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Physical science, or physics as it is most often called,
is a very interesting and exciting topic
...
Physics allows us to
explain how engines work, both piston and gas turbine;
how airplanes and helicopters fly; and countless other
things related to the field of aviation and aerospace
...
For example, through the use of physics we can explain
what the concept of thrust means for a jet engine, and
then follow it up by mathematically calculating the
pounds of thrust being created
...
It does not attempt to determine why matter and energy behave as they do in their relation to
physical phenomena, but rather how they behave
...
Matter
Matter is the foundation or the building blocks for any
discussion of physics
...
According to the Law of Conservation,
matter cannot be created or destroyed, but it is possible to change its physical state
...
Although it no longer exists in the state
of liquid gasoline, the matter still exists in the form of
the gases given off by the burning fuel
...
In other words, how many molecules are in the object,
or how many atoms, or to be more specific, how many
protons, neutrons, and electrons
...
The only way to change the mass of an object is
to add or take away atoms
...
2
feet per second per second (32
...
An object
weighing 32
...
A slug is a quantity of mass that will
accelerate at a rate of 1 ft/s2 when a force of 1 pound
is applied
...
2) a mass of one slug
is equal to 32
...
Weight is a measure of the pull of gravity acting on the
mass of an object
...
Because it is not possible for the mass of an object to
go away, the only way for an object to be weightless
is for gravity to go away
...
Even though the shuttle is quite a few miles above
the surface of the earth, the force of gravity has not
gone away, and the astronauts are not weightless
...
Mathematically, weight can be stated
as follows:
Weight = Mass × Gravity
Attraction
Attraction is the force acting mutually between particles of matter, tending to draw them together
...
”
He showed how each particle of matter attracts every
other particle, how people are bound to the earth, and
how the planets are attracted in the solar system
...
This
is sometimes referred to as granular — consisting or
appearing to consist of small grains or granules
...
Thus, two portions of
matter cannot at the same time occupy the same space
...
The unit volume selected for use in the English system
of measurement is 1 cubic foot (ft3 )
...
Therefore, density
is expressed in pounds per cubic foot ( lb⁄ft3) or in grams
per cubic centimeter ( g ⁄cm3 )
...
Its weight is then divided
by its volume to find the weight per unit volume
...
6 lb
...
Its volume is 24 ft3 (4 ft × 3 ft × 2 ft)
...
6 lb, then 1 ft3 weighs
1,497
...
4 lb
...
4 lb/ft3
...
In the metric
system, the density of water is 1 g ⁄cm3
...
Changes in temperature will not
change the weight of a substance, but will change the
volume of the substance by expansion or contraction,
thus changing its weight per unit volume
...
Pressure
is more critical when measuring the density of gases
than it is for other substances
...
Standard conditions for the measurement of the
densities of gases have been established at 0°C for
temperature and a pressure of 76 cm of mercury (Hg)
...
) Density is computed based on these conditions
for all gases
...
For this purpose, a standard
3-2
is needed
...
For gases, air is most commonly used
...
In physics, the word “specific” implies a ratio
...
The terms
“specific weight” or “specific density” are sometimes
used to express this ratio
...
Specific Gravity =
Weight of the substance
Weight of an equal volume of water
OR
Specific Gravity =
Density of the substance
Density of water
The same formulas are used to find the density of gases
by substituting air or hydrogen for water
...
For example, if a certain hydraulic fluid has
a specific gravity of 0
...
8
times as much as 1 ft3 of water: 62
...
8, or
49
...
Specific gravity and density are independent of the size
of the sample under consideration and depend only
upon the substance of which it is made
...
A device called a hydrometer is used for measuring
specific gravity of liquids
...
[Figure 3-2] The larger glass tube provides the container for the liquid
...
There must be enough
liquid to raise the float and prevent it from touching
the bottom
...
To determine specific gravity, the scale
is read at the surface of the liquid in which the float is
immersed
...
When immersed in a liquid
of greater density, the float rises, indicating a greater
specific gravity
...
An example of the use of the hydrometer is to determine the specific gravity of the electrolyte (battery
Specific
Gravity
0
...
7
Helium
0
...
789
Titanium
4
...
898
0
...
1
Nitrogen
0
...
82
Iron
7
...
000
Lube Oil
0
...
4
Oxygen
1
...
928
Copper
8
...
528
Water
1
...
4
Sulfuric
Acid
1
...
3
Mercury
13
...
5
1150
Discharged
1275
Charged
11 50
12 00
13 00
Figure 3-1
...
12 50
Jet Fuel
Jp-4
Ethyl
Alcohol
Jet Fuel
Jp-5
11 00
0
...
917
12 50
Ice
13 00
0
...
When a battery is discharged, the calibrated float immersed in the electrolyte
will indicate approximately 1150
...
The values
1150, 1275, and 1310 actually represent 1
...
275,
and 1
...
The electrolyte in a discharged battery is 1
...
275
to 1
...
Energy
Energy is typically defined as something that gives us
the capacity to perform work
...
Energy can be classified
as one of two types: either potential or kinetic
...
Potential energy may be classified into three groups: (1) that due to position, (2) that
due to distortion of an elastic body, and (3) that which
produces work through chemical action
...
Figure 3-2
...
To calculate the potential energy of an object due to its
position, as in height, the following formula is used:
Potential Energy = Weight × Height
A calculation based on this formula will produce an
answer that has units of foot-pounds (ft-lb) or inchpounds (in-lb), which are the same units that apply
to work
...
It can be seen that potential energy
and work have a lot in common
...
How much potential
energy does the airplane possess because of this raised
position?
Potential Energy = Weight × Height
PE = 450,000 lb × 4 ft
PE = 1,800,000 ft-lb
As mentioned previously, aviation gasoline possesses
potential energy because of its chemical nature
...
One pound of
aviation gas contains 18,900 BTU of heat energy, and
each BTU is capable of 778 ft-lb of work
...
Imagine the potential energy in the completely serviced
fuel tanks of an airplane
...
An airplane rolling down the runway or a rotating flywheel on an engine are both examples of kinetic energy
...
To calculate the
kinetic energy for something in motion, the following
formula is used:
Kinetic Energy = 1⁄2 Mass × Velocity 2
To use the formula, we will show the mass as weight
÷ gravity and the velocity of the object will be in feet
per second
...
Example: A Boeing 777 weighing 600,000 lb is moving
down the runway on its takeoff roll with a velocity of
200 fps
...
2 × 200 2
KE = 372,670,000 ft-lb
Force, Work, Power, and Torque
Force
Before the concept of work, power, or torque can be
discussed, we must understand what force means
...
For example,
if we apply a force to an object, the tendency will be
3-4
Figure 3-3
...
for the object to move
...
The unit for force in the English system of measurement is pounds, and in the metric system it is newtons
...
448 newtons
...
The GE90-115
turbofan engine (powerplant for the Boeing 777-300),
for example, has 115,000 pounds of thrust
...
This is true because all machines transfer input energy,
or the work done on the machine, to output energy, or
the work done by the machine
...
Two factors are involved:
(1) force and (2) movement through a distance
...
Two men push against it for a period of time, but the
aircraft does not move
...
By definition, work is accomplished only when
an object is displaced some distance against a resistive force
...
Notice
Figure 3-4
...
these are the same units that were used for potential
and kinetic energy
...
One pound of force is equal to 4
...
28 feet
...
36 ft-lb
...
Work = Force × Distance
= 5,000 lb × 80 ft
= 400,000 ft-lb
In this last example, notice the force does not equal the
weight of the airplane
...
In
virtually all cases, it takes less work to move something horizontally than it does to lift it vertically
...
Friction and Work
In calculating work done, the actual resistance overcome is measured
...
[Figure 3-5] A 900-lb load
is being pulled a distance of 200 ft
...
This is because the person pulling
the load is not working against the total weight of the
load, but rather against the rolling friction of the cart,
which may be no more than 90 lb
...
Without friction it would be impossible to walk
...
Yet friction is a liability as well as an asset, and
Force
90 lb
Work = force × distance
= 90 lb × 200 ft
= 18,000 ft-lb
Gravity
200 ft
Resistance
Figure 3-5
...
3-5
requires consideration when dealing with any moving
mechanism
...
One force is required to start a body moving,
while another is required to keep the body moving at
constant speed
...
Thus, the three kinds of friction may be classified as:
(1) starting (static) friction, (2) sliding friction, and
(3) rolling friction
...
Once in motion, it slides more easily
...
The force necessary to start
the body moving slowly is designated “F,” and “F'” is
the normal force pressing the body against the surface
(usually its weight)
...
The nature of the surfaces is indicated by
the coefficient of starting friction which is designated
by the letter “k
...
Thus, when the load (weight of the object) is
known, starting friction can be calculated by using the
following formula:
F = kF'
For example, if the coefficient of sliding friction of a
smooth iron block on a smooth, horizontal surface is
0
...
Starting friction for objects equipped with wheels
and roller bearings is much smaller than that for sliding objects
...
Therefore, the couples between the cars are
purposely made to have a few inches of play
...
Then, with a quick start
forward the first car is set in motion
...
It would be
impossible for the engine to start all of the cars at the
same instant, for static friction, which is the resistance
3-6
of being set in motion, would be greater than the force
exerted by the engine
...
Sliding Friction
Sliding friction is the resistance to motion offered by
an object sliding over a surface
...
The amount of
sliding resistance is dependent on the nature of the
surface of the object, the surface over which it slides,
and the normal force between the object and the surface
...
F = mN
In the formula above, “F” is the resistive force due to
friction expressed in pounds; “N” is the force exerted
on or by the object perpendicular (normal) to the
surface over which it slides; and “m” (mu) is the coefficient of sliding friction
...
The area
of the sliding object exposed to the sliding surface has
no effect on the results
...
Therefore, area does not enter into
the equation above
...
The force of friction for
objects mounted on wheels or rollers is called rolling
friction
...
For example, the
value of “m” for rubber tires on concrete or macadam
is about 0
...
The value of “m” for roller bearings is
very small, usually ranging from 0
...
003 and
is often disregarded
...
What force must be
exerted by the towing vehicle to keep the airplane
rolling after once set in motion?
F = mN
= 0
...
In other words, how long does it take
to accomplish the work
...
This person would probably assume that he or she is to
lift it all at once
...
If the weight is to be lifted in a
shorter period of time, it will take more power
...
The units
depend on how distance and time are measured
...
People wanted to know how many horses the
steam engine was equivalent to
...
It was found that the average horse could
lift a weight of 550 lb, one foot off the ground, in one
second
...
1 hp = 375 mile pounds per hour (mi-lb/hr)
1 hp = 746 watts (electricity conversion)
To convert power to horsepower, divide the power by
the appropriate conversion based on the units being
used
...
Power = Force × distance ÷ time
= 19,000 lb × 4 ft ÷ 2 min
...
Hp = 38,000 ft-lb/min
...
Hp = 1
...
15 hp in
their arms for the necessary 2 minutes
...
Whereas work is
described as a force acting through a distance, torque
is described as a force acting along a distance
...
If we push on an object with a force of 10 lb and
it moves 10 inches in a straight line, we have done
100 in‑lb of work
...
If the bolt was already tight and did not
move as we pushed down on the wrench, the torque of
100 lb‑in would still exist
...
Notice that with torque nothing had to move, because
the force is being applied along a distance and not
through a distance
...
The units of work were inch-pounds and the units of
torque were pound-inches, and that is what differentiates the two
...
Both types of engines create torque in advance
of being able to create work or power
...
The piston is
attached to the connecting rod, which is attached to
the crankshaft at an offset
...
[Figure 3-6]
Example: A Cessna 172R has a Lycoming IO-360 engine
that creates 180 horsepower at 2,700 rpm
...
The connecting rod
attaches to the crankshaft at an offset distance of 4 in
...
Torque = 180 × 5,252 ÷ 2,700
= 350 lb-ft
In a turbine engine, the turbine blades at the back of the
engine extract energy from the high velocity exhaust
gases
...
The number of inches from the
turbine blades to the center of the shaft would be like the
length of the wrench discussed earlier
...
The formula that shows this relationship is as follows:
Torque = Horsepower × 5,252 ÷ rpm
Simple Machines
A machine is any device with which work may be
accomplished
...
1
...
