Student - Private Pilot Ground School
—by Rod Machado
Okay, here's the deal. I'm going to stick you in an airplane that's capable
of doing 120 knots—twice as fast as most cars on the freeway below—and I
have only one request: I want you to fly as slow as you can. Sounds
reasonable, right? Not really. This is like asking an Indy 500 racecar driver
not to take his machine out of first gear. There is, however, a good reason
for flying slowly.
The practice of slow flight is the proving ground in which you prepare for
aviation's biggest event: landing. After all, you don't want to land at cruise
speeds, because airplanes weren't designed to maneuver on the surface at high
velocities. You don't want to burn the tires off the rims, do you? (Just
kidding, but it's not far from the truth.) In general, the slower you are upon
touchdown, the easier it is to control the airplane on the runway.
Additionally, airplanes can't fly too slowly, or they'll cease flying and
start falling (this is called stalling, but it has nothing to do with the
engine stopping, as you'll later learn). That's why I want you to feel
comfortable operating at slower speeds so you'll know where the dangers are.
And, as you'll eventually discover, it's sometimes necessary to follow slower
airplanes. You need to know how to adjust your airspeed to prevent chewing up
their tail feathers. These are only a few of the reasons we practice slow
flight. It's an important maneuver. Let's get started by discussing how
airplane wings develop lift.
The Wing and Its Things
In ground school many years ago, my instructor asked me about the origin
and definition of the word "wing." I replied, "Ma'am, I think
it means 'the arm of a bird'." She mumbled something about why many
animals eat their young at birth and then went to the dictionary to look up
the definition. Wing was defined as "a moveable, paired appendage for
flying." She looked at me and said, "Well, what does that sound like
to you?" I said, "Well, ma'am, that sounds like the arm of a bird to
me." We agreed to disagree, even though I was right.
The wing has several distinct parts: the upper cambered surface, lowered
cambered surface, leading edge, trailing edge, and chord line (Figure 4-1).
Figure 4-1 The Five Components of a Wing.
Notice that the upper cambered (meaning curved) surface seems to have a
greater curve to it than the lower cambered surface. This isn't accidental. In
fact, this is so important that we'll talk about it in detail shortly.
Perhaps the only term whose definition isn't intuitively obvious is the chord
line. The chord line is an imaginary line connecting the wing's leading edge
with its trailing edge. Believe me, there is no line inside the wing that looks
like this. It's only imaginary, just like the arrows showing the four forces in
Lesson 1. When the shoe salesperson points to your foot and says, "Your toe
is here," you want to respond by saying, "Thanks, I've been looking
for that." In reality, he or she is pointing out the position of something
not visually obvious. The chord line does something similar. Given the wing's
curved surfaces, it's difficult to tell which way the wing points. Since
engineers don't like uncertainty, they agreed that the chord line will represent
the general shape of a wing.
How the Wing Works
To understand lift, you must visualize how the wing attacks the air.
Aeronautical engineers talk about the wing contacting, or attacking, the air at
a specific angle. This occurs in much the same way a pit bull attacks a
mailman—mouth first. What part of the wing does the attacking? Is it the
leading edge? Is it the trailing edge? Or is it the bottom of the wing? This is
where the definition of chord line becomes useful.
Because wings come in variable sizes and shapes (just like pilots), it is
sometimes difficult to determine exactly how and where the wind strikes the
wing. Fortunately, the chord line substitutes as a general reference for the
shape of the wing. If I say that the wind blows onto the wing at an 18-degree
angle, I'm saying that the angle between the wind and the chord line is 18
degrees (Figure 4-2).
Figure 4-2 Angle of Attack. The angle of attack is the
angle between the chord line and the relative wind
(this is the wind that is blowing on the wing).
This distinction, although seemingly trite, is as important to an engineer as
tightly stitched pant seams are to a matador.
Only one more definition needs be absorbed before the secrets of lift are
revealed. That term is called the relative wind (which is not a reference to an
uncle who tells long stories without inhaling).
Movement of an airplane generates wind over the wing. This wind is called the
relative wind because it is relative to (or results from) motion. For instance,
in Figure 4-3, no matter which way the jogger runs, he feels wind in his face
that's relative (opposite and equal) to his motion.
Figure 4-3 Relative Wind.
The relative wind is wind resulting from an object's motion.
Despite the actual wind blowing from behind, the jogger
feels wind on his face as a result of his running motion.
Relative wind is relative (opposite and equal) to the
movement of an object.
