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PRINCIPLES OF FLIGHT

 

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CHAPTER – 1

UNITS OF MEASUREMENT

There are several different qualities we can measure and

for each different quality we assign a unit of measurement .

This sounds much more technical than it really is. When you

talk about time, you speak about seconds, minutes, hours,

days, etc... These are just some of the units of measurement

for time.

You keep track of, or measure, time with a clock or a

calendar and say things like "there is 20 minutes until

recess" or "It takes 10 days to drive across the country."

These are measurements just like any an engineer or

scientist would make and they always include two things: a

number, (20 and 10 in the two statements above) and a unit

(minutes and days) that puts the number in context. The

following physical qualities are important in aeronautics

and give some examples of the units of measurement that go

with them

Length: Length is another basic measurement that we all

use in everyday life. We say things like "I have to ride 3

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miles to school" or "I am 5 feet tall." The units of length

are inches, feet, meters, miles, etc...

Area: Area is a measure of how much room there is on a

surface like the floor space in a classroom or the size of

something that is mostly flat like a wing. Area is usually

measured in square feet (a one foot by one foot square) or

square meters in science and engineering. Other units of

area you know are acres and square miles.

Volume: Volume is how much apace there is in a container

like in a fuel tank or an airplane cabin. Cubic feet (one

foot by one foot by one foot cubes) or cubic meters,

gallons, litres, and cubic centimetres (CC's for short) are

all commonly used units of volume

Velocity: How fast you travel is measured by velocity. If you

run 10 miles in 1 hour your velocity was 10 mile per hour.

In aeronautics we usually use feet per second or meters

per second. Like these examples, units of velocity are

usually expressed as a unit of length per a unit of time. An

exception to this rule are knots, which are also often used

in aeronautics. One knot is about 1.7 feet per second.

Acceleration: Acceleration is a measure of how quickly

velocity is changing if your car goes from zero to sixty in 5

seconds, the acceleration would be (60 miles per hour) per

(5 seconds) or 12 miles per hour per second. A more typical

unit that an engineer would use is "meters per second per

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second" or "meters per second squared". Acceleration is

also a measure of how fast you slow down even though we

use the word deceleration in conversation.

Mass: Mass is the measure of how much stuff (matter) is in

an solid object or contained in a volume. Mass resists being

lifted. It also resists acceleration. If you put a lot of

people in your car you are adding a lot of mass and you will

not be able to accelerate as quickly. Mass is measured in

kilograms or sometimes as slugs. Mass is very different

from weight. Two things with the exact same mass will have

different weights on the Earth and the Moon.

Density: Density is a measure of how tightly packed

together the mass in a volume or object is. If you have a 10

gallon fish tank with water in it and another 10 gallonfish tank with air in it the one with the water is heavier

because water is much more dense than air. The units of

density are always mass per volume. kilograms per cubic

meter, slugs per gallon, slugs per cubic foot....

Force: Force is how hard an object is pushed or pulled. The

most well known force is the force of gravity called

weight. Forces are most often measured in pounds or

Newton’s.

Pressure: Pressure is a force spread out over an area

versus one that is applied at a single point. Pressure is a

very important concept in aeronautics as it is a source of

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the lift that gets an aircraft off the ground. Pressure can

be a deceivingly powerful quantity. A small pressure

applied over a large area can add up to a very large force.

pressure is measured in units of pounds per square foot,

Newton’s per square meter (or Pascals for short),

atmospheres, and bars among other things.

Temperature: Temperature is a measurement of how hot

something is. It is the same thing you hear about on the

news every day. It is measured in degrees Fahrenheit or

degrees Celsius. Everything has a temperature - the air

around you (That's what they are talking about on the

weather every morning), your body, rocks, trees,

clouds...everything.

Viscosity: Viscosity is probably the most obscure physicalquality on this list. It is a measure of how much a fluid will

resist flowing. Honey is very viscous, water is less viscous.

