One fine spring day I was instructing a student who had about 5 hours experience. This was her first lesson in slow flight, but she was doing really well: she was maintaining the assigned altitude, the assigned heading, and the assigned airspeed (a couple of knots above the stalling speed). She was also doing a good job of keeping the inclinometer ball in the center, which required considerable pressure on the right rudder pedal because of the high power and low airspeed. I was really enjoying the flight, but suddenly I developed a feeling that there was something wrong. Gradually it dawned on me what the problem was. The problem was that the airplane was upside down.
Here’s what had happened: her right foot had gotten tired, so she just removed it from the pedal — all at once. This produced a sudden yaw to the left. Naturally the left wing dropped, so she applied full right aileron. The nose was dropping, too, so she pulled back sharply on the yoke. The next thing anybody knew, we were upside down.
I took the controls and rolled the plane right-side-up. (See section 16.22 for more about this.) We lost about 500 feet of altitude during the maneuver. The student asked “What was THAT?” and I said “That was a pretty nice snap roll”.
This is indeed the recipe for a snap roll: starting from a speed slightly above the stall, apply a sudden yaw with the rudder, apply opposite aileron, and pull back on the yoke. SNAP! — One wing stalls and the plane rolls over. In our case, we didn’t roll exactly 180 degrees — “only” about 135 degrees — but that’s upside down enough for most people. It took a fraction of a second.
In due course the student completed her training and got her license. She’s even still speaking to me. There are a number of points to be learned from this adventure:
Let me reiterate: Piloting an airplane at low speeds requires using the rudder pedals. If you don’t know how to do this correctly, you have no business trying to land, take off, or anything else.
First, let’s clarify the concepts and the terminology a little bit:
|Slipping Situation||Slipping Maneuver|
|A slipping situation is any situation where the air happens to be flowing somewhat crosswise, leftward or rightward across the fuselage. We quantify this by saying there is a nonzero slip angle. A formal definition of the slip angle can be found in section 19.7.3.||A slipping maneuver involves a nonzero slip angle at some point during the maneuver. The constant-course slip maneuver (aka forward slip) is the most common example; see figure 11.1.|
|The slip angle is a very simple concept.||The forward slip maneuver is complex. It involves a sequence of steps, observing multiple things, using multiple controls, and achieves multiple objectives.|
Your primary means of controlling the slip angle is by deflecting the rudder ... although there are six or seven other things that can contribute to increasing or decreasing the slip angle, as discussed in chapter 8.
Alas, when somebody uses the word “slip” it is not always clear whether it refers to a slipping situation in the narrow sense or to a slipping maneuver in the broad sense. Sometimes you can figure it out from context, and sometimes not.
The easiest and most direct way to perceive the slip angle is to use a slip string, as we now discuss.
To create a slip string, tape a piece of yarn to the nose of the airplane, on the centerline, in front of the window where you can see it. Leave a foot or so of yarn dangling free, so it will align itself with the airflow.
To a good approximation, the deflection of the string is proportional to the airplane’s slip angle, that is, the angle of the X-axis relative to the free-stream relative wind. The deflection is not strictly equal to the slip angle, merely proportional, because the fuselage disturbs the local airflow pattern. The sideways component of the flow is increased more than the fore-to-aft component, so the sensitivity of the string is increased. That is, the angle of the string is larger than the actual slip angle.
In a single-engine aircraft, the propwash interferes somewhat with the slip string, in a couple of ways: The straight-back component of propwash decreases the sensitivity somewhat, and the helical component of propwash biases the string slightly to one side. Even so, the slip string gives valuable information.
The slip string is commonly referred to as a “yaw string”, but that is a misnomer, because it measures the slip angle, not the yaw angle (i.e. heading) or yaw rate. The slip angle measures the angle between the fuselage and the relative wind, whereas yaw is defined relative to some fixed spatial direction. Heading and heading change (i.e. yaw rate) are easy to perceive by looking out the window, while it is not easy to perceive slip angle except by reference to a slip string. Heading can also be perceived using the directional gyro (heading indicator), and yaw rate can be perceived using the rate-of-turn gyro or by observing the rate of motion of the directional gyro.
