Aviation Stack Exchange is a question and answer site for aircraft pilots, mechanics, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
I am wondering what turn coordination really means and what makes a turn uncoordinated?
I know that when the turn is coordinated, there is no slip and skid, an aircraft is flying a perfect circle etc. and that in one case there is too much rudder input or too little bank etc. etc. But I am interested about forces during the turn. Can someone tell me which force during an uncoordinated turn is too big or too small and literally what makes a turn uncoordinated in terms of forces?
It's important not to overthink it. I'll keep it simple. A coordinated turn means you are keeping the tail lined up with the nose in the airstream. If you are uncoordinated, you are flying sideways in the airstream to some degree or another; the side of the fuselage is being presented to the airflow.
If you learn to fly in gliders, it's obvious because you have a yaw string on the canopy to tell you that the tail is lined up behind the nose in the airstream, since the string itself allows a direct visualization of the airstream. In power planes with a propeller up front, you are forced to use an inclinometer to tell you the same thing, because it detects the lateral acceleration relative to the vertical axis that is present if the tail is not lined up behind the nose.
Practically speaking, whether you are banked in a turn or in level flight, it really doesn't matter. The fuselage is lined up in the airstream or it's not. The ball is centered, or it's not. Just use your feet to keep it lined up as required using the skid ball.
Can someone tell me which force during an uncoordinated turn is too big or too small and literally what makes a turn uncoordinated in terms of forces?
The slip-skid ball (inclinometer ball) will be off-center whenever the component of the net aerodynamic force that we see when looking at the airplane in a head-on view is tilted "sideways" in the aircraft's reference frame, rather than acting "straight up" perpendicular to the wingspan.
That's really all there is to it.
To a first approximation, this only can happen when the fuselage is flying sideways through the air and generating an aerodynamic sideforce, although there are some additional nuances to be considered (e.g. the helicopter case and the twin engine aircraft w/ one engine out case.)
The direction and magnitude of the "load" vector is irrelevant-- the perceived "load" is nothing more than the mirror image of the real aerodynamic force vector acting on the aircraft.
The direction and magnitude of the "weight" or "gravity" vector is irrelevant-- gravity exerts an equal force per unit mass (acceleration) on every molecule of the aircraft and contents, including the slip-skid ball and its surrounding glass tube, and the pilot and the seat he or she is sitting in. Gravity doesn't pull the pilot to one side of the aircraft, or pull the slip-skid ball to one side of its tube.
The magnitude of the "centrifugal force" vector is irrelevant-- "centrifugal force" is not a real force and is only a valid concept from the point of view of the (accelerated) reference frame of the aircraft itself. The "centrifugal force" vector is simply the mirror image of the centripetal force vector, which is the component of the real aerodynamic force that acts in the centripetal direction.
The magnitude of the lift vector is irrelevant-- moving the stick aggressively forward or aft will dramatically change the magnitude of the lift vector and net aerodynamic force and G-loading (which is just another word for net aerodynamic force, or for the lift vector), and also will dramatically change the turn rate, but has little effect on the slip-skid ball.
It's a basically a mistake or at least an un-useful concept to think uncoordinated flight (slip or skid) is CAUSED by flying at the wrong turn rate for the bank angle. Rather, allowing (or forcing) the aircraft to fly sidewise through the air CAUSES the yaw string to be off-center and ALSO creates an aerodynamic sideforce which CAUSES the turn rate to be wrong for the bank angle and ALSO causes the inclinometer ball to deflect off-center. Hence rudder usage is key to avoiding (or causing) slips or skids. WHY a given rudder input would be required to prevent slip or skid, is a complicated aerodynamic question. In some cases no rudder input is required at all.