2
...
3
...
The pulley system enables the load to be raised by
exerting a force that is smaller than the weight of
the load
...
Machines can be used to multiply speed
...
Torque = F × d
Torque = 500 × 4
Torque = 2,000 lb-in
Torque = 166
...
Machines can be used to change the direction of
a force
...
A downward force on one side of the rope exerts
an upward force on the other side, raising the flag
toward the top of the pole
...
They are the lever,
the pulley, the wheel and axle, the inclined plane, the
screw, and the gear
...
The pulley (block and tackle), the
wheel and axle, and gears operate on the machine
principle of the lever
...
Connecting rod
Crankshaft
An understanding of the principles of simple machines
provides a necessary foundation for the study of compound machines, which are combinations of two or
more simple machines
...
Piston engine and torque
...
It cannot, however, multiply force
and speed at the same time
...
To do otherwise would mean the
machine has more power going out than coming in,
and that is not possible
...
If there is a
mechanical advantage in terms of force, there will be
a fractional disadvantage in terms of distance
...
Mechanical Advantage = Force Out ÷ Force In
Or
Mechanical Advantage = Distance Out ÷ Distance In
The Lever
The simplest machine, and perhaps the most familiar
one, is the lever
...
There are three basic
parts in all levers
...
” Shown in Figure 3-7
are the pivot point “F” (fulcrum), the effort “E” which
is applied at a distance “L” from the fulcrum, and a
resistance “R” which acts at a distance “l” from the
fulcrum
...
The concept of torque was discussed earlier in this
chapter, and torque is very much involved in the
operation of a lever
...
This combination of force and distance
creates torque, which tries to cause rotation
...
As mentioned earlier, the
seesaw is a good example of a lever, and it happens
to be a first class lever
...
Increasing the distance from the applied effort to the
Resistance “R”
Effort “E”
“I”
“L”
Fulcrum “F”
fulcrum, compared to the distance from the fulcrum
to the weight being moved, increases the advantage
provided by the lever
...
The proper
balance of an airplane is also a good example, with the
center of lift on the wing being the pivot point (fulcrum)
and the weight fore and aft of this point being the effort
and the resistance
...
Effort (E) × Effort Arm (L) = Resistance (R) × Resistance Arm (l)
What this formula really shows is the input torque
(effort times effort arm) equals the output torque
(resistance times resistance arm)
...
Example: A first class lever is to be used to lift a 500-lb
weight
...
How much force is required to lift the
weight?
Effort (E) × Effort Arm (L) = Resistance (R) × Resistance Arm (l)
E × 60 in = 500 lb × 12 in
E = 500 lb × 12 in ÷ 60 in
E = 100 lb
The mechanical advantage of the lever in this example
would be:
Mechanical Advantage = Force Out ÷ Force In
= 500 lb ÷ 100 lb
= 5, or 5 to 1
An interesting thing to note with this example lever is
if the applied effort moved down 10 inches, the weight
on the other end would only move up 2 inches
...
The reason for this is the concept of work
...
Second Class Lever
The second class lever has the fulcrum at one end and
the effort is applied at the other end
...
A wheelbarrow
is a good example of a second class lever, with the
wheel at one end being the fulcrum, the handles at the
opposite end being the applied effort, and the bucket
Figure 3-7
...
3-9
Effort “E”
Effort “E”
“L”
“I”
“L”
“I”
Fulcrum “F”
Resistance “R”
Fulcrum “F”
Resistance “R”
Figure 3-8
...
Figure 3-9
...
in the middle being where the weight or resistance is
placed
...
The hydraulic actuator that makes the
gear retract is attached somewhere in the middle, and
that is the applied effort
...
The first class lever, however, is more
versatile
...
The second class lever can
only be made to gain force
...
The weight
in the bucket is 18 inches from the center of the wheel
...
33, or 3
...
Levers that help accomplish
this are third class levers
...
Third class levers
are easily recognized because the effort is applied
between the fulcrum and the resistance
...
The top of the landing gear, where
it attaches to the airplane, is the pivot point
...
The
wheel, or disk, is normally grooved to accommodate a
rope
...
The frame that supports the wheel
is called a block
...
Each block contains one or more pulleys and
a rope connecting the pulley(s) of each block
...
In Figure 3-10, the arm from point “R” to
point “F” is equal to the arm from point “F” to point “E”
(both distances being equal to the radius of the pulley)
...
Thus, the force of the pull on the rope
must be equal to the weight of the object being lifted
...
Single Movable Pulley
A single pulley can be used to magnify the force
exerted
...
This single movable pulley
acts like a second class lever, with the effort arm (EF)
being the diameter of the pulley and the resistance arm
(FR) being the radius of the pulley
...
In use, if someone pulled in 4 ft of the effort rope, the
weight would only rise off the floor 2 ft
...
With this type of pulley, the effort will always
be one-half of the weight being lifted
...
Single fixed pulley
...
In Figure 3-12, the
block and tackle is made up of four pulleys, the top
two being fixed and the bottom two being movable
...
The number of weight supporting
ropes determines the mechanical advantage of a block
and tackle, so in this case the mechanical advantage is
four
...
The Gear
Two gears with teeth on their outer edges, as shown in
Figure 3-13, act like a first class lever when one gear
drives the other
...
The effort arm is the diameter of the driven gear, and
the resistance arm is the diameter of the drive gear
...
Single movable pulley
...
The gear on top (yellow) is 9 inches in
diameter and has 45 teeth, and the gear on the bottom
(blue) is 12 inches in diameter and has 60 teeth
...
The mechanical
advantage in terms of force would be the effort arm
divided by the resistance arm, or 9 ÷ 12, which is 0
...
This would actually be called a fractional disadvantage,
because there would be less force out than force in
...
Spur gears
...
Block and tackle
...
33
...
In order to be a force gaining machine,
the small gear needs to turn the large one
...
The end result is an
increase in force, and ultimately torque
...
The size of the gears and their number of teeth
determine the mechanical advantage, and whether force
is being increased or rpm is being increased
...
[Figure 3-14]
The worm gear has an extremely high mechanical
advantage
...
One complete revolution of the worm gear only makes the spur gear turn an
amount equal to one tooth
...
This is a force gaining machine, to the
tune of 25 times more output force
...
The
power output shaft of the engine would drive the sun
gear in the middle, which rotates the planetary gears
and ultimately the ring gear
...
To figure how much gear
reduction is taking place, the number of teeth on the
Sun Gear
Ring Gear
Figure 3-14
...
Planetary
Gears
Figure 3-16
...
Cessna 172 right main gear is sitting on an electronic
scale
...
With an inclined plane, the length of the incline is the
effort arm and the vertical height of the incline is the
resistance arm
...
The Cessna 172 in
Figure 3-17 weighed 1,600 lb on the day of the weighing
...
Worm gear
...
In this case, the gear reduction is 2
...
93 times greater than the propeller
...
Some
familiar examples of the inclined plane are mountain
highways and a loading ramp on the back of a moving
truck
...
A ramp can be seen in Figure 3-17, where a
Figure 3-17
...
3-13
like a weight pushing on the end of a lever, a reaction
will occur inside the object which is known as stress
...
An external force acting on an object causes the stress
to manifest itself in one of five forms, or combination
of those five
...
Figure 3-18
...
arm) and the length of the ramp is 24 inches (effort
arm)
...
A
bolt, for example, has a spiral thread that runs around
its circumference
...
The circumference
of the bolt is the effort arm and the distance between
the threads is the resistance arm
...
A chisel is a good example of a wedge
...
The 8-inch length is the effort
arm and the 1⁄2-inch width is the resistance arm
...
Stress
Whenever a machine is in operation, be it a simple
machine like a lever or a screw, or a more complex
machine like an aircraft piston engine or a hydraulically operated landing gear, the parts and pieces of
that machine will experience something called stress
...
In
the block and tackle system discussed earlier in this
chapter, the upper block that housed the two fixed pulleys was secured to an overhead beam
...
The weight being lifted would cause the ropes and the
blocks to be under tension
...
Compression
Compression is a force that tries to crush an object
...
The rivet passes through a hole drilled in
the pieces of aluminum, and then a rivet gun on one
side and a bucking bar on the other apply a force
...
[Figure 3-19]
Torsion
Torsion is the stress an object experiences when it is
twisted, which is what happens when torque is applied
to a shaft
...
When a shaft is
twisted, tension is experienced at a diagonal to the
shaft and compression acts 90 degrees to the tension
...
The turbine blades extract energy from
the high velocity air as a force in pounds
...
[Figure 3-21]
Bending
An airplane in flight experiences a bending force on the
wing as aerodynamic lift tries to raise the wing
...
Turbofan engine, torque creating
torsion in the shaft
...
When the airplane is on the
ground sitting on its landing gear, the force of gravity
tries to bend the wing downward, subjecting the bottom
of the wing to compression and the top of the wing to
tension
...
Shear
When a shear stress is applied to an object, the force
tries to cut or slice through, like a knife cutting through
butter
...
As shown in Figure 3-23, a fork fitting is secured
Wing top is
under Tension
Figure 3-19
...
Wing bottom is
under Compression
Figure 3-22
...
Tension stress
Clevis Bolt
Force
Rotation
Compression
Figure 3-20
...
Figure 3-23
...
3-15
to the end of the cable, and the fork attaches to an eye
on the airframe with the clevis bolt
...
This bolt would be
designed to take very high shear loads
...
One characteristic of matter is that it tends to
be elastic, meaning it can be forced out of shape when
a force is applied, and then return to its original shape
when the force is removed
...
On turbine engine test cells, the thrust of the engine
is typically measured by what are called strain gages
...
A deflecting beam style of torque wrench uses the
strain on the drive end of the wrench and the resulting
distortion of the beam to indicate the amount of torque
on a bolt or nut
...
” In a more specific sense, the relationship between velocity, acceleration, and distance is known as kinematics
...
When an object is at different points
in space at different times, that object is said to be in
motion, and if the distance the object moves remains
the same for a given period of time, the motion may
be described as uniform
...
Speed and Velocity
In everyday conversation, speed and velocity are often
used as if they mean the same thing
...
Speed refers to
how fast an object is moving, or how far the object will
travel in a specific time
...
For
example, if the information is supplied that an airplane
leaves New York City and travels 8 hours at a speed
of 150 mph, this information tells nothing about the
direction in which the airplane is moving
...
Velocity is that quantity in physics which denotes
both the speed of an object and the direction in which
the object moves
...
Velocity is also
described as being a vector quantity, a vector being a
line of specific length, having an arrow on one end or
the other
...
Figure 3-24
...
3-16
Two velocity vectors, such as one representing the
velocity of an airplane and one representing the
velocity of the wind, can be added together in what is
called vector analysis
...
With no wind, the speed and direction
of the airplane would be that shown by vector “A
...
”
fps, multiply by 1
...
Velocity is calculated the same
way, the only difference being it must be recalculated
every time the direction changes
...
Vec
tor
C=
Mov
eme
nt o
f
airp
lane
Vector A = Velocity of airplane
Acceleration
Acceleration is defined as the rate of change of velocity
...
If the
increase in velocity is 10 mph in 5 seconds, the rate of
change in velocity is 10 mph in 5 seconds, or 2 mph per
second
...
467, it could also
be expressed as an acceleration of 2
...
By comparison, the acceleration due
to gravity is 32
...
Acceleration (A) =
Velocity Final (Vf) − Velocity Initial (Vi)
Time (t)
Example: An Air Force F-15 fighter is cruising at 400
mph
...
What is
the average acceleration in mph/s and fps/s?
A=
Vf − Vi
t
A=
1200 − 400
20
A = 40 mph⁄s , or by multiplying by 1
...
7 fps⁄s
In the example just shown, the acceleration was found
to be 58
...
Since 32
...
2 to find out how many G forces the pilot is
experiencing
...
82 Gs
...
Vector analysis for airplane velocity
and wind velocity
...
Because of the circular pattern, the airplane is constantly changing direction,
which means the airplane is constantly changing
velocity
...
To calculate the speed of an object, the distance it
travels is divided by the elapsed time
...
If the distance is
measured in feet and the time in seconds, the units of
speed will be feet per second (fps)
...
Inertia is responsible for the discomfort felt when an airplane is brought to a sudden
halt in the parking area and the passengers are thrown
forward in their seats
...