Beware: The inclinometer ball is often referred to as the slip/skid instrument, but that is another misnomer. It measures inclination, not slip. As we shall see in section 17.1.4, it is quite possible for the airplane to be inclined but not slipping. To repeat: there is no good way to determine the slip angle without a slip string.
The term skid denotes a particular type of slip that occurs when the airplane is in a bank and the uncoordinated airflow is coming from the side with the raised wing. Typically this happens because you have tried to speed up a turn using “bottom rudder”, that is, pressing the rudder pedal on the same side as the lowered wing.
We use the term proper slip to denote a slip that is not a skid.
If you have plenty of airspeed, the aerodynamics of a skid is the same as the aerodynamics of a proper slip. In both cases there is air flowing crosswise over the fuselage. However, you should form the habit of not skidding the airplane, for the following reason.
If the aircraft stalls, any slight crosswise flow will cause one wing to stall before the other. In particular, having the rudder deflected to the right means the aircraft will suddenly roll to the right. If the aircraft is in a 45 degree bank to the right and rolls another 45 degrees in the same direction (because you were applying right rudder pressure), it will reach the knife-edge attitude (wings vertical). If on the other hand you were holding top rudder (still holding right rudder but banking to the left this time), a sudden roll of 45 degrees would leave you with wings level (which is a big improvement over wings vertical).
If the wings are level, you can make a proper slip to the left or to the right; a skid is impossible by definition.
It is appallingly easy to set up a situation that leads to an unintentional skid. Suppose you are ready to make a left turn from base to final. You start the turn improperly, by applying a little left rudder. The crosswise airflow pattern acting on the dihedral of the wings will cause the airplane to bank to the left and make a relatively normal turn in the desired direction. You absent-mindedly maintain the left rudder pressure, so the bank continues to steepen. You decide to apply right aileron to prevent further steepening of the turn. That’s all you need: you are in a skidding left turn, holding left rudder and right aileron, at low altitude. If you stall, you’ll never be heard from again. Seriously, folks, this could happen to you.
There are only a few cases where bottom rudder is appropriate, for example:
Note that in all these cases you apply only enough bottom rudder to maintain coordinated flight. Do not skid!
There are only a handful of practical situations that call for slipping the airplane. The most common situations are:
Slipping is a very common, normal technique during the approach to landing in aircraft that don’t have flaps, such as Piper Cubs and some Citabrias. Slipping also comes in handy in situations where you might want an abnormally steep descent, due to an emergency (or due to bad planning, as mentioned in section 7.7.1).
The goal is to make sure, despite the crosswind, that the direction of flight and the axis of the airplane are both aligned with the runway. The rudder is used to yaw the airplane so that its axis is aligned with the runway, and the ailerons are used to lower the upwind wing so that the slip is a nonturning slip. To say the same thing another way: the rudder mainly determines the direction the airplane is pointing (with a small side-effect on the direction it is going), while the bank mainly determines the direction the airplane is going. For details, see section 12.9.
This is a relatively easy maneuver, so pilots tend not to think about it much, but it does need to be done right. See section 13.5.
Note that in all three of the situations itemized above, the slipping maneuver is performed in such a way that the direction of motion does not change. We call this a constant-course slip, or equivalently a nonturning slip. It is also called a forward slip, but don’t ask me why – the terminology is not very descriptive. This is diagrammed in figure 11.1. This is the normal, everyday, workhorse slip.
Meanwhile, many other types of slip are possible. For example, you could add a little bit of turn to an established slip, or add a little bit of slip to an established turn. Turning slips can be useful on rare occasions; see section 11.5.5. The most important thing to remember about a turning slip is to make sure it is a proper slip, not a skid, for reasons discussed in section 11.4.
Also, it is amusing to practice wings-level boat turns every so often. This maneuver has no direct relevance to ordinary flying, but it makes a good lesson, because it sheds some light on the basic aerodynamics. See section 8.11.
