Copyright © 1996-2005 jsd
During flight, you have quite a number of tasks and responsibilities:
The first three items on this list are what I call the “fundamentals” of maneuvering.2 Simple maneuvers (including plain old straight and level flight) and even some quite complex maneuvers can be broken down into combinations of these three fundamental tasks. Of course, while you are maneuvering you still remain responsible for all the other items on the list.
Some of the maneuvers in this chapter are important parts of everyday flying. For instance, final approach requires lining up on a “front window” ground reference. Flying the downwind leg of the airport traffic pattern requires paralleling a “side window” ground reference. Oftentimes you or your passengers want to get a good view of some landmark, which requires turning around a point. If there is some wind (as there almost always is) you will need to correct for it.
The other maneuvers in this chapter, even though they are not directly practical, serve important pedagogical purposes. Chandelles and lazy eights are good illustrations of several of the points made in this book, including (a) the importance of angle of attack, (b) the relationship between angle of attack and pitch attitude, and (c) the behavior of the plane when its airspeed doesn’t equal its trim speed. Some of these maneuvers may seem daunting at first, because they require doing several things at once. Fortunately, though, the ingredients are not particularly hard and can be learned separately.
Mid-air collisions are overwhelmingly most likely to occur at low altitudes, in the vicinity of an airport, in good VFR weather.
Alas there is no easy way to scan for traffic. There are right ways and wrong ways, but even if you do it right it isn’t easy.
Airliners all have electronic traffic-detection / collision-avoidance systems. Probably the day will come when even the simplest light aircraft will have them too. In the meantime, your eyes are your primary defense. You must use them wisely.
The objective is to spot conflicting traffic while it is still a good ways away, while you still have time to take evasive action. However, when traffic is far away it is hard to see. Trying to spot a typical single-engine airplane two nautical miles away, end-on, is like trying to spot a peppercorn or BB on a shag rug about 55 feet away. (That’s a 6mm diameter, 17 meters away.) If a moderately-fast light aircraft is overtaking a slow one, a two-mile separation could be less than 90 seconds of flight time. If two moderately-fast aircraft are approaching head-on, a two-mile separation is less than 30 seconds of flight time.
In the central part of your visual field, there is tremendously high acuity. Unfortunately, the acuity falls off steeply as you move away from the center. Just 10 degrees off-center, the acuity is tenfold less than it is in the center, and it keeps getting rapidly worse after that.
Your periperhal vision can see extremely dim objects, quite a bit dimmer than can be seen with your central vision, but this is nearly useless for the task at hand. At night, other aircraft have lights. Spotting traffic is actually easier during the night than during the day.
Also note that your peripherhal vision excels at detecting motion. However, that, too, is nearly useless for the task at hand. Traffic that is steadily moving across your field of view is not a threat. You need to be concerned about something that just sits there and gets bigger. (At night the it sits there and gets brighter.) In addition, you need to be concerned if nearby traffic is maneuvering.
Peripheral vision is good for noticing strobe lights, so it’s not completely useless.
All this leaves us with a dilemma:
Therefore a compromise is recommended: Divide the sky into chunks about ten degrees across, so that no point is more than five degrees from the center. Check each chunk separately. This gives you a marginally-manageable number of chunks, and marginally-decent acuity within each chunk.
Scan along the horizon. Traffic at your altitude will appear at the level of the horizon. Traffic that is climbing or descending toward your altitude will be within a few degrees of the horizon. Similarly, if you are climbing or descending, you need to be particularly concerned about traffic slightly above or below the horizon, respectively.
The FAA recommends that you dwell on each chunk for at least one second. (That is, you should not try to scan by sweeping your eyes smoothly along the horizon.) At that rate, it will take you at least 18 seconds to scan a 180-degree stretch of horizon. That’s a long time.
Beware: traffic that is below the horizon can be exceedingly hard to see. Also, the end-on view is a lot smaller than the side view. Once I spent about 10 minutes following another airplane, two miles in trail. We were both descending toward the same airport. I knew exactly where the other aircraft was. It showed up on our fish-finder, and I was talking to the pilot on the radio. I looked and I looked, but I didn’t see anything until the other airplane flared for landing.
Beware: something like 80% of all mid-air collisions involve one airplane overtaking another one traveling in the same direction. (You might have guessed that head-on collisions would be more prevalent, but just the opposite is true. Evidently we are getting a big payoff from the rule that keeps opposite-direction traffic at different cruising altitudes, and the rule that keeps everybody going the same direction in the traffic pattern.)
It is also worth knowing that your eyes won’t focus properly if they don’t have anything in particular to look at. This is called empty-field myopia. This can become relevant if you are flying between layers, or below a featureless ceiling above featureless terrain or water. Haze of course makes it worse. When you switch from looking inside the cockpit to looking outside, you should take a moment to focus on something far away – a wingip, perhaps – before you begin scanning a featureless sky.
Some other bits of advice:
The aforementioned scanning techiques are important, but they are worthless if you don’t put them into practice. The biggest threat comes from people who know all the techniques, and perform a fine scan on those rare occasions when they remember to scan at all.
If your last scan was a long time ago, it doesn’t matter whether that scan was super-excellent or merely passable. What matters is how long it has been since your last good scan. (This is the minimax principle, as discussed in section 21.11.) Make sure you always have a good scan, without lapses.
This is a very important maneuver which has not always been sufficiently stressed during pilot training. The idea is to change speed while maintaining constant altitude, constant heading, et cetera.
Here’s a good exercise: Start from level cruising flight. Slow down to VY, while maintaining constant altitude. When you reach the new speed, set the engine controls and trim so that the plane will maintain the new speed. After you have flown in this configuration long enough to convince yourself that everything is stable, slow down to a speed well below VY (but with a reasonable margin above the stall). Again, stabilize the plane at the new speed, still maintaining constant altitude. Then increase speed back to VY and stabilize. Then increase speed to cruise and stabilize. Iterate this a few times until you are sure you’ve got the hang of it.
You will have an easier time understanding how to use the throttle (especially at speeds below VY) if you keep in mind the concepts of kinetic energy and power curve. These are discussed at length in section 7.2.
You will also want to keep in mind the relationship between trim and airspeed, as discussed in section 2.6.
An interesting variation of this maneuver is to practice speeding up and slowing down with the flaps extended. (Make sure you observe the speed limit for flaps-extended operations, which is typically quite a bit lower than for flaps-retracted operations.) This is interesting because on some planes, adding power with flaps extended causes a huge nose-up trim change; you will need to roll in some nose-down trim to compensate.
In flight it is fairly common for the airplane to find itself at an airspeed rather different from its trim speed. This situation will result in a phugoid oscillation, as discussed in section 6.1.14. It is definitely worth seeing this behavior for yourself.
Start with an airspeed, say, halfway between VY and cruise. Pull back on the yoke until the airplane slows down about ten knots, and then let go. As discussed in section 6.1.14, the airplane tries “too hard” to return to its original airspeed, altitude, and attitude; it will overshoot and oscillate for several cycles.
From time to time during this maneuver, look at the airspeed indicator and altimeter. This will provide a good illustration of the law of the roller coaster (9 feet per knots, per hundred knots). See section 1.2.1. This maneuver is also a good illustration of the principle of angle of attack stability, as discussed in chapter 6.
Practice “catching” the phugoid at various points in the cycle. That is, by pushing or pulling on the yoke, maintain constant altitude until the airspeed returns to normal. It is particularly interesting to catch it right when the airspeed equals the trim speed. By returning it to normal attitude at that moment, you can instantly end the oscillations.
If you use the wrong procedure (pushing on the yoke when the altitude is highest and pulling on the yoke when the altitude is lowest) you will just make the situation worse. This an example of a pilot-induced oscillation (PIO). It is more common than you might think, and can cause serious trouble if it happens near the ground, as discussed in connection with evil zooms in section 12.11.9 and section 16.21.6.
In this section, we restrict attention to steady, coordinated turns. (We exclude, for now, such things as boat turns, which are discussed in section 8.11.)
Flying around in an established turn is relatively simple. For perfect coordination, you ought to deflect the rudder toward the inside of the turn (to compensate for the long-tail slip effect, as discussed in section 8.10). Then you need to deflect the ailerons toward the outside of the turn (to compensate for the overbanking tendency, section 9.4). This is remarkably unlike a car, in which you must keep the wheel deflected to the inside, and you can judge the tightness of the turn by the deflection of the wheel. In the airplane, don’t look at the yoke. Judge the tightness of the turn by looking at the bank angle. Then do whatever you need to do with the yoke to maintain the chosen bank angle.
There are four main factors involved in such a turn:
There are strict relationships involving these four quantities. If you know any two of them, the other two are automatically determined.
We can derive one such relationship by balancing the centrifugal force against the turning force, i.e. the horizontal component of lift. The latter is given by:
|Fhorizontal = m g tan(θ) (16.1)|
where m is the mass, g is the acceleration of gravity, and θ is the bank angle. This formula comes from the trigonometry of the situation, as shown in figure 16.1.
Meanwhile, in a steady turn the centrifugal force is m v2 / r, where r is the turning radius. You can understand this as follows: r / v is the amount of times it takes to fly one radian around the circle. That means v / r is the rate at which the heading is changing. Therefore we have:
Equating the two forces gives us:
The mass drops out. That makes sense, because we can think of equation 16.3 as a trigonometric relationship between the centrifugal acceleration (not force) and the acceleration of gravity. The result is also independent of altitude. The result applies to every make and model of airplane in the world. It’s just trigonometry.