Relative wind is movement-generated wind that's equal to and opposite to the
motion of the airplane. To illustrate this point, stick your hand out the window
of a moving automobile (keep all other body parts inside, please). You'll feel
wind blowing opposite the motion of the car. Drive a car backwards on the
freeway, and you will feel wind and hear a lot of horns blowing from directly
behind you (you'll also attract the police).
Move the airplane forward, as shown by Airplane A in Figure 4-4, and wind
blows on its nose.
Figure 4-4 All illustrations show the relative wind
is opposite and equal to the motion of the airplane.
As a airplane climbs or descends (that is, moves up or down the imaginary
hill we discussed in Lesson 2), relative wind still blows on its nose (Airplanes
B and C). But if an airplane drops straight down, without a change in pitch, the
relative wind blows on its belly (Airplane D). As far as Airplane D is
concerned, the wind is blowing on its belly despite the level attitude. As for
the passengers, they're probably underneath the front seat in the fetal position
making spiritual transmissions that don't require a radio. Try not to scare your
passengers. It isn't nice, and they don't like it).
The following point is so important, I want you to put one finger in your
ear. Go ahead, do it before reading any further! I want you to do this because I
don't want this information to go in one ear and out the other. The important
principle to remember is that relative wind is independent of which way the
airplane's nose is pointed. Relative wind is opposite in direction and equal to
the airplane's velocity. Let's see how the wing actually attacks the wind to
Attacking the Air
Hunting is a sport to some people. It's also a sport where your opponent
doesn't know it's a participant. Attacking an animal means that the hunter must
point his weapon precisely at the prey. The hunter looks though the gun sight
and sees the path of the bullet. An airplane is unlike a gun (and a car) in that
its vertical climb path is different from its incline (the direction it points
Remember that 750-foot tower off the end of the runway? On takeoff, if you
point your airplane slightly above the top of that obstacle (like a rifle
sight), it's unlikely that you're going to clear it. In fact, the only thing
being cleared is the area—as the firemen try to talk you down from the side of
that tower. Remember, airplanes with limited thrust have shallower climb
paths—unlike some fighter jets.
The most important principle to understand here (put that finger back in the
ear) is that the nose (therefore the wing) can be pointed on an incline that's
different from the actual climb path. An angle exists between the amount the
wing is inclined and its climb path (you'll soon see why). Remembering that the
relative wind is always equal and opposite to the flight path, it's more precise
to say that an angle exists between the chord line and the relative wind. This
angle is known as the angle of attack (Figure 4-5).
Figure 4-5 The Angle of Attack.
Figure 4-6 shows the wing (chord line) of Airplane A making a 5-degree angle
to the relative wind.
Figure 4-6 Angle of Attack. How Lift Develops.
A more common way of saying this is that the wing's angle of attack is 5
degrees. Airplanes B, C, and D show increasing angles of attack of 10 degrees,
30 degrees, and 45 degrees, respectively. The greater the difference between the
wing and the relative wind, the greater the angle of attack. And, as you're
about to see, the wing's lift is directly associated with its angle of attack.
How Lift Develops
The wing is the ultimate air slicer. As powerful as any Samurai sword or
karate chop, it's a precision device for slicing air in a specific way. Wings
are expressly built to plow through air molecules, separating them either above
or below, while offering little resistance in the horizontal direction. Any
horizontal resistance slows the wing down. This horizontal resistance is called
drag, and it's definitely a case of less being better.
Figure 4-7 shows how the airfoil (a fancy name for a wing) splits the wind
when it's at a 10-degree angle of attack.
Figure 4-7 Airflow Over and under a Wing.
Lift from an airfoil is produced by air flowing over and under the wing
Airflow strikes the leading edge of the wing forcing some air over (and some
under) the airfoil. Both the air flowing over and the air flowing under the wing
are responsible for generating lift. Let's first examine how the airflow
striking the bottom of the wing produces some of the total lift that is
Impact vs. Pressure Lift
Sticking your hand out the window of a moving automobile does two things: it
demonstrates how a relatively flat surface develops lift, and it signals a left
turn. Figure 4-8 shows how wind is deflected downward when it strikes your hand.
Figure 4-8 impact Lift. Airflow striking the hand is
downward. This imparts an equal and opposite upward force
to the hand. High pressure is created on the bottom of
the hand by impacting air molecules.