This property of matter is described by Newton’s first
law of motion, which states:
Objects at rest tend to remain at rest and objects in
motion tend to remain in motion at the same speed and
in the same direction, unless acted on by an external
force
...
A body that has great momentum has a strong tendency
to remain in motion and is therefore hard to stop
...
Newton’s second law
applies to this property
...
The rate of change of momentum is
proportional to the applied force
...
Earlier in this chapter, it was
determined that mass equals weight divided by gravity,
and acceleration equals velocity final minus velocity initial divided by time
...
The air enters going
100 fps and leaves going 1,200 fps
...
2 (1)
F = 5,124 lb of thrust
Third Law
Newton’s third law of motion is often called the law
of action and reaction
...
This means
that if a force is applied to an object, the object will
supply a resistive force exactly equal to and in the
opposite direction of the force applied
...
For example,
as a man stands on the floor, the floor exerts a force
against his feet exactly equal to his weight
...
Forces always occur in pairs
...
3-18
The string exerts a centripetal force on the object, and
the object exerts an equal but opposite force on the
string, obeying Newton’s third law of motion
...
For example,
if one end of a string is tied to an object and the other
end is held in the hand, the object can be swung in a
circle
...
When the weight is at point A, due to
inertia it wants to keep moving in a straight line and
end up at point B
...
WT
“C”
Centripetal Force
F=
When an aircraft propeller pushes a stream of air backward with a force of 500 lb, the air pushes the blades
forward with a force of 500 lb
...
A turbofan engine
exerts a force on the air entering the inlet duct, causing
it to accelerate out the fan duct and the tailpipe
...
A
e
orc
lF
a
ug
trif
en
C
Figure 3-26
...
force that is equal to centripetal force, but acting in an
opposite direction, is called centrifugal force
...
Thus, if the
mass of the object in Figure 3-26 is doubled, the pull
on the string must be doubled to keep the object in its
circular path, provided the speed of the object remains
constant
...
If the string in
Figure 3-26 is shortened and the speed remains constant, the pull on the string must be increased since the
radius is decreased, and the string must pull the object
from its linear path more rapidly
...
Centripetal
force is thus directly proportional to the square of the
velocity of the object
...
Example: What would the centripetal force be if a 10
pound weight was moving in a 3-ft radius circular path
at a velocity of 500 fps?
Centripetal Force = Mass (Velocity 2) ÷ Radius
Centripetal Force = 10 (500 2) ÷ 32
...
It can also be said that the object is experiencing 2,588 Gs (force of gravity)
...
Heat
Heat is a form of energy
...
Heat
may also be defined as the total kinetic energy of the
molecules of any substance
...
This includes all methods of
producing increased motion of molecules such
as friction, impact of bodies, or compression of
gases
...
Electrical energy is converted
to heat energy when an electric current flows
through any form of resistance such as an electric
iron, electric light, or an electric blanket
...
Most forms of chemical reaction
convert stored potential energy into heat
...
• Radiant Energy
...
• Nuclear Energy
...
• The Sun
...
When a gas is compressed, work is done and the gas
becomes warm or hot
...
In the first case, work was converted
into energy in the form of heat; in the second case
heat energy was expended
...
Also, when two surfaces are rubbed
together, the friction develops heat
...
Thus,
heat can be regarded as a form of energy
...
In a hot body, these small
particles possess relatively large amounts of kinetic
energy, but in cooler bodies they have less
...
Mechanical energy apparently is transformed,
and what we know as heat is really kinetic energy of
the small molecular subdivisions of matter
...
They are the calorie and the BTU
...
3-19
This term “calorie” (spelled with a lower case c) is
1/1,000 of the Calorie (spelled with a capital C) used in
the measurement of the heat energy in foods
...
The calorie and the gram are seldom used in
discussing aviation maintenance
...
A device known as the calorimeter is used to measure
quantities of heat energy
...
A given weight of the
fuel is burned in the calorimeter, and the heat energy
is absorbed by a large quantity of water
...
A
definite relationship exists between heat and mechanical energy
...
Since each BTU
is capable of 778 ft-lb of work, 1 lb of aviation gasoline
is capable of 14,704,200 ft-lb of work
...
The formula for calculating
thermal efficiency is:
Thermal Efficiency =
Horsepower Produced ÷ Potential Horsepower in Fuel
For example, consider the piston engine used in a
small general aviation airplane, which typically consumes 0
...
Imagine that the engine is creating 200 hp
...
5 by the horsepower of 200, we find the
engine is consuming 100 lb of fuel per hour, or 1
...
Earlier in this chapter, one horsepower
was found to be 33,000 ft-lb of work per minute
...
67 lb per minute × 18,900 BTU per lb × 778 ft lb per BTU
Hp = 744
3-20
33,000 ft-lb/min
The example engine is burning enough fuel that it has
the potential to create 744 horsepower, but it is only
creating 200
...
2688 or 26
...
The wasted
energy is in the form of friction and heat
...
Heat Transfer
There are three methods by which heat is transferred
from one location to another or from one substance to
another
...
Conduction
Heat transfer always takes place by areas of high heat
energy migrating to areas of low heat energy
...
Everyone knows from experience that the metal handle
of a heated pan can burn the hand
...
The metal transmits
the heat more easily than the wood because it is a better
conductor of heat
...
Some metals are much better conductors
of heat than others
...
Woods and plastics are used for handles because they
conduct heat very slowly
...
Of those listed, silver is the
best conductor and lead is the poorest
...
It is interesting to note that silver, copper, and aluminum are also
excellent conductors of electricity
...
Notice that the ice in the test tube shown in Figure 328 is not melting rapidly even though the water at the
top is boiling
...
1
...
00
1
...
94
...
80
...
40
MAGNESIUM
...
08
IRON
...
20
SILVER
ALUMINUM
...
30
0
COPPER
...
10
Ice
...
60
Figure 3-27
...
Gases are even poorer conductors of heat than liquids
...
Since conduction is a process whereby the increase
in molecular energy is passed along by actual contact,
gases are poor conductors
...
These molecules
strike adjacent molecules causing them to become agitated
...
Because
molecules are farther apart in gases than in solids, the
gases are much poorer conductors of heat
...
A
wooden handle on a pot or a soldering iron serves as
a heat insulator
...
These materials are therefore used for many types of
insulation
...
For
example, an incandescent light bulb will, when heated,
become increasingly hotter until the air surrounding it
begins to move
...
This
upward motion of the heated air carries the heat away
from the hot light bulb by convection
...
Water as a poor conductor
...
The rate
of cooling of a hot electronics component, such as the
CPU in a computer, can be increased if it is provided
with copper fins that conduct heat away from the hot
surface
...
A convection process may take place in a liquid as well
as in a gas
...
The bottom of the pan becomes
hot because it conducts heat from the surface it is in
contact with
...
As the heated water
starts to rise and cooler water moves in to take its place,
the convection process begins
...
In some installations, pumps are used to circulate
water or oil to help cool large equipment
...
An aircraft air-cooled piston engine is a good example
of convection being used to transfer heat
...
This engine
does not depend on natural convection for cooling, but
rather forced air convection coming from the propeller
on the engine
...
Once
the heat gets to the fins, forced air flowing around the
cylinders carries the heat away
...
Aircraft piston engine cooled by convection
...
For example, the heat one feels when sitting in front
of an open fire cannot be transferred by convection
because the air currents are moving toward the fire
...
Therefore, there
must be some way for heat to travel across space other
than by conduction and convection
...
Since conduction and convection take place only
through some medium, such as a gas or a liquid, heat
from the sun must reach the earth by another method,
since space is an almost perfect vacuum
...
The term “radiation” refers to the continual emission
of energy from the surface of all bodies
...
It is in the form of electromagnetic waves, radio waves, or x-rays, which are
all alike except for a difference in wave length
...
Most forms of energy can
3-22
be traced back to the energy of sunlight
...
These electromagnetic heat waves
are absorbed when they come in contact with nontransparent bodies
...
The differences between conduction, convection, and
radiation may now be considered
...
This fact is evident at
the time of an eclipse of the sun when the shutting off
of the heat from the sun takes place at the same time as
the shutting off of the light
...
For example,
the air inside a greenhouse may be much warmer than
the glass through which the sun’s rays pass
...
For example, radiation can
be cut off with a screen placed between the source of
heat and the body to be protected
...
Each substance requires a quantity of heat,
called its specific heat capacity, to increase the temperature of a unit of its mass 1°C
...
Specific heat is
expressed as a number which, because it is a ratio,
has no units and applies to both the English and the
metric systems
...
The larger bodies of water on the earth keep the air
and solid matter on or near the surface of the earth at a
fairly constant temperature
...
Therefore, when the temperature falls below that
of such bodies of water, they give off large quantities
of heat
...
The specific heat values of some common materials
are listed in Figure 3-30
...
It is of particular concern when
calculating changes in the state of gases
...
The Centigrade scale is constructed by using the freezing and boiling points of
water, under standard conditions, as fixed points of
zero and 100, respectively, with 100 equal divisions
between
...
The absolute or Kelvin
scale is constructed with its zero point established as
minus 273°C, meaning 273° below the freezing point
of water
...
When working with temperatures, always make sure
which system of measurement is being used and know
how to convert from one to another
...
8 × Degrees Celsius) + 32
Degrees Celsius = (Degrees Fahrenheit – 32) × 5⁄9
Degrees Kelvin = Degrees Celsius + 273
Degrees Rankine = Degrees Fahrenheit + 460
For purposes of calculations, the Rankine scale is
commonly used to convert Fahrenheit to absolute
...
Thus, 72°F equals 460° plus 72°, or 532° absolute
...
Thus −40°F equals 460° minus 40°, or
420° absolute
...
The Kelvin and Centigrade scales are used more
extensively in scientific work; therefore, some technical manuals may use these scales in giving directions
and operating instructions
...
Therefore, the Fahrenheit scale is
used in most areas of this book
...
031
Mercury
0
...
195
Alcohol
0
...
712
Water
0
273
32
492 Pure water freezes
−273
0
−460
0
0
...
094
Copper
212
0
...
000
Celsius
(Centigrade)
Kelvin
Fahrenheit
Molecular motion
ceases at absolute
zero
Rankine
Figure 3-31
...
Figure 3-30
...
3-23
Thermal Expansion/Contraction
Thermal expansion takes place in solids, liquids, and
gases when they are heated
...
Because the molecules of solids are much closer
together and are more strongly attracted to each other,
the expansion of solids when heated is very slight in
comparison to the expansion in liquids and gases
...
Thermal expansion in solids must be explained in
some detail because of its close relationship to aircraft
metals and materials
...
The amount that a unit length of
any substance expands for a one degree rise in temperature is known as the coefficient of linear expansion for
that substance
...
To estimate the expansion of any object, such as a steel
rail, it is necessary to know three things about it: its
length, the rise in temperature to which it is subjected,
and its coefficient of expansion
...
Expansion = (11 × 10 −6 ) × (9 feet) × 34°
Expansion = 0
...
4 × 10− 6
Silver
19 × 10− 6
Figure 3-32
...
3-24
This amount, when added to the original length of the
rod, makes the rod 9
...
Its length has only
increased by 4⁄100 of an inch
...
Thus, thermal expansion must
be taken into consideration when designing airframes,
powerplants, or related equipment
...
The force is typically measured in pounds and the surface area in square inches,
making the units of pressure pounds per square inch
or psi
...
When atmospheric pressure is being measured, in addition to psi, other means of pressure measurement can be
used
...
Standard day atmospheric pressure is
equal to 14
...
92 inches of mercury ("Hg), 760
millimeters of mercury (mm Hg), or 1013
...
The relationship between these units of measure is
as follows:
1 psi = 2
...
7 mm Hg
1 psi = 68
...
The test tube is then
turned upside down and placed in an open container
of mercury, and the top is uncovered
...
Atmospheric pressure pushing down
on the mercury in the open container tries to make the
mercury stay in the test tube
...
Under standard day atmospheric conditions,
the air in a 1-in2 column extending all the way to the
top of the atmosphere would weigh 14
...
A 1 in2
column of mercury, 29
...
7 lb
...
7 psi is equal to 29
...
Figure 3-33 demonstrates this point
...
