3-1

Introduction

This chapter examines the fundamental physical laws 

governing the forces acting on an aircraft in flight, and 

what effect these natural laws and forces have on the 

performance characteristics of aircraft. To control an aircraft, 

be it an airplane, helicopter, glider, or balloon, the pilot 

must understand the principles involved and learn to use or 

counteract these natural forces.

Structure of the Atmosphere

The atmosphere is an envelope of air that surrounds the 

Earth and rests upon its surface. It is as much a part of the 

Earth as the seas or the land, but air differs from land and 

water as it is a mixture of gases. It has mass, weight, and 

indefinite shape.

The atmosphere is composed of 78 percent nitrogen, 21 

percent oxygen, and 1 percent other gases, such as argon 

or helium. Some of these elements are heavier than others. 

The heavier elements, such as oxygen, settle to the surface 

of the Earth, while the lighter elements are lifted up to the 

region of higher altitude. Most of the atmosphere’s oxygen 

is contained below 35,000 feet altitude.

Air, like fluid, is able to flow and change shape when 

subjected to even minute pressures because it lacks strong 

molecular cohesion. For example, gas completely fills any 

container into which it is placed, expanding or contracting 

to adjust its shape to the limits of the container.

Principles of 

Flight

Chapter 3
3-2

Figure 2-1. Standard sea level pressure.

30

25

20

15

10

 5

 0

Inches of 

Mercury

Millibars

1016

847

677

508

339

 170

 0

29.92

Standard

Sea Level

Pressure

Hg 1013

Standard

Sea Level

Pressure

mb

A t m o s p h e r i c P r e s s u r e

Figure 3-1. Standard sea level pressure.

Figure 2-2. Properties of standard atmosphere.

Altitude (ft) Pressure (Hg)

Temperature

(°C) (°F)

Standard Atmosphere

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

10,000

11,000

12,000

13,000

14,000

15,000

16,000

17,000

18,000

19,000

20,000

29.92

28.86

27.82

26.82

25.84

24.89

23.98

23.09

22.22

21.38

20.57

19.79

19.02

18.29

17.57

16.88

16.21

15.56

14.94

14.33

13.74

15.0

13.0

11.0

9.1

7.1

5.1

3.1

1.1

-0.9

-2.8

-4.8

-6.8

-8.8

-10.8

-12.7

-14.7

-16.7

-18.7

-20.7

-22.6

-24.6

59.0

55.4

51.9

48.3

44.7

41.2

37.6

34.0

30.5

26.9

23.3

19.8

16.2

12.6

9.1

5.5

1.9

-1.6

-5.2

-8.8

-12.3

Figure 3-2. Properties of standard atmosphere.

Atmospheric Pressure

Although there are various kinds of pressure, pilots are 

mainly concerned with atmospheric pressure. It is one of 

the basic factors in weather changes, helps to lift an aircraft, 

and actuates some of the important flight instruments. These 

instruments are the altimeter, airspeed indicator, vertical 

speed indicator, and manifold pressure gauge.

Air is very light, but it has mass and is affected by the 

attraction of gravity. Therefore, like any other substance, it 

has weight, and because of its weight, it has force. Since it is 

a fluid substance, this force is exerted equally in all directions, 

and its effect on bodies within the air is called pressure. Under 

standard conditions at sea level, the average pressure exerted 

by the weight of the atmosphere is approximately 14.70 

pounds per square inch (psi) of surface, or 1,013.2 millibars 

(mb). Its thickness is limited; therefore, the higher the altitude, 

the less air there is above. For this reason, the weight of the 

atmosphere at 18,000 feet is one-half what it is at sea level. 

The pressure of the atmosphere varies with time and location. 

Due to the changing atmospheric pressure, a standard 

reference was developed. The standard atmosphere at sea 

level is a surface temperature of 59 °F or 15 °C and a surface 

pressure of 29.92 inches of mercury ("Hg), or 1,013.2 mb. 