Remember that we are only considering steady, coordinated turns.
We see that for any particular bank attitude, the turning radius depends on the square of the speed. If this is a ground reference maneuver, the speed that matters is the groundspeed, as you can see by analyzing the maneuver from an earthbound observer’s point of view. A turn that consumes a tenth of a mile at 60 knots will consume nearly a mile at 180 knots.
speed rate radius bank load (knots) (∘/sec) (nm) (degrees) factor 60 10.5 0.09 30 1.15 75 8.4 0.14 30 1.15 90 7.0 0.20 30 1.15 105 6.0 0.28 30 1.15 120 5.3 0.36 30 1.15 135 4.7 0.46 30 1.15 150 4.2 0.57 30 1.15 165 3.8 0.69 30 1.15 180 3.5 0.82 30 1.15Table 16.1: Constant-Bank Turn
You can re-arrange equation 16.3 to find how much bank you need to achieve a given turning radius. For example, if you want the radius to be one third of a nautical mile, the bank angle is given in table 16.2.
speed rate radius bank load (knots) (∘/sec) (nm) (degrees) factor 60 2.9 0.33 9 1.0 75 3.6 0.33 14 1.0 90 4.3 0.33 19 1.1 105 5.0 0.33 26 1.1 120 5.7 0.33 32 1.2 135 6.4 0.33 39 1.3 150 7.2 0.33 45 1.4 165 7.9 0.33 50 1.6 180 8.6 0.33 55 1.7Table 16.2: Constant-Radius Turn
A standard rate turn is defined to be three degrees per second. This is what ATC expects when you’re on an instrument clearance. It is also called a two-minute turn, because at that rate it takes two minutes to make a complete 360∘ turn.
The time is related to the airspeed and the turning radius, because the distance flown during one complete turn is simply 2 π r. Therefore:
|t = 2 π r / v (16.4)|
We can plug equation 16.4 into equation 16.3 to find how bank is related to airspeed in a two-minute standard-rate turn:
where t is the time for one complete turn.
The rate of turn (in degrees per unit time) is:
These relationships are tabulated in table 16.3. You can see that for a standard-rate turn, the required bank angle grows in proportion to the airspeed, provided the bank angle is not too large. The radius of turn grows in proportion to the airspeed (not the square thereof).
You should figure out the bank angle that corresponds to a standard-rate turn for the airspeed(s) you normally use.
speed rate radius bank load (knots) (∘/sec) (nm) (degrees) factor 60 3 0.32 9.4 1.01 75 3 0.40 11.6 1.02 90 3 0.48 13.9 1.03 105 3 0.56 16.1 1.04 120 3 0.64 18.2 1.05 135 3 0.72 20.3 1.07 150 3 0.80 22.4 1.08 165 3 0.88 24.4 1.10 180 3 0.95 26.3 1.12Table 16.3: Standard-Rate Turn
Here is a good maneuver for learning about your plane’s roll-wise inertia and adverse yaw, called “coordinated wing rocking”. The procedure is: roll rather rapidly into a 45 degree bank to the left. Pause for a moment, then roll to wings level. Pause again, then roll 45 degrees to the right. Pause again, roll wings level, and repeat.
Refer to chapter 11 for a discussion of various techniques for perceiving whether or not your maneuvers are accurately coordinated.
The rolls should be done sufficiently rapidly that significant aileron deflection is required. Do the maneuver at cruise airspeed, and then do it at approach speed and even slower speeds, so you can see how the amount of rudder required increases as the speed decreases. Do the maneuver while looking out the side (wings should go up and down like a flyswatter, with no slicing) and while looking out the front (rate of turn proportional to amount of bank, no backtracking on roll-in, no overshoot on roll-out). Pay attention to the seat of your pants.
You should do the maneuver two ways: once with large aileron deflection applied gradually, and once with large aileron deflection applied suddenly. The difference between the two demonstrates adverse yaw.
Unlike the exercise in section 16.6 (which involved coordinated wing rocking) this one involves intentionally uncoordinated wing rocking. Put the airplane in a slight bank (15 degrees or so), then apply top rudder to keep it from turning. Hold it there for a few seconds, then roll back to wings level, hold it there, then roll to the other side, etc., maintaining constant heading throughout. This is the same as the “side-slip maneuver” discussed in section 11.5.7. This is grossly uncoordinated, but it is amusing and educational because it lets you learn the feel of the controls and the response of the airplane.
To establish a slip, you begin by putting the airplane into a bank. At this point, there is a sideways force but not yet any sideways motion, so there is no weathervaning tendency and no need to apply top rudder. It takes a couple of seconds for the airplane to build up sideways velocity, during which time you feed in progressively more top rudder.
The logic here has to do with the physics of inertia, as discussed near the end of section 16.9. The same logic applies in reverse when you roll out: The sideways motion will remain for a little while, even after you have removed the sideways force. Therefore, in order to maintain heading, you will need to keep the rudder deflected during the roll-out and immediately afterwards. Then, as the sideways velocity goes away, the need for rudder pressure goes away.
This exercise is of some limited value in preparation for crosswind landings. Consider the contrast:
In a purely academic sense, it is always good to know how the
airplane responds to the controls, and constant-heading slip connects
a particular combination of control-inputs to a particular
combination of results.
Also: There are certain special situations where a rolling into a slip without changing the heading is the right thing to do. For example, suppose you are properly aligned for landing in no-wind conditions, and then a crosswind suddenly springs up. If you notice immediately, all you need to do is establish a slip.
|You should not become overly enamored of the constant-heading slip, because there are plenty of situations where that’s not the right thing to do. For example, on final approach, if your position is offset from the runway centerline, a constant-heading slip is not the right way to make the correction. As discussed in section 12.9.2, it is far more important to keep the airplane’s axis aligned with the direction of travel than to keep it aligned with the runway centerline.|
Slipping along a road (section 16.9) is another relevant exercise. It tells you another part of the story.
When rolling out of a constant-heading slip, be sure to do it smoothly and gradually. Otherwise a skid is likely to develop, as discussed in section 11.5.7.
Constant-heading slips are essentially the same as the top three “points” of an an aerobatic 8-point roll. These are sometimes improperly called Dutch rolls, but they are not the same as the natural aerodynamic Dutch roll oscillations discussed in section 10.6.1. Both involve slipping to one side and then the other, like a Dutch kid on skates, making a series of slips (left, right, left, right) without much change in “direction”, depending on what you mean by “direction”. However, note the differences:
Another amusing and educational exercise is called “drawing with the nose”. It goes like this: keeping the wings level at all times, yaw the nose to the left with the rudder. Then raise the nose with the flippers. Then yaw the nose to the right with the rudder. Then lower the nose with the flippers, and repeat. Imagine you are drawing a rectangle on the sky in front of you, using the axis of the airplane as your pencil.
Because of the slip-roll coupling described in section 9.2, while pressing right rudder you will need to apply left aileron to keep the wings level. The purpose of this exercise is to illustrate yaw-wise inertia, yaw-wise stability, and yaw-wise damping. Among other things, you will notice that if you make a sudden change in rudder deflection, the nose will overshoot before settling on it steady-stage heading. (Once again, the combination of controls used here is very different from proper turning procedure.)
One of the most basic maneuvers involves choosing a ground reference such as a long, straight road and flying along it. A long, straight powerline or fenceline can also serve as a ground reference, although a road is particularly convenient if you ever need to make an emergency landing. The point of the maneuver is to practice perceiving and correcting for crosswinds, so choose a road that has a significant crosswind component.
Actually, correcting for the crosswind is the easy part. If the plane starts getting blown off to the left of the road, you will instinctively turn the plane a little to the right to compensate. The tricky part is to notice that you have done so. The situation shown on the left side of figure 16.2 (crosswind from the left) seems quite normal. Similarly, the situation shown on the other side (crosswind from the right) also seems quite normal. It is important to be able to perceive the difference. The outside world looks the same in both cases; the difference is that the alignment of the airplane has changed relative to the outside world.
You should always make a point of noting your direction of flight (which is aligned with the road in this case) relative to bolts on the cowling, marks on the windshield,3 and other parts of the airplane. In particular, in figure 16.2, there are short red and green lines on the windshield, and blue X on the cowling. Pay attention to how these line up relative to the course line you are following.
Figure 16.3 show bird’s eye views of the same two situations, to help you understand what’s going on ... but remember, when you are piloting the plane, such views are not available to you.
You should be especially alert to these perceptions during final approach, since you need information about the wind in order to prepare for a proper crosswind landing.
It also pays to notice the crosswind during the base leg. If the crosswind is trying to blow you toward the airport then you will have a tailwind on final and (most likely) a tailwind during landing. You might want to break off the approach and take a good look at the windsock before trying again. See section 12.7.4.
A less-common possibility is that you have a tailwind on final that shears to a headwind at runway level. This is the opposite of the decreasing headwind that you normally encounter on approach. For details on this, see section 16.18.3.
These perceptions can give you precise information about the amount of crosswind. It is proportional to the wind-correction angle and airspeed:
This maneuver is good practice in preparation for crosswind landings. As discussed in section 12.9.2, the goals and priorites are as follows:
It should go without saying that while doing this you should uphold all the usual pilot responsibilities, such as seeing and avoiding other traffic, maintaining a safe altitude and airspeed, checking the engine gauges every so often, et cetera.
Slipping along a road is excellent preparation for crosswind landings (section 12.9).