Flowing fluids exert a force on what they are flowing

over. This force (measured in Newton’s) is a factor of how

fast the fluid is flowing (measured in meters per second),

how thick the layer of fluid is (measured in meters), and

the viscosity. When you put all these factors together you

find that the units of viscosity are "Newton-seconds per

square meter".

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Introduction

Man has dreamed of flight for thousands of years. From the

beginning have watched birds fly and imagined the

possibilities of being free from the earth's surface.However, it took hundreds of years for us to understand

our world and the forces in it well enough to make it

possible for humans to fly through the air.

First, we needed to understand the stuff the earth and air

were made of. We invented and discovered ways of

measuring things so that we could classify and sort them.

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With this knowledge man has developed theories to explain

and predict how and why the earth and air behaves the way

it does and eventually learned to harness their power so

that we could build the engines and wings needed to get

off the ground. The science of building an airplane is

called Principles of Flight.

Before we begin to learn about it let's define it. It is the

science of designing an airplane. There are four basic

subjects that controllers must understand.

1. How the airplane and its wing slips easily through the

air and can lift itself off the ground. This is called

aerodynamics.

2. How to control the airplane so that it will be able to

turn but not spin out of control. This is calledstability and control.

3. How to build an engine (be it a jet or a propeller) so

that airplane can push its way through the wind. This

is called propulsion.

CHAPTER - 2

Aerodynamics is the study of the forces acting on an object

due to air moving past it.

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FORCES ACTING ON AN AIRPLANE

The airplane in straight-and-level unaccelerated flight is

acted on by four forces. The four forces lift , gravity,

thrust and drag.

LIFT: The upward acting force;

WEIGHT OR GRAVITY: The downward acting force;

THRUST: The forward acting force;

DRAG: The backward acting, or retarding force of wind

resistance.

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Lift opposes Weight.

Thrust opposes drag.

Drag and weight are forces inherent in anything lifted from

the earth and moved through the air. Thrust and lift are

artificially created forces used to overcome the forces of

nature and enable an airplane to fly. The engine and

propeller combination is designed to produce thrust to

overcome drag. The wing is designed to produce lift to

overcome the weight (or gravity).

In straight-and-level, unaccelerated flight, (Straight-and-

level flight is coordinated flight at a constant altitude

and heading) lift equals weight and thrust equals drag,

though lift and weight will not equal thrust and drag. Any

inequality between lift and weight will result in the

airplane entering a climb or descent. Any inequality

between thrust and drag while maintaining straight-and-

level flight will result in acceleration or deceleration

until the two forces become balanced.

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AIRFOILS

An airfoil is a device which gets a useful reaction from

moving over its surface. We will consider an airfoil a

device which, when moved through the air, is capable of

producing lift. Wings, horizontal tail surfaces, vertical tail

surfaces, and propellers are all examples of airfoils.

We will use a cross-sectional view of a wing in our

discussion. Generally the wing of the type of aircraft the

pilot will fly looks in cross section like the one in the

figure below.

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The forward part of an airfoil is rounded and is called the

leading edge. The aft part is narrow and tapered and is

called the trailing edge. A reference line often used in

discussing airfoils is the chord, an imaginary straight line

joining the extremities of the leading and trailing edges.

FORCES ACTING ON AN AIRCRAFT

A - Lift

Lift is produced by a lower pressure created on the upper

surface of an airplane's wing compared to the pressure on

the wing's lower surface, causing the wing to be "lifted"

upward. The special shape of the airplane wing (airfoil) is

designed so that air flowing over it will have to travel a

greater distance faster, resulting in a lower pressure area

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(see illustration) thus lifting the wing upward. Lift is that

force which opposes the force of gravity (or weight).

B- ThrustThrust is a force created by a power source which gives an

airplane forward motion. It can either "pull" or "push" an

airplane forward. Thrust is that force which overcomes

drag. Conventional airplanes utilize engines as well as

propellers to obtain thrust. 