[Figure 3-1]

A standard temperature lapse rate is one in which the 

temperature decreases at the rate of approximately 3.5 °F 

or 2 °C per thousand feet up to 36,000 feet which is 

approximately -65 °F or -55 °C. Above this point, the 

temperature is considered constant up to 80,000 feet. A 

standard pressure lapse rate is one in which pressure decreases 

at a rate of approximately 1 "Hg per 1,000 feet of altitude gain 

to 10,000 feet. [Figure 3-2] The International Civil Aviation 

Organization (ICAO) has established this as a worldwide 

standard, and it is often referred to as International Standard 

Atmosphere (ISA) or ICAO Standard Atmosphere. Any 

temperature or pressure that differs from the standard lapse 

rates is considered nonstandard temperature and pressure. 

Since aircraft performance is compared and evaluated with 

respect to the standard atmosphere, all aircraft instruments 

are calibrated for the standard atmosphere. In order to account 

properly for the nonstandard atmosphere, certain related 

terms must be defined.

Pressure Altitude

Pressure altitude is the height above a standard datum plane 

(SDP), which is a theoretical level where the weight of the 

atmosphere is 29.92 "Hg (1,013.2 mb) as measured by a 

barometer. An altimeter is essentially a sensitive barometer 

calibrated to indicate altitude in the standard atmosphere. If 

the altimeter is set for 29.92 "Hg SDP, the altitude indicated 

is the pressure altitude. As atmospheric pressure changes, the 

SDP may be below, at, or above sea level. Pressure altitude 

is important as a basis for determining airplane performance, 

as well as for assigning flight levels to airplanes operating at 

or above 18,000 feet.
3-3

The pressure altitude can be determined by either of two 

methods:

1. Setting the barometric scale of the altimeter to 29.92 

and reading the indicated altitude.

2. Applying a correction factor to the indicated altitude 

according to the reported altimeter setting.

Density Altitude

SDP is a theoretical pressure altitude, but aircraft operate in a 

nonstandard atmosphere and the term density altitude is used 

for correlating aerodynamic performance in the nonstandard 

atmosphere. Density altitude is the vertical distance above sea 

level in the standard atmosphere at which a given density is 

to be found. The density of air has significant effects on the 

aircraft’s performance because as air becomes less dense, 

it reduces:

• Power because the engine takes in less air. 

• Thrust because a propeller is less efficient in thin 

air.

• Lift because the thin air exerts less force on the 

airfoils.

Density altitude is pressure altitude corrected for nonstandard 

temperature. As the density of the air increases (lower density 

altitude), aircraft performance increases and conversely 

as air density decreases (higher density altitude), aircraft 

performance decreases. A decrease in air density means 

a high density altitude; an increase in air density means a 

lower density altitude. Density altitude is used in calculating 

aircraft performance, because under standard atmospheric 

conditions, air at each level in the atmosphere not only has 

a specific density, its pressure altitude and density altitude 

identify the same level.

The computation of density altitude involves consideration 

of pressure (pressure altitude) and temperature. Since aircraft 

performance data at any level is based upon air density 

under standard day conditions, such performance data 

apply to air density levels that may not be identical with 

altimeter indications. Under conditions higher or lower than 

standard, these levels cannot be determined directly from 

the altimeter.

Density altitude is determined by first finding pressure 

altitude, and then correcting this altitude for nonstandard 

temperature variations. Since density varies directly with 

pressure, and inversely with temperature, a given pressure 

altitude may exist for a wide range of temperature by allowing 

the density to vary. However, a known density occurs for any 

one temperature and pressure altitude. The density of the air 

has a pronounced effect on aircraft and engine performance. 

Regardless of the actual altitude at which the aircraft is 

operating, it will perform as though it were operating at an 

altitude equal to the existing density altitude.

Air density is affected by changes in altitude, temperature, 

and humidity. High density altitude refers to thin air while 

low density altitude refers to dense air. The conditions that 

result in a high density altitude are high elevations, low 

atmospheric pressures, high temperatures, high humidity, or 

some combination of these factors. Lower elevations, high 

atmospheric pressure, low temperatures, and low humidity 

are more indicative of low density altitude. 