As you go from point B to point D in the diagram, you establish a slip. This is a constant-course slip, in accordance with the terminology introduced in section 11.5.1.
During the slip, e.g. at point D, you must maintain a bank toward the upwind side and maintain a rudder deflection toward the downwind side.
This is a proper slip (not a skid), as you can tell from the fact that you are applying top rudder.
This is a nonturning slip. As with any nonturning slip, you can think of it as an ordinary turn in one direction (due to the bank, and the force of the sideways component of lift) canceled by a boat turn in the other direction (due to the slip angle, and the force of the wind hitting the side of the fuselage).
We can begin to understand the situation with the help of figure 16.5. It’s a feedback loop. You observe the bank angle, and think about it. If it’s not to your liking, you deflect the ailerons, and over time that affects the bank angle. You observe the new bank angle, and the cycle continues.
To perform a slipping maneuver, we need to control the bank angle and the slip angle, so the situation is even more complicated, as shown in figure 16.6. It’s another feedback loop. You observe three things (heading, direction of travel, and left-right position) and think about them. If they are not to your liking, you change two things (the bank angle and the rudder deflection). Over time, this affects the things you care about. You observe the new situation, and the cycle continues.
We can understand the situation even better with the help of figure 16.7. As is so often the case in aviation, you get to perceive two things, you get to control two things, and you achieve two results ... but the things you control are not directly matched to the things you perceive or the results you want to achieve. Therefore you have to combine things.
|As shown on the left side of the diagram, if you use more left bank and more right rudder together, in the correct proportions, you can increase the amount of crosswind correction (while maintaining the same direction of travel).||As shown on the right side of the diagram, if you use more left bank and less right rudder, in the correct proportions, you can change the direction of travel while (while maintaining the same amount of crosswind correction).|
Here’s the procedure: In preparation for the maneuver, choose a long straight road with a crosswind. Ten or fifteen knots of crosswind component will serve the purpose nicely. To simplify the discussion, let’s assume the crosswind is coming from the left, as in figure 16.4.
Let’s assume you start out with the direction of travel aligned with the road, as at point B in the diagram. The next step is to perceive the crosswind correction angle, i.e. the angle between the axis of the airplane and the direction of the road. Then roll into a left bank and deflect the rudder to the right. By doing these things together you can establish a slip without changing the direction of travel, as suggested by the left side of figure 16.7. In this way you achieve the primary goal, i.e. getting the airplane’s axis aligned with its direction of travel relative to the ground.
Now suppose that the direction of travel and the axis are aligned with each other, but not aligned with the direction of the road. This can happen if there is a bend in the road, or if a temporary gust of wind has given you a sideways push, or (gasp) if there has been a momentary lapse in pilot technique. To turn to the left, temporarily use a little bit more left bank, as suggested by the right side of figure 16.7. If you do this just right, you can change the direction of travel while keeping the same amount of crosswind correction. You are keeping the airplane’s axis aligned with its direction of travel, even as both are changing. In this way you achieve the second goal, i.e. getting both the axis and the direction of travel aligned with the ground reference.
This turning maneuver “feels” similar to an ordinary coordinated turn, and indeed it is similar, in the following sense: In a coordinated turn, both the heading and the direction of travel change together, maintaining a constant (zero) slip angle. During a slipping turn of the kind we are considering here, both the heading and the direction of travel change together, maintaining a constant (nonzero) slip angle. Attaching a slip string to the airplane may help you appreciate this point.
By the same token, if you want to turn right, temporarily use a little less left bank. Beware: When decreasing the amount of left bank, don’t go past zero bank. Don’t set up a bank to the right, because that would constitute a skid.
|In reality, the first priority is to keep the axis aligned with the direction of travel ... even during a course-correction maneuver when the direction of travel is not quite aligned with the ground reference.||This is worth emphasizing, because some people seem to have the mistaken idea that the first priority is to yaw the airplane to keep its axis aligned with the runway (or other ground reference).|
Let’s be clear: The first priority is to keep the axis aligned with your actual course over the ground, not necessarily the desired course. The reason for this has to do with what happens when you touch down during a crosswind landing, as discussed in section 12.9.2.
During any slip, keep in mind that in addition to the sideways forces, there will be lots of rearward drag. You will need to add power to maintain altitude. For goodness sake don’t pull back on the yoke when practicing slips. You will be at a fairly low altitude (since this is a ground-reference maneuver) and you really don’t want to stall in such a situation. Maintain a constant angle of attack by watching the angles as described in chapter 2. The angles are more reliable than the airspeed indicator, because the slip perturbs the pressure at the static port. I’ve seen situations where the indicated airspeed differed from the calibrated airspeed by 40 knots (due to a pedal-to-the-floor slip).
Make a note of how much bank angle and how much rudder pressure are needed for a given amount of crosswind. This varies considerably from one type of airplane to another. This knowledge comes in handy during crosswind landings; you don’t want to wait until you are in the midst of a landing to figure it out.
As discussed in section 11.5.7 and section 16.7, it is generally not a good idea to practice side-slipping, i.e. slipping so as to change the direction of travel while maintaining constant heading. Such a maneuver is generally not a good strategy for changing course, and generally not a good strategy for losing altitude. Worst of all, it tempts you to make a skidding turn at the end of the maneuver. Don’t practice something that might cause you to develop bad habits.
Constructive suggestions: Practice slipping with constant direction of travel (section 16.9) rather than constant heading. Also practice slipping turns, with constant slip angle. A slip string might help with this. Also practice wings-level boat turns (section 8.11).
Imagine you are not completely familiar with the aircraft you are flying. You are have just flown an instrument approach, and have broken out of the clouds about 150 feet above the runway. You are flying at 100 knots. Within the next 15 seconds or so, you need to slow down to 71 knots in preparation for landing. To deal with this situation, you take the following actions:
Now imagine that those actions do not cause the airplane to slow down! You discover that on this airplane, each of those actions causes a nose-down trim change. The airplane pitches over and dives toward the ground at high speed. This is not good.
Therefore, in this airplane, a much better procedure would be to take the following actions:
For any given airplane, you need to know how much trim it takes to compensate for each configuration change. This information is typically not provided by the Pilot’s Operating Handbook. You need to obtain it empirically. Go to the practice area and do some experiments at a safe altitude.
First, just fly around for a while at normal cruise airspeed. This lets you see what the cruise angle of attack looks like; this information comes in handy on final approach, as discussed in section 12.11.3.
You should also take this opportunity to learn how the airplane responds. Practice the basic maneuvers as described in previous sections of this chapter. Speed changes are worth practicing; some airplanes are much harder to slow down than others. Coordinated turns are worth practicing; different airplanes require different patterns of rudder usage. Nonturning slips are important for landings; you need to know how much yaw and how much drag is produced by a given amount of rudder pressure. Phugoids are definitely worth investigating; different airplanes respond differently.
Next, investigate the effect of the trim wheel. The wheel has bumps on it, which we can use as our unit of measurement. Move the wheel one bump, and see what effect that has on the airspeed. If you have electric trim, figure out how fast it moves (how many bumps per second).
Next, slow down to the airspeed you normally use in the traffic pattern. Again, get the airplane nicely trimmed and just fly around a while. Make a note of the angle of attack.
After the airplane is once again flying along, nicely trimmed at pattern speed, extend one notch of flaps. Maintain the same speed. Make careful note of how many bumps of trim it takes to maintain constant speed, compensating for the flap extension. Do not bother to maintain level flight. Leave the power setting alone, and make a note of how much rate of descent is caused by the drag of the flaps. Also note how the pitch attitude changes; remember that extending the flaps changes the angle of incidence, as discussed in section 2.4.
Do the same for each successive notch of flaps. In each case, make careful note of how much you have to move the trim wheel to maintain constant speed. Also observe the resulting rate of descent, and observe the change in incidence.
Do the same for other possible configuration changes (landing gear, speed brakes, et cetera).
After you have done that, investigate the effect of power changes. Determine how many RPM (or how many inches of manifold pressure) you need to remove in order to change from level flight to a 500 fpm descent. Also observe the effect that such a power change has on the trim speed.
Now, during the descent, check the effects of configuration changes again. You need two sets of observations: one using a power setting appropriate for level flight in the traffic pattern, and one using a power setting appropriate for final descent. In an ideal airplane, configuration changes would not affect the trim, but in a real airplane they do, by an amount that depends on the power setting.
At this point, you should be able to construct a crib card along the following lines:
Each of the blanks gets filled in with some positive number (for nose-up trim application) or negative number (for nose-down trim application). The exact values aren’t important; the idea is to have enough information to prevent nasty surprises like the situation described at the beginning of this section.
Finally, fly around for a while slightly above minimum controllable airspeed, with flaps extended. See section 16.20 for more discussion of slow flight procedures. Practice rocking the wings. Make sure you can bank the plane left or right, with reflexively correct use of ailerons and rudder.
Additional familiarization exercises are discussed in connection with landings in section 12.11.4.
Familiarizing yourself with a new type of airplane can take a goodly amount of time, especially if you have modest total pilot experience. On the other hand, if you are just re-familiarizing yourself with the plane after a period of inactivity, you can run through the maneuvers fairly quickly.
Pilots who have been properly trained in a slow, light, simple aircraft should be able to transition to a fast, heavy, complex single, or a light twin, or even a three-engine jet – with only a few surprises. I’ve seen it done. In contrast, though, far too many pilots have picked up a load of bad habits and dirty tricks that only work in one type of aircraft, so for them transitioning to other types will be traumatic.