7 psi
Atmospheric
pressure
80
10
760 mm
29
...
Psig read on a fuel pressure gauge
...
Atmospheric pressure as inches of mercury
...
This can be seen on the fuel pressure gauge shown in
Figure 3-34
...
Absolute Pressure
A gauge that includes atmospheric pressure in its reading is measuring what is known as absolute pressure,
or psia
...
If someone hooked up a
psia indicating instrument to an engine’s oil system,
the gauge would read atmospheric pressure when the
engine was not running
...
For the manifold pressure on a piston engine, a psia gauge does make good
sense
...
The only gauge that has the flexibility to show this variety of readings is the absolute
pressure gauge
...
Remember that 29
...
35
30
25
30 28
29
...
Press
...
0 psi
15
6
10
Figure 3-35
...
Differential Pressure
Differential pressure, or psid, is the difference between
pressures being read at two different locations within
a system
...
These two readings are sent to a
transmitter which powers a light located on the flight
deck
...
If
the filter starts to clog, the pressure drop will become
3-25
a long drive on a hot day, the pressure in the tires of
an automobile increases, and a tire which appeared to
be somewhat “soft” in cool morning temperature may
appear normal at a higher midday temperature
...
Figure 3-36
...
greater, eventually causing the advisory light on the
flight deck to come on
...
In this case,
the difference in pressure is between the inside and
the outside of the airplane
...
Gas Laws
The simple structure of gases makes them readily
adaptable to mathematical analysis from which has
evolved a detailed theory of the behavior of gases
...
The theory assumes
that a body of gas is composed of identical molecules
which behave like minute elastic spheres, spaced relatively far apart and continuously in motion
...
Since the molecules are
continuously striking against each other and against
the walls of the container, an increase in temperature
with the resulting increase in molecular motion causes
a corresponding increase in the number of collisions
between the molecules
...
If the container were an open vessel, the gas would
expand and overflow from the container
...
For instance, when making
3-26
Boyle’s Law
As previously stated, compressibility is an outstanding
characteristic of gases
...
” By direct measurement he discovered that when the temperature of
a combined sample of gas was kept constant and the
absolute pressure doubled, the volume was reduced to
half the former value
...
From
these observations, he concluded that for a constant
temperature the product of the volume and absolute
pressure of an enclosed gas remains constant
...
” The following formula is used for Boyle’s law calculations
...
Volume 1 × Pressure 1 = Volume 2 × Pressure 2
Or
V1P1 = V2P2
Example: 10 ft 3 of nitrogen is under a pressure of 500
psia
...
29 psia
The useful applications of Boyle’s law are many and
varied
...
Charles’ Law
The French scientist, Jacques Charles, provided much
of the foundation for the modern kinetic theory of
gases
...
The formula which is used to
express the general gas law is as follows:
Force pushing down on gas
Pressure 1 (Volume 1) Pressure 2 (Volume 2)
=
Temperature 2
Temperature 1
Or
P1 (V1) (T2) = P2 (V2) (T1)
When using the general gas law formula, temperature
and pressure must be in the absolute
...
Boyle’s law example
...
As a
formula, this law is shown as follows:
Volume 1 × Absolute Temperature 2 =
Volume 2 × Absolute Temperature 1
Or
V1T2 = V2T1
Charles’ law also works if the volume is held constant,
and pressure and temperature are the variables
...
Example: A 15-ft 3 cylinder of oxygen is at a temperature of 70°F and a pressure of 750 psig
...
What would be the new pressure
in psig?
70 degrees Fahrenheit = 530 degrees Rankine
140 degrees Fahrenheit = 600 degrees Rankine
750 psig + 14
...
7 psia
P1T2 = P2T1
764
...
7 (600) ÷ 530
P2 = 865
...
The gas starts out at a temperature of 60°F
and a pressure of 1,000 psig
...
What would its new pressure
be in psig?
60 degrees Fahrenheit = 520 degrees Rankine
90 degrees Fahrenheit = 550 degrees Rankine
1,000 psig + 14
...
7 psia
P1 (V1) (T2) = P2 (V2) (T1)
1,014
...
3 psig
Dalton’s Law
If a mixture of two or more gases that do not combine
chemically is placed in a container, each gas expands
throughout the total space and the absolute pressure of
each gas is reduced to a lower value, called its partial
pressure
...
The pressure of the mixed gases is equal to the
sum of the partial pressures
...
”
Fluid Mechanics
A fluid, by definition, is any substance that is able to
flow if it is not in some way confined or restricted
...
One significant difference comes into play when a force is applied to these
fluids
...
Many of the principles that aviation is based on, such as the theory of
lift on a wing and the force generated by a hydraulic
3-27
system, can be explained and quantified by using the
laws of fluid mechanics
...
Buoyancy
A solid body submerged in a liquid or a gas weighs
less than when weighed in free space
...
An object will
float if this upward force of the fluid is greater than
the weight of the object
...
A person can lift a
larger weight under water than he or she can possibly
lift in the air
...
C
...
As a result, he discovered that the buoyant
force which a fluid exerts upon a submerged body is
equal to the weight of the fluid the body displaces
...
This principle applies to all fluids, gases as well as
liquids
...
The following experiment is illustrated in Figure 3-38
...
The
heavy metal cube is first weighed in still air and weighs
10 lb
...
The difference between
the two weights is the buoyant force of the water
...
The volume of water
which overflows equals the volume of the cube
...
) If this experiment is performed carefully,
the weight of the water displaced by the metal cube
The amount of buoyant force available to an object can
be calculated by using the following formula:
Buoyant Force = Volume of Object × Density of Fluid Displaced
If the buoyant force is more than the object weighs, the
object will float
...
For the object that
sinks, its measurable weight will be less by the weight
of the displaced fluid
...
Will the object float? If the object sinks,
what is its measurable weight in the submerged condition? If the object floats, how many cubic feet of its
volume is below the water line?
Buoyant Force = Volume of Object × Density of Fluid Displaced
= 10 (62
...
The difference between
the buoyant force and the object’s weight will be its
measurable weight, or 76 lb
...
Catch
bucket
0
...
11 cu'
Overflow can
7
Figure 3-38
...
3-28
Two good examples of buoyancy are a helium filled
airship and a seaplane on floats
...
That means both have more buoyant force
than weight
...
At
a minimum, the floats on this airplane must be large
enough to displace a weight in water equal to the
airplane’s weight
...
For this airplane, the necessary size of the
floats would be calculated as follows:
Divide the airplane weight by the density of water
...
4 = 200
...
200
...
2 ft 3
Add the two volumes together to get the total volume of the floats
...
3 + 160
...
5 ft 3
By looking at the Twin Otter in Figure 3-39, it is
obvious that much of the volume of the floats is out
of the water
...
Some of the large Goodyear airships have a volume
of 230,000 ft 3
...
07651 lb⁄ ft3)
...
Figure 3-40 shows an inside view
of the Goodyear airship
...
Through the air scoop, item
9, air can be pumped into the ballonets or evacuated
from the ballonets in order to control the weight of the
airship
...
Although the
airship is classified as a lighter-than-air aircraft, it is in
fact flown in a condition slightly heavier than air
...
This can be seen in
Figure 3-41, where three different shapes and sizes of
containers are full of colored water
...
Because
of this height, each one would exert a pressure on the
bottom of 8
...
The container on the left, with a
surface area of 1 in 2, contains a volume of 231 in 3 (one
gallon)
...
34 lb, which is
why the pressure on the bottom is 8
...
Still thinking about Figure 3-41, if the pressure was
measured half way down, it would be half of 8
...
17 psi
...
Pressure based on
the column height of a fluid is known as static pressure
...
For example, if a carburetor needs to have 2 psi supplied to its inlet (head of
pressure), this could be accomplished by having the
Figure 3-39
...
Nose cone
support
Suspension
cables
Light sign
Neoprene
cover
Control surfaces
Air valves
Forward
Aft balloonet
balloonet
Passenger car Engines
Air scoops
Figure 3-40
...
Each container is filled with colored water to a height of 231 inches
...
34 psi
Figure 3-41
...
3-29
fuel tank positioned the appropriate number of inches
higher than the carburetor
...
When a fluid is in motion, and its velocity
is converted to pressure, that pressure is known as
ram
...
In the
inlet of a gas turbine engine, for example, total pressure is often measured to provide a signal to the fuel
metering device or to provide a signal to a gauge on
the flight deck
...
This pressure acts at right angles to containing surfaces
...
The concept of the pressure
set up in a fluid, and how it relates to the force acting
on the fluid and the surface area through which it acts,
is Pascal’s law
...
If
the additional pressure is 100 psi, this 100 psi will act
equally and undiminished from the top of the cylinder
all the way to the bottom
...
34 psi, and if a gauge were positioned
half way down the cylinder, it would read 104
...
34)
...
F
A
P
Figure 3-42
...
An easy and convenient way to remember the formulas
for Pascal’s law, and the relationship between the variables, is with the triangle shown in Figure 3-42
...
For example, if the “A” (area) is covered
up, what remains is the “F” on the top and the “P” on the
bottom, meaning force divided by pressure
...
Based on Pascal’s law, the pressure in the system would
be equal to the force applied divided by the area of the
piston, or 10 psi
...
The hydraulic system in Figure 3-44 is a little more
complex than the one in Figure 3-43
...
The input cylinder and
piston is connected to a second cylinder, which contains
a 5‑in2 piston
...
By transposing the original formula,
we have two additional formulas, as follows:
Pressure = Force ÷ Area
And
Area = Force ÷ Pressure
10 psi
10 psi
Force = 5 lb
Piston area = 1⁄2 in2
Pressure = Force ÷ Area
Pressure = 5 ÷ 1⁄2
Pressure = 10 psi
Figure 3-43
...
3-30
Distance moved = 1 in
Force = 50 lb
10 psi
50 lb
Force = 5 lb
1
Piston area = ⁄2 in
2
Piston area
= 5 in2
Force = Pressure × Area
Force = 10 psi × 5 in2
Force = 50 lb
Distance moved = 20 in
Input piston area = 1⁄4 in2
10 psi
Output
piston
area =
15 in2
Piston area (distance) = Piston area (distance)
1
⁄4 in2 (20 in) = 5 in (distance)
5 ÷ 5 = Distance
Distance = 1 in
Figure 3-44
...
Figure 3-45
...
input piston pushes on the piston in the second cylinder,
creating an output force of 50 pounds
...
In a simple two-piston hydraulic system, the
relationship between the piston area and the distance
moved is shown by the following formula
...
In Figure 3-44, the input force is 5 lb
and the output force is 50 lb, or 10 times greater
...
The mechanical advantage in
Figure 3-44 would be 50 divided by 5, or 10
...
Mechanical Advantage = Force Out ÷ Force In
Or
Mechanical Advantage = Distance Out ÷ Distance In
Earlier in this chapter when simple machines, such as
levers and gears were discussed, it was identified that
no machine allows us to gain work
...
Since work is equal to force times distance, if we gain
force with a hydraulic system, we must lose distance
...
In order to think about the distance that the output
piston will move in response to the movement of the
input piston, the volume of fluid displaced must be
considered
...
So when a piston
of 2 in2 moves down in a cylinder a distance of 10 in,
it displaces a volume of fluid equal to 20 in3 (2 in2 ×
10 in)
...
This concept is shown in Figure 3-45, where a small input piston moves a distance
of 20 inches, and the larger output piston only moves
a distance of 1 inch
...
An input force of 50 lb is applied, and the input piston
moves 30 inches
...
Before the
numbers are plugged into the formulas, it is often possible to analyze the variables in the system and come
to a realization about what is happening
...
That comparison tells us
that the output force will be 20 times greater than the
input force, and also that the output piston will only
move 1/20 as far
...
Bernoulli’s Principle
Bernoulli’s principle was originally stated to explain
the action of a liquid flowing through the varying
cross-sectional areas of tubes
...
A tube constructed in this manner is called a “venturi,”
or “venturi tube
...
As the passageway starts to spread out, it is
referred to as a diverging duct
...
The venturi in Figure 3-46 can be used to illustrate
Bernoulli’s principle, which states that: The static
pressure of a fluid (liquid or gas) decreases at points
where the velocity of the fluid increases, provided no
energy is added to nor taken away from the fluid
...