C- Weight

Force resulting from the effect of the Earth’s gravity

acting on the aircraft’s mass; the force of the engines must

overpower this to keep the aircraft in the air.

D- Drag

Force opposite to thrust that creates resistance to the

aircraft’s forward movement and must be reduced.

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In aerodynamics, lift-induced drag, induced drag, vortex drag,

or sometimes drag due to lift, is a drag force that occurs

whenever a moving object redirects the airflow coming at

it. This drag force occurs in airplanes due to wings or a

lifting body redirecting air to cause lift that redirect air

to cause a down force. With other parameters remaining

the same, as the angle of attack increases, induced drag

increases.

Lift is produced by accelerating airflow over the upper

surface of a wing, creating a pressure difference between

the air flowing over the wing upper and lower surfaces. On

a wing of finite span, some air flows around the wingtip

from the lower surface to the upper surface producing

wingtip vortices. 

Wingtip vortices form the major component of wake 

turbulence.

Wake turbulence is turbulence that forms behind an

aircraft as it passes through the air. This turbulence

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includes various components, the most important of which

are wingtip vortices and jet wash. Jet wash refers simply to

the rapidly moving gasses expelled from a jet engine; it is

extremely turbulent, but of short duration. Wingtip

vortices, on the other hand, are much more stable and can

remain in the air for up to three minutes after the passage

of an aircraft. Wingtip vortices make up the primary and

most dangerous component of wake turbulence.

Wake turbulence is especially hazardous during the landing 

and take-off phases of flight, for three reasons. The first

is that during take-off and landing, aircraft operate at low

speeds and high angle of attack. This flight attitude

maximizes the formation of dangerous wingtip vortices.

Secondly, takeoff and landing are the times when a plane is

operating closest to its stall speed and to the ground -

meaning there is little margin for recovery in the event of

encountering another aircraft's wake turbulence. Thirdly,

these phases of flight put aircraft closest together and

along the same flight path, maximizing the chance of

encountering the phenomenon.

CHAPTER – 3

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Structural Components and Control of an Aircraft

The main structural components of an aircraft are:

• Rotary and fixed wing

A fixed-wing aircraft is a heavier-than-air craft whose lift is

generated not by wing motion relative to the aircraft, but

by forward motion through the air. The term is used to

distinguish from rotary-wing aircraft where the movement

of the wing surfaces relative to the aircraft generates

lift. 

Fixed wing aircraft

Fixed-wing aircraft range from small training and

recreational aircraft to wide-body aircraft and military

cargo aircraft.

Rotary-wing aircraft

A helicopter is an aircraft that is lifted and propelled by

one or more horizontal  rotors, each rotor consisting of

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two or more rotor blades. Helicopters are classified as

rotorcraft or rotary-wing aircraft  to distinguish them from

fixed-wing aircraft because the helicopter achieves lift

with the rotor blades which rotate around a mast.

The wings are the most important lift-

producing part of the aircraft. Wings

vary in design depending upon the

aircraft type and its purpose. Most

airplanes are designed so that the

outer tips of the wings are higher than

where the wings are attached to the

fuselage. This upward angle is called

the dihedral and helps keep the airplane from rolling

unexpectedly during flight. Wings also carry the fuel for

the airplane.

DIFFERENT TYPES OF WINGS

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Rectangular Straight Wing

Tapered Straight Wing

Rounded or Elliptical Straight Wing

Slight Sweepback Wing

Moderate Sweepback Wing

Great Sweepback Wing

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• Tail Plane

A tailplane, also known as horizontal stabilizer, is a small

lifting surface located behind the main lifting surfaces of

a fixed-wing aircraft as well as other non-fixed wing

aircraft such as helicopters.

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Tail Plane

or

Horizontal

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The empennage or tail assembly provides stability and

control for the aircraft. The empennage is composed of two

main parts: the vertical stabilizer (fin) to which the rudder

is attached; and the horizontal stabilizer to which the

elevators are attached. These stabilizers of the airplane

help to keep the airplane pointed into the wind. When the

tail end of the airplane tries to swing to either side, the

wind pushes against the tail surfaces, returning it to its

proper place. The rudder and elevators allow the pilot to

control the yaw and pitch motion of the airplane,

respectively.