Here are some of the things to watch out for.
Let’s talk more about trimming. Suppose you are leveling off after a climb, in a high-performance airplane. You let it speed up for a few seconds, and then trim it — but then it will speed up some more and you will need to trim it some more. You should plan on prolonged acceleration and repeated trimming.
If you take any non-turbocharged airplane up to a typical cruising altitude, the throttle will be wide open at cruise. This means that when you level off after a climb, the airspeed will converge only asymptotically to the final value. This is the mirror-image of the problem shown in figure 7.1. It could easily require several minutes for the airspeed to get “close enough” and you will have to re-trim repeatedly during the process.
It would be a mistake to think you can just trim the aircraft and then move on to other tasks. Rather, you must carry out other tasks while the airplane gradually speeds up, while you continually adjust pitch and trim. Turbulence and/or passengers shifting their weight around make trimming a never-ending task. Ideally, trimming is like breathing: it’s important, you do it all the time, and it doesn’t distract you from other tasks. See chapter 2 for a discussion of the basic ideas of angle of attack. See section 7.2 for the particular case of speed-changing maneuvers. See section 16.11 for other trim-related issues.
Turns are more challenging if you are trying to turn around a specific ground reference, maintaining a constant distance from it. If there is any significant wind (which there almost always is), this requires constantly changing bank angles.
The best way to analyze this situation is to begin by considering what happen if you do not make any correction for the wind. Figure 16.8 shows three complete turns made using a constant bank angle.
In the absence of wind, you would have performed three perfect circles around the southeasternmost tree in the orchard. However, since there is some wind, we can use the principle of relativity. Relative to the air, you have still made three perfect circles. However, the air itself has moved during the maneuver, carrying the whole pattern downwind. Therefore relative to the ground, we see the cycloid pattern shown in the figure.
To transform this pattern into one that is circular relative to the ground, you need a steeper bank at the points where you are headed downwind (e.g. point A and neighboring points), and a shallower bank at the points where you are headed upwind (e.g. point C and neighboring points). As you can see from table 16.2, the effect can be fairly large.
If you fly the maneuver at 90 KIAS, your groundspeed will vary from 105 (downwind) to 75 (upwind). That’s a ratio of 1.4 to 1. Let’s assume you remain 1/3rd of a mile from the landmark, since that is the distance to which the table applies. The speed in the left-hand column of the table should be taken as a ground speed, since we want the radius to remain constant as seen from the ground. The table tells us the required bank angle will vary from 26 degrees at point A to 14 degrees at point C.
At points B and D in the figure, the bank angle will be the same as in the no-wind case — but you will need apply wind corrections to your heading, as discussed in section 16.8.
Eights around pylons are performed by flying turns around a point clockwise around one pylon, and counterclockwise around another pylon, as shown in figure 16.9.
If you can do turns around a point, you can learn eights around pylons very quickly. The techniques for wind correction etc. are just the same.
The only new element in this maneuver is choosing the right place to roll out of the turn and begin the straightaway section, so that the two circles will be the same size. It may help to visualize the desired figure-eight shaped ground track on the ground, and then just follow that track.
It is best to enter on a downwind heading, so that the first turn will be the steepest.
Note: This maneuver is not to be confused with eights on pylons (which are discussed in section 16.17.3).
A chandelle is a stylized climbing turn. The key elements are:
You must choose what entry speed to use. Here are some considerations to guide your choice:
The maneuver emphasizes headings and attitudes. You should use ground references to judge the correct headings, but you shouldn’t bother to remain over a particular point or to correct headings for wind drift.
You have some discretion when selecting the initial bank angle. Usually 30 degrees works fine. If the bank is too shallow, during the second half of the maneuver you will find that the airplane has slowed to its final speed before the turn is completed; ideally the final speed and the final heading should be reached simultaneously. Happily, since the airspeed is changing only rather slowly at the end, this is relatively easy to arrange.
The end of the maneuver depends on airplane performance:
If you want to learn to do chandelles, it may help to divide the maneuver into separate “climb” and the “turn” components. It is sometimes useful to analyze and practice these components separately.
The second half of the climb contains an interesting lesson. The pitch attitude and power setting are constant, but the result is very far from being constant performance. The angle of attack is increasing, the airspeed is decreasing, and the rate of climb is decreasing. (I mention this because some flight schools used to emphasize the rule “pitch+power=performance”, which is not a good rule.)
This second part of the maneuver begins with the airplane climbing rapidly. The climb angle is, intentionally, unsustainable. The airplane will nevertheless climb in the short run. For a while, it can climb by cashing in airspeed, according to the law of the roller coaster.
As the airspeed decreases, the airplane must fly at an ever-higher angle of attack in order to support its weight. Since the pitch attitude is being held constant, this means that the direction of flight must be bending over. This is illustrated in figure 2.11 in section 2.10.
This should drive home the lesson that pitch attitude is not the same as angle of attack, and that angle of attack, not pitch attitude, is what directly determines performance.
You should not attempt to micro-manage the altitude during a chandelle. You should maintain the chosen pitch attitude and let the airplane’s intrinsic vertical damping (and energy budget) take care of the vertical motion.
The choice of pitch attitude with which you begin the second half of the chandelle is obviously critical, since you will be stuck with it for the rest of the maneuver. If it is too nose-high, the airplane will slow down too quickly and you will run out of airspeed before the turning part of the maneuver is completed. Conversely, if the pitch attitude is too low, you will have airspeed left over at the end of the turn. The right answer depends on the performance of the airplane (and on the timing of the turning part of the chandelle). The answer can be determined by trial and error. About 15 degrees is a good initial guess for typical training airplanes.
Now let’s examine the turning component of the chandelle. Again, the second half is the interesting part. It will take a certain amount of time, and during this time you must roll the wings level, using a uniform roll rate. If you roll too slowly, the airplane will turn through 90 degrees before the rollout is completed. Conversely, if you roll too quickly you will run out of bank before the 90 degree turn is completed. At each instant, you should estimate the amount of turn remaining and the amount of bank remaining, and fudge the roll-rate accordingly. As always, a small correction early is better than a large correction late. It is useful to practice this a couple of times in level flight, before combining it with the climbing component.
When performing the complete maneuver (climbing and turning together) there is one more wrinkle: Remember that rate of turn depends partly only on bank angle but also depends inversely on airspeed. Since the airspeed is decreasing during the maneuver, you must take this into account when planning the roll rate for the complete maneuver.
Also, as the airspeed decreases you will need progressively more right rudder to compensate for the helical propwash, and progressively more right aileron to compensate for the rotational drag on the propeller blades. Furthermore, remember that adverse yaw and the effects of yaw-wise inertia become more pronounced at low airspeeds (as always). Maintain proper coordination (zero slip) at all times.
The lazy eight derives its name from the motion of the airplane’s axis during the maneuver. In particular, imagine that the airplane is at a very high altitude, so we don’t need to worry about the ground getting in the way. Further imagine that the airplane is centered in a cylinder of paper, 10 miles in diameter and 5 miles high. Also imagine that the airplane carries a very long pencil sticking out the front, aligned with airplane’s axis. During the course of a lazy eight, the pencil will draw a giant figure eight, sideways, on the paper. The very long pencil provides lots of leverage, so that the drawing depends on attitude, not altitude.
We now discuss the basic elements of the maneuver, by reference to figure 16.10. Start at point A, in level flight. Pull the nose up. Gradually start banking to the right. At point B, stop pulling the nose up; let it start going down. Keep the bank; keep turning to the right. At point C, the pencil slices through the horizon. The body of the pencil is horizontal, while its tip is moving down and to the right. Start rolling out the bank. Point D is the lowest pitch attitude. The bank is about half gone; keep rolling it out. At point E the pitch attitude and the bank attitude should be level. Pull the pencil straight up through the horizon. Start rolling to the left. At point F, start letting the pitch attitude back down again. At point G, the pencil-point slices through the horizon again, this time moving down and to the left. Start rolling out the bank. Point H is the lowest point in the leftward stroke. By the time you return to point A, the pitch and bank attitudes should be level again. Pull the pencil straight up through the horizon again, and repeat the maneuver.
For the next level of refinement, arrange the timing and the bank angles so that point B is 45 degrees of heading away from point A; point C is at 90 degrees, point D is at 135 degrees, and point E is at 180 degrees.
For the next level of refinement, arrange the push/pull forces so that points B and F are about 20 degrees above the horizon, and points H and D are about 20 degrees below the horizon.
Note that up to this point we have not mentioned anything about altitude or airspeed. This is primarily an attitude maneuver, and you should learn it in terms of attitudes.
When learning the maneuver, it helps to separate the “up/down” part from the “left/right” part. The left/right part of the maneuver is quite simple. You just very gradually roll into a turn to the right, then very gradually roll out. You continue the roll so it becomes a turn to the left, and then gradually roll out.
The up/down part of the maneuver is almost as simple. You just pull the nose above the horizon for a while, then lower it to the horizon; let it go below the horizon, then pull it back to the horizon and repeat.
One tricky part about combining the left/right part with the up/down part: the vertical motion goes through two cycles (ascending, descending, ascending, descending) while the horizontal motion is going through only one (rightward, leftward).
To get a deeper understanding of the maneuver, we must think a little about the altitudes and airspeeds.
During the whole quadrant from A to C, the nose is above the horizon. The airplane is climbing and slowing down. Therefore C is the point with the highest altitude and the lowest airspeed. Point C has a high altitude even though we (correctly) drew it in the figure on the same line as point A. That is because the maneuver is defined in terms of attitude, not altitude, and we imagine that the paper on which the lazy eight is drawn is so far away that the pencil has lots of leverage — the angle matters a lot, and the altitude matters hardly at all.
