Aircraft Aerodynamics
Introduction
These topics that are directly related Aerodynamics of
aircraft. By studying aerodynamics, a person becomes familiar with the
fundamentals of aircraft flight.
Basic Aerodynamics
Aerodynamics is the study of the dynamics of gases,
the interaction between a moving object and the atmosphere being of primary
interest for this handbook. The movement of an object and its reaction to the
air flow around it can be seen when watching water passing the bow of a ship.
The major difference between water and air is that air is compressible and
water is incompressible. The action of the airflow over a body is a large part
of the study of aerodynamics. Some common aircraft terms, such as rudder, hull,
water line, and keel beam, were borrowed from nautical terms. Many textbooks
have been written about the aerodynamics of aircraft flight. It is not
necessary for an airframe and power plant (A&P) mechanic to be as
knowledgeable as an aeronautical engineer about aerodynamics. The mechanic must
be able to understand the relationships between how an aircraft performs in
flight and its reaction to the forces acting on its structural parts.
Understanding why aircraft are designed with particular types of primary and
secondary control systems and why the surfaces must be aerodynamically smooth
becomes essential when maintaining today’s complex aircraft. The theory of
flight should be described in terms of the laws of flight because what happens
to an aircraft when it flies is not based upon assumptions, but upon a series
of facts. Aerodynamics is a study of laws which have been proven to be the
physical reasons why an airplane flies. The term aerodynamics is derived from
the combination of two Greek words: “aero,” meaning air, and “dyne,” meaning
force of power. Thus, when “aero” joins “dynamics” the result is “aerodynamics”—the
study of objects in motion through the air and the forces that produce or
change such motion. Aerodynamically, an aircraft can be defined as an object
traveling through space that is affected by the changes in atmospheric
conditions. To state it another way, aerodynamics covers the relationships
between the aircraft, relative wind, and atmosphere.
The Atmosphere
Before examining the fundamental laws of flight,
several basic facts must be considered, namely that an aircraft operates in the
air. Therefore, those properties of air that affect the control and performance
of an aircraft must be understood. The air in the earth’s atmosphere is
composed mostly of nitrogen and oxygen. Air is considered a fluid because it
fits the definition of a substance that has the ability to flow or
Assume the
shape of the container in which it is enclosed. If the container is heated,
pressure increases; if cooled, the pressure decreases. The weight of air is
heaviest at sea level Where it has been compressed by all of the air above.
This compression of air is called atmospheric pressure.
Pressure
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| Fig 1. Barometer used to measure atmospheric pressure. |
Atmospheric
pressure is usually defined as the force exerted against the earth’s surface by
the weight of the air above that surface. Weight is force applied to an area
that results in pressure. Force (F) equals area (A) times pressure (P), or F =
AP. Therefore, to find the amount of pressure, divide area into force (P =
F/A). A column of air (one square inch) extending from sea level to the top of
the atmosphere weighs approximately 14.7 pounds; therefore, atmospheric
pressure is stated in pounds per square inch (psi). Thus, atmospheric pressure
at sea level is 14.7 psi. Atmospheric pressure is measured with an instrument
called a barometer, composed of mercury in a tube that records atmospheric
pressure in inches of mercury ("Hg). [Figure 1] The standard measurement in
aviation altimeters and U.S. weather reports has been "Hg. However,
world-wide weather maps and some non-U.S. manufactured aircraft instruments
indicate pressure in millibars (mb), a metric unit.
At sea level, when the average atmospheric pressure is
14.7 psi, the barometric pressure is 29.92 "Hg, and the metric measurement
is 1013.25 mb. An important consideration is that atmospheric pressure varies
with altitude. As an aircraft ascends, atmospheric pressure drops, oxygen
content of the air decreases, and temperature drops. The changes in altitude
affect an aircraft’s performance in such areas as lift and engine horsepower.
The effects of temperature, altitude, and density of air on aircraft
performance are covered in the following paragraphs.