• Fuselage

The fuselage is an aircraft's main body section that holds crew and

passengers.

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1: Subsonic 

2: High-speed / supersonic 

3: High-capacity subsonic

4: Highly-maneuverable supersonic

5: Flying boat 6: Hypersonic 

• Flaps

Flaps are hinged surfaces on the trailing edge of the wings of a fixed-wing aircraft. As

flaps are extended, the stalling speed of the aircraft is reduced. Flaps are also used on

the leading edge of the wings of some high-speed jet aircraft.

• Ailerons

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PLAIN FLAP

SPLIT FLAP

SLOTTED FLAP

FOWLER FLAP

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Ailerons are hinged control surfaces attached to the

trailing edge of the wing of a fixed-wing aircraft. The

ailerons are used to control the aircraft in roll. The two

ailerons are typically interconnected so that one goes

down when the other goes up: the down-going aileron

increases the lift 

POSITION OF AILERON AND FLAPS IN JET AIRCRAFTS

on its wing while the up-going aileron reduces the lift on

the other wing, producing a rolling moment about the

aircraft's longitudinal axis.

• Elevator

Elevators are control surfaces, usually at the rear of an aircraft,

which control the aircraft's orientation by changing the pitch of the

aircraft, and so also the angle of attack of the wing.

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• Rudder

The rudder is usually attached to the fin (or vertical 

stabilizer) which allows the pilot to control yaw in the

vertical axis, i.e. change the horizontal direction in which

the nose is pointing. The rudder's direction is manipulated

with the movement of foot pedals by the pilot

• Landing Gear

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Landing gear usually includes wheels equipped with shock 

absorbers for solid ground, but some aircraft are equipped

with skis for snow or floats for water.

FLIGHT ENVELOPE

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In aerodynamics, the flight envelope or performance envelope

of an aircraft refers to the capabilities of a design in

terms of speed and altitude.

Maximum Speeds

Airspeed is the speed of an aircraft relative to the air.

There are several different measures of airspeed: indicated

airspeed, calibrated airspeed, equivalent airspeed and true

airspeed.

Minimum and Stall Speeds

Ceiling

The service ceiling attempts to capture the maximum usable

altitude of an aircraft. Specifically, it is the density 

altitude at which flying in a clean configuration, at the

best rate of climb airspeed for that altitude and with all

engines operating and producing maximum continuous power,

will produce a 100 feet per minute climb. The highest

altitude an airplane can sustain level flight, or altitude

above which the cabin pressurization system can no longer

maintain a sufficient oxygen level for passengers and crew,

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and where the pressure differential is so great as to put

severe stress on the pressure cabin of the aircraft. Most

commercial jetliners have a ceiling of about 42,000 feet

(12,802 m) while some business jets can reach 52,000 feet

or higher.

Critical angle of attack

The critical angle of attack is the angle of attack at which

the air no longer flows smoothly over the upper surface

of the airfoil. At this point, the aircraft is said to be in a

stall. A fixed-wing aircraft typically stalls around a

certain critical angle of attack (rather than at the same

airspeed). The airspeed at which the aircraft stalls is

variable, depending on the weight of the aircraft, the load 

factor at the time and the thrust from the engine. The

critical angle of attack is typically around 15° for many

airfoils.

Modern airliners that have fly-by-wire technology avoid

the critical angle of attack by means of software in the

computer systems that govern the flight controls.

Maximum Rate of Climb

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Best rate of climb (Vy) is performed at airspeed where the

most excess power is available over that required for

level flight. This condition of climb will produce the most

gain in altitude in the least amount of time (maximum rate

of climb in feet per minute). The best rate of climb made at

full allowable power is a maximum climb. It must be fully

understood that attempts to obtain more climb performance

than the airplane is capable of by increasing pitch attitude

will result in a decrease in the rate of altitude gain.