Here are a few things to keep in mind during any slip, to help prevent the sort of snap roll mentioned in section 11.1.
|Make sure you have plenty of airspeed during a slip.||An uncoordinated stall is a good way to produce a snap roll or a spin. An inadvertent snap roll that leaves the airplane upside-down on short final is not good for your health.|
|Maintaining proper airspeed is trickier than it may sound, because the slip raises the stall speed (as discussed in section 11.5.3), and the slip messes with the airspeed indicator (as discussed in section 11.5.4).|
|When beginning or ending any slip, apply the rudder smoothly and gradually.||A rapid yawing motion (especially at low airspeed) is one of the things that can cause a snap roll, because it speeds up one wingtip and slows down the other wingtip.|
|Any intentional slip should be a proper slip (as opposed to a skid).||A snap roll toward the side with the raised wing is bad enough, but a snap roll toward the side with the already-lowered wing is even worse, for reasons discussed in section 11.4.|
A nonzero slip angle has two primary effects:
We now turn from the simple topic (aerodynamical slip angle) to a much more complicated topic (various slipping maneuvers).
The basic procedure for establishing a proper slip is to lower one wing with the ailerons while applying opposite rudder. We say a proper slip uses “top rudder” because you are pressing the rudder pedal on the same side as the raised wing.
As a special case, if you match the rudder deflection to the bank angle just right, no net turn results. This is called a nonturning slip. Loosely speaking, this can be considered an ordinary turn to one side (because of the bank angle) plus a boat turn to the other side (because of the rudder deflection).
In a slip, the fuselage interferes with the airflow over the wing. That is, part of the wing is in the aerodynamic “shadow” of the fuselage. This is particularly pronounced in high-wing aircraft, as shown in figure 11.2. For more on this, see section 9.2.
We can understand the consequences of this by reference to equation 11.1, which is a clone of equation 4.2.
|lift force = ½ρV2 × coefficient of lift × area (11.1)|
In particular, let’s apply this equation to the situation where the wing is flying at the critical angle of attack, which corresponds to the maximum coefficient of lift. The “shadow” effect reduces the useful area of the wing. The total lift must remain the same, to support the weight of the aircraft. The air density ρ remains the same, so the only remaining variable is the true airspeed, V. When the effective area goes down, the airspeed must go up. In other words, the slip increases the true stall speed.
Beware: In some aircraft, the airspeed indicator is grossly perturbed by a slip, as mentioned in section 2.13.7. In a typical Cessna 152/172/182, depending on the amount of slip, the airspeed can easily be off by 20%, which means the energy is off by 40%. This is enough to cause real trouble. In some less-common aircraft, you can send the airspeed needle below zero.
Suppose the static port is on the left side of the fuselage (as it is on many aircraft), and suppose you are in a slip to the left (the kind that requires pressing on the right rudder pedal). In this situation, the left (upwind) side of the fuselage is a high-pressure point. This high pressure cancels some of the dynamic pressure in the Pitot tube, so the airspeed indicator will lose airspeed.
Now suppose you are in a slip to the right. The static port is now on the downwind side. This will not be a low-pressure point. In fact, it could have almost as much high pressure as the other side, because of pressure recovery, as indicated in figure 4.13. More likely, there will be will be only partial pressure recovery, as illustrated figure 4.12, and the static port will measure something partway between the real static pressure and the high pressure observed on the upwind side of the fuselage.
Therefore: In a slip toward the side with the static port, expect the airspeed indicator to lose a lot of airspeed. In a slip toward the other side, expect a smaller loss.
Better yet, don’t use the airspeed indicator during the slip. Use it before the slip to make sure you have the desired angle of attack, and then ignore it during the slip. Maintain the desired angle of attack by looking out the window. Observe the pitch attitude relative to the direction of flight. Don’t forget that because of drag caused by the slip, the new direction of flight will be angled more downward. Find a new landmark that remains a fixed angle below the horizon.
The slip also has an effect on the altimeter and vertical speed indicator, since they use the same static pressure port, but this is usually not as obnoxious as the effect on the airspeed indicator.
There are also issues with the Pitot tube, but they are usually hard to notice, because they are small relative to the static port issues.
Slipping during a turn (or turning during a slip) is relatively unusual, but not unheard-of.
Let’s consider the scenario shown in figure 11.3. Upon reaching the vicinity of the airport, you notice that you need to slip to lose altitude. You also need to maneuver to get lined up for landing. At point B, the question arises: should you slip by lowering the left wing or the right wing?
Let’s assume you want to continue slipping, including slipping during the turn at point D, where you turn to join the base leg. During the turn, you really want to have a proper slip, not a skid. That means you want to be holding top rudder during the turn. Therefore you might as well get a head start by applying right rudder and lowering the left wing, as shown at points B and C in the diagram. That is, lower the wing that is going to be on the inside of the upcoming turn.