Density
Density is weight per unit of volume. Since air is a
mixture of gases, it can be compressed. If the air in one container is under
half as much pressure as an equal amount of air in an identical container, the
air under the greater pressure weighs twice as much as that in the container
under lower pressure. The air under greater pressure is twice as dense as that
in the other container. For the equal weight of air, that which is under the
greater pressure occupies only half the volume of that under half the pressure.
The density of gases is governed by the following rules:
1. Density varies in direct proportion with the
pressure.
2. Density varies inversely with the temperature.
Thus, air at high altitudes is less dense than air at
low altitudes, and a mass of hot air is less dense than a mass of cool air.
Changes in density affect the aerodynamic performance of aircraft with the same
horsepower. An aircraft can fly faster at a high altitude where the density is
low than at a low altitude Where the density is greater. This is because air
offers less resistance to the aircraft when it contains a smaller number of air
particles per unit of volume.
Humidity
Humidity is the amount of water vapor in the air. The
maximum amount of water vapor that air can hold varies with the temperature.
The higher the temperature of the air, the more water vapor it can absorb.
1. Absolute humidity is the weight of water vapor in a
unit volume of air.
2. Relative humidity is the ratio, in percent, of the
moisture actually in the air to the moisture it would hold if it were saturated
at the same temperature and pressure.
Bernoulli’s
Principle
Bernoulli’s principle states that when a fluid (air) flowing through
a tube reaches a constriction, or narrowing, of the tube, the speed of the
fluid flowing through that constriction is increased and its pressure is
decreased. The cambered (curved) surface of an airfoil (wing) affects the
airflow exactly as a constriction in tube affects airflow. [Figure 2] Diagram
A of Figure 2 illustrates the effect of air passing through
a constriction in a tube. In B, air is flowing past a cambered surface, such as
an airfoil, and the effect is similar to that of air passing through a
restriction
 |
Fig 2. Bernoulli’s Principle. |
`As the air
flows over the upper surface of an airfoil, its velocity increases and its
pressure decreases; an area of low pressure is formed. There is an area of
greater pressure on the lower surface of the airfoil, and this greater pressure
tends to move the wing upward. The difference in pressure between the upper and
lower surfaces of the wing is called lift. Three-fourths of the total lift of
an airfoil is the result of the decrease in pressure over the upper surface.
The impact of air on the under surface of an airfoil produces the other
one-fourth of the total lift.
Airfoil
An airfoil is a surface designed to obtain lift from
the air through which it moves. Thus, it can be stated that any part of the
aircraft that converts air resistance into lift is an airfoil. The profile of a
conventional wing is an excellent example of an airfoil. [Figure 3] Notice that the top surface of the wing profile has greater
curvature than the lower surface. The difference in curvature of the upper and
lower surfaces of the wing builds up the lift force. Air flowing over the top
surface of the wing must reach the trailing edge of the wing in the same amount
of time as the air flowing under the wing. To do this, the air passing over the
top surface moves at a greater velocity than the air passing below the wing
because of the greater distance it must travel along the top surface.
 |
| Fig 3 Airflow over wing section |
This increased velocity, according to Bernoulli’s
Principle, means a corresponding decrease in pressure on the surface. Thus, a
pressure differential is created between the upper and lower surfaces of the
wing, forcing the wing upward in the direction of the lower pressure. Within
limits, lift can be increased by increasing the angle of attack (AOA), wing
area, velocity, density of the air, or by changing the shape of the airfoil.
When the force of lift on an aircraft’s wing equals the force of gravity, the
aircraft maintains level flight.
Shape of the Airfoil
Individual airfoil section properties differ from
those properties of the wing or aircraft as a whole because of the effect of
the wing planform. A wing may have various airfoil sections from root to tip,
with taper, twist, and sweepback. The resulting aerodynamic properties of the wing
are determined by the action of each section along the span. The shape of the
airfoil determines the amount of turbulence or skin friction that it produces,
consequently affecting the efficiency of the wing. Turbulence and skin friction
are controlled mainly by the fineness ratio, which is defined as the ratio of
the chord of the airfoil to the maximum thickness. If the wing has a high
fineness ratio, it is a very thin wing.