To you, the low airspeed at C is more immediately noticeable than anything else. The airplane is below its trim speed, so the nose wants to drop all by itself. At this point you will not need to push on the yoke; you just need to reduce the back pressure to let the nose go down at the desired rate.
During the whole quadrant from C to E, the nose is below the horizon. The airplane is descending and speeding up. Therefore point E has a much lower altitude than point C, and indeed should be level with point A.
The second ascending/descending cycle (from E back to A) should be pretty similar to the first.
The commercial-pilot Practical Test Standard requires that you return to your initial altitude and airspeed every time you pass point A and point E. You might hope that this would happen automatically if you leave the throttle setting alone, relying on the law of the roller coaster. Alas, that hope is in vain, for the following reason: Normally you start the maneuver at a speed well above VY, with a power setting appropriate for level flight at this speed. Now suppose you fly a nice smooth symmetric maneuver that returns to the original airspeed. The maneuver starts with a pull, and at all times you will have an airspeed at or below the initial airspeed. You will be flying the maneuver at more-efficient airspeeds, closer to VY.6 You will gain energy. You will gain altitude. If you try to fix the altitude by diving, you will end up with excess airspeed. The only way to make things come out even is to fudge the power setting; usually you need slightly less power than for level flight. This is most noticeable in airplanes with big engines and long wings, where the normal operating speeds are large compared to VY.
This maneuver contains a very nice lesson about the principles of flight. Much of the vertical part of the maneuver can be considered a “controlled phugoid”. In particular, during the phase from B to D the nose is dropping but you are not pushing it down — indeed you are maintaining back pressure as you gently lower the nose. The feeling is sort of like the feeling you get when lowering a heavy object on a rope, and is quite striking.
This should drive home the message that the airplane is definitely not trimmed for a definite pitch attitude — it is trimmed for a definite angle of attack (or, approximately, a definite airspeed). At point C, among others, the airplane is well below its trim speed, so it wants to dive and rebuild its airspeed.
You have considerable discretion as to the steepness of the banks. Increasing it just speeds up the whole maneuver. A typical choice is to have 30 degrees of bank at points C and G (the points of maximum bank). A lesser bank is also fine, but then you will want to choose a lesser nose-high attitude at points B and F. This is because you will be spending more time ascending, and you don’t want to run out of airspeed. Make sure the airspeed at points C and G is 5 or 10 percent above the stall.
As with the chandelle, you will have to work a bit to maintain proper coordination. There is nothing surprising — just a wide range of roll rates and a wide range of airspeeds.
The “eights on pylons” maneuver is required on the commercial and flight instructor practical tests. Being able to do this maneuver well, especially if there is a wind, definitely demonstrates that you can control the airplane around all axes at once. This maneuver is not to be confused with eights “around” pylons (which are discussed in section 16.14). The ambiguous term “pylon eights” should be avoided.
Before we can discuss this maneuver, we need to have a clear idea what it means to have a sight line that runs past your eye, side to side, parallel to the Y axis, as shown in figure 16.11. You can establish such a sight line as follows: The idea is that the crosshairs should be symmetric on each side, and each should be “eye high”. That is, a line from the left-side crosshairs to the right-side crosshairs should run past your eye.
If you have a clear view of the horizon, you can double-check your crosshairs as follows: Take a look out the side, and see how high the horizon is relative to the wingtip. You ought to know this information by heart anyway, since it is needed when maneuvering the aircraft by reference to the wingtip, for instance when sightseeing or when scanning for traffic. In normal wings-level flight, i.e. when the bank angle is zero, the crosshairs ought to be aimed at the horizon. Check both the left side and the right side.
It is also possible to check the crosshairs during initial taxi. For example, if you are taxiing directly across a long straight horizontal runway, it is fairly easy to look out the side and imagine what a good lateral sight line looks like.
Once you have constructed a good set of crosshairs, you can use them to maintain a good sight line, always parallel to the Y axis, even when the aircraft is in a bank, as shown in figure 16.12.
Section 16.17.2 discusses turns on a single pylon. Then section 16.17.3 discusses how to combine those to form eights on a pair of pylons.
The fundamental objective of the ‘turns on a pylon” maneuver is simple: Imagine a sight line parallel to the airplane’s Y axis, as shown in figure 16.12. The objective is to keep this sight line aimed directly at the base of the pylon. This requirement heavily constrains the attitude of the airplane. At each moment during the maneuver your bank and heading are completely determined by your altitude and position relative to the pylon. The only thing that makes the maneuver possible at all is that you are free to adjust your altitude.
In the absence of wind, the maneuver will work at a particular altitude — the so-called pivotal altitude — and not otherwise. Interestingly, the pivotal altitude does not depend on what you choose as your distance from the pylon. As shown in figure 16.13, if you start close to the pylon, you will have a large bank angle and therefore a lot of Gs. However, since you are close to the pylon, the circle will be small, and you will need a lot of Gs in order to change the airplane’s velocity (from northbound to southbound and back) in the small time available. In contrast, if you start out far from the pylon, the bank will be shallow, and you will pull a smaller number of Gs for a longer time.
The pivotal altitude is proportional to the square of the airspeed: 0.0885 feet per knot squared, or 885 feet per (hundred knots) squared. This number applies to all aircraft. It’s just the inverse acceleration of gravity (1/g), expressed in aviation units.7 That is:
If you happen to be above the pivotal altitude, the airplane will be banked too steeply and will turn too quickly. Your sight-line past your wingtip, which is supposed to be pointed at the pylon, will be swept backward and will appear to fall behind the pylon. Or to say it the other way, the pylon will appear to be moving ahead of where you want it to be. The solution is to descend. At the lower altitude your bank will be less, and the problem will correct itself. Any airspeed you gain during the descent can only help you by further reducing the rate of turn.
Conversely, if you are too low, the bank will be too shallow and the pylon will appear to fall behind where you want it to be.
The main rule is simple: go down to speed up and “catch” the pylon; go up to slow down and “wait for” the pylon.
You may be tempted to use the rudder to swing one wingtip a little bit forward or backward, but this defeats the purpose of the maneuver and is not the correct procedure. Maintain coordinated flight.
Observe your ground track. In the no-wind case it should be a perfect circle. If on the other hand you find yourself gradually spiraling outward away from the pylon, it means your bank is too shallow, presumably because your crosshairs are too low. This introduces a parallax error as shown in red in figure 16.12 and figure 16.11. To fix the problem, choose better (higher) crosshairs.
Conversely, if you find yourself gradualy spiraling in toward the pylon, it means you have systematically too much bank. This presumably means your crosshairs are too high, so that your sight line is misaligned relative to the Y axis. To solve the problem, choose better (lower) crosshairs.
In the presence of wind, the pattern is no longer a perfect circle. In fact, the ground track is an ellipse with the pylon at one focus. You are nearest the pylon when the airplane is headed directly downwind. This gives max bank when flying downwind, which makes a certain amount of sense — you want to bank more steeply when the groundspeed is highest. This is shown in figure 16.14.
The wind also prevents you from flying the pattern at constant altitude (for reasons that will be discussed below). The altitude is highest when the airplane is headed directly downwind. This is shown in figure 16.15. Once again, this contributes to creating max bank when flying downwind, which makes sense.
There are two strategies, depending on how much the plane speeds up when it descends.
The typical case will lie somewhere in between; fortunately the answers in the two cases are not very different.
When going upwind, you need to have a much gentler rate of turn. There are three factors at work:
The first two factors are diagrammed in figure 16.16. In the constant-airspeed case factor 1 does half the job and factor 2 does the other half. In the constant-energy case they all three divide the job, roughly in the ratio 50% : 20% : 30%.
By geometry, the angle of bank is inversely proportional to the distance r from the pylon. It is also proportional to height. In the constant-airspeed case, the height is itself inversely proportional to r. Combining these, you get that the airplane is “attracted” toward the pylon with an acceleration that goes like 1/r2. (Remember that the horizontal acceleration is one G times the tangent of the bank angle, which is simply proportional to the bank angle when the angle is not too large.)
You may recognize this situation as analogous to astronomy: Whenever you have an inverse-square central force, you get an elliptical orbit. What’s more, the analogy says you can apply Kepler’s law of equal areas in equal time, which is equivalent to saying the airplane’s angular momentum about the pylon will be constant. This allows you to figure out how much the ellipse differs from a circle: Suppose the wind is 10% of your groundspeed. Then when you are going directly downwind, you will have to be 10% closer to the pylon. Similarly when you are going directly upwind, you will have to be 10% farther from the pylon.
As previously mentioned, in the zero-wind case, the pivotal altitude is simply proportional to groundspeed squared. Several well-known books try to argue that on the upwind leg of the turn on pylon, the groundspeed is lower, so the altitude should be lower. That is a false explanation (even though the altitude is indeed lower there). The actual altitude change is much less than you would predict by the groundspeed argument (by a factor of 2 in the constant-airspeed case and by a factor of 4 or so in the constant-energy case).
I worked out the correct expression for the altitude. It depends on one factor of airspeed and one factor of groundspeed:
You can easily verify that in the no-wind case, this expression gives the same answer as equation 16.7, as it should.
Notice that we are taking the dot product between the airspeed vector and the groundspeed vector. As always, these satisfy the vector equation VA + W = VG.