When it comes time to turn, at point D, keep the same amount of right rudder and increase the amount of left bank.
Theoretically, if you were concerned with point B and nothing else, you could slip to either side and it wouldn’t matter in terms of losing altitude, and it wouldn’t matter in terms of following your intended flight path. However, the story changes when we consider point D also. Lowering the outside wing at point B would not be good strategy, because of the mess it creates at point D.
In the middle of the base leg, at point F in the diagram, you want to fly for at least a little ways with no slip, for multiple reasons. For one thing, you need to see if there is any wind drift, which is hard to do while slipping. If the wind is drifting you toward the airport on base leg, that’s a very bad sign, because it means you will have a tailwind on final. It means you should break off the approach and strategize an approach to the reciprocal runway.
At point G, you are anticipating the next turn. As always, if this is to be a slipping turn, you want it to be a proper slip, not a skid. Therefore the slip at point G should involve lowering the wing that will be on the inside of the upcoming turn.
At point J, if you have done everything right, you will have the correct altitude and the correct airspeed, so you can discontinue the slip. You can proceed with a normal flare and touchdown.
As a separate matter, on short final, if there is any wind, you should establish a slip to compensate. The direction of this slip will be determined by the wind, whereas the direction of the previous slips was determined by the direction of the turns.
As previously mentioned, always use the rudders smoothly and gently. In particular, if you are slipping to the left and want to transition to slipping to the right, or vice versa, take it easy. Most airplanes have tremendous amounts of rudder authority, enough to get you into trouble if you overdo things. For that matter, it is good practice to perform all maneuvers smoothly and gently (with rare exceptions, as mentioned in section 18.7).
Tangential remark: Whether or not you are slipping, intercept the final approach course far enough from the runway so that you have plenty of stabilized, straight-in final approach.
The definition of a slip “to the right” versus a slip “to the left” is a bit arbitrary and hard to remember. The following table may help. A slip to the right corresponds to a positive slip angle β. It is sometimes called simply a “right slip”
slip to the right,
|The airplane is moving toward a point that is somewhere to the right of the nose.||The nose is pointing to the left of the direction of flight.|
|The air is hitting the right side of the fuselage.||You are applying left rudder.|
|The inclinometer ball is displaced to the right.||The slip string is displaced to the left.|
|If this is a nonturning slip, or a proper turning slip, you are banked to the right.||If you are not banked, you are making a boat turn to the left.|
Because of the potential for confusion, I try to avoid terms such as “left slip” and “slip to the left” entirely. Instead, I might say “let’s make a wings-level boat turn to the right” or “let’s perform a nonturning slip, banked to the left”.
Once again (as in section 11.2), the meaning of the term “side slip” (or equivalently sideslip) depends on whether we are talking about the angle or the maneuver.
|In some of the older engineering literature, the slip angle is called the “sideslip angle”. The two terms are synonymous. It’s the same angle, no matter what you call it. Calling it simply the slip angle is preferable, because we don’t want anybody to get the idea that the “side” slip angle is somehow different from some “other” slip angle.||There is also something called a “side slip maneuver”. (Again, don’t ask me where this terminology comes from; it is not very descriptive.) Another name for the same maneuver is “constant-heading slip”, as discussed in section 16.7.|
|The slip angle is important. Ordinary cruising flight calls for zero slip, but in special situations you might want a nonzero slip angle; for an overview see section 11.5.1.||The stylized side slip maneuver shown in figure 11.4 has no direct practical application that I know of. However, it has some academic interest, and it has some limited value in preparation for the special situation shown in figure 11.5.|
The side slip maneuver is shown in figure 11.4. The dotted black line shows your path over the ground. For simplicity, we assume no-wind conditions. The maneuver has three main phases: At point C in the diagram, the maneuver begins with a slipping turn, namely a slip to the right and an MV-turn1 to the right. Then there is a steady phase, including points D and E, where we have a nonturning slip situation. Finally, at point F the maneuver ends with another slipping turn. This time it is a slip to the right and an MV-turn to the left ... which is likely to be a skid.
A side slip on final approach would be a bad idea for multiple reasons.