A thick wing has a low fineness ratio. A wing with a
high fineness ratio produces a large amount of skin friction. A wing with a low
fineness ratio produces a large amount of turbulence. The best wing is a
compromise between these two extremes to hold both turbulence and skin friction
to a minimum. The efficiency of a wing is measured in terms of the lift to drag
ratio (L/D). This ratio varies with the AOA but reaches a definite maximum
value for a particular AOA. At this angle, the wing has reached its maximum
efficiency. The shape of the airfoil is the factor that determines the AOA at
which the wing is most efficient; it also determines the degree of efficiency.
Research has shown that the most efficient airfoils for general use have the
maximum thickness occurring about one-third of the way back from the leading
edge of the wing. High-lift wings and high-lift devices for wings have been
developed by shaping the airfoils to produce the desired effect. The amount of
lift produced by an airfoil increases with an increase in wing camber. Camber
refers to the curvature of an airfoil above and below the chord line surface.
Upper camber refers to the upper surface, lower camber to the lower surface,
and mean camber to the mean line of the section. Camber is positive when
departure from the chord line is outward and negative when it is inward. Thus,
high-lift wings have a large positive camber on the upper surface and a
slightly negative camber on the lower surface. Wing flaps cause an ordinary
wing to approximate this same condition by increasing the upper camber and by
creating a negative lower camber.
It is also
known that the larger the wingspan, as compared to the chord, the greater the
lift obtained. This comparison is called aspect ratio. The higher the aspect
ratio, the greater the lift. In spite of the benefits from an increase in aspect
ratio, it was found that definite limitations were defined by structural and
drag considerations.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 aircraft off the ground. Thus, modern aircraft have airfoils which
strike a medium between extremes, the shape depending on the purposes of the
aircraft for which it is designed
Angle
of Incidence
The acute angle the wing chord makes with the longitudinal axis of
the aircraft is called the angle of incidence, or the angle of wing setting. [Figure 4] The
angle of incidence in most cases is a fixed, built in angle. When the leading
edge of the wing is higher than the trailing edge, the angle of incidence is said
to be positive. The angle of incidence is negative when the leading edge is
lower than the trailing edge of the wing.
 |
| Figure 4. Angle of incidence. |
Angle
of Attack (AOA)
Before beginning the discussion on AOA and its effect on airfoils,
first consider the terms chord and center of pressure (CP) as illustrated in Figure 5. The chord of an airfoil or
wing section is an imaginary
Straight line that passes through the section from the leading edge
to the trailing edge, as shown in Figure 5. The chord line provides one side of an
angle that ultimately forms the AOA. The other side of the angle is formed by a
line indicating the direction of the relative airstream. Thus, AOA is defined
as the angle between the chord line of the wing and the direction of the
relative wind. This is not to be confused with the angle of incidence,
illustrated in Figure 4, which is the angle between the chord line of the wing and the longitudinal
axis of the aircraft. On each part of an airfoil or wing surface, a small force
is present.
 |
| Fig 5. Airflow over a wing section |
This force is of a different magnitude and direction from any
forces acting on other areas forward or rearward from this point. It is
possible to add all of these small forces mathematically. That sum is called
the “resultant force” (lift). This resultant force has magnitude, direction,
and location, and can be represented as a vector, as shown in Figure -5. The point of intersection of
the resultant force
line with the chord line of the airfoil is called the center of pressure
(CP). The CP moves along the airfoil chord as the AOA changes. Throughout most
of the flight range, the CP moves forward with increasing AOA and rearward as
the AOA decreases. The effect of increasing AOA on the CP is shown in Figure 6.The AOA changes as the
aircraft’s attitude changes. Since the AOA has a great deal to do with determining
lift, it is given primary consideration when designing airfoils. In a properly designed
airfoil, the lift increases as the AOA is increased. When the AOA is increased
gradually toward a positive AOA, the lift component increases rapidly up to a
certain point and then suddenly begins to drop off. During this action the drag
component increases slowly at first, then rapidly as lift begins to drop off.When
the AOA increases to the angle of maximum lift, the burble point is reached.