According to Sir Isaac Newton, who knew about such things, for every action
there is an equal and opposite reaction. Wind deflected downward by the airfoil
creates an upward (opposite) movement of the wing. This upward movement is
caused by the impact energy of billions of tiny air molecules striking the
underside of the wing. Also, higher pressure on the bottom surface of the wing
results from this molecular impact. The wing moves upward as if it were being
pushed from below.
This type of lift is known as barn door lift, or impact lift. It generally
contributes only a small portion of the total lift produced by the wings, which
means that man and woman do not fly by barn-door lift alone. If we could, it
would mean eccentric people would report flying barn doors instead of UFOs.
A more subtle and powerful form of lift occurs from curved airflow over the
top of the wing.
Bending the Wind with the Wing
The Japanese invented the art of paper bending and called it origami. They
then experimented with people-bending and called it judo. This art was not
perfected, however, until the airlines adopted the practice, which is referred
to as "flying coach."
Airliners (indeed all airplanes) bend something else—they use their wings
to bend the wind. Wind bending did not sound sophisticated enough to explain why
airplanes fly, so it was given a fancy Greek title. We call wind bending
aerodynamics. Simply stated, the wing is a precision device for bending or
curving the wind downward.
But how does bending the wind over the wing create lift? Let's find out.
Figure 4-9 shows a cross section of an airfoil.
Figure 4-9 Airflow Above and Below the Wing at a Small
Angle of Attack.
At low angles of attack, the air above the wing
is curved while the air below the wing is relatively straight.
Examine its shape carefully. At small angles of attack, air flowing above the
wing is bent, or curved, with great precision as it follows the upper cambered
surface. A rather straight surface on the bottom of the wing leaves the air
underneath relatively unbent. Bending, or curving, the wind above the wing
forces air to travel a greater distance than the straighter airflow below. If
the wind above is to reach the trailing edge at nearly the same time as the wind
below (science and experiments say that it does), it must speed up to cover the
For example, assume you are walking your pit bull terrier (named Bob) on a
leash. You are on the sidewalk, and Bob is walking in the gutter (Figure 4-10).
Figure 4-10 Different Distances in Curvature Above Than
the Car (Wing too).
Bob encounters a parked Volkswagen and decides to walk over the car rather
than around it (remember, he's a pit bull... they're stubborn). Obviously, the
distance over the car is greater than the distance you will travel on the
sidewalk. In order for Bob to avoid being choked by the leash, he will have to
speed up slightly as he covers this greater distance.
Do you notice the resemblance of the VW's profile to a wing? It's curved on
top and rather straight on the bottom. As air flows over the wing, it curves and
Most wings are designed with their upper surface curved and their lower
surface relatively straight. Because of the wing's shape, even at a small angle
of attack, a cambered wing still adds a slight curve and acceleration to the
wind. This produces the lift you learn to love, particularly if you think an
airplane should fly.
Angle of Attack and the Generation of Lift
During takeoff on a commercial airliner, have you ever noticed that the pilot
always raises the nose slightly to begin the climb after attaining a minimum
forward speed? This is called rotation, and it isn't something that's done to
the airplane's tires.
As the airplane accelerates for takeoff, it eventually reaches a sufficient
speed to begin flying. At this relatively slow speed, however, the wing's
engineered curve isn't capable of curving, or deflecting, enough air downward to
produce the necessary lift for flight. This is why the airplane doesn't hop off
the ground like a grasshopper that just landed on a hot barbecue. The pilot must
do something extra to add an additional curve to the wind. Raising the nose
slightly increases the angle of attack. This forces the air to undergo an
additional curve greater than that which the engineered shape of the airfoil can
produce. Figure 4-11 depicts this process.
Figure 4-11 Two Forms of Lift.
A-Lift from low pressure. At large angles of attack, the
airflow is forced to curve beyond the engineered shape
of the wing. B-Impact lift on the bottom of the wing
increases at a high angle of attack.
With this additional curvature, air travels a greater distance, its speed
increases, pressure lowers on top of the airfoil, and sufficient lift to begin
flying is produced at a slower airspeed (thanks for the lift, Bernoulli!).
Greater impact lift results from increased exposure of the wing's lower surface
to the relative wind. The result is that an increasing angle of attack permits
the airplane to produce the necessary lift for flight at a slower airspeed.
Now you know how airfoils generate the required lift at slower airspeeds. You
also know why airplanes taking off or landing at slower speeds seem to have a
rather nose-high attitude. But what happens at higher airspeeds? Have you
noticed that in cruise flight at cruise airspeeds, airplanes fly at near-level
Figure 4-12 shows an airplane at several different angles of attack.