It is instructive to consider the special situation shown in figure 11.5. The control inputs are exactly the same as in figure 11.4, but the ground track is quite different, because of the gusty crosswind. It is hard to practice the maneuver shown here (except in a simulator), because it is hard to call up a gust when you want one. If this maneuver is done right, there is no risk of a skid during roll-out, because there is no sideways momentum to contend with. That is to say, there is no MV-turn involved.
There are lots of situations where you want your heading to be aligned with your motion through the air (e.g. normal coordinated flight) and there are some situations where you want your heading to be aligned with your direction of motion over the ground (e.g. crosswind landing, as discussed in section 12.9, especially section 12.9.2). In contrast, it’s hard to imagine a general-aviation situation2 that calls for maintaining a constant heading while the direction of travel is changing. (In figure 11.5, the direction of motion relative to the air is changing, but the direction of travel over the ground is not.)
Note that the concepts of “forward slip” and “side slip” do not cover all the bases. I’m pretty sure that a wings-level boat turn is neither a forward slip nor a side slip ... although the terminology in this area is not universally agreed upon.
Also note the following contrast:
|The situation at point E in figure 11.4 is aerodynamically indistinguishable from the situation at point E in figure 11.1. It is a nonturning slip situation. The air doesn’t care how you established this slip angle, or what you intend to do next.||The side slip maneuver in figure 11.4 is different from the forward slip maneuver in figure 11.1. The beginning is different, the ending is different, and the ground-track is different. As the pilot, you care about what is happening right now, but you also care about what happens next.|
As discussed in chapter 8, there are six or seven things that can cause the airplane to yaw. Your job is to use the rudders to eliminate the unwanted yaw, so that the airplane is always pointing the way it is going (except for intentional slip).
The objective is to anticipate how much rudder is required in various circumstances, so you aren’t constantly correcting for errors.
The hardest thing to deal with is yaw-wise inertia. The rule is: rolling to the left requires left rudder; rolling to the right requires right rudder. The amount of rudder pressure should be proportional to the rate of roll. Adverse yaw complicates the situation, and requires rudder deflection whenever the roll rate does not match the aileron deflection. To a fair approximation the two effects can be covered by the rule: “rudder deflection proportional to aileron deflection”.
Note that (unlike yaw-wise inertia) adverse yaw occurs even if you aren’t turning. Suppose that a wind gust causes the left wing to drop. You immediately use right aileron to raise the wing. Right rudder is required. Don’t get the idea that rudder is only required when you intend to turn.
Another tricky case arises when you make your first left turn after takeoff. You are holding a large amount of right rudder pressure because of the helical propwash, and you need to apply left aileron. Rather than using actual left rudder pressure, it probably suffices to use a reduction in right rudder pressure. This is harder to learn than it sounds. You may find it more convenient to maintain whatever right-rudder pressure is required to compensate for the helical propwash, and to make left turns by applying countervailing force on the left rudder pedal.
If you have a rudder trim control, you are encouraged to use it. For example, during the transition from cruise to a steady climb, roll in some right rudder trim to compensate for the helical propwash effect. This saves you the trouble of holding steady rudder pressure all during the climb. Remember how much trim you put in, to make it easier to take it back out when you end the climb and resume normal cruise.
To learn good coordination, first practice looking out the side. When you roll into a turn, you should see the wing go up or down like a flyswatter. If it slices down-and-forward, or up-and-backward, you are not using enough rudder.
You can control the airplane quite nicely while looking out the side. You can judge pitch attitude by the angle the wing chord makes with the lateral horizon. You can judge bank angle by the height of the wingtip above or below the horizon. And, as just mentioned, you can judge coordination by watching for forward or backward slicing motions when you roll into or out of a turn.
It is really important to be able to do this. Just for starters, there is no way you can do a decent job of scanning for traffic if you can’t control the airplane precisely while looking out the side.
I even have my students do fancy things like stalls (and stall recoveries) while looking out the side.
Don’t be a “gauge junkie” — the sort of pilot who can’t even fly a rectangular traffic pattern except by reference to the directional gyro. When making a 90 degree turn, identify a landmark 90 degrees from your original heading and turn toward it. No gauges are required.
The next step is to learn how to perceive correct coordination while looking out the front. This requires having a precise visual reference. There are several ways to arrange this.