Velocity
Pressure
Velocity
Pressure
Low High
Low High
Low High
Low High
“A”
“B”
“C”
Low High
Low High
Velocity
Pressure
Figure 3-46
...
3-32
In the wide section of the venturi (points A and C of
Figure 3-46), the liquid moves at low velocity, producing a high static pressure, as indicated by the pressure
gauge
...
In this
narrow section, the liquid moves at a higher velocity,
producing a lower pressure than that at points A and C,
as indicated by the velocity gauge reading high and the
pressure gauge reading low
...
As the air flows through the carburetor on its way to
the engine, it goes through a venturi, where the static
pressure is reduced
...
Bernoulli’s principle is extremely important in understanding how some of the systems used in aviation
work, including how the wing of an airplane generates lift or why the inlet duct of a turbine engine on
a subsonic airplane is diverging in shape
...
The curved top
surface acts like half of the converging shaped middle
of a venturi
...
The static pressure on the bottom of the wing is now
greater than the pressure on the top, and this pressure
difference creates the lift on the wing
...
Sound
Sound has been defined as a series of disturbances in
matter that the human ear can detect
...
There are three elements
which are necessary for the transmission and reception
of sound
...
Anything which moves
back and forth (vibrates) and disturbs the medium
around it may be considered a sound source
...
When the bell is struck and
begins to vibrate, the particles of the medium (the surrounding air) in contact with the bell also vibrate
...
The eardrum, acting as detector, is set in motion by
the vibrating particles of air, and the brain interprets
the eardrum’s vibrations as the characteristic sound
associated with a bell
...
When an
object is thrown into a pool, a series of circular waves
travel away from the disturbance
...
In the cross-section perspective in Figure 3-47, notice that the water waves are
a succession of crests and troughs
...
Water waves are known as transverse waves
because the motion of the water molecules is up and
down, or at right angles to the direction in which the
waves are traveling
...
Sound travels through matter in the form of longitudinal wave motions
...
[Figure 3-48] When the tine of a tuning fork moves in
an outward direction, the air immediately in front of the
tine is compressed so that its momentary pressure is raised
above that at other points in the surrounding medium
...
Tuning fork
Rarefaction
Compression
Amplitude
Wave
length
Figure 3-48
...
When the tine returns and moves in an inward direction, the air in front of the tine is rarefied so that its
momentary pressure is reduced below that at other
points in the surrounding medium
...
The progress of any wave involves two distinct motions: (1) The wave itself moves forward with
constant speed, and (2) simultaneously, the particles
of the medium that convey the wave vibrate harmonically
...
Speed of Sound
In any uniform medium, under given physical conditions, sound travels at a definite speed
...
Even in the same medium under different conditions
of temperature, pressure, and so forth, the velocity of
sound varies
...
Pressure waves
traveling outward
Round object dropped
into the water
Crest
Crest
Trough
Figure 3-47
...
In general, a difference in density between two substances is sufficient to indicate which one will be the
faster transmission medium for sound
...
However, there are some
surprising exceptions to this rule of thumb
...
Sound travels at 16,700 fps in
3-33
Mach Number
In the study of aircraft that fly at supersonic speeds,
it is customary to discuss aircraft speed in relation
to the velocity of sound (approximately 760 miles
per hour (mph) at 59°F)
...
If the speed of sound at sea level
is 760 mph, an aircraft flying at a Mach number of
1
...
2 = 912 mph
...
The outstanding recognizable difference
between the tones produced by two different keys on a
piano is a difference in pitch
...
A
good example of frequency is the noise generated by
a turbofan engine on a commercial airliner
...
Loudness
When a bell rings, the sound waves spread out in all
directions and the sound is heard in all directions
...
A stronger blow
produces vibrations of greater amplitude in the bell,
and the sound is louder
...
Hence, the
loudness of the sound depends on the amplitude of the
vibrations of the sound waves
...
3-34
Measurement of Sound Intensity
Sound intensity is measured in decibels, with a decibel
being the ratio of one sound to another
...
A faint whisper would have an
intensity of 20 dB, and a pneumatic drill would be 80
dB
...
A 110 dB noise, by
comparison, would sound twice as loud as the jetliner’s
engine
...
Doppler Effect
When sound is coming from a moving object, the
object’s forward motion adds to the frequency as
sensed from the front and takes away from the frequency as sensed from the rear
...
The velocity of sound
in air at 0°C (32°F) is 1,087 fps and increases by 2 fps
for each Centigrade degree of temperature rise (1
...
As the sound wave advances, variations in pressure
occur at all points in the transmitting medium
...
The intensity is proportional to the square
of the pressure variation regardless of the frequency
...
THRESHOLD OF AUDIBILITY
aluminum at 20°C, and only 4,030 fps in lead at 20°C,
despite the fact that lead is much more dense than aluminum
...
180
160
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
Turbojet engine at takeoff
at 150 ft
Automobile horn
Riveting machine, as heard
by operator
An express train passing
close by
Modern turbo engine at
takeoff at 150 ft
Subway train at 20 ft
(1⁄2 the loudness of 110 dB noise)
Pneumatic drill at 50 ft
Vacuum cleaner at 50 ft
Nearby freeway auto traffic
Private business office
Residential area in the evening
Soft whisper at 5 ft
Radio studio
Faint whisper
Figure 3-49
...
why the sound from an airplane seems different as
it approaches compared to how it sounds as it flies
overhead
...
As it flies away, the loudness and
pitch both decrease noticeably
...
The sound energy being created by the
airplane piles up, and attaches itself to the structure
of the airplane
...
When the sound
of the airplane is heard, it will be in the form of what
is called a sonic boom
...
If two pieces of matter have the same natural frequency, and one of them
starts to vibrate, it can transfer its wave energy to the
other one and cause it to vibrate
...
Some piston engine powered
airplanes have an rpm range that they are placarded to
avoid, because spinning the prop at that rpm can cause
vibration problems
...
At that particular rpm, stresses can be set
up that could lead to the propeller coming apart
...
Data available about the atmosphere may determine
whether a flight will succeed, or whether it will even
become airborne
...
Pascan and Torricelli have been credited with developing the barometer, an instrument for measuring atmospheric pressure
...
They determined that air has weight
which changes as altitude is changed with respect to
sea level
...
Composition of the Atmosphere
The atmosphere is a complex and ever changing
mixture
...
In addition to a number of gases, it
contains quantities of foreign matter such as pollen,
dust, bacteria, soot, volcanic ash, spores, and dust
from outer space
...
Six
miles up, for example, it is too thin to support respiration, and 12 miles up, there is not enough oxygen to
support combustion, except in some specially designed
turbine engine powered airplanes
...
Physicists disagree as to the boundaries of the
outer fringes of the atmosphere
...
There are also certain nonconformities at various levels
...
This ozone filters
out a portion of the sun’s lethal ultraviolet rays, allowing only enough to come through to give us sunburn,
kill bacteria, and prevent rickets
...
Also in this region, all the atoms
become ionized
...
Then the temperature begins to rise to a peak value
of 77° Centigrade (350° Kelvin) at the 55 mile level
...
From the 50 mile level upward, a man or any other
living creature, without the protective cover of the
3-35
atmosphere, would be broiled on the side facing the
sun and frozen on the other
...
The atmosphere is divided into concentric layers
or levels
...
However, one
boundary, the tropopause, exists between the first and
second layer
...
The four atmosphere
layers are the troposphere, stratosphere, ionosphere, and
the exosphere
...
Atmospheric pressure is often measured by a mercury
barometer
...
It is then inverted and the open end placed in a dish of
mercury
...
Gravity acting on the mercury in the tube will try to
make the mercury run out
...
At some point these
two forces (gravity and atmospheric pressure) will
equilibrate and the mercury will stabilize at a certain
height in the tube
...
7 lb
...
92 inches tall, would also weigh
14
...
That is why 14
...
92 "Hg
...
The troposphere extends from the earth’s surface to
about 35,000 ft at middle latitudes, but varies from
28,000 ft at the poles to about 54,000 ft at the equator
...
Nearly all cloud formations are within the
troposphere
...
The stratosphere extends from the upper limits of the
troposphere (and the tropopause) to an average altitude
of 60 miles
...
Little is known about the
characteristics of the ionosphere, but it is thought that
many electrical phenomena occur there
...
The exosphere (or mesosphere) is the outer layer of the
atmosphere
...
In this layer,
the temperature is fairly constant at 2,500° Kelvin, and
propagation of sound is thought to be impossible due
to lack of molecular substance
...
This pressure is due to the
weight of the atmosphere
...
7 lb
...
7 pounds per square inch (14
...
Since atmospheric pressure at any altitude is due to
the weight of air above it, pressure decreases with
increased altitude
...
This mechanical instrument
is a much better choice than a mercury barometer for
use on airplanes
...
The calibrations are
made in thousands of feet rather than in psi or inches
of mercury
...
7 psi
Atmospheric
pressure
760 mm
29
...
Atmospheric pressure as inches of mercury
...
Density altitude
is a calculated altitude obtained by correcting pressure
altitude for temperature
...
It
contains water vapor in one of two forms: (1) fog or
(2) water vapor
...
Clouds are composed of fog
...
The presence of water vapor in the air is quite evident in Figure
3‑52, with a military F-18 doing a high-speed fly-by at
nearly Mach 1
...
Figure 3-51
...
level is 29
...
7 psi
...
58 "Hg, or 10
...
Altimeters are calibrated so that if the pressure exerted
by the atmosphere is 10
...
[Figure 3-51]
Atmospheric Density
Since both temperature and pressure decrease with
altitude, it might appear that the density of the atmosphere would remain fairly constant with increased
altitude
...
The result is that density decreases
with increased altitude
...
Since standard pressure and
temperatures have been associated with each altitude,
the density of the air at these standard temperatures
and pressures must also be considered standard
...
This gives rise to the expression “density altitude,” symbolized “Hd
...
Remember,
however, that density altitude is not necessarily true
altitude
...
In this case, at
an actual altitude of 12,000 ft, we have air that has the
As a result of evaporation, the atmosphere always
contains some moisture in the form of water vapor
...
Moisture does not consist of tiny particles of liquid held
in suspension in the air as in the case of fog, but is an
invisible vapor truly as gaseous as the air with which it
mixes
...
In flight, at cruising power, the effects
are small and receive no consideration
...
Two things
are done to compensate for the effects of humidity on
takeoff performance
...
F-18 high-speed fly-by and a vapor cloud
...
Second, because the power output
of reciprocating engines is decreased by humidity, the
manifold pressure may need to be increased above that
recommended for takeoff in dry air in order to obtain
the same power output
...
Since
water vapor is incombustible, its pressure in the atmosphere is a total loss as far as contributing to power
output
...
This mixture of water vapor, air,
and fuel enters the combustion chamber where it is
ignited
...
The
resulting horsepower loss under humid conditions can
therefore be attributed to the loss in volumetric efficiency due to displaced air, and the incomplete combustion due to an excessively rich fuel⁄air mixture
...
There are several types of charts in use
...
The effect of fog on the performance of an engine
is very noticeable, particularly on engines with high
compression ratios
...
However,
on a foggy day it is difficult to cause detonation to
occur
...
When these particles enter the cylinders, they absorb
a tremendous amount of heat energy in the process of
vaporizing
...
Fog will generally cause a decrease in horsepower
output
...
Absolute Humidity
Absolute humidity is the actual amount of the water
vapor in a mixture of air and water
...
The
amount of water vapor that can be present in the air
is dependent upon the temperature and pressure
...
When
3-38
air has all the water vapor it can hold at the prevailing
temperature and pressure, it is said to be saturated
...
This ratio is usually multiplied by 100 and expressed as a percentage
...
This indicates that the
air holds 56 percent of the water vapor required to saturate it at 75°F
...
This is because less water vapor is required
to saturate the air at the lower temperature
...
If the temperature drops below the dew point,
condensation occurs
...
This happens because the glasses were below
the dew point temperature of the air in the room
...
This principle is applied in
determining the dew point
...
The temperature
at which this occurs is the dew point
...
The dew point for a
given condition depends on the amount of water pressure present; thus, a direct relationship exists between
the vapor pressure and the dew point
...