Effect of Pressure on Density

Since air is a gas, it can be compressed or expanded. When 

air is compressed, a greater amount of air can occupy a given 

volume. Conversely, when pressure on a given volume of air 

is decreased, the air expands and occupies a greater space. 

At a lower pressure, the original column of air contains 

a smaller mass of air. The density is decreased because 

density is directly proportional to pressure. If the pressure is 

doubled, the density is doubled; if the pressure is lowered, the 

density is lowered. This statement is true only at a constant 

temperature.

Effect of Temperature on Density

Increasing the temperature of a substance decreases its density. 

Conversely, decreasing the temperature increases the density. 

Thus, the density of air varies inversely with temperature. 

This statement is true only at a constant pressure. 

In the atmosphere, both temperature and pressure decrease 

with altitude, and have conflicting effects upon density. 

However, the fairly rapid drop in pressure as altitude is 

increased usually has the dominating effect. Hence, pilots 

can expect the density to decrease with altitude.

Effect of Humidity (Moisture) on Density

The preceding paragraphs refer to air that is perfectly dry. In 

reality, it is never completely dry. The small amount of water 

vapor suspended in the atmosphere may be almost negligible 

under certain conditions, but in other conditions humidity 

may become an important factor in the performance of an 

aircraft. Water vapor is lighter than air; consequently, moist 

air is lighter than dry air. Therefore, as the water content 

of the air increases, the air becomes less dense, increasing 

density altitude and decreasing performance. It is lightest or 

least dense when, in a given set of conditions, it contains the 

maximum amount of water vapor. 

Humidity, also called relative humidity, refers to the amount 

of water vapor contained in the atmosphere, and is expressed 
3-4

as a percentage of the maximum amount of water vapor the 

air can hold. This amount varies with temperature. Warm air 

holds more water vapor, while colder air holds less. Perfectly 

dry air that contains no water vapor has a relative humidity 

of zero percent, while saturated air, which cannot hold any 

more water vapor, has a relative humidity of 100 percent. 

Humidity alone is usually not considered an important factor 

in calculating density altitude and aircraft performance, but 

it does contribute. 

As temperature increases, the air can hold greater amounts 

of water vapor. When comparing two separate air masses, 

the first warm and moist (both qualities tending to lighten 

the air) and the second cold and dry (both qualities making 

it heavier), the first must be less dense than the second. 

Pressure, temperature, and humidity have a great influence 

on aircraft performance because of their effect upon density. 

There are no rules of thumb that can be easily conveyed 

but the affect of humidity can be determined using online 

formulas. In the first case, the pressure is needed at the 

altitude for which density altitude is being sought. Using 

Figure 3-2, select the barometric pressure closest to the 

associated altitude. As an example, the pressure at 8,000 feet 

is 22.22 "Hg. Using the National Oceanic and Atmospheric 

Administration (NOAA) website (http://www.srh.noaa.gov/

elp/wxcalc/densityaltitude.html) for density altitude, enter the 

22.22 for 8,000 feet in the station pressure window. Entering 

a temperature of 80° and a dew point of 75°. The result is a 

density altitude of 11,564 feet. With no humidity, the density 

altitude would be almost 500 feet lower.

Another site (http://wahiduddin.net/calc/density_altitude.

htm) provides a more straight forward method of determining 

the effects of humidity on density altitude without using 

additional interpretive charts. In any case, the effects of 

humidity on density altitude include a decrease in overall 

performance in high humidity conditions.

Theories in the Production of Lift

Newton’s Basic Laws of Motion

The formulation of lift has historically been the adaptation 

over the past few centuries of basic physical laws. These 

laws, although seemingly applicable to all aspects of lift, 

do not answer how lift is formulated. In fact, one must 

consider the many airfoils that are symmetrical, yet produce 

significant lift. 

The fundamental physical laws governing the forces acting 

upon an aircraft in flight were adopted from postulated 

theories developed before any human successfully flew 

an aircraft. The use of these physical laws grew out of the 

Scientific Revolution, which began in Europe in the 1600s. 