Start by getting the airplane trimmed for straight and level flight at a reasonable airspeed, headed toward a definite point. In the figure, the plane is headed toward a point a couple of degrees to the right of the mountains.
You can now use your finger as a reference, as shown in figure 11.6. Rest your hand on the top of the instrument panel and align your finger with the straight-ahead point on the horizon.
Another option is to use a mark on the windshield, as shown by the red wedge in the figure. It really helps to have a mark that falls very close to the line from your dominant eye to the aim point on the horizon, so if you can’t find a scratch or a bug-corpse in just the right point, you should make a mark. You can use a grease pencil, a washable marking pen, a bit of tape, or whatever.
A single reference of this sort only works if your head is in the right position — wherever it was when you established the reference. Since you need to move around to look for traffic, be careful to move back into position before using the reference.
If you want to get really fancy, you can use both a finger and a mark on the windshield. That makes it easy to detect if your head is out of position. This also helps rule out the image from your non-dominant eye (although the easiest thing is to close that eye if it is confusing you).
Figure 11.7 shows how the situation should look after rolling smoothly into a turn to the right with 30 degrees of bank.
You should use the visual reference as your primary indicator of pitch attitude and heading. Throughout the roll-in, turn, and roll-out, the rate of turn (i.e. the rate of heading change) should be proportional to the amount of bank. As the bank increases, the rate of turn should increase.
It is common mistake to think that the airplane should simply pivot on its axis (roll-wise) and then start turning (horizontal-wise). If you look closely, you will see in figure 11.7 that the sight line has already moved to the right a little. This represents the amount of turn that occurred during the roll-in. (If you roll in more slowly, this amount will increase.) Remember: The rate of turn should be proportional to the amount of bank. If the sight mark initially stands still (or backtracks!) and only later starts turning in the proper direction, it means you aren’t applying enough rudder to compensate for yaw-wise inertia (and adverse yaw).
During the roll-out, the same rule still applies: the rate of turn should be proportional to the amount of bank. As the bank goes away, the rate of turn should go away. (Of course, the proportionality factor always depends on airspeed, but at each airspeed there is a definite proportionality between bank and rate of turn.) If you neglect to compensate for yaw-wise inertia, the nose will overshoot (yawing toward the continuation of the turn).
Summary: Don’t let the nose backtrack on roll-in. Don’t let the nose overshoot on roll-out. The rate of turn should be proportional to the amount of bank. The yaw-wise inertia and adverse yaw lead to the rule: rudder deflection should be proportional to aileron deflection.
You can see in figure 11.7 why we went to the trouble of putting a mark on the windshield, rather than using, say a bolt on the cowling at the location marked by the cross in the figure (near the end of your thumb). Such an off-axis reference will not exhibit a rate of turn proportional to the amount of bank. As you can see, the problem is that the cross necessarily rotates a little ways to the outside of the properly-coordinated turn. If you tried to prevent this reference from swinging to the outside of the turn, you would be applying too much rudder during the roll-in. The amount of the error would depend on the angle between the bogus sight line and the actual roll axis — which depends on the shape of the airplane and the height of the pilot.
Once you have learned to make really good turns using the roll-axis sight mark, you should gradually learn to do without it. Make a point of imagining where the mark would be relative to other visual references such as the cowling, the window-frame, et cetera.
Later (after making a few hundred coordinated turns) you should be able to do it with your eyes closed, just by knowing how the controls should feel.
By the way: as you may have noticed, the sight line in figure 11.7 is slightly above the horizon. This is because you need to pitch up a little bit to deal with the load factor in the turn.
Notes: (1) If you make a mark on the windshield, use a bright color, since black is too hard to distinguish from traffic. (2) The best time to make the mark is before takeoff. Taxi into position at the end of a long taxiway, and make a mark that lines up with the horizon at the far end. Even if the pre-flight mark is not perfect, it will facilitate making a better mark later.