Let’s do an example to see how this works. Suppose you are maintaining a constant 100 knots of airspeed, and that there is 20 knots of wind. We start by considering the two points where you are directly upwind or directly downwind of the pylon. At these points, your heading is necessarily directly crosswind. (This is required by the fact that your wing is always pointing at the pylon.) At these points, VA·VG = VA·VA. The wind drops out of the dot product, since the wind is perpendicular to the airspeed vector. Therefore, at these special points, you fly the on-pylon maneuver at the same altitude you would have predicted using the overly-simple equation 16.7, just based on your airspeed. Let’s call this the “nominal” altitude.
Meanwhile, at the “high” end of the ellipse, where you are nearest the pylon and headed directly downwind, equation 16.8 tells you the altitude for the on-pylon maneuver is greater by a factor of 1.2 compared to the nominal altitude. This is 20% greater than you would have expected from squaring your airspeed, but 20% less than you would have expected from squaring your groundspeed, if you tried to rely on the overly-simple equation 16.7.
By the same token, at the “low” end of the ellipse, where you are farthest from the pylon and headed directly upwind, equation 16.8 tells you that the altitude for the on-pylon maneuver is 0.8 of the nominal altitude. Again the right answer is not what you would have gotten by squaring your airspeed, nor is it what you would have gotten by squaring your groundspeed.
Next we should try to understand why the center of the pattern is shifted crosswind from the pylon. (Some other books show it blown downwind, or just centered on the pylon with no shift at all, even in the presence of wind.) For sake of discussion, let’s divide the pattern in half along the long axis (which includes the pylon). If the airplane is positioned to windward of this line, it is subject to a crosswind from outside the pattern, which tends to drift the plane sideways closer to the pylon, making the bank steeper. This effect occurs throughout the windward half, so the plane is closest and steepest when it crosses from the windward to the leeward half (at which point it is headed directly downwind).
For these turns on pylons (unlike turns around pylons), there is nothing you can do to prevent the plane from being blown sideways. Consider the point where the plane is directly upwind of the pylon. The heading is constrained to be directly across the wind. Therefore the plane will be blown toward the pylon.
By the same token, whenever the airplane is on the leeward side of dividing line, it is subject to a crosswind from inside the pattern, which tends to drift the plane sideways farther from the pylon and hence make the bank shallower. The effect is cumulative, so the plane is farthest and shallowest when it crosses from the leeward to windward half (at which point it is headed directly upwind).
Also, draw a line from the pylon to a generic point on the ellipse. The wings of the plane, at that point, will lie on that line; the heading of the plane will be perpendicular to that line. Except for the two special points at the ends of the ellipse, the heading will not be tangent to the ellipse; the angle between the heading and the tangent is precisely the crosswind correction angle. You will note that the plane is always crabbed into the wind, as it should be, to maintain coordinated flight with the correct crosswind correction. This can be seen in figure 16.14.
In flight, you can follow these simple rules:
In principle, these rules are all you need to know. However, the other information in this section makes your job 1000% easier. It allows you to anticipate the required altitude changes and the elliptical ground track. Anticipating the required actions is easier than waiting until there is an error and then making corrections.
Note: The rules of the game require you to maintain coordinated flight (zero slip) during this maneuver. You might think that if the pylon is just a little ahead or behind, you could make things “look” better by yawing the aircraft just a little. However, this is against the rules. If you try it on a checkride, the examiner will notice. In any case, slipping doesn’t pay. It doesn’t solve the fundamental problem, and it might interfere with your ability to perceive the problem. You need to change altitude in order to actually catch the pylon.
The eights-on-pylon maneuver consists of a turn on one pylon followed by an opposite-direction turn on another pylon, as shown in figure 16.17. The two-pylon maneuver adds the complexity of planning when to shift from one pylon to the other, but is actually easier to perform because you can use the straightaway between turns to recover from any small errors.
You don’t want to pick pylons that are too close together. You do want pylons that are crosswind from each other, so that the pattern will be symmetric. As usual, it is best to enter on a downwind heading, as shown in the figure, so that your first turn will be your steepest turn. Maintain coordination; don’t fudge things with the rudder.
In some ways, an airplane performs differently when going downwind as opposed to upwind — and in other ways it doesn’t. There are a lot of misconceptions about both halves of this statement.
Let us first consider the situation where there is a steady wind; that is, a wind that does not vary with time or with altitude.
|Maneuvers relative to a ground reference will be different when headed downwind as opposed to upwind.||Maneuvers that do not involve a ground reference will be unaffected by the wind.|
|For instance, the airplane will climb and descend at a steeper angle (in terms of altitude per unit distance over the ground) when headed upwind.||For instance, the airplane will climb and descend at a rate (in terms of altitude per unit time) that is independent of the wind.|
|Similarly, a constant-radius turn relative to a ground reference will require a steeper bank on downwind and a shallower bank on upwind.||Similarly, a constant-radius turn relative to a cloud will require the same angle of bank throughout the maneuver.|
|The point is that the airplane, the cloud, and the airmass are one big uniform moving system. By Galileo’s principle of relativity, the overall uniform motion doesn’t matter.|
Note that obstacle clearance is an important ground-reference maneuver. Your rate of climb is unaffected by the wind, but your angle of climb is affected. You can climb at a steeper angle on an upwind heading.
Finally, consider ground observers’ perceptions. There are some maneuvers, such as an aerobatic loop, that should not be corrected for the wind. Imagine you are using a smoke generator. You want the smoke to form a nice round loop. Like the cloud mentioned above, the smoke is comoving with the air, so the overall wind speed shouldn’t matter. However, especially if the smoke generator is turned off, the maneuver will appear different to an observer on the ground. This appearance does not (and should not) matter to the pilot in the cockpit, but it does matter if you are on the ground piloting a radio-controlled model, or judging an aerobatic contest.
There are several good reasons for being aware of your groundspeed, including:
On the other hand, during turns and other maneuvers, it would make absolutely no sense to try to maintain constant groundspeed.
We shall have more to say about the effects (or non-effects) of a steady wind in section 16.18.4, in connection with the infamous “downwind turn”.
In the real world, the wind almost always changes with altitude. In particular, it is very common to find that the wind at ground level is blowing in the same general direction as the wind at 3000 feet AGL, but at a much lower speed. This is because of friction between the air and the surface.
Most of this frictional windshear is concentrated at the lowest altitudes. At low altitudes, it is common to see a frictional windshear of several knots per hundred feet, while at enroute altitudes (several thousand feet AGL) it is more typical to see a frictional windshear of a few knots per thousand feet.
Wooded areas, tall buildings, and/or steep hills upwind of your position can create a particularly sharp shear layer, i.e. a situation where wind speed changes with altitude. Similarly, the transition between a wooded or built-up area and an adjacent open field can create a sharp shear of the other kind, such that wind speed changes depending on your horizontal position.
In addition to the aforementioned frictional windshear, frontal activity (especially warm fronts) can cause very large windshears that are more complicated and less predictable than the normal, every-day frictional wind shear. This can be very significant when you’re on approach, as discussed in section 16.18.3.
Let’s analyze how windshear affects the airplane. Suppose you start out at point A, and fly to point B where there is more headwind or less tailwind. If the windshear is sudden, you will notice a sudden increase in airspeed. The windshear has added something to your energy8 budget. If the shear is more gradual, the airplane (because it is trimmed for a definite angle of attack) will probably convert the extra airspeed into extra altitude, but you will still wind up at point B with more energy than you would have without the windshear. It makes it look like your engine is putting out more power than it actually is. (Section 16.18.3 discusses how this affects approach and departure.)
Here’s a famous example: An albatross is a huge bird that spends its life flying over the oceans of the world. It rarely needs to flap its wings, but it doesn’t soar in updrafts the way hawks do. Instead, the albatross flies a figure-eight pattern in the shear zone near the surface, climbing into an increasing headwind on the upwind legs and descending into a decreasing tailwind on the downwind legs — gaining energy both ways. I use the term albatross effect for any situation where energy is extracted from windshear. Doing it intentionally is called dynamic soaring.
We can apply the same line of reasoning to the reverse process: Suppose you start out at point C and fly to a point D where you have less headwind or more tailwind. This means you will arrive at point D with less energy than you would have without the windshear.
Think for a moment how you would handle the following scenario:
You are trying to land at Smallville Municipal Airfield, which is rather short and obstructed. The windsock indicates that you have five or ten knots of headwind on the chosen runway. The airplane is acting “funny” on final. That is, even with zero engine power and full flaps you cannot get the airplane to descend steeply enough to stay on the glide slope. Three approaches in a row have ended in go-arounds (which allowed you to carefully check the windsock three times).
Obviously something nasty is happening — something that’s not easy to figure out, especially if you’ve never seen it before, so I might as well tell you:
We are talking about a situation where a tailwind shears to a headwind on final. There is a decreasing tailwind followed by an increasing headwind. Both add energy to your energy budget, via the albatross effect.
This scenario is fairly uncommon yet still common enough to cause trouble. By that I mean that it is sufficiently uncommon that you probably won’t encounter it during training, but eventually you will encounter it. So you’d better think about the situation, figure out how to recognize it, and plan what you’re going to do about it. (You can contrast this scenario with the normal situation, as discussed at the end of this section.)
There are many cues that you should be using to make sure you land at the right spot with the right airspeed. See section 12.7.4 for details. The cues most directly helpful in the present scenario (windshear on final) are
By way of contrast, let’s take another look at the normal approach situation. Ordinarily you expect to see a headwind on final, in particular a decreasing headwind. The surface wind has the same direction as the wind aloft, but its magnitude is reduced due to surface friction.