Driven by the belief the universe operated in a predictable 

manner open to human understanding, many philosophers, 

mathematicians, natural scientists, and inventors spent their 

lives unlocking the secrets of the universe. One of the best 

known was Sir Isaac Newton, who not only formulated the 

law of universal gravitation, but also described the three 

basic laws of motion. 

Newton’s First Law: “Every object persists in its state of rest 

or uniform motion in a straight line unless it is compelled to 

change that state by forces impressed on it.”

This means that nothing starts or stops moving until some 

outside force causes it to do so. An aircraft at rest on the ramp 

remains at rest unless a force strong enough to overcome 

its inertia is applied. Once it is moving, its inertia keeps 

it moving, subject to the various other forces acting on it. 

These forces may add to its motion, slow it down, or change 

its direction.

Newton’s Second Law: “Force is equal to the change in 

momentum per change in time. For a constant mass, force 

equals mass times acceleration.” 

When a body is acted upon by a constant force, its resulting 

acceleration is inversely proportional to the mass of the body 

and is directly proportional to the applied force. This takes 

into account the factors involved in overcoming Newton’s 

First Law. It covers both changes in direction and speed, 

including starting up from rest (positive acceleration) and 

coming to a stop (negative acceleration or deceleration). 

Newton’s Third Law: “For every action, there is an equal 

and opposite reaction.”

In an airplane, the propeller moves and pushes back the 

air; consequently, the air pushes the propeller (and thus the 

airplane) in the opposite direction—forward. In a jet airplane, 

the engine pushes a blast of hot gases backward; the force of 

equal and opposite reaction pushes against the engine and 

forces the airplane forward. 

Magnus Effect

In 1852, the German physicist and chemist, Heinrich Gustav 

Magnus (1802–1870), made experimental studies of the 

aerodynamic forces on spinning spheres and cylinders. 

(The effect had already been mentioned by Newton in 

1672, apparently in regard to spheres or tennis balls). These 

experiments led to the discovery of the Magnus Effect, which 

helps explain the theory of lift. 
3-5

Flow of Air Against a Nonrotating Cylinder

If air flows against a cylinder that is not rotating, the flow of 

air above and below the cylinder is identical and the forces 

are the same. [Figure 3-3A]

A Rotating Cylinder in a Motionless Fluid

In Figure 3-3B, the cylinder is rotated clockwise and observed 

from the side while immersed in a fluid. The rotation of the 

cylinder affects the fluid surrounding the cylinder. The flow 

around the rotating cylinder differs from the flow around a 

stationary cylinder due to resistance caused by two factors: 

viscosity and friction. 

Viscosity

Viscosity is the property of a fluid or semifluid that causes it 

to resist flowing. This resistance to flow is measurable due 

to the molecular tendency of fluids to adhere to each other to 

some extent. High-viscosity fluids resist flow; low-viscosity 

fluids flow easily.

Similar amounts of oil and water poured down two identical 

ramps demonstrate the difference in viscosity. The water seems 

to flow freely while the oil flows much more slowly. (An 

excellent website to demonstrate types of viscosity is found at 

the Cornell University website on viscosity, located at http://

atlas.geo.cornell.edu/education/student/viscosity.html.)

Since molecular resistance to motion underlies viscosity, 

grease is very viscous because its molecules resist flow. 

Hot lava is another example of a viscous fluid. All fluids are 

viscous and have a resistance to flow whether this resistance 

is observed or not. Air is an example of a fluid whose 

viscosity can not be observed.

Since air has viscosity properties, it will resist flow to some 

extent. In the case of the rotating cylinder within an immersed 

fluid (oil, water, or air), the fluid (no matter what it is) resists 

flowing over the cylinder’s surface. 

Friction

Friction is the second factor at work when a fluid flows 

around a rotating cylinder. Friction is the resistance one 

surface or object encounters when moving over another and 

exists between a fluid and the surface over which it flows. 

If identical fluids are poured down the ramp, they flow in the 

same manner and at the same speed. If one ramp’s surface 

is coated with small pebbles, the flow down the two ramps 

differs significantly. The rough surface ramp impedes the 

flow of the fluid due to resistance from the surface (friction). 

