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Copyright © 1996-2001 jsd

11   Slips, Skids, and Snap Rolls


You should learn from the mistakes of others, because you'll never have enough time to make all those mistakes yourself.
— Ben Franklin

11.1   A Lesson on Snap Rolls

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. 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.

11.2   Intentional Slips

A slip is performed by lowering one wing with the ailerons, and then applying opposite rudder. This is called ``top rudder'' because you are pressing the rudder pedal on the same side as the raised wing.

Because the rudder is deflected, the air will be flowing somewhat across the fuselage. This creates much more drag than the usual streamlined flow along the fuselage. This is often useful for dissipating energy, e.g. for making the aircraft descend more rapidly on approach. This is always done at an airspeed well above the stall.

The crosswise flow not only creates drag (a rearward force) but also creates a sideways force that tends to change the direction of flight, as discussed in section 8.10. A slip, therefore, can be viewed as a boat turn in one direction (because of the crosswise flow), possibly combined with an ordinary turn in the other direction (because of the bank angle).

If you match the rudder deflection to the bank angle just right, no net turn results. This is called a nonturning slip.

Nonturning slips are used during crosswind landings, as discussed in section 12.9. The upwind wing is lowered using the ailerons and the opposite rudder is used to prevent the aircraft from turning. The idea is that the bank determines the direction the airplane is going, while the rudder determines the direction the airplane is pointing. The idea 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 definition of a slip ``to the left'' versus a slip ``to the right'' is a bit arbitrary and hard to remember. The following table may help. In a slip to the left:

``left'' slip, matching statement
  mismatching statement
the airplane is moving toward a point that is somewhere to the left of the nose   the nose is pointing to the right of the direction of flight
the air is hitting the left side of the fuselage   you are applying right rudder
the inclinometer ball is displaced to the left   the slip string is displaced to the right
if this is a nonturning slip, you are banked to the left   if you are not banked, you are making a boat turn to the right

Because of the potential for confusion, I try to avoid the term ``slip to the left'' entirely. Instead, I might say ``let's make a boat turn to the right'' or ``let's lower the left wing and perform a nonturning slip''.

Some folks try to define the terms side slip and forward slip, but it is not worth making such a distinction. In figure 11.1, depending on your intentions, you could be making a side slip to runway 11 or a forward slip to runway 9. But you could change your intentions at any point before the flare. During the approach, you are performing a nonturning slip toward the runway intersection, and that's all that need be said. Aerodynamically speaking, there is no meaningful difference between forward slip and side slip. Both are synonymous with nonturning slip.

sideslip
Figure 11.1: Side Slip Indistinguishable From Forward Slip

In some aircraft the airspeed indicator is grossly perturbed by a slip, as mentioned in section 2.12.7.

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, 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 4.9. More likely, there will be will be relatively little pressure recovery, as illustrated figure 4.10, and the static port will measure something rather close to the real static pressure.

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, ignore the airspeed indicator and determine the 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.

11.3   Skids

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.

I will 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.

*   Bottom Rudder: Right vs. Wrong

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.

Never apply more bottom rudder
than is needed to center the inclinometer ball.

There are only a few cases where bottom rudder is appropriate, for example:
  1. If you are already turning to the left and use left aileron deflection to steepen the turn, you will need left rudder deflection in proportion to the aileron deflection, because of adverse yaw and yaw-axis inertia, as discussed in section 8.8.
  2. In a steady bank to the right during a low-airspeed, high-power climb, you may need some right rudder deflection to compensate for engine torque effects (mainly the effect of the helical propwash hitting the vertical fin and rudder, as discussed in section 8.4).
  3. In a long-winged airplane in a steep turn at low airspeed, the long-tail slip effect will require you to hold bottom rudder to maintain coordination, as discussed in section 8.9.
  4. In a multi-engine airplane with one engine inoperative, you need to bank the airplane toward the side with the working engine, and deflect the rudder toward the same side, as discussed in section 17.2.5.
Note that in all these cases you apply only enough bottom rudder to maintain coordinated flight. Do not skid!

11.4   Anticipate Correct Rudder Usage

As discussed in chapter 8, there are four or five 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.

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-axis 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-axis 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, by all means use it to compensate for the helical propwash effect.

11.5   Perceiving Slip, Perceiving Coordination

11.5.1   Looking Out the Side

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.

11.5.2   Looking Out the Front

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.

hud-finger
Figure 11.2: Finger Used as Heads-Up Display

You can now use your finger as a reference, as shown in figure 11.2. 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.

On the other hand, if you use both a finger and a mark on the windshield, you can easily 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 11.3 shows how the situation should look after rolling smoothly into a turn to the right with 30 degrees of bank.

hud-turn
Figure 11.3: Heads-Up Display, Starting a Turn

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.

The rate of turn should be proportional to the amount of bank.

It is common mistake to think that the airplane should simply pivot around the roll axis and then start turning. If you look closely, you will see in figure 11.3 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-axis 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-axis 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-axis inertia and adverse yaw lead to the rule: rudder deflection should be proportional to aileron deflection.

You can see in figure 11.3 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 11.3 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.

11.5.3   Using the Inclinometer Ball

The inclinometer ball will remain centered throughout the roll-in, turn, and roll-out if everything is done correctly. There are two drawbacks: (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 not worth the trouble. (2) In general, anything that can be done by outside references should be done by outside references.

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.

11.5.4   Using the Seat of Your Pants

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.1

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. But 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 11.5.1 and section 11.5.2 are the typically the easiest way to learn proper coordination. But 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 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-axis 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.

11.5.5   Intentional Slips

The previous sections concentrated on how to maintain coordinated flight. Sometimes, though, you want to perform a slip. You might want to get rid of some energy, or to align the airplane for a crosswind landing. The procedure is straightforward.

  1. Make a note of the pitch attitude and direction of flight, since in some aircraft the airspeed indicator is perturbed by a slip, as discussed in section 11.2. You will have to maintain angle of attack by looking at the angles themselves.
  2. Make a note of which way the airplane is going. For a crosswind landing, maneuver so that the motion is aligned with the runway.
  3. Make a note of which way the airplane is pointing. Call this heading A.
  4. Using the rudder, yaw the nose to a new direction. Call this heading B. For a crosswind landing, choose this to be aligned with the runway.
  5. Bank the airplane as required to keep it going the same direction as before.
The difference between heading A and heading B is the slip angle.

11.5.6   Slip Angle versus Bank Angle

Do not confuse slip angle with bank angle. In fact, they are perpendicular. That is, slip involves a rotation around the yaw axis, while bank involves a rotation around the roll axis, as defined in section 19.6.1.

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 8.10.

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 17.2.

Causes of slip include: Causes of turn include:

*   Coordination Exercises

Section 16.7 discusses some good coordination exercises.

11.6   Summary

· A proper slip results from applying more top rudder
  (or less bottom rudder) than required for coordinated flight.
· A skid results from applying more bottom rudder
  (or less top rudder) than required for coordinated flight.

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.

1
In common usage, the phrase is a metaphor for any situation where the practitioner has such a good feel for the situation that quantitative information is superfluous — or where the practitioner is forced to rely on imprecise indications because quantitative information is unavailable.

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Copyright © 1996-2001 jsd