in this context to mean not the actual direction and rate of yaw rotation, but rather the condition of flying in such a manner that a yaw string would stream toward the inside or outside of the turn. I.e., the condition of flying with the nose pointing toward the inside or the outside of the aircraft's actual instantaneous direction of travel.

According to the FAA handbook: “In a slipping turn, the aircraft is not turning at the rate appropriate to the bank being used, since the aircraft is yawed toward the outside of the turning flight path.

I'm actually surprised that the FAA published such an enlightened statement. In the past, in flight training materials, including materials from the FAA, the emphasis is entirely on the turn rate being "wrong" for the bank angle. But the statement quoted above seems to recognize that the fundamental cause of this "mismatch" between the turn rate and the bank angle is the fact that the aircraft is being flown in a "yawed" condition (in the sense described above), i.e. the nose is not aligned with the actual instantaneous direction of the flight path, so the airflow is striking one side of the fuselage, generating a real aerodynamic force that causes an alteration in the the turn rate.

The aircraft is banked too much for the ROT, so the horizontal lift component is greater than the centrifugal force.”

Is that statement true?

This is where things get really confusing. That final quoted statement is technically true, but very misleading. We'll revisit that at the end of this answer.

Ultimately I'd suggest that the "causal chain" goes like this--

1. Airplane nose is not fully aligned, yaw-wise, with actual instantaneous direction of travel, generally due to some aerodynamic yaw torque (for example due to "adverse yaw" from the deflected ailerons and/or from a non-zero roll rate, or due to an intentional rudder input by the pilot) that counteracts the aircraft's natural "weathervane stability" or "directional stability". Yaw rotational inertia can also play a contributing role as a turn is being initiated (or exited), but this effect is generally slight.

(This "misalignment" between nose and instantaneous direction of flight path is sort of but not exactly the same as the yaw rate or turn rate being "wrong" for the bank angle.)

2. Side of fuselage is exposed to free-stream relative wind

3. Airflow hits side of fuselage and generates a real, tangible aerodynamic force

4. The net aerodynamic force vector is no longer pointing straight "up" in the reference frame of the banked aircraft, as it would be if the wings' lift vector were the only unbalanced aerodynamic force at play.

5. This causes a reduction (or increase) in turn rate so the turn rate (and likewise the yaw rate) is now "too low" (or "too high") for the bank angle.

6. This also causes the slip-skid ball to ride off-center.

Note that either item 4 or item 5 can be viewed as the fundamental "cause" of item 6, depending what reference frame we use. Item 4 rather than item 5 is best viewed as the cause of item 6 if we are using a valid inertial reference frame -- e.g. the earth, not the moving airplane-- in which case our explanation should not invoke the fictitious "centrifugal force".

Note that for the exact same degree of "misalignment" between the aircraft nose and the actual instantaneous direction of the flight path-- i.e. the exact same displacement of a "yaw string"-- the resulting amount of aerodynamic sideforce generated by the airflow against the fuselage, and the resulting displacement of the slip-skid ball, and the resulting change in the turn rate, will vary according to the shape of the fuselage and the amount of side area therefore presented to the airflow. So a given deflection of the yaw string will cause a different deflection of the slip-skid ball in various different aircraft.

Note that by putting item #1 at the very front of the "causal chain", we're well equipped to understand why appropriate rudder inputs are the key to preventing slips and skids.

Note also that this perspective works well even for extreme cases, even including 90-degree-banked "knife edge" linear flight such as we see at airshows. Some other "explanations" that we read in pilot training materials tend to break down in such extreme cases.

Now, returning to --

The aircraft is banked too much for the ROT, so the horizontal lift component is greater than the centrifugal force.”

This statement is technically true, but unhelpful. The centrifugal force is nothing more and nothing less than the exact mirror-image of the actual centripetal force, and the centripetal force is simply the horizontal component of the net aerodynamic force generated by the aircraft.

So to say that the "horizontal lift component is greater than the centrifugal force", is just a really complicated and obtuse way of saying that the aircraft is generating some other aerodynamic horizontal force component that is opposing the horizontal force from the lift vector, and also is slowing the turn rate. That other horizontal force component is the airflow striking the side of the fuselage, on the side that is inside the turn. So the specific situation being described here is a slip. And the point that is left out of the quoted description, is the simple fact that the net aerodynamic force vector is no longer pointing "straight up" in the aircraft's own reference frame, as it would be if the lift vector from the wing, and gravity, were the only forces at play.

NOTE that this aerodynamic sideforce from the airflow striking the side of the fuselage is missing from all the diagrams depicting slips and skids in this related ASE question, even though the resulting "imbalance" between centrifugal force and the "horizontal component of lift" is depicted. This is a fundamentally flawed way of illustrating the forces at play in slips and skids.

(Link: What is missing from these diagrams of the forces in slips and skids? )

Answer 2

The pilots answer might be: use your rudder to fix it. Step on the ball.

If you are uncomfortable with your bank angle, you can also roll out of it a bit. Ailerons and rudder work together here.

But the theory may be more difficult to wrap your head around, but I will try:

The wing is "boss" when it comes to generating flight forces. Lift is the most powerful force your plane creates. Thrust is second. (And gravity is there too).

"the horizontal component is greater than the centrifugal force"

The horizontal lift component creates the centripetal force. The opposite "centrifugal force" is the result of the plane being pulled (accelerated) to the side. If the plane is banked too steeply and not yawing enough, it is slipping. The ball is to the inside. If the plane is yawing too much and not banked enough, it is "skidding" and the ball is to the outside. You want the correct yawing and banking combination to center "centrifugal force" right through the floor$^1$, which puts the ball right between the marks on the curved tube inclinometer.

Adverse yaw, from the outside aileron being deflected down, tends to pull the nose outside the turn. Now you are in a "slipping" turn. Ball is to the inside. "Coordinate" by stepping on the ball.

The tail of a properly designed airplane should "weathervane" the nose into the wind, and the rudder helps, if needed.

Some aircraft have more adverse yaw tendency than others. But centering the ball in turns with the rudder is very good because it reduces drag and is most comfortable for passengers.

$^1$ the gravity component also affects the position of the "ball". It is the resultant vector of gravity and centrifugal force that centers the ball. This is what you feel "in the seat of your pants".