Yet in truth the situation is a little more nuanced -- a helicopter tail rotor can create a sideforce that will offset the inclinometer ball even the yaw string is centered (no slip or skid) and the fuselage is not flying sideways through the air, and so for that matter can a deflected rudder when it is opposing some other yaw torque such as that due to having only one engine running in a twin-engine aircraft. So from another perspective it can be both true and useful to realize that the inclinometer ball will be off-center if the turn rate is "mismatched" to the bank angle and G-loading-- even if the yaw string is centered. Whether the aircraft is "coordinated" or "uncoordinated" in such a case is a matter of semantics. Certainly an aerodynamicist would not say it is "slipping" or "skidding", because there is no sideways airflow over the aircraft.
Even in these nuanced cases, the basic rule applies-- the slip-skid ball (inclinometer ball) will be off-center whenever the component of the net aerodynamic force that we see when looking at the airplane in a head-on view is tilted "sideways" in the aircraft's reference frame, rather than acting "straight up" perpendicular to the wingspan. In the case of the helicopter tail rotor, the sideforce is coming from the tail rotor itself rather than from airflow striking the side of the fuselage. In the case of the aircraft with strongly deflected rudder (to compensate for a failed engine in a twin-engine aircraft, or for any other reason) the sideforce is coming from the rudder itself rather than from the airflow striking the side of the fuselage.
In most cases-- a twin-engine aircraft with one failed engine and strongly deflected rudder being a notable exception-- the sideforce generated directly by an aircraft's rudder can be ignored. Note that the sideforce from the rudder itself acts to move the slip-skid ball to the left when the rudder is deflected to the left, because the rudder is pushing air toward the left and creating an aerodynamic sideforce toward the right. In most cases when we move the rudder to the left we actually see the slip-skid ball move toward the right, because we are exposing the right side of the fuselage to the airflow, pushing air toward the right and creating a leftwards aerodynamic sideforce that dominates over the small rightwards aerodynamic sideforce created by the rudder itself.
One thing is certain-- if the plane is moving sideways through the air, and your analysis of the forces at play isn't considering the real, tangible aerodynamic sideforce generated by the resulting sideways airflow impacting against the side of the fuselage, then you aren't seeing the whole picture. We find this mistake in many pilot ground school materials, as well as in FAA "written" test questions and in study guides for FAA "written" tests.
IF you are defining uncoordinated flight as the inclinometer ball being off center, THEN you can say the cause is that the turn rate, and the resulting apparent "centrifugal force" (which is not a real force), are mismatched to the bank angle and G-load. To a FIRST APPROXIMATION, this means that aerodynamic sideforce generated by the fuselage is not zero, i.e. the fuselage is being allowed to fly sideways through the air, but as noted above this is not ALWAYS the case.
IF you are defining a slip or skid as the yaw string being off center, THEN you can say that the cause is that the rudder is not being applied as needed to bring the net aerodynamic yaw torque to zero when the nose of the aircraft is pointing directly into the oncoming airflow (relative wind). Thus causing the aircraft to adopt a different orientation to the oncoming airflow (relative wind), where the net yaw torque is zero.
WHY some rudder deflection is often needed to prevent the plane from flying slightly sideways through the air in turning flight, is a complicated matter best addressed in another question. However we can start by noting that the outboard wingtip must move faster through the air, and thus tends to experience more drag, than the inboard wingtip. We can also note that the fuselage can't curve itself like a banana to accomodate the curving relative wind (airflow) in the turn, so when the vertical fin "weathervanes" into alignment with the direction of the airflow at the rear of the aircraft, more forward locations such as the CG and the nose tend to experience a sideways component in the airflow.
If you are asking about the forces involved in an uncoordinated turn, Thrust, Drag, Lift, and Load (weight) still apply. But, let us define Lift as acting perpendicular to to the wings in a direction opposite Load. While Load is acting perpendicular to the wings in the direction of gravity plus the aircraft’s momentum. When you are banked, or you are in a turn, Load is acting still perpendicular to the wings. Except, now the direction is not straight down. It is split between the downward component and the sideways or lateral component. If the Lift components are not kept in balance with the Load component, you will experience uncoordinated flight. In a slip, you, the pilot, will feel the lateral forces pull your body toward the center of the turn. In a skid, you will feel the lateral forces pulling you to the outside of the turn. In coordinated flight, the pilot feels the forces increase in a direction that, to them, feels straight down into their seat.