It is important to remember that all surfaces, no matter 

how smooth they appear, are not smooth and impede the 

flow of a fluid. Both the surface of a wing and the rotating 

cylinder have a certain roughness, albeit at a microscopic 

level, causing resistance for a fluid to flow. This reduction 

in velocity of the airflow about a surface is caused by skin 

friction or drag.

When passing over a surface, molecules actually adhere 

(stick, cling) to the surface, illustrated by the rotating cylinder 

in a fluid that is not moving. Thus, 

1. In the case of the rotating cylinder, air particles near 

the surface that resist motion have a relative velocity 

near zero. The roughness of the surface impedes their 

motion.

2. Due to the viscosity of the fluid, the molecules on the 

surface entrain, or pull, the surrounding flow above it 

in the direction of rotation due to the adhesion of the 

fluid to itself.

There is also a difference in flow around the rotating cylinder 

and in flow around a nonrotating cylinder. The molecules at 

the surface of the rotating cylinder are not in motion relative 

to the cylinder; they are moving clockwise with the cylinder. 

Due to viscosity, these molecules entrain others above 

them resulting in an increase in fluid flow in the clockwise 

direction. Substituting air for other fluids results in a higher 

velocity of air movement above the cylinder simply because 

more molecules are moving in a clockwise direction.

A Rotating Cylinder in a Moving Fluid

When the cylinder rotates in a fluid that is also moving, 

the result is a higher circulatory flow in the direction of the 

rotating cylinder. [Figure 3-3C] By adding fluid motion, the 

magnitude of the flow increases.

The highest differences of velocity are 90° from the relative 

motion between the cylinder and the airflow. Additionally, 

and as shown in Figure 3-4, at point “A,” a stagnation point 

exists where the air stream impacts (impinges) on the front 

of the airfoil’s surface and splits; some air goes over and 

some under. Another stagnation point exists at “B,” where 

the two airstreams rejoin and resume at identical velocities. 

When viewed from the side, an upwash is created ahead of 

the airfoil and downwash at the rear.

In the case of Figure 3-4, the highest velocity is at the top of 

the airfoil with the lowest velocity at the bottom. Because 

these velocities are associated with an object (in this case, 

an airfoil) they are called local velocities as they do not exist 

outside the lift-producing system, in this case an airfoil. This 

concept can be readily applied to a wing or other lifting 

surface. Because there is a difference of velocity above and 

below the wing, the result is a a higher pressure at the bottom 

of the wing and a lower pressure on the top of the wing.
3-6

A. Flow of air against a nonrotating cylinder B. Rotating cylinder in a fluid

C. Rotating cylinder in a moving fluid D. Leading edge of wing 

E. Leading edge of wing under 1,500x magnification 

A B

A B

Leading edge

Figure 3-3. A illustrates uniform circulation. B illustrates the increased airflow over the top of a rotating cylinder. The airflow speed is 

further increased when the rotating cylinder is in a moving stream of air (C). The air molecules near the surface of an object are slowed 

and almost stationary. D is an example of typical aircraft grade aluminum used in aircraft construction to include wings and leading 

edges of wings as shown in E (left). When magnified at 1,500x (E, right), polished aluminum is visibly rough. This demonstrates why 

airflow is affected by molecular irregularities of the surface.
3-7

Figure 2-3. Air circulation.

B

A

Trailing edge stagnation point

Leading edge stagnation point

Figure 3-4. Air circulation around an airfoil occurs when the front 

stagnation point is below the leading edge and the aft stagnation 

point is beyond the trailing edge.

Figure 2-4. Air pressure decreases in a venturi.

8

4 6

2

0 I0

VELOCITY

8

4 6

2

0 I0

PRESSURE

8

4 6

2

0 I0

VELOCITY

8

4 6

2

0 I0

PRESSURE PR

8

4 6

2

0 I0

VELOCITY

8

4 6

2

0 I0

PRESSURE

Figure 3-5. Air pressure decreases in a venturi tube.