In aerodynamics, the rate of climb RoC is the speed at which

an aircraft increases its altitude.

The rate of decrease in altitude is referred to as the rate of

descent or sink rate.

There are two traditional airspeeds for most aircraft

relating to rates of ascent, referred to as Vx and Vy.

Vx refers to the "best angle of climb speed," which is

usually less than Vy, the "best rate of climb speed." The

difference between these speeds relating to the rate of

climb lies in the climb objective.

In aerodynamics, angle of climb is the ratio between distance

travelled over the ground and altitude gained. The angle

of climb can be expressed as the angle between a plane

horizontal to the earth's surface and the actual flight

path followed by the aircraft during its ascent. See

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motion. There may be one or more pistons. Each piston is

inside a cylinder, into which a gas is introduced, either

already hot and under pressure, or heated inside the

cylinder either by ignition of a fuel air mixture (internal 

combustion engine) or by contact with a hot heat

exchanger in the cylinder. The hot gases expand, pushing

the piston to the bottom of the cylinder. The piston is

returned to the cylinder top either by a flywheel or the

power from other pistons connected to the same shaft. In

most types the expanded or "exhausted" gases are removed

from the cylinder by this stroke.

Continental O-520 is a six-cylinder, horizontally-opposed aircraft engine produced by  Continental Motor 

It’s most common application (the Cessna 404 and Cessna 421 twin-engine aircraft)

 

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Type of propellersIn designing propellers, the maximum performance of the

airplane for all condition of operation from takeoff, climb,

cruising, and high speed. The propellers may be classified

under eight general types as follows:

1. Fixed pitch: The propeller is made in one piece. Only one

pitch setting is possible and is usually two blades

propeller and is often made of wood or metal.

2. Ground adjustable pitch: The pitch setting can be adjusted

only with tools on the ground before the engine is running.

This type of propellers usually has a split hub. The blade

angle is specified by the aircraft specifications. The

adjustable - pitch feature permits compensation for the

location of the flying field at various altitudes and also

for variations in the characteristics of airplanes using the

same engine. Setting the blade angles by loosened the

clamps and the blade is rotated to the desired angle and

then tighten the clamps.

3. Two-position: A propeller which can have its pitch

changed from one position to one other angle by the pilot

while in flight.

4. Controllable pitch: The pilot can change the pitch of the

propeller in flight or while operating the engine by mean

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of a pitch changing mechanism that may be operated by

hydraulically.

5. Constant speed: The constant speed propeller utilizes a

hydraulically or electrically operated pitch changing

mechanism which is controlled by governor. The setting of

the governor is adjusted by the pilot with the rpm lever in

the cockpit. During operation, the constant speed propeller

will automatically change its blade angle to maintain a

constant engine speed. If engine power is increase, the

blade angle is increased to make the propeller absorb the

additional power while the rpm remain constant. At the

other position, if the engine power is decreased, the blade

angle will decrease to make the blades take less bite of air

to keep engine rpm remain constant. The pilot selects the

engine speed required for any particular type of operation.

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6. Full Feathering: A constant speed propeller which has the

ability to turn edge to the wind and thereby eliminate drag

and wind milling in the event of engine failure. The term

Feathering refers to the operation of rotating the blades

of the propeller to the wind position for the purpose of

stopping the rotation of the propeller to reduce drag.

Therefore, a Feathered blade is in an approximate in-line-

of-flight position, streamlined with the line of flight

(turned the blades to a very high pitch). Feathering is

necessary when the engine fails or when it is desirable to

shutoff an engine in flight.

7. Reversing: A constant speed propeller which has the

ability to assume a negative blade angle and produce a

reversing thrust. When propellers are reversed, their

blades are rotated below their positive angle, that is,

through flat pitch, until a negative blade angle is obtained

in order to produce thrust acting in the opposite direction

to the forward thrust. Reverse propeller thrust is used

where a large aircraft is landed, in reducing the length of

landing run.8. Beta Control: A propeller which allows the manual

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repositioning of the propeller blade angle beyond the

normal low pitch stop. Used most often in taxiing, where

thrust is manually controlled by adjusting blade angle

with the power lever.