A decreasing headwind makes the angle of descent steeper in two ways:
By the same logic, you ordinarily expect to see an increasing headwind on a straight-out departure, which helps you climb steeply.
In section 16.18.2 and section 16.18.3 we discussed how you could gain or lose energy due to a windshear. In this section, we return to considering only a steady wind, and discuss what happens if you convert a headwind into a tailwind simply by turning the airplane.
Let’s consider the scenario described in table 16.4.
true airspeed 100 knots initial heading north final heading south time spent turning 1.2 min = .02 hour mass of airplane 1 ton wind speed 20 knots wind direction from the northTable 16.4: Downwind Turn Scenario
Let’s calculate the energy and momentum twice, as shown in table 16.5. In the “balloon” column everything is measured relative to an observer in a balloon (comoving with the air mass), and in the “ground” column everything is measured relative to an observer on the ground.
balloon ground initial momentum 100 80 final momentum -100 -120 change in momentum -200 -200 average N-S force 10000 10000 initial energy 5000 3200 final energy 5000 7200 change in energy required 0 4000 N-S distance during turn 0 .4 energy provided by wind 0 4000Table 16.5: Downwind Turn Analysis
The first four rows of table 16.5 have to do with the momentum balance. The momentum is calculated using the usual formula: mass times velocity. (The units here are rather strange, tons times knots, but it’s OK as long as consistent units are used throughout the calculation.) The North-South component of the average force is just the change in momentum divided by the time. We see that although the initial and final momenta appear different in the two columns, the change in momentum is the same. This upholds Galileo’s principle of relativity: the force required to turn the airplane is independent of the frame of reference.
The last five rows of table 16.5 have to do with the energy balance. The energy is calculated using the usual formula: one half of the mass times velocity squared. According to the ground observer, the airplane needs to gain quite a lot of energy during the turn. You may be wondering where this energy comes from. Obviously it does not come from the airplane’s engine. Actually it gains energy the same way a baseball gains energy when it is struck by a bat. You know that although a ball does not gain any energy when it bounces off a stationary wall, it does gain energy when it bounces off a fast-moving bat. The energy gain is force times distance (counting only distance in the same direction as the force). According to the observer in the balloon, the force of the turn is (at every instant) perpendicular to the direction of the force, so there is no energy gain. Meanwhile, according to the observer on the ground, the wind moves the airplane 0.4 miles in the North-South direction during the turn, and turning the airplane requires a huge force in this direction. This effect — the airplane being batted by the wind — supplies exactly the needed energy. Again, we see that the principle of relativity is upheld: the energy budget works out OK no matter what frame of reference is used.
Note that if you overlooked the bat effect you would fool yourself into thinking that turning downwind caused a huge energy deficit. It doesn’t. Don’t worry about it.
Throughout each flight — and certainly before starting any ground reference maneuvers — you should have in mind a good estimate of the speed and direction of the wind.
There are various ways you can figure this out:
It is a good idea to know the wind before starting a maneuver (rather than trying to figure it out “on the fly”). It really helps to be able to plan the maneuver and anticipate the necessary wind corrections.
It is a good idea to begin ground-reference maneuvers such (as turns around a point) a downwind heading, as shown in figure 16.8, so that your first bank will be your steepest bank. You don’t want to be a position where (late in the maneuver) you must choose between abandoning the effort or using an excessive bank angle.
It really helps to have a precise visual reference for pitch and yaw, as discussed in section 11.7.2.
You can use your finger and/or a mark on the windshield, as illustrated in figure 11.7. If you can’t find a suitable mark on the windshield, you can make one.
The reference should be directly in front of your dominant eye. It is a common mistake to choose a mark on the cowling. Such a mark is below where it should be, and tempts you to use too much rudder when rolling into right turns, and too little rudder when rolling into left turns. It is another common mistake to choose a reference point that is on the centerline of the airplane. Assuming your eye is quite a bit to the left of the centerline, your sight line through this point is very far from being parallel to the axis of the airplane. This tempts you to make diving left turns and climbing right turns.
As you become more experienced, you won’t need to use your finger or an explicit mark on the windshield; you can just imagine where the reference point must be. Just make sure you use a point directly in front of your dominant eye.
You want to take a systematic approach to all maneuvers. I learned the following “maneuver checklist” from John Beck:
Repeat this list to yourself over and over again as you do the maneuver. Chant it aloud if you wish. Doing each thing as you say it not only keeps you from overlooking something, but also gives a nice rhythm to the work.
If you are not proficient in handling the plane at low speeds, you have no business trying to land the plane.
To begin a practice session, go up to a safe altitude and make sure there are no other aircraft nearby. Slow down to a speed, say, 15 knots above the stall speed. Once you are comfortable with this, reduce the speed another 5 knots. Again, once you are comfortable, reduce the speed another 5 knots.
During the maneuver, you should
As discussed in section 7.3 and elsewhere, it would be OK to use the yoke to control altitude if you were on the front side of the power curve and you were willing to accept an airspeed excursion. However, during this slow flight maneuver, you definitely are not on the front side of the power curve and you definitely cannot tolerate airspeed excursions. Therefore you will need to use the yoke (and trim) to control airspeed, and once you’ve got the desired airspeed, you will need to use the throttle to control altitude. (To adjust airspeed at constant altitude, you will need to use the throttle and yoke together, as discussed in section 16.3.)
Remember that the airplane is optimized for cruise flight. During cruise, you can fly straight and level with little or no control force, and you can make gentle turns with little or no use of the rudders, using ailerons alone.
In contrast, during slow flight
Because (as discussed in section 5) there will be very little roll damping, you will need to apply lots of little aileron deflections to maintain wings-level flight, especially in the presence of turbulence.
Make a note of the pitch attitude that corresponds to level flight at minimum controllable airspeed (with and without flaps). Note the pitch attitude of the nose against the forward horizon, and the wingtip against the lateral horizon. This information will come in very handy during landing, as discussed in section 12.11.3.
Practice rocking the wings. Make sure you can bank the plane left or right, with reflexively correct use of ailerons and rudder. Practice making turns to a precise heading.
Practice diving 50 feet. That is, push the nose down a few degrees (not so much that you experience negative G loads), dive for a few seconds, and then pull back and level out. Make a note of how much airspeed you gain by diving 50 feet. This information will come in handy during stall recoveries, as discussed in the next section.
There are many variations on the stall maneuver. You can stall the airplane with or without flaps extended, with or without power, during straight or turning flight, while pulling one or multiple Gs, and during level, climbing, or descending flight.
To keep the discussion simple, let’s first go through one specific scenario, and discuss the possible variations later.
Scenario #1: Start out in level flight at a typical traffic-pattern speed, in the landing configuration (full flaps extended,10 landing gear extended, carb heat on, et cetera). Then reduce the power to idle. As the airplane slows down, pull back on the yoke at a steady rate, cashing in airspeed to pay for drag, maintaining altitude. Maintain constant heading. Maintain coordination. When the airspeed gets low enough, you may observe a sudden, distinct stall. The nose will drop, even though you are pulling back on the yoke. Obviously it is time to begin your stall recovery, as discussed below.
However, it is quite possible you will not always observe a sudden, distinct stall. In particular, if your airplane is loaded so that its center of mass is right at the forward edge of the weight and balance envelope, you may be unable to deflect the elevator enough to cause a stall using the procedure described above.11 At this point you are at a very low airspeed, unable to stall the airplane, and unable maintain altitude by pulling back on the yoke. At this point you should declare an end to the attempted stall and begin your stall recovery procedure. The ability to recognize the low-speed limit of performance in this situation is valuable, and should be practiced, but you should practice full-blown stalls also.
The most elegant way to improve your chances of observing a full-blown stall is to move the center of mass farther aft, using ballast. As described in section 6.1.11, 100 pounds of water stowed securely in the back of the airplane12 should make it a whole lot easier to raise the nose.
Another trick that might increase your control authority is to use a little bit of engine power, a few hundred RPM above idle. On many airplanes this extra propwash flowing over the elevator increases the control authority just enough to permit a quite distinct stall. On other airplanes (including those with high T-tails) this trick doesn’t work at all — the propwash over the wings lowers the stalling speed more than the propwash over the tail improves the control authority.
A third way to provoke a distinct stall is to zoom a little bit. That is, you maintain constant altitude while you slow down most of the way. Keep track of how far back you have pulled back on the yoke. When you have used up most of the available backward motion, use the last inch or so to pull back faster than would be needed to maintain 100% level flight. The airplane will rotate to a more nose-high attitude, climb a few feet, then stall.
Stall recovery, especially for poorly-trained pilots, poses psychological problems. In particular, if you are laboring under the dangerous misconception that the yoke is the up/down control, your instincts will be all wrong: the nose is dropping and the airplane is losing altitude, so you will be tempted to pull back on the yoke. This makes a bad situation much worse.
The correct way to think about the stall is to realize that the shortage of airspeed is your biggest problem. You need to push on the yoke and dive to regain airspeed.
In addition to the airspeed problem, you also have an energy problem. Therefore, while you are pushing on the yoke with one hand, you should be pushing on the throttle with the other hand.