To oversimplify it, think of it like turning your car. A coordinated turn will see your back wheels always following your front wheels (albeit a little more to the inside of the turn). If you overcome friction to break traction between your front tires and the ground, you will perform a slip called snow plowing. If you loose traction in your back tires, you will perform a skid called fishtailing. You might not feel too much of the lateral forces in a slip. It may feel more like the momentum of the car. You will feel more of the lateral forces toward the outside of the turn in a skid. Either way, your nose is not following your tail. It is uncoordinated.
The same is true in an airplane. And just like in a car, uncoordinated flight in an airplane can be advantageous depending on your circumstances and desired outcome. The problem with an airplane is that you need a certain velocity (direction and intensity) of airspeed over each wing to continue flight. If you fishtail (swing your tail) to the outside of the turn, you slow down the wing on the inside of the turn and speed up the wing on the outside of the turn. With enough of a change in velocity, you can stall the inside wing while simultaneously creating an excess of Lift in the outside wing. This will lift the outside wing and drop the inside wing creating a dangerous spin scenario.
Another concept to consider is the unintended and undesirable yaw created by the downward pointing aileron creating more Lift than the upward pointing aileron. This creates more drag on the side of the downward pointing aileron. This adverse yaw is also a contributing factor in making a turn uncoordinated. The nose will turn in a direction opposite that of the bank and intended turn in a type of snowplowing. This will cause excess drag due to the airplane’s aerodynamics no longer being streamlined. This increase in total Drag will cause the aircraft to Slow down or not fly as fast as it should in a given flight regime.
The Slip/Skid Indicator on your Turn Coordinator acts like a calibrated pendulum. It will move in the direction that the forces acting on the plane will move any object in the plane (including the pilot). In other words, it will move to the outside of the turn in a skid, and to the inside of the turn in a slip. Applying rudder pressure in the same direction (or easing/reducing rudder pressure in the opposite direction) will correct the slip or skid.
It isn't all that difficult if you understand the three different axis, and the corresponding rotation about each axis. (roll, pitch, and yaw)
In your question you described a coordinated turn. (no slip or skid) An uncoordinated turn is simply when there IS slip or skid. This occurs where there is either not enough, or too much yaw. This happens when you apply either not enough, or too much rudder input.
In a coordinated turn, the weight of your body is always downwards on your seat. That makes it comfortable for the pilot as well as passengers. You might have noticed while travelling in an aircraft that, say your plane is taking a left turn, but you don't feel the plane taking any turn, only to realize it when you look out the window (the landscape is tilted way too much!). That's because the left rolling action (which would have made your weight shift to the left) is compensated by the left rudder (which makes the plane have an imbalanced counterclockwise torque at the tail, makes the plane turn left, and the centrifugal force makes your weight shift to the right). In cars having stiffer suspensions, taking a turn even at slower speeds makes passengers experience centrifugal forces. This is due to the restricted rolling movement of a car's body.
A simple way to describe it is coordinated flight is one in which the fuselage of the aircraft is aligned with, or is tangent to, the direction of flight. This may or may not result in a centered ball on a quality-of-turn indicator, but will always result in a centered yaw string as the fuselage is aligned to the relative wind. Lack of coordination in a turn follows when the the fuselage is not aligned with the relative wind. Lack of coordination results when the horizontal component of lift from the wings is not balanced with the centrifugal force imposed on the aircraft from the turn and a net side load is imposed. It can also be a byproduct of an asymmetric drag loading on the airframe eg adverse yaw caused by aileron deflection. This causes the aircraft to slip or skid in the turn. This can be corrected by with the use of rudder or anti-torque input as in the case of a helicopter. Large deflections of rudder to counter asymmetrical drag, such as the case of an engine failure in a multiengine aircraft also require a shallow bank angles away from the direction of slip to maintain coordinated flight. This will result in an slightly uncentered ball, but an aligned yaw string.