The
conditions in the atmosphere vary continuously, and
it is generally not possible to obtain exactly the same
set of conditions on two different days or even on two
successive flights
...
The set of
standard conditions presently used in the United States
is known as the U
...
Standard Atmosphere
...
The standard
sea level conditions are:
Pressure at 0 altitude (P0) = 29
...
174 fps/s
The U
...
Standard Atmosphere is in agreement with
the International Civil Aviation Organization (ICAO)
Standard Atmosphere over their common altitude
range
...
Aircraft Theory of Flight
Before a technician can consider performing maintenance on an aircraft, it is necessary to understand the
pieces that make up the aircraft
...
For helicopters, names like main rotor, anti-torque
rotor, and autorotation come to mind as a small portion
of what needs to be understood about rotorcraft
...
Four Forces of Flight
During flight, there are four forces acting on an airplane
...
[Figure 3-53] Lift is the upward force created by the
wing, weight is the pull of gravity on the airplane’s
mass, thrust is the force created by the airplane’s propeller or turbine engine, and drag is the friction caused
by the air flowing around the airplane
...
Any
time the forces are not in balance, something about
the airplane’s condition is changing
...
When an airplane is accelerating, it has more thrust
than drag
...
When an airplane is decelerating, it has less thrust
than drag
...
When an airplane is at a constant velocity, thrust
and drag are equal
...
Four forces acting on an airplane
...
When an airplane is climbing, it has more lift than
weight
...
When an airplane is descending, it has more weight
than lift
...
When an airplane is at a constant altitude, lift and
weight are equal
...
Bernoulli’s principle, as we
refer to it today, states that “as the velocity of a fluid
increases, the static pressure of that fluid will decrease,
provided there is no energy added or energy taken
away
...
A converging shape is one whose cross-sectional area
gets progressively smaller from entry to exit
...
Figure 3-54
shows a converging shaped duct, with the air entering
on the left at subsonic velocity and exiting on the right
...
Because a unit of air must exit the duct when another
unit enters, the unit leaving must increase its velocity
as it flows into a smaller space
...
From the entry point to the exit point, the duct is spreading out and the area is getting larger
...
The total energy in the air has not changed
...
Venturi with a superimposed wing
...
Bernoulli’s principle and a converging duct
...
Bernoulli’s principle and a diverging duct
...
In the discussion of Bernoulli’s principle earlier in
this chapter, a venturi was shown in Figure 3-46
...
There are two arrows
showing airflow
...
3-40
In Figure 3-56, the air reaching the leading edge of
the wing separates into two separate flows
...
The air going over the top, because
of the curvature, has farther to travel
...
The higher velocity on the top
causes the static pressure on the top to be less than it
is on the bottom, and this difference in static pressures
is what creates lift
...
If the difference in static pressure between
the top and bottom is 0
...
Since there are
10,800 in 2 of surface area, there would be 1,080 lb of
lift (0
...
Subsonic
400
mph
In the converging part of the venturi, velocity would
increase and static pressure would decrease
...
Lift and Newton’s Third Law
Newton’s third law identifies that for every force there
is an equal and opposite reacting force
...
As the
air travels around a wing and leaves the trailing edge,
the air is forced to move in a downward direction
...
In this case, the reacting force is what we call lift
...
The mass would be
the weight of air flowing over the wing every second,
and the acceleration would be the change in velocity
the wing imparts to the air
...
They are just two different ways to describe the same
thing, namely the lift on a wing
...
An airfoil
can be the wing of an airplane, the blade of a propeller, the rotor blade of a helicopter, or the fan blade
of a turbofan engine
...
By
comparison, a propeller blade, helicopter rotor blade,
or turbofan engine fan blade rotates through the air
...
The rotating wing can be
viewed as a device that creates lift, or just as correctly,
it can be viewed as a device that creates thrust
...
The
terms and their meaning are as follows:
Camber
The camber of a wing is the curvature which is present
on top and bottom surfaces
...
The bottom of the wing, more often than not,
is relatively flat
...
The bottom of the wing has less
velocity and more static pressure, which is why the
wing generates lift
...
The
angle between the chord line and the longitudinal axis
of the airplane is known as the angle of incidence
...
If the airplane is flying
due north, and someone in the airplane is not shielded
from the elements, that person will feel like the wind
is coming directly from the south
...
Wing terminology
...
As the angle of attack increases,
the lift on the wing increases
...
When this occurs, a
condition known as a stall takes place
...
A wing in the shape
of a rectangle is very common on small general aviation airplanes
...
For airplanes that operate at high subsonic speeds, sweptback wings are common, and for
supersonic flight, a delta shape might be used
...
If a wing has a long span and a very narrow
chord, it is said to have a high aspect ratio
...
The angle of incidence of a wing is the angle formed
by the intersection of the wing chord line and the horizontal plane passing through the longitudinal axis of
the aircraft
...
This feature causes
the inboard part of the wing to stall before the outboard
part, which helps maintain aileron control during the
initial stages of a wing stall
...
Because air has viscosity, this
layer of air tends to adhere to the wing
...
Here the flow is called the laminar layer
...
As the air flows to the top of the
wing, it is directed along the wing’s surface at a high
velocity and helps keep the boundary layer from becoming turbulent and separating from the wing’s surface
...
Wing boundary layer separation
...
Here it is called
the turbulent layer
...
Where the boundary layer becomes
turbulent, drag due to skin friction is relatively high
...
As the angle of attack increases, the transition
point also tends to move forward
...
At this point, the lift of
the wing is destroyed and a condition known as a stall
has occurred
...
View B shows an extreme angle of attack and
the airflow separating and becoming turbulent on the
top of the wing
...
Boundary Layer Control
One way of keeping the boundary layer air under
control, or lessening its negative effect, is to make
the wing’s surface as smooth as possible and to keep
it free of dirt and debris
...
With a smooth and
clean wing surface, the onset of a stall is delayed and
the wing can operate at a higher angle of attack
...
On a high-speed airplane, even a few bugs
splattered on the wing’s leading edge can negatively
affect boundary layer air
...
3-42
Another way of controlling boundary layer air is to
create a suction on the top of the wing through a large
number of small holes
...
Vortex generators are used on airplanes that fly at high
subsonic speed, where the velocity of the air on the
top of the wing can reach Mach 1
...
Vortex
generators are short airfoils, arranged in pairs, located
on the wing’s upper surface
...
Wingtip Vortices
Wingtip vortices are caused by the air beneath the wing,
which is at the higher pressure, flowing over the wingtip and up toward the top of the wing
...
This vortex is also referred to as
wake turbulence, and is a significant factor in determining how closely one airplane can follow behind another
on approach to land
...
Vortices from
the wing and from the horizontal stabilizer are quite
visible on the MD-11 shown in Figure 3-59
...
Upwash is the deflection of
the oncoming airstream, causing it to flow up and over
the wing
...
This downward deflection is
what creates the action and reaction described under
lift and Newton’s third law
...
These axes of rotation are the
longitudinal, lateral, and vertical
...
As
the airplane pivots on one of these axes, it is in essence
Figure 3-59
...
pivoting around the center of gravity (CG)
...
On the brightly colored airplane shown in Figure 3‑60,
the three axes are shown in the colors red (vertical
axis), blue (longitudinal axis), and orange (lateral axis)
...
The rudder, in red, causes the airplane to move around
the vertical axis and this movement is described as
being a yaw
...
The ailerons, in blue,
cause the airplane to move around the longitudinal axis
and this movement is described as being a roll
...
The flight control that produces motion around the
indicated axis is a matching color
...
The three axes of an airplane
...
If that straight-and-level flight is
disrupted by a disturbance in the air, such as wake
turbulence, the airplane might pitch up or down, yaw
left or right, or go into a roll
...
has the nose pitch up because of wake turbulence, the
tendency will be for the nose to continue to pitch up
even after the turbulence goes away
...
Static Stability
The initial response that an airplane displays after
its equilibrium is disrupted is referred to as its static
stability
...
If the static stability is
negative, the airplane will continue to move away
from its original position after the disruptive force is
removed
...
Dynamic stability involves the oscillations
that typically occur as the airplane tries to return to its
original position or attitude
...
3-43
(A)
Positive static and
positive dynamic
stability
(B)
Positive static and
negative dynamic
stability
(C)
Positive static and
negative dynamic
stability
Time
Aft CG
limit
Center of
gravity
Time
Forward
CG limit
Center of
lift
Time
Figure 3-61
...
Figure 3-62
...
Imagine that an airplane in straight-and-level flight is
disturbed and pitches nose up
...
If it immediately returns to
straight-and-level flight, it is also said to have positive
dynamic stability
...
This
pitching up and then down is known as an oscillation
...
If the
oscillations increase over time, the airplane is classified
as having negative dynamic stability
...
airplane is concentrated
...
This gives
the airplane a nose-down pitching tendency, which is
balanced out by the force generated at the horizontal
stabilizer and elevator
...
If it is too far forward, the
forces at the tail might not be able to compensate and
it may not be possible to keep the nose of the airplane
from pitching down
...
In view A, the displacement from equilibrium goes
through three oscillations and then returns to equilibrium
...
In view C, the displacement from equilibrium is staying the same with each oscillation
...
If an
airplane is longitudinally stable, it will return to a
properly trimmed angle of attack after the force that
upset its flightpath is removed
...
There is a point on the wing of
an airplane, called the center of pressure or center of
lift, where all the lifting forces concentrate
...
This center of lift runs from wingtip to
wingtip
...
It can be seen that
the center of gravity is not only forward of the center
of lift, it is also forward of the center of gravity limit
...
This
airplane would be highly unstable longitudinally, especially at low speed when trying to land
...
The airplane will now have a tendency to pitch nose
up, which can lead to the wing stalling and possible
loss of control of the airplane
...
If one wing is lower than the other, good lateral
stability will tend to bring the wings back to a level
flight attitude
...
Dihedral is an upward angle for the wings with respect
to the horizontal, and it is usually just a few degrees
...
When the left wing drops, this
will cause the airplane to experience a sideslip toward
the low wing
...
The dihedral of a wing
...
Directional stability caused by
distance to vertical stabilizer
...
The
dihedral on a wing is shown in Figure 3-63
...
The vertical fin gives the airplane this
stability, causing the airplane to align with the relative
wind
...
The distance from the pivot point on a
weather vane to its tail is greater than the distance from
its pivot point to the nose
...
On an airplane, the same is
true
...
[Figure 3-64]
Dutch Roll
The dihedral of the wing tries to roll the airplane in
the opposite direction of how it is slipping, and the
vertical fin will try to yaw the airplane in the direction of the slip
...
If the wing
dihedral has the greatest effect, the airplane will have
a tendency to experience a Dutch roll
...
Although this condition is not
considered dangerous, it can produce an uncomfortable
feeling for passengers
...
Flight Control Surfaces
The purpose of flight controls is to allow the pilot to
maneuver the airplane, and to control it from the time
it starts the takeoff roll until it lands and safely comes
to a halt
...
In flight, and to some
extent on the ground, flight controls provide the airplane with the ability to move around one or more of
the three axes
...
Flight Controls and the Lateral Axis
The lateral axis of an airplane is a line that runs below
the wing, from wingtip to wingtip, passing through the
airplane’s center of gravity
...
The flight control that handles this
job is the elevator attached to the horizontal stabilizer,
a fully moving horizontal stabilizer, or on a v-tail configured airplane, it is called ruddervators
...
In Figure
3-66, a fully moving horizontal stabilizer, known as
a stabilator, can be seen on a Piper Cherokee Arrow,
and Figure 3-67 shows a ruddervator on a Beechcraft
Bonanza
...
Elevator on a Cessna 182 provides pitch control
...
Moving horizontal stabilizer, known as a
stabilator, on a Piper Cherokee Arrow provides pitch control
...
Ruddervators on a Beechcraft Bonanza
provide pitch control
...
Cessna 182 control wheel and rudder pedals
...
On the airplanes
shown in Figures 3-65, 3-66, and 3-67, a control wheel
or yoke is used
...
Movement of the elevator causes the nose of the airplane to pitch up or pitch down by rotating around the
lateral axis
...
3-46
On the Piper Cherokee Arrow shown in Figure 3-66,
pulling back on the control wheel causes the entire
horizontal surface (stabilator) to move, with the trailing edge deflecting upward
...