As a further step to improve the energy situation, remove unnecessary drag. On most airplanes with N notches of flaps, the first several notches are somewhat helpful, because they allow you to fly slowly without stalling. The Nth notch, however, typically doesn’t contribute much to lowering the stall speed, and just adds a lot of drag. This would be useful if you were trying to descend, but since we are trying to climb at the moment, you should retract the Nth notch of flaps as early as possible during the stall recovery. If the maneuver began with less than full flaps extended, leave the flaps alone, dive to regain airspeed, and then gradually retract the flaps.
While all this is going on, you should use the rudder and ailerons to keep the wings level and maintain a more-or-less constant heading.
You don’t need to dive very far to regain a reasonable flying speed. According to the law of the roller coaster (as discussed in section 1.2.1), if you start out at 45 knots and dive 45 feet, you will wind up at 55 knots. If you start out at 50 knots and dive 80 feet, you will wind up at 65 knots.13
At the bottom of the dive, perform a nice gentle pull-out. If you pull too rapidly, you put a big G load on the wings, which will cause them to stall at a speed that would otherwise have been just fine.
After you have leveled out at the bottom of the dive, speed up in horizontal flight until you reach best-climb airspeed. Retract any remaining flaps as you speed up. Then climb at VY to a safe altitude.
To summarize: the key elements of stall recovery include
A non-pilot might have thought that it would be hard to stall an airplane with the engine at full power, but in fact it is quite possible, and the accident statistics show that it happens fairly frequently. Therefore let’s consider another scenario:
At a safe altitude in the practice area, set up for a power-off descent in the landing configuration. In particular, let this be a short-field approach, with the airplane trimmed to fly at the lowest practical airspeed. Then apply full power, as if for a go-around. In some airplanes (including the widely-used C-152, C-172, and C-182), and depending on where the center of mass is, this combination of trim, flaps, and power will cause the nose to pitch up quite dramatically. The airplane will climb very steeply and then stall. You don’t need to pull back at all. Indeed, you may want to push a little bit so that the stall won’t be too extreme.
In airplanes with better go-around characteristics (including a C-172 with the flaps retracted) you will need to work a little harder to perform a power-on stall. A possible — but not very stylish — way to perform this maneuver would be to start from cruising flight, add full power, and pull back until you get a stall. This is perhaps worth doing once, but it is not the recommended way of demonstrating a power-on stall, because it results in climbing an unnecessarily long way. That is, it just isn’t logical to apply full power while you are trying to slow down. Therefore the conventional procedure is this: At a safe altitude, reduce power and slow down in level flight to a speed a few knots above the stall. Then add power. (Use partial power the first time, and then use progressively more power as you learn how the airplane behaves.) Then gradually pull back some more.
As the airspeed bleeds off, you will need to apply more and more right14 rudder to maintain coordination (i.e. to compensate for the helical propwash). Coordination is very important, because even a slight slip angle will cause one wing to stall before the other. This could easily result in a spin, and even if you don’t get a full-blown spin, the sudden change in bank angle is pretty unpleasant.
Also, in this high-power low-airspeed situation, you will need to apply steady right aileron (to compensate for the rotational drag of the propeller). Note that (as discussed in section 5.4.2) the roll damping goes to zero at about the same point where the stall occurs, so you will need to intervene rather actively to keep the wings level. The standard advice applies: make sure you use the ailerons and rudder together. Because the airspeed is low, you will need a whole lot of rudder deflection to coordinate with a small amount of aileron deflection, and indeed right near the stall you can quite nicely control the bank angle using the rudder alone. Imagine that the left wing is about to stall. By stepping on the right rudder pedal, you can swing the nose to the right, causing the left wing to speed up and become unstalled. During this maneuver, you might want to lower the nose a tiny bit, so the right wing, which is swinging backwards, doesn’t stall.
If you manage to maintain perfect coordination and perfectly level wings right up to the point of the power-on stall, you can still expect that the airplane will want to yaw and roll to the left just after the stall. There are several factors at work:
Of course, you can anticipate this, and apply additional right rudder as the nose drops. With a little experience, you can arrange that the wings remain level and the nose drops without yawing. If the left wing starts to drop, you can pick it up by using right aileron (coordinated with right rudder) and/or using uncoordinated right rudder to swing the left wing forward.
The recovery from a power-on stall is basically the same: dive to regain airspeed, add power (if you were not already at full power), maintain wings level, reduce drag, and climb back to a safe altitude.
The practical test standard calls for performing power-on stalls with the flaps in the takeoff configuration and gear down (the takeoff configuration) or gear retracted (departure configuration) which simulates a stall happening shortly after takeoff. It is well worth practicing other configurations, too — particularly the approach configuration, which simulates what might happen if you mishandle a go-around.
The stall occurs at a definite angle of attack. This is not quite the same as a definite airspeed, for reasons discussed in section 2.13.5. At any speed below maneuvering speed, if you pull the yoke back far enough, you will stall.15
Suppose you are in a dive, and you want to pull up into a climb, as shown in figure 16.18. If you pull back back on the yoke to the point where you are developing 2 Gs, the stalling speed will be 41% higher than it would be in unaccelerated flight. The rule is: stalling speed goes like the square root of the load factor.
In an aerobatic loop, you are pulling about 4 Gs at the bottom, so the stalling speed is about twice what it would be in ordinary unaccelerated flight. Also, since you might be rapidly approaching the ground at this point, you may be tempted to pull back extra-sharply ... but be careful, because this would be a really inopportune time to stall. Make sure you have plenty of altitude and plenty of airspeed before attempting any high-G maneuvers.
Any stall that happens during the recovery from a previous stall is called a secondary stall. It is not uncommon for secondary stalls to be accelerated stalls.
An even more-common type of accelerated stalls occurs during turns. If you are in a nice steady turn with 45 degrees of bank, the load factor is 1.4, so the stalling speed will be 20% higher than it would be in ordinary one-G flight. Therefore, if you are relying on the airspeed indicator to warn you of an impending stall, you will be fooled.
To a first approximation, the recovery procedure for an accelerated stall is the same as for any other stall: reduce the back pressure, dive far enough to obtain a reasonable airspeed, roll the wings level, add power, reduce drag, and climb back to a safe altitude. One helpful difference is that because you had extra airspeed at the time of the stall, you might not need to dive very far, if at all.
During a turn, if you stall inadvertently, it is common (but not guaranteed) for the outside wing to stall. That’s because if you were paying so little attention to the airspeed that you stalled, you probably weren’t paying attention to coordination, either. That means it was probably a slipping turn, due to the long-tail slip effect (section 8.10). A little slip goes a long way toward determining which wing will stall first.
In contrast, if you stall during a coordinated turn, the inside wing ought to stall first, because it has less airspeed and higher angle of attack, as discussed in section 9.4. Additional complicating factors are discussed in section 16.21.4. See section 16.22 for how to recover from this.
Another thing that makes accelerated stalls a bit more challenging has to do with perception of the stall. Imagine an airplane where the stall doesn’t exhibit a sudden “break”. Then as you approach an ordinary, straight-ahead stall, you have a constant heading and everything looks fairly normal. Nothing is changing much, so any change stands out.
Now contrast that with a stall during a 60-degree bank. The pitch-wise direction of rotation is far from vertical, so any pitch change will move the nose mostly along the horizon, and might not stand out relative to the already-rapid turning motion.
During accelerated-stall practice, a student once complained “I can’t get this thing to stall”. I replied “We’re going down more than 2000 feet per minute. This is stalled enough for me”. The point is, it is possible to be very deeply stalled and not realize it, if you don’t know what to watch for.
Note to instructors: You can demonstrate this using the following technique. First, do an extra-super good job of clearing the area, including the airspace below you. Then ask the student to demonstrate a steep turn, emphasizing the use of outside references. As the turn begins, you surreptitiously reduce engine power. The student may try to maintain altitude by pulling back on the yoke. The student is expecting a steep turns exercise, but it rapidly turns into a stall recognition and recovery exercise.
As discussed in section 12.11.9, it is fairly easy to get into a situation where you have a nose-high pitch attitude, very little airspeed, and very little altitude. In this situation, the usual stall-recognition and stall-recovery techniques will do you no good whatsoever. You need to recover before the airplane stalls, and you need to recover with zero loss of altitude.
Therefore it is a good idea to practice recovering from this situation. The procedure is:
Practice this over and over, until you are confident that you can recover from a pitch excursion with zero loss of altitude.
If you start out steeply banked, and then for any reason the inside wing drops, you could wind up in a knife-edge attitude, or even inverted. This is probably not what you wanted.
You should take the opportunity right now to think about how to recover from such a situation. You want to dive to gain airspeed, but in an unfamiliar bank attitude, banked 90 degrees or more, it might not be 100% obvious how to accomplish this.
You should start by pushing on the yoke. Push to the position that corresponds to zero angle of attack, so there is no load on the wings. (As part of the check-out process, make a point of figuring out where that point is. On most non-aerobatic planes, it is pretty close to all the way foward.) Then just sit there for two or three seconds. The airplane will fly like a dart – like any other object that flies at zero angle of attack. This works whether the airplane is upright, inverted, or anything in between. Gravity ensures the airplane will soon be descending (in addition to whatever horizontal velocity remains). The rudder and horizontal tail guarantee that it fly nose-forward. After the airplane has dived a couple dozen feet, you will have enough airspeed that the ailerons are effective. At this point, use the ailerons to roll upright and level the wings. Then pull out of the dive and proceed with normal stall recovery.
If you ever find yourself upside-down, you might think you have the choice of performing a half-loop or a half-roll. In theory, either one will do, but in practice you should roll, because it is quicker and easier, and puts less stress on the airplane.