This low-pressure area produces an upward force known as 

the Magnus Effect, the physical phenomenon whereby an 

object’s rotation affects its path through a fluid, to include 

air. Two early aerodynamicists, Martin Kutta and Nicolai 

Joukowski, eventually measured and calculated the forces for 

the lift equation on a rotating cylinder (the Kutta-Joukowski 

theorem).

To summarize the Magnus effect, an airfoil with a positive 

AOA develops air circulation about the upper surface of the 

wing. Its sharp trailing edge forces the rear stagnation point 

to be aft of the trailing edge, while the front stagnation point 

falls below the leading edge. [Figure 3-4]

Bernoulli’s Principle of Differential Pressure

A half-century after Newton formulated his laws, Daniel 

Bernoulli, a Swiss mathematician, explained how the pressure 

of a moving fluid (liquid or gas) varies with its speed of 

motion. Bernoulli’s Principle states that as the velocity of a 

moving fluid (liquid or gas) increases, the pressure within 

the fluid decreases. This principle explains what happens to 

air passing over the curved top of the airplane wing.

A practical application of Bernoulli’s Principle is the venturi 

tube. The venturi tube has an air inlet that narrows to a throat 

(constricted point) and an outlet section that increases in 

diameter toward the rear. The diameter of the outlet is the 

same as that of the inlet. At the throat, the airflow speeds up 

and the pressure decreases; at the outlet, the airflow slows 

and the pressure increases. [Figure 3-5]

Since air is recognized as a body and it is accepted that it must 

follow the above laws, one can begin to see how and why an 

airplane wing develops lift. As the wing moves through the 

air, the flow of air across the curved top surface increases in 

velocity creating a low-pressure area. 

Although Newton, Magnus, Bernoulli, and hundreds of other 

early scientists who studied the physical laws of the universe 

did not have the sophisticated laboratories available today, 

they provided great insight to the contemporary viewpoint 

of how lift is created.

Airfoil Design

An airfoil is a structure designed to obtain reaction upon its 

surface from the air through which it moves or that moves 

past such a structure. Air acts in various ways when submitted 

to different pressures and velocities; but this discussion 

is confined to the parts of an aircraft that a pilot is most 

concerned with in flight—namely, the airfoils designed to 

produce lift. By looking at a typical airfoil profile, such as 

the cross section of a wing, one can see several obvious 

characteristics of design. [Figure 3-6] Notice that there is 

a difference in the curvatures (called cambers) of the upper 

and lower surfaces of the airfoil. The camber of the upper 

surface is more pronounced than that of the lower surface, 

which is usually somewhat flat.

NOTE: The two extremities of the airfoil profile also differ in 

appearance. The end, which faces forward in flight, is called 
3-8

Figure 2-6. Airfoil designs.

Double wedge airfoil

(Supersonic)

Circular arc airfoil

(Supersonic)

Laminar flow airfoil

(Subsonic)

Clark 'Y' airfoil

(Subsonic)

Later airfoil

Early airfoil

Figure 3-7. Airfoil designs.

Camber of upper surface

Camber of lower surface

Trailing edge

Leading edge

Mean camber line

Chord line

Figure 3-6. Typical airfoil section.

the leading edge, and is rounded; the other end, the trailing 

edge, is quite narrow and tapered.

A reference line often used in discussing the airfoil is the chord 

line, a straight line drawn through the profile connecting the 

extremities of the leading and trailing edges. The distance 

from this chord line to the upper and lower surfaces of the 

wing denotes the magnitude of the upper and lower camber at 

any point. Another reference line, drawn from the leading edge 

to the trailing edge, is the mean camber line. This mean line is 

equidistant at all points from the upper and lower surfaces.

An airfoil is constructed in such a way that its shape takes 

advantage of the air’s response to certain physical laws. 

This develops two actions from the air mass: a positive 

pressure lifting action from the air mass below the wing, 

and a negative pressure lifting action from lowered pressure 

above the wing.