Control and Operation 

Basic requirement: For flight operation, an engine is

demanded to deliver power within a relatively narrow band

of operating rotation speeds. During flight, the speed-

sensitive governor of the propeller automatically

controls the blade angle as required to maintain a

constant r.p.m. of the engine.

Three factors tend to vary the r.p.m. of the engine

during operation. These factors are power, airspeed, and air

density. If the r.p.m. is to maintain constant, the blade angle

must vary directly with power, directly with airspeed, and

inversely with air density. The speed-sensitive governor

provides the means by which the propeller can adjust itself

automatically to varying power and flight conditions while

converting the power to thrust.

JET ENGINES

A jet engine is a gas turbine engine. A jet engine develops

thrust by accelerating a relatively small mass of air to

very high velocity, as opposed to a propeller, which

develops thrust by accelerating a much larger mass of air

to a much slower velocity.

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One of the advantages  of the jet engine over the piston

engine is the jet engine’s capability of producing much

greater amounts of thrust horsepower at the high

altitudes and high speeds. In fact, turbojet engine

efficiency increases with altitude and speed.

Basic turbojet engine

In theory, the jet engine is simpler and more directly

converts thermal energy (the burning and expansion of

gases) into mechanical energy (thrust). The piston or

reciprocating engine, with all of its moving parts, must

convert the thermal energy into mechanical energy and

then finally into thrust by rotating a propeller.

Although the propeller driven airplane is not nearly as

efficient as the jet, particularly at the higher altitudes

and cruising speeds required in modern aviation, one of the

few advantages the propeller driven airplane has over the

jet is that maximum thrust is available almost at the start

of the takeoff roll. Initial thrust output of the jet engine

on takeoff is relatively lower and does not reach peak

efficiency until the higher speeds. The fanjet or turbofan 

engine was developed to help compensate for this problem

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The airspeed indicator or airspeed gauge is an instrument

used in an aircraft to display the craft's airspeed, typically

in knots, to the pilot.

The face of a true airspeed indicator typical for a faster single engine aircraft

A redline mark indicates V NE, or velocity (never exceed) . This is

the maximum demonstrated safe airspeed that the aircraft

must not exceed under any circumstances. The red line is

preceded by a yellow band which is the caution area, which

runs from V NO (maximum structural cruise speed ) to V NE. A green

band runs from V S1 to V NO. V S1 is the stall speed with flaps 

and landing gear retracted. A white band runs from V SO to

V FE. V SO is the minimum steady flight speed with flaps 

extended, and V FE is the highest speed at which flaps can be

extended. Airspeed indicators in multi-engine aircraft show

a short radial red line near to the bottom of green arc for

V mc, the minimum indicated airspeed at which the aircraft can

be controlled with the critical engine inoperative and a

blue line for V YSE, the speed for best rate of climb with the

critical engine inoperative.

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mounted in a curved sealed glass tube. No pitch information

is provided. A more correct term for this instrument is Turn  

and Slip because it is not the aircraft angle of bank that is

sensed.

Artificial Horizon

An attitude indicator (ADI), also known as  gyro horizon or

artificial horizon, is an instrument used in an aircraft to

inform the pilot of the orientation of the aircraft relativeto earth. It indicates pitch (fore and aft tilt), bank or roll

(side to side tilt) and yaw (left to right, right to left), and

is a primary instrument for flight in instrument 

meteorological conditions. Attitude indicators also have

significant application under visual flight rules, though

some light aircraft do not have them installed.

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FLIGHT CONTROL SURFACES

The three primary flight controls are the ailerons, elevator and rudder 

Ailerons: The two ailerons, one at the outer trailing edge

of each wing, are movable surfaces that control movement

about the longitudinal axis. The movement is roll. Lowering

the aileron on one wing raises the aileron on the other.