The inclinometer ball will remain almost centered throughout the roll-in, turn, and roll-out if everything is done correctly. There are several reasons why you should not over-emphasize this instrument: (1) The response of the ball to coordination errors is sluggish and complex, so you have to be quite an expert to get useful feedback from it. In particular, I see lots of pilots who apply approximately the right amount of rudder, but apply it too late or too early. Diagnosing such errors using just the ball is nearly impossible; other references are more informative. (2) In general, anything that can be done by outside references should be done by outside references. (3) When rolling into or rolling out of a turn, there will be a force on the rudder which must be balanced by a horizontal component of lift (i.e. a slight bank) in order to maintain zero slip. See section 17.1.3 for an explanation of why having the ball in the center is not exactly what you want (but nearly so).
The inclinometer ball definitely is helpful for providing information about a long-term slip — in particular, for telling you how much rudder trim to dial in during a high-power / low-speed climb. Especially in an unfamiliar airplane, it can be hard to tell whether one wing is down a little bit, without referring to the ball.
The phrase “flying by the seat of your pants” has become such a common cliché that people forget its real meaning: you can use the sense of touch in your rear end to determine whether or not your control usage is properly coordinated.3
The idea of using your rear end as an inclinometer might sound trivial or obvious; after all, even non-pilots can notice immediately if they sit down on a park bench that is inclined. However, the non-pilots are probably cheating, using their sense of sight (referring to the horizon) and their sense of which way is up (based on the acceleration-detecting organs in the inner ear). In the airplane, as you roll into a turn, the situation is much more challenging. First of all, you want the load vector (gravity plus centrifugal force) to be directed straight down into your seat (perpendicular to the wings, not to the horizon) — so visual reference to the horizon doesn’t tell you what you need to know about inclination. Secondly, the organs of your inner ear are sensitive not only to the load vector but also to the rate of roll — so they don’t tell you what you need to know, either.
This is a good illustration of why learning to fly an airplane is hard: the airplane is inclined (relative to the horizon) but it is not inclined (relative to the load vector). One sense (sight) conflicts with two others (inner ear and seat of pants).
Because the sense of sight is so dominant, the visual references discussed in section 11.7.1 and section 11.7.2 are typically the easiest way to learn proper coordination. Be that as it may, you should also pay attention to what the seat of your pants is telling you. If you are being sloshed side to side as you roll into a turn, there is something wrong. It may help to close your eyes so you can concentrate on the seat of your pants while the instructor makes a series of coordinated and uncoordinated turns.
While we are on the subject of the sense of touch: as you get experience with a particular airplane, you will learn how much force is required on the rudder to go with a certain amount of force on the ailerons (depending on airspeed, of course). Once you’ve got the feel of the controls, you should be able to make a decent turn without much thought or effort.
Flying by the seat of your pants may sound like a throwback to the days when airmail was carried in fabric-covered biplanes, but it is a useful technique even in modern instrument flying. Proper coordination is still important, and modern airplanes still suffer from yaw-wise inertia and adverse yaw, especially at approach speeds. As you maneuver to stay on the localizer, you don’t want to be looking at the inclinometer ball — you’ve got too many other things competing for your visual attention.
The previous sections concentrated on how to maintain coordinated flight. Sometimes, though, there is good reason to perform an intentional slip. One reason might be to set up for a crosswind landing, as discussed in section 12.9. Another reason might be to get rid of some energy, in which case the procedure is straightforward.
The difference between heading A and heading B is the slip angle.
Do not confuse slip angle with bank angle. In fact, they are perpendicular. That is, slip involves a yaw-wise rotation, while bank involves a roll-wise rotation, as defined in section 19.7.1.
Although in a non-turning slip you can perhaps judge the amount of slip by the amount of bank, in general perceiving the bank angle is a rather poor substitute for perceiving the slip angle.
If you don’t use the rudder, then
Using the rudder and ailerons, you can perform a wings-level boat turn, which involves a slip angle with zero bank. See section 8.11.
In a twin with an engine out, you can have a turn with no bank and no slip, or a slip with no bank and no turn, or (preferably) a bank with no turn and no slip. See section 17.1.
Causes of slip include:
Causes of turn include:
A skid is more dangerous than a proper slip, because it is more likely to flip you upside down if anything goes wrong. Therefore, never apply excess bottom rudder (no exceptions). To say it another way, never try to speed up a turn with the rudder (no exceptions). Never try to roll out of a turn without applying coordinated rudder (no exceptions). Right aileron deflection requires right rudder deflection; left aileron deflection requires left rudder deflection.
Section 16.6 discusses a good coordination exercise.