As the air stream strikes the relatively flat lower surface of 

a wing or rotor blade when inclined at a small angle to its 

direction of motion, the air is forced to rebound downward, 

causing an upward reaction in positive lift. At the same time, 

the air stream striking the upper curved section of the leading 

edge is deflected upward. An airfoil is shaped to cause an 

action on the air, and forces air downward, which provides 

an equal reaction from the air, forcing the airfoil upward. If 

a wing is constructed in such form that it causes a lift force 

greater than the weight of the aircraft, the aircraft will fly.

If all the lift required were obtained merely from the 

deflection of air by the lower surface of the wing, an aircraft 

would only need a flat wing like a kite. However, the balance 

of the lift needed to support the aircraft comes from the flow 

of air above the wing. Herein lies the key to flight.

It is neither accurate nor useful to assign specific values to 

the percentage of lift generated by the upper surface of an 

airfoil versus that generated by the lower surface. These are 

not constant values and vary, not only with flight conditions, 

but also with different wing designs. 

Different airfoils have different flight characteristics. Many 

thousands of airfoils have been tested in wind tunnels and in 

actual flight, but no one airfoil has been found that satisfies 

every flight requirement. The weight, speed, and purpose 

of each aircraft dictate the shape of its airfoil. The most 

efficient airfoil for producing the greatest lift is one that has 

a concave, or “scooped out” lower surface. As a fixed design, 

this type of airfoil sacrifices too much speed while producing 

lift and is not suitable for high-speed flight. Advancements 

in engineering have made it possible for today’s high-speed 

jets to take advantage of the concave airfoil’s high lift 

characteristics. Leading edge (Kreuger) flaps and trailing 

edge (Fowler) flaps, when extended from the basic wing 

structure, literally change the airfoil shape into the classic 

concave form, thereby generating much greater lift during 

slow flight conditions.

On the other hand, an airfoil that is perfectly streamlined 

and offers little wind resistance sometimes does not have 

enough lifting power to take the airplane off the ground. 

Thus, modern airplanes have airfoils that strike a medium 

between extremes in design. The shape varies according to 

the needs of the airplane for which it is designed. Figure 3-7

shows some of the more common airfoil sections.

Low Pressure Above

In a wind tunnel or in flight, an airfoil is simply a streamlined 

object inserted into a moving stream of air. If the airfoil profile 

were in the shape of a teardrop, the speed and the pressure 

changes of the air passing over the top and bottom would be 

the same on both sides. But if the teardrop shaped airfoil were 

cut in half lengthwise, a form resembling the basic airfoil 
3-9

Figure 2-8. Pressure distribution on an airfoil & CP changes 

with an angle of attack.

High angle of attack

Normal angle of attack

Low angle of attack

at at ck of

Angle

-8°

CP

attack

Angle

of

+10°

CP

attack of Angle

+4°

CP

Figure 3-8. Pressure distribution on an airfoil and CP changes 

with AOA.

travel is very important, since it affects the position of the 

air loads imposed on the wing structure in both low and high 

AOA conditions. An airplane’s aerodynamic balance and 

controllability are governed by changes in the CP.

(wing) section would result. If the airfoil were then inclined so 

the airflow strikes it at an angle (angle of attack (AOA)), the 

air moving over the upper surface would be forced to move 

faster than the air moving along the bottom of the airfoil. This 

increased velocity reduces the pressure above the airfoil.

Applying Bernoulli’s Principle of Pressure, the increase in 

the speed of the air across the top of an airfoil produces a 

drop in pressure. This lowered pressure is a component of 

total lift. The pressure difference between the upper and 

lower surface of a wing alone does not account for the total 

lift force produced.

The downward backward flow from the top surface of an 

airfoil creates a downwash. This downwash meets the flow 

from the bottom of the airfoil at the trailing edge. Applying 

Newton’s third law, the reaction of this downward backward 

flow results in an upward forward force on the airfoil.