Without
this tab, the stabilator might be too easy to move and
a pilot could overcontrol the airplane
...
As the name implies, these
surfaces also act as the rudder for this airplane
...
Movement around this
axis is known as roll, and control around this axis is
called lateral control
...
The ailerons move as a result of the pilot rotating the
control wheel to the left or to the right, much the same
as turning the steering wheel on an automobile
...
Turning the control wheel to the left causes the trailing edge of the aileron on the left wing to rise up into
the airstream, and the aileron on the right wing lowers down into the airstream
...
In Figure 3-69, an Air Force F-15 can be seen doing
an aileron roll
...
Flight Controls and the Vertical Axis
The vertical axis of an airplane runs from top to bottom
through the middle of the airplane, passing through the
center of gravity
...
Movement around this axis is controlled by
the rudder, or in the case of the Beechcraft Bonanza
in Figure 3-67, by the ruddervators
...
Air Force F-15 doing an aileron roll
...
When the right rudder pedal is pushed,
the trailing edge of the rudder moves to the right, and the
nose of the airplane yaws to the right
...
Even though the rudder of the airplane will make the
nose yaw to the left or the right, the rudder is not what
turns the airplane
...
Let’s say we want to turn the airplane to the right
...
The increased lift on the left
wing also increases the induced drag on the left wing,
which tries to make the nose of the airplane yaw to
the left
...
Once the nose of the airplane is pointing in
the right direction, pressure on the rudder is no longer
needed
...
Tabs
Trim Tabs
Trim tabs are small movable surfaces that attach to
the trailing edge of flight controls
...
Trim tabs can be installed
on any of the primary flight controls
...
In order to be stable in flight, most
airplanes have the center of gravity located forward of
the center of lift on the wing
...
To relieve the pilot of the need to hold back
pressure on the control wheel, a trim tab on the elevator can be adjusted to hold the elevator in a slightly
deflected position
...
Anti-servo Tab
Some airplanes, like a Piper Cherokee Arrow, do not
have a fixed horizontal stabilizer and movable elevator
...
Because of the location of the
pivot point for this movable surface, it has a tendency
to be extremely sensitive to pilot input
...
As the trailing edge
of the stabilator moves down, the anti-servo tab moves
down and creates a force trying to raise the trailing
edge
...
The anti-servo tab on a Piper Cherokee Arrow is shown
in Figure 3-70
...
Elevator trim tab on a Cessna 182
...
Rudder on a Piper Cherokee Arrow
...
In these cases, a balance
tab can be used to generate a force that assists in the
movement of the flight control
...
Split flap
Slotted flap
Servo Tab
On large airplanes, because the force needed to move
the flight controls is beyond the capability of the pilot,
hydraulic actuators are used to provide the necessary
force
...
When
the control wheel is pulled back in an attempt to move
the elevator, the servo tab moves and creates enough
aerodynamic force to move the elevator
...
Like the
balance tab, the servo tab moves in the opposite direction of the flight control’s trailing edge
...
During normal flight, the servo
tabs act like balance tabs
...
To give the wing the ability to
produce maximum low speed lift without being drag
prohibitive, retractable high lift devices, such as flaps
and slats, are utilized
...
Flaps can be
installed on the leading edge or trailing edge, with the
leading edge versions used only on larger airplanes
...
The four different types of flaps in use are the plain,
split, slotted, and Fowler
...
When deployed downward, they
3-48
Fowler flap
Figure 3-72
...
increase the effective camber of the wing and the
wing’s chord line
...
The split flap attaches to the bottom of the wing, and
deploys downward without changing the top surface of
the wing
...
The slotted flap is similar to the plain flap, except
when it deploys, the leading edge drops down a small
amount
...
This additional airflow over the top of the flap
produces additional lift
...
When it deploys, it moves
aft in addition to deflecting downward
...
This type of flap is the most
effective of the four types, and it is the type used on
commercial airliners and business jets
...
This ducted air flows over the top of the wing at a high
velocity and helps keep the boundary layer air from
becoming turbulent and separating from the wing
...
Stabilizer
Elevator
Upper rudder
Elevator tab
Lower rudder
Anti-balance tabs
Ground spoilers
Inboard flap
Inboard aileron
Inboard aileron tab
Leading edge flaps
(extended)
Outboard flap
Balance tab
Leading edge slats
(extended)
Outboard aileron
Flight spoilers
Figure 3-73
...
Leading Edge Slats
Leading edge slats serve the same purpose as slots,
the difference being that slats are movable and can be
retracted when not needed
...
On most of today’s
commercial airliners, the leading edge slats deploy
when the trailing edge flaps are lowered
...
The controls by color are
as follows:
High-Speed Aerodynamics
Compressibility Effects
When air is flowing at subsonic speed, it acts like
an incompressible fluid
...
In a converging shaped passage, subsonic
air speeds up and its static pressure decreases
...
[Figure 3-74] At supersonic flow, air acts like
a compressible fluid
...
All aerodynamic tabs are shown in green
...
All leading and trailing edge high lift devices are
shown in red (leading edge flaps and slats, trailing
edge inboard and outboard flaps)
...
The tail mounted primary flight controls are in
yellow (rudder and elevator)
...
The wing mounted primary flight controls are in
purple (inboard and outboard aileron)
...
Supersonic airflow through a venturi
...
The Speed of Sound
Sound, in reference to airplanes and their movement
through the air, is nothing more than pressure disturbances in the air
...
As an airplane
flies through the air, every point on the airplane that
causes a disturbance creates sound energy in the form
of pressure waves
...
The speed
of sound in air changes with temperature, increasing
as temperature increases
...
Altitude in Feet
Temperature (°F)
Speed of Sound
(mph)
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
15,000
20,000
25,000
30,000
35,000
* 36,089
40,000
45,000
50,000
55,000
60,000
65,000
70,000
75,000
80,000
85,000
90,000
95,000
100,000
59
...
43
51
...
30
44
...
17
37
...
04
30
...
90
23
...
51
–12
...
15
–47
...
82
–69
...
70
–69
...
70
–69
...
70
–69
...
70
–69
...
70
–64
...
57
–48
...
11
761
758
756
753
750
748
745
742
740
737
734
721
707
692
678
663
660
660
660
660
660
660
660
660
660
660
664
671
678
684
* Altitude at which temperature stops decreasing
Figure 3-75
...
3-50
Subsonic, Transonic, and Supersonic Flight
When an airplane is flying at subsonic speed, all of the
air flowing around the airplane is at a velocity of less
than the speed of sound (known as Mach 1)
...
How fast
an airplane can fly and still be considered in subsonic
flight varies with the design of the wing, but as a Mach
number, it will typically be just over Mach 0
...
When an airplane is flying at transonic speed, part of
the airplane is experiencing subsonic airflow and part
is experiencing supersonic airflow
...
The shock wave forms 90 degrees to the airflow and
is known as a normal shock wave
...
The shock wave also causes the center of lift to
shift aft, causing the nose to pitch down
...
Transonic speed is typically between
Mach 0
...
20
...
At this speed, the shock wave which formed on top
of the wing during transonic flight has moved all the
way aft and has attached itself to the wing trailing
edge
...
20 to 5
...
If
an airplane flies faster than Mach 5, it is said to be in
hypersonic flight
...
For a slow moving airplane,
the pressure waves travel out ahead of the airplane,
traveling at the speed of sound
...
At this point the sound energy starts to
pile up, initially on the top of the wing, and eventually
attaching itself to the wing leading and trailing edges
...
If the shock waves reach the ground, and cross the path
of a person, they will be heard as a sonic boom
...
Normal shock wave
...
Sound energy in subsonic and supersonic flight
...
View B is the wing of
an airplane in supersonic flight, with the sound pressure
waves piling up toward the wing leading edge
...
If
the leading edge of the wing is blunted, instead of being
rounded or sharp, a normal shock wave will also form
in front of the wing during supersonic flight
...
The velocity of the air behind a normal shock wave is
subsonic, and the static pressure and density of the air
are higher
...
Oblique Shock Wave
An airplane that is designed to fly supersonic will have
very sharp edged surfaces, in order to have the least
amount of drag
...
These
shock waves are known as oblique shock waves
...
Figure 3-78 shows an oblique
Figure 3-78
...
shock wave on the leading and trailing edges of a
supersonic airfoil
...
For this reason, supersonic air,
when given the opportunity, wants to expand outward
...
At the point where the direction of flow changes,
an expansion wave will occur
...
An expansion wave is not a shock
wave
...
High-Speed Airfoils
Transonic flight is the most difficult flight regime for
an airplane, because part of the wing is experiencing
subsonic airflow and part is experiencing supersonic
airflow
...
In supersonic
flight, the aerodynamic center moves back to 50 percent
of the wing’s chord, causing some significant changes
in the airplane’s control and stability
...
60
View A
Airflow over the wing
reaches Mach
...
The velocity has surpassed the critical Mach
number, and a normal shock wave has formed on
the top of the wing
...
Normal shock
Subsonic
Mach number =
...
88
View D
Normal
shock
Normal
shock
Mach number =
...
05
View F
Figure 3-79
...
3-52
D
...
A normal shock wave is
now forming on the bottom of the wing as well
...
E
...
Some airflow separation is still
occurring
...
The Mach number is fairly low, and the entire wing
is experiencing subsonic airflow
...
The velocity has reached the critical Mach number,
where the airflow over the top of the wing is
reaching Mach 1 velocity
...
82
(Critical Mach no
...
80, flies too fast and enters transonic flight, some noticeable changes will take place
with respect to the airflow over the wing
...
The scenario for the
six views is as follows:
F
...
If the
wing has a sharp leading edge, the shock wave
will attach itself to the sharp edge
...
The bow wave
in front of the wing leading edge of view F would be
attached to the leading edge, if the wing was a double
wedge or biconvex design
...
Aerodynamic Heating
One of the problems with airplanes and high-speed
flight is the heat that builds up on the airplane’s surface
because of air friction
...
5, skin temperatures
on its surface range from 450°F to over 1,000°F
...
Main components of a helicopter
...
Double wedge and biconvex
supersonic wing design
...
The supersonic transport Concorde was
originally designed to cruise at Mach 2
...
0 because of structural
problems that started to occur because of aerodynamic
heating
...
Helicopter Aerodynamics
The helicopter, as we know it today, falls under the
classification known as rotorcraft
...
The rotating wing of a rotorcraft can be
thought of as a lift producing device, like the wing of
an airplane, or as a thrust producing device, like the
propeller on a piston engine
...
[Figure 3-81]
Main Rotor Systems
The classification of main rotor systems is based on
how the blades move relative to the main rotor hub
...
In the fully articulated rotor system, the blades are
attached to the hub with multiple hinges
...
Rotor systems
using this type of arrangement typically have three
or more blades
...
The hinge that
allows the blades to move fore and aft is called a drag
or lag hinge
...
These hinges and their
associated movement are shown in Figure 3-82
...
The semi-rigid rotor system is used with a two blade
main rotor
...
The teetering action allows the blades to flap, with one
blade dropping down while the other blade rises
...
Figure 3-84 shows a Bell Jet Ranger helicopter
in flight
...
With a rigid rotor system, the blades are not hinged
for movement up and down (flapping) or for movement fore and aft (drag)
...
The rigid
3-53
Lead or lag
Flap
Drag hinge
Pitch
Flap hinge
Pitch change rod
Axis of rotation
Figure 3-82
...
rotor system uses blades that are very strong and yet
flexible
...
The Eurocopter model 135 uses a rigid rotor
system
...
Eurocopter 725 main rotor head
...
Bell Jet Ranger with semi-rigid main rotor
...
If the helicopter’s main rotor system rotates
clockwise when viewed from the top, the helicopter
will try to rotate counterclockwise
...
For this reason, a helicopter
uses what is called an anti-torque system to counteract
the force trying to make it rotate
...
These blades are called a tail rotor or
anti-torque rotor, and their purpose is to create a force
(thrust) that acts in the opposite direction of the way
the helicopter is trying to rotate
...
Figure 3-85
...
Tail rotor
Figure 3-86
...
Figure 3-86 shows a three bladed tail rotor on an
Aerospatiale AS-315B helicopter
...