This is known as the critical angle. When the critical angle is reached, the
air ceases to flow smoothly over the top surface of the airfoil and begins to burble
or eddy. This means that air breaks away from the upper camber line of the
wing. What was formerly the area of decreased pressure is now filled by this
burbling air.When this occurs, the amount of lift drops and drag becomes excessive.
The force of gravity exerts itself, and the nose of the aircraft drops. This is
a stall. Thus, the burble point is the stalling angle.
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| Figure 6. Effect on increasing angle of attack. |
As
previously seen, the distribution of the pressure forces over the airfoil
varies with the AOA. The application of the resultant force, or CP, varies
correspondingly. As this angle increases, the CP moves forward; as the angle
decreases, the CP moves back. The unstable travel of the CP is characteristic of
almost all airfoils.
Boundary Layer
In
the study of physics and fluid mechanics, a boundary layer is that layer of
fluid in the immediate vicinity of a bounding surface. In relation to an
aircraft, the boundary layer is the part of the airflow closest to the surface
of the aircraft. In designing high-performance aircraft, considerable attention
is paid to controlling the behavior of the boundary layer to minimize pressure
drag and skin friction drag.
Thrust and Drag
An
aircraft in flight is the center of a continuous battle of forces. Actually,
this conflict is not as violent as it sounds, but it is the key to all
maneuvers performed in the air. There is nothing mysterious about these forces;
they are definite and known. The directions in which they act can be
calculated, and the aircraft itself is designed to take advantage of each of
them. In all types of flying, flight calculations are based on the magnitude
and direction of four forces: weight, lift, drag, and thrust. [Figure -7]
 |
| Figure 7. Forces in action during flight. |
An
aircraft in flight is acted upon by four forces:
1.
Gravity or weight—the force that pulls the aircraft toward the earth. Weight is
the force of gravity acting
downward
upon everything that goes into the aircraft, such as the aircraft itself, crew,
fuel, and cargo.
2.
Lift—the force that pushes the aircraft upward. Lift acts vertically and
counteracts the effects of weight.
3.
Thrust—the force that moves the aircraft forward. Thrust is the forward force
produced by the powerplant that
overcomes the force of drag.
4.
Drag—the force that exerts a braking action to hold the aircraft back. Drag is
a backward deterrent force and is caused by the disruption of the airflow by
the wings, fuselage, and protruding objects.
These
four forces are in perfect balance only when the aircraft is in
straight-and-level unaccelerated flight.
The
forces of lift and drag are the direct result of the relationship between the
relative wind and the aircraft. The force of lift always acts perpendicular to
the relative wind, and the force of drag always acts parallel to and in the
same direction as the relative wind. These forces are actually the components
that produce a resultant lift force on the wing. [Figure-8]
 |
| Figure 8. Resultant of lift and drag. |
Weight
has a definite relationship with lift, and thrust with drag. These
relationships are quite simple, but very important in understanding the
aerodynamics of flying. As stated previously, lift is the upward force on the
wing perpendicular to the relative wind. Lift is required to counteract the
aircraft’s weight, caused by the force of gravity acting on the mass of the
aircraft. This weight force acts downward through a point called the center of
gravity (CG). The CG is the point at which all the weight of the aircraft is
considered to be concentrated.When the lift force is in equilibrium with the
weight force, the aircraft neither gains nor loses altitude. If lift becomes less
than weight, the aircraft loses altitude. When the lift is greater than the
weight, the aircraft gains altitude.