High Pressure Below

A certain amount of lift is generated by pressure conditions 

underneath the airfoil. Because of the manner in which air 

flows underneath the airfoil, a positive pressure results, 

particularly at higher angles of attack. But there is another 

aspect to this airflow that must be considered. At a point 

close to the leading edge, the airflow is virtually stopped 

(stagnation point) and then gradually increases speed. At 

some point near the trailing edge, it again reaches a velocity 

equal to that on the upper surface. In conformance with 

Bernoulli’s principle, where the airflow was slowed beneath 

the airfoil, a positive upward pressure was created i.e., as the 

fluid speed decreases, the pressure must increase. Since the 

pressure differential between the upper and lower surface 

of the airfoil increases, total lift increases. Both Bernoulli’s 

Principle and Newton’s Laws are in operation whenever lift 

is being generated by an airfoil.

Pressure Distribution

From experiments conducted on wind tunnel models and on 

full size airplanes, it has been determined that as air flows 

along the surface of a wing at different angles of attack, there 

are regions along the surface where the pressure is negative, 

or less than atmospheric, and regions where the pressure is 

positive, or greater than atmospheric. This negative pressure 

on the upper surface creates a relatively larger force on the 

wing than is caused by the positive pressure resulting from 

the air striking the lower wing surface. Figure 3-8 shows the 

pressure distribution along an airfoil at three different angles 

of attack. The average of the pressure variation for any given 

angle of attack is referred to as the center of pressure (CP). 

Aerodynamic force acts through this CP. At high angles of 

attack, the CP moves forward, while at low angles of attack 

the CP moves aft. In the design of wing structures, this CP 
3-10

vortex

Tip

Figure 3-9. Tip vortex.

Airfoil Behavior

Although specific examples can be cited in which each of 

the principles predict and contribute to the formation of lift, 

lift is a complex subject. The production of lift is much more 

complex than a simple differential pressure between upper 

and lower airfoil surfaces. In fact, many lifting airfoils do 

not have an upper surface longer than the bottom, as in the 

case of symmetrical airfoils. These are seen in high-speed 

aircraft having symmetrical wings, or on symmetrical rotor 

blades for many helicopters whose upper and lower surfaces 

are identical. In both examples, the relationship of the airfoil 

with the oncoming airstream (angle) is all that is different. A 

paper airplane, which is simply a flat plate, has a bottom and 

top exactly the same shape and length. Yet these airfoils do 

produce lift, and “flow turning” is partly (or fully) responsible 

for creating lift.

As an airfoil moves through air, the airfoil is inclined 

against the airflow, producing a different flow caused by the 

airfoil’s relationship to the oncoming air. Think of a hand 

being placed outside the car window at a high speed. If the 

hand is inclined in one direction or another, the hand will 

move upward or downward. This is caused by deflection, 

which in turn causes the air to turn about the object within 

the air stream. As a result of this change, the velocity about 

the object changes in both magnitude and direction, in turn 

resulting in a measurable velocity force and direction.

A Third Dimension

To this point the discussion has centered on the flow across 

the upper and lower surfaces of an airfoil. While most of the 

lift is produced by these two dimensions, a third dimension, 

the tip of the airfoil also has an aerodynamic effect. The highpressure area on the bottom of an airfoil pushes around the tip 

to the low-pressure area on the top. [Figure 3-9] This action 

creates a rotating flow called a tip vortex. The vortex flows 

behind the airfoil creating a downwash that extends back to the 

trailing edge of the airfoil. This downwash results in an overall 

reduction in lift for the affected portion of the airfoil. 

Manufacturers have developed different methods to 

counteract this action. Winglets can be added to the tip of 

an airfoil to reduce this flow. The winglets act as a dam 

preventing the vortex from forming. Winglets can be on the 

top or bottom of the airfoil. Another method of countering 

the flow is to taper the airfoil tip, reducing the pressure 

differential and smoothing the airflow around the tip. 

Chapter Summary

Modern general aviation aircraft have what may be considered 

high performance characteristics. Therefore, it is increasingly 

necessary that pilots appreciate and understand the principles 

upon which the art of flying is based. For additional 

information on the principles discussed in this chapter, visit 

the National Aeronautics and Space Administration (NASA) 

Beginner’s Guide to Aerodynamics at http://www.grc.nasa.

gov/WWW/K-12/airplane/index.html.