Figure 4-12 Relationship Between Angle of Attack and
With speed variations in level flight, the relationship between the
angle of attack and airspeed is clearly shown.
With increasing airspeed, the airplane requires a smaller angle
of attack to remain airborne. As the airplane's airspeed
decreases, a larger angle of attack in necessary.
At higher speeds, airplanes can fly at lower angles of attack because the
wing's shape generates sufficient lift. Slow the airplane, and the wing must
artificially bend the wind by increasing its angle of attack.
An intimate and sizzling relationship exists between angle of attack and
lift. If lift and angle of attack were Rhett Butler and Scarlett O'Hara, Atlanta
wouldn't be the only thing on fire. At small angles of attack (such as during
cruise flight), the engineered shape of the airfoil generates sufficient lift
for flight as long the airspeed is high. The impact of air underneath the wing
doesn't play as big a role in lift development at higher (cruise) speeds because
less of the wing's underside is exposed to the wind.
In summary, the slower an airplane moves, the greater the angle of attack
needed for flight. There is, however, such a thing as too much of a good thing.
Bend the air too much, and instead of flowing smoothly over the wing and
creating lift, it bubbles and burbles and pretty much fails to be uplifting. We
call this condition a stall, and this will be covered in a future class.
Now it's time to talk about the details of entering and leaving slow flight
as it's done in the air.
Slow Flight in Action
In straight-and-level flight at cruise power, the airplane moves through the
air at approximately 110 knots. Our pitch attitude at this airspeed is
approximately 4 degrees nose up, as seen on the attitude indicator. From this
condition, let's discuss how you'll enter slow flight. Let's make this realistic
by supposing that you're preparing to land and must slow the airplane to 75
knots to keep from weed-whacking the airplane ahead of you.
When entering slow flight, your airplane should look something like the one
in Figure 4-13.
Leaving Slow Flight
Let's suppose we're following an airplane and the tower controller wants you
to increase your speed from 75 to 85 knots. How do you accomplish this? Simply
reverse the process used to enter slow flight.
When exiting slow flight, your airplane should look something like the one in
Maintaining Altitude at Cruising Speed
So far, you've examined how to fly the airplane at several different speeds.
At this stage of your training, you should be aware that the throttle is best
used to maintain your altitude or rate of descent. The airspeed is maintained by
adjusting the airplane's pitch attitude. But what about when you're not trying
to maintain a specific speed, such as in cruise flight? After all, in cruise
flight, you don't maintain your altitude using throttle adjustments, do you? No,
you don't. Here's why.
In cruise flight, you typically set the throttle to a power setting that
won't harm the engine (for simplicity in teaching, we'll assume that the
application of full throttle in any of our simulations won't hurt the engine).
Then, for the most part, you leave the throttle alone. You're not necessarily
concerned with maintaining a specific airspeed in cruise flight. In this case,
power is fixed at a specific setting, and you make slight adjustments in pitch
attitude to hold or modify your altitude. In slow flight, however, you'll use
power to control your altitude and pitch (joystick) to control your airspeed.
This might be the opposite of what you'd guess. As you'll soon see, however,
this is the basic technique I want you to use when landing an airplane.
You're on Your Own
Now I want you to proceed to the interactive lesson and practice slow flight
in the airplane. Your ultimate objective is to maintain altitude and heading
while trying to fly at various slow flight-speeds. At first, you'll find it a
bit tricky to maintain airspeed and altitude while flying a precise heading. So
establish your priorities as follows: First, adjust pitch to give you the
airspeed you want. Then, while maintaining that pitch attitude, make small
adjustments in power to hold your altitude.
If you feel lucky, try slow flight in turns. But be careful in those turns.
Remember from Lesson 2 that a slight increase in pitch attitude was necessary to
maintain altitude in a turn. Now that you know how to use the throttle, you'll
want to add a little bit of power, if necessary, to help maintain altitude in a
turn. The steeper the turn, the more power you'll need. Be generous with your
use of trim in slow flight (although it's best not to trim in turns since turns
are transient conditions). This prevents the airplane slinking away from the
pitch attitude you want if your attention is diverted from the instrument panel.
Above all, have fun!
Click the Fly This Lesson Now link to practice what you've just
THIS LESSON IS AVAILABLE IN THE ACTIVE FLIGHT