Wing
area is measured in square feet and includes the part blanked out by the
fuselage. Wing area is adequately described as the area of the shadow cast by
the wing at high noon. Tests show that lift and drag forces acting on a wing are
roughly proportional to the wing area. This means that if the wing area is
doubled, all other variables remaining the same, the lift and drag created by
the wing is doubled. If the area is tripled, lift and drag are tripled.
Drag
must be overcome for the aircraft to move, and movement is essential to obtain
lift. To overcome drag and move the aircraft forward, another force is
essential. This force is thrust. Thrust is derived from jet propulsion or from a
propeller and engine combination. Jet propulsion theory is based on Newton’s
third law of motion. The turbine engine causes a mass
of air to be moved backward at high velocity causing a reaction that moves the
aircraft forward.
In a propeller/engine combination, the
propeller is actually two or more revolving airfoils mounted on a horizontal
shaft. The motion of the blades through the air produces lift similar to the
lift on the wing, but acts in a horizontal direction, pulling the aircraft
forward.
Before
the aircraft begins to move, thrust must be exerted. The aircraft continues to
move and gain speed until thrust and drag are equal. In order to maintain a
steady speed Weight drag are equal. In order to maintain a steady
speed, thrust and drag must remain equal, just as lift and weight must be equal
for steady,
horizontal flight. Increasing the lift means that the
aircraft moves
upward, whereas decreasing the lift so that it is less than the weight causes
the aircraft to lose altitude. A similar rule applies to the two forces of
thrust and drag. If the revolutions per minute (rpm) of the engine is reduced,
the thrust is lessened, and the aircraft slows down. As long as the thrust is
less than the drag, the aircraft travels more and more slowly until its speed
is insufficient to support it in the air.
Likewise, if the rpm of the engine is
increased, thrust becomes greater than drag, and the speed of the aircraft
increases. As long as the thrust continues to be greater than the drag, the aircraft
continues to accelerate. When drag equals thrust, the aircraft flies at a
steady speed.
The relative
motion of the air over an object that produces lift also produces drag. Drag is
the resistance of the air to objects moving through it. If an aircraft is
flying on a level course, the lift force acts vertically to support it while
the drag force acts horizontally to hold it back. The total amount of drag on
an aircraft is made up of many drag forces, but this handbook considers three:
parasite drag, profile drag, and induced drag.
Parasite drag
is made up of a combination of many different drag forces. Any exposed object
on an aircraft offers some resistance to the air, and the more objects in the
airstream,the more parasite drag. While parasite drag can be reduced by
reducing the number of exposed parts to as few as practical and streamlining
their shape, skin friction is the type of parasite drag most difficult to
reduce. No surface is
perfectly
smooth. Even machined surfaces have a ragged uneven appearance when inspected
under magnification. These ragged surfaces deflect the air near the surface
causing resistance to smooth airflow. Skin friction can be reduced by using
glossy smooth finishes and eliminating protruding rivet heads, roughness, and
other irregularities.
Profile drag may be considered the parasite
drag of the airfoil. The various components of parasite drag are all of the
same nature as profile drag.
The action of
the airfoil that creates lift also causes induced drag. Remember, the pressure
above the wing is less than atmospheric pressure, and the pressure below the
wing is equal to or greater than atmospheric pressure. Since fluids always move
from high pressure toward low pressure, there is a spanwise movement of air
from the bottom of the wing outward from the fuselage and upward around the
wing tip. This flow of air results in spillage over the wing tip, thereby setting
up a whirlpool of air called a “vortex.” [Figure 9]
 |
| Figure 9. Wingtip vortices. |
The air on the
upper surface has a tendency to move in toward the fuselage and off the
trailing edge. This air current forms a similar vortex at the inner portion of
the trailing edge of the wing. These vortices increase drag because of the
turbulence produced, and constitute induced drag.
Just as lift increases with an increase in
AOA, induced drag also increases as the AOA becomes greater. This occurs because,
as the AOA is increased, the pressure difference between the top and bottom of
the wing becomes greater. This causes more violent vortices to be set up,
resulting in more turbulence and more induced drag.