Some potential problems with this tail rotor
system are as follows:
• The spinning blades are deadly if someone walks
into them
...
Fenestron on a Eurocopter Model 135
...
The fan forces air
into the tail boom, where a portion of it exits out of slots
on the right side of the boom and, in conjunction with
the main rotor downwash, creates a phenomenon called
the “Coanda effect
...
The higher pressure on the left side of the boom creates the primary force that counteracts the torque of
the main rotor
...
The air exits the nozzle at a high velocity, and creates
an additional force (thrust) that helps counteracts the
torque of the main rotor
...
For helicopters with two main rotors, such as the Chinook that has a main rotor at each end, no anti-torque
• When the helicopter is in forward flight and a
vertical fin may be in use to counteract torque,
the tail rotor robs engine power and creates drag
...
Because the
rotating blades in this design are enclosed in a shroud,
they present less of a hazard to personnel on the ground
and they create less drag in flight
...
This system uses a high
Figure 3-88
...
3-55
Low pressure side
Air exit slots
High pressure side
Rotating nozzle
Figure 3-89
...
rotor is needed
...
Helicopter Axes of Flight
Helicopters, like airplanes, have a vertical, lateral, and
longitudinal axis that passes through the helicopter’s
center of gravity
...
Figure 3-90 shows the three axes of a
helicopter and how they relate to the helicopter’s movement
...
Notice in the figure that the vertical axis passes
Longitudinal
axis
Lateral
axis
Vertical axis
Figure 3-90
...
3-56
almost through the center of the main rotor, because
the helicopter’s center of gravity needs to be very close
to this point
...
Like in an airplane, rotation around this axis
is known as yaw
...
To make the nose of the
helicopter yaw to the right, the pilot pushes on the right
anti-torque pedal
...
By using the anti-torque pedals,
the pilot can intentionally make the helicopter rotate in
either direction around the vertical axis
...
Some helicopters have a vertical stabilizer, such as
those shown in Figures 3-90 and 3-92
...
Cyclic pitch
control
Collective
pitch control
Anti-torque
pedals
Figure 3-91
...
Control Around the Longitudinal and Lateral Axes
Movement around the longitudinal and lateral axes is
handled by the helicopter’s main rotor
...
The collective pitch lever is on the side of the pilot’s seat, and
the cyclic pitch lever is at the front of the seat in the
middle
...
The grip on the end of the collective pitch control is the throttle for the engine, which is
rotated to increase engine power as the lever is raised
...
The collective pitch lever may have adjustable
friction built into it, so the pilot does not have to hold
upward pressure on it during flight
...
When the cyclic pitch lever
is pushed forward, the rotor blades create more lift as
they pass through the back half of their rotation and less
lift as they pass through the front half
...
The pitch change rods that were
seen earlier, in Figures 3-82 and 3-83, are controlled
by the cyclic pitch lever and they are what change the
pitch of the rotor blades
...
The end
result is the helicopter moves in the forward direction
...
If the cyclic pitch lever is moved to the left or the right,
the helicopter will bank left or bank right
...
Just the opposite is true if the helicopter is
banking to the left
...
In Figure 3-92, an Agusta A-109 can
be seen in forward flight and banking to the right
...
Agusta A-109 banking to the right
...
Air Force CH-53 in a hover
...
Force CH-53 is seen in a hover, with all the rotor blades
flapping up as a result of creating equal lift
...
A horizontal stabilizer
can be seen on the Agusta A-109 in Figure 3-92
...
The early attempts at forward
flight resulted in the helicopter rolling over when it
tried to depart from the hover and move in any direction
...
Helicopters in Flight
Hovering
For a helicopter, hovering means that it is in flight at
a constant altitude, with no forward, aft, or sideways
movement
...
The engine of the helicopter
must be producing enough power to drive the main
rotor, and also to drive whatever type of anti-torque
system is being used
...
For a helicopter to experience ground effect, it typically
needs to be no higher off the ground than one half of its
main rotor system diameter
...
Being close to
the ground affects the velocity of the air through the
rotor blades, causing the effective angle of attack of
the blades to increase and the lift to increase
...
On
a windy day, the positive influence of ground effect is
lessened, and at a forward speed of 5 to 10 mph the
positive influence becomes less
...
When the
helicopter starts to move, the velocity of airflow seen
by the rotor blades changes
...
Viewed
from the top, as the blades move around the right side
of the helicopter, they are moving toward the nose; as
they move around the left side of the helicopter, they
are moving toward the tail
...
This causes the
blade on the right side to create more lift and the blade
on the left side to create less lift
...
In Figure 3-94, blade number 2 would be called the
advancing blade, and blade number 1 would be called
the retreating blade
...
The increased lift created
by the blade on the right side will try to roll the helicopter to the left
...
Blade rotation
Direction
of relative
wind
#1
#2
Direction
of flight
(100 mph)
Blade tip
speed— 400 mph
Blade #1 experiences 300 mph airflow (Tip speed – airspeed)
Blade #2 experiences 500 mph airflow (Tip speed + airspeed)
Figure 3-94
...
Blade Flapping
To solve the problem of dissymmetry of lift, helicopter
designers came up with a hinged design that allows the
rotor blade to flap up when it experiences increased
lift, and to flap down when it experiences decreased
lift
...
This upward motion of the blade changes the direction
of the relative wind in relation to the chord line of the
blade, and causes the angle of attack to decrease
...
The retreating blade experiences a reduced
velocity of airflow and reduced lift, and flaps down
...
The
end result is the lift on the blades is equalized, and the
tendency for the helicopter to roll never materializes
...
The rigid type
of rotor system has blades that are flexible enough to
bend up or down with changes in lift
...
Eventually, the velocity of the air over the rotor blade will
reach sonic velocity, much like the critical Mach number for the wing of an airplane
...
As the helicopter’s forward speed increases, the relative
wind over the retreating blade decreases, resulting in
a loss of lift
...
At
a high enough forward speed, the angle of attack will
increase to a point that the rotor blade stalls
...
When approximately 25 percent of the rotor system
is stalled, due to the problems with the advancing and
retreating blades, control of the helicopter will be lost
...
Autorotation
The engine on a helicopter drives the main rotor system by way of a clutch and a transmission
...
Having the
rotor system disconnect from the engine in the event of
an engine failure is necessary if the helicopter is to be
capable of a flight condition called autorotation
...
This
flight condition is similar to an airplane gliding if its
engine fails while in flight
...
The altitude of the helicopter, which equals
potential energy, is given up in order to have enough
energy (kinetic energy) to keep the rotor blades turning
...
With the airspeed bled off,
and the helicopter now close to the ground, the final
step is to use the collective pitch control to cushion the
landing
...
In Figure 3-96, a Bell Jet
Ranger is shown approaching the ground in the final
stage of an autorotation
...
Weight-shift control aircraft in level flight
...
Rotor blade airflow during normal flight
and during autorotation
...
Bell Jet Ranger in final stage of autorotation
...
The wing structure
is also tubular, with the fabric covering creating the
airfoil shape
...
Within the weight-shift control aircraft community, these aircraft are typically referred to as trikes
...
There is also a support
tube, known as a king post, extending up from the top
of the wing, with cables running down and secured to
the tubular wing structure
...
The lines that run from the king post to the trailing edge
of the wing are known as reflex cables
...
If
the aircraft goes into an inadvertent stall, having the
trailing edge of the wing in a slightly raised position
helps raise the nose of the aircraft and get it out of the
stall
...
Unlike a traditional airplane, the trike does not have
a rudder, elevator, or ailerons
...
In
Figure 3-98, the pilot’s hand is on a control bar that is
connected to a pivot point just forward of where the
wing attaches
...
Weight-shift control aircraft getting ready for flight
...
Running from the wing leading edge to trailing
edge are support pieces known as battens
...
The names of some of the primary parts of the
trike are shown in Figure 3-98, and these parts will be
referred to when the flight characteristics of the trike
are described in the paragraphs that follow
...
As the groundspeed of the aircraft
reaches a point where flight is possible, the pilot pushes
forward on the control bar, which causes the wing to
pivot where it attaches to the mast and the leading
edge of the wing tilts up
...
With sufficient lift, the trike rotates and starts
climbing
...
Once the trike is in level flight,
airspeed can be increased or decreased by adding or
taking away engine power by use of the throttle
...
Because the trike does not have a horizontal stabilizer
or elevator, it must create stability along the longitudinal axis in a different way
...
Pressure acting on the tips of the delta wing creates the force
that balances out the nose heavy tendency
...
This is possible because
the frame leading edges and the sail are flexible, which
is why they are sometimes referred to as flexible wing
aircraft
...
A traditional small airplane, like a Cessna
172, turns or banks by using the ailerons, effectively
altering the camber of the wing and thereby generating differential lift
...
The cross-bar
(wing spreader) of the wing frame is allowed to float
slightly with respect to the keel, and this, along with
some other geometric considerations allows the sail to
“billow shift
...
The throttle is typically
controlled with a foot pedal, like a gas pedal in an
automobile
...
Weight shift to the left causing a left-hand turn
...
When it is time to land, the pilot reduces engine power
with the foot-operated throttle, causing airspeed and
wing lift to decrease
...
When the trike is almost to the point of touchdown, the
engine power will be reduced and the angle of attack
of the wing will be increased, to cushion the descent
and provide a smooth landing
...
When landing in a cross wind, the
pilot will land in a crab to maintain direction down the
runway
...
wing and lifting up on it
...
A trike getting ready to touch down can be seen in
Figure 3-101
...
Push out
Figure 3-99
...
Aircraft banks to the left
because of increased lift on
the right wing
...
If the pilot pushes the bar to the right, the wing pivots
with the left wingtip dropping down and the right
wingtip rising up, causing the aircraft to bank to the
left
...
The shift in weight to the
left increases the wing loading on the left, and lessens
it on the right
...
The increased load on the left wing causes the left
wing to billow, which causes the fabric to tighten on the
right wing and the angle of attack and lift to increase
...
Billow on the left wing is depicted in Figure 3-100
...
The weight of the trike and its occupants acts
like a pendulum, and helps keep the aircraft stable in
flight
...
Once the trike is in flight and flying straight and level,
the pilot only needs to keep light pressure on the bar
that controls the wing
...
The same as with any airplane, increasing engine
power will make the aircraft climb and decreasing
3-62
Powered Parachute Aerodynamics
A powered parachute has a carriage very similar to
the weight-shift control aircraft
...
In Figure 3-102, a powered parachute is on
its approach to land with the wing fully inflated and
rising up above the aircraft
...
In between all the cells
there are holes that allow the air to flow from one cell
to the next, in order to equalize the pressure within the
inflated wing
...
The weight of the aircraft acting on these lines
and their individual lengths cause the inflated wing to
take its shape
...
As in weight-shift control aircraft, the powered parachute does not have the traditional flight controls of
a fixed-wing airplane
...
Weight-shift control aircraft landing
...
Once the groundspeed is
sufficient for the wing’s lift greater than the weight of
the aircraft, the aircraft lifts off the ground
...
The powered parachute
will typically lift off the ground at a speed somewhere
between 28 and 30 mph, and will have an airspeed in
flight of approximately 30 mph
...
Advancing the throttle makes the aircraft climb, and
retarding the throttle makes it descend
...
The throttle, for controlling engine power,
is typically located on the right-hand side of the pilot
...
These bars can be seen in Figure
3-103
...
When the right foot pedal is
pushed, the line pulls down on the trailing edge of the
right wingtip
...
When pressure is taken off the foot
Wing attach point
adjustable for CG
Steering bars to
control turning
Figure 3-102
...
Nosewheel
steering
Throttle
Figure 3-103
...
3-63
Figure 3-104
...
pedal, the drag on the entire airfoil equalizes and the
aircraft resumes its straight-and-level flight
...
To land a powered parachute, the first action the pilot
takes is to reduce engine power and allow the aircraft
to descend
...
As
the aircraft approaches the ground, the descent rate
can be lessened by increasing the engine power
...
This action increases the drag on the wing
uniformly, causing the wing to pivot aft, which raises
In Figure 3-104, the pilot is pushing on both foot pedals
and the left and right wing trailing edges are deflected
downward
...
The increase in lift reduces the descent rate to
almost nothing, and provides for a gentle landing
...
In that case, it might be
a relatively hard landing