The wing with the lowered aileron goes up because of its

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increased lift, and the wing with the raised aileron goes

down because of its decreased lift. Thus, the effect of

moving either aileron is aided by the simultaneous and

opposite movement of the aileron on the other wing.

Rods or cables connect the ailerons to each other and to

the control wheel (or stick) in the cockpit. When pressure

is applied to the right on the control wheel, the left

aileron goes down and the right aileron goes up, rolling

the airplane to the right. This happens because the down

movement of the left aileron increases the wing camber

(curvature) and thus increases the angle of attack. The

right aileron moves upward and decreases the camber,

resulting in a decreased angle of attack. Thus, decreased

lift on the right wing and increased lift on the left wing

cause a roll and bank to the right.

Elevators: The elevators control the movement of the

airplane about its lateral axis. This motion is pitch. The

elevators form the rear part of the horizontal tail

assembly and are free to swing up and down. They arehinged to a fixed surface--the horizontal stabilizer.

Together, the horizontal stabilizer and the elevators form

a single airfoil. A change in position of the elevators

modifies the camber of the airfoil, which increases or

decreases lift.

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Like the ailerons, the elevators are connected to the

control wheel (or stick) by control cables. When forward

pressure is applied on the wheel, the elevators move

downward. This increases the lift produced by the

horizontal tail surfaces. The increased lift forces the tail

upward, causing the nose to drop. Conversely, when back

pressure is applied on the wheel, the elevators move

upward, decreasing the lift produced by the horizontal tail

surfaces, or maybe even producing a downward force. The

tail is forced downward and the nose up. 

The elevators control the angle of attack of the wings.

When back pressure is applied on the control wheel, the

tail lowers and the nose raises, increasing the angle of

attack. Conversely, when forward pressure is applied, the

tail raises and the nose lowers, decreasing the angle of

attack.

Rudder: The rudder controls movement of the airplane

about its vertical axis. This motion is yaw. Like the other

primary control surfaces, the rudder is a movable surface

hinged to a fixed surface which, in this case, is the vertical

stabilizer, or fin. Its action is very much like that of the

elevators, except that it swings in a different plane--from

side to side instead of up and down. Control cables connect

the rudder to the rudder pedals.

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Trim Tabs: A trim tab is a small, adjustable hinged surface

on the trailing edge of the aileron, rudder, or elevator

control surfaces. Trim tabs are labor saving devices that

enable the pilot to release manual pressure on the primary

controls.

Some airplanes have trim tabs on all three control

surfaces that are adjustable from the cockpit; others have

them only on the elevator and rudder; and some have them

only on the elevator. Some trim tabs are the ground-

adjustable type only.

The tab is moved in the direction opposite that of the

primary control surface, to relieve pressure on the

control wheel or rudder control. For example, consider

the situation in which we wish to adjust the elevator trim

for level flight. ("Level flight" is the attitude of the

airplane that will maintain a constant altitude.) Assume

that back pressure is required on the control wheel to

maintain level flight and that we wish to adjust the

elevator trim tab to relieve this pressure. Since we areholding back pressure, the elevator will be in the "up"

position. The trim tab must then be adjusted downward so

that the airflow striking the tab will hold the elevators in

the desired position. Conversely, if forward pressure is

being held, the elevators will be in the down position, so

the tab must be moved upward to relieve this pressure. In

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this example, we are talking about the tab itself and not

the cockpit control.

Rudder and aileron trim tabs operate on the same principle

as the elevator trim tab to relieve pressure on the rudder

pedals and sideward pressure on the control wheel,

respectively.

AXIS OF ROTATION

Axis of an Airplane in Flight . 

An airplane may turn about three axes. Whenever the

attitude of the airplane changes in flight (with respect to

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the ground or other fixed object), it will rotate about one

or more of these axes. Think of these axes as imaginary

axles around which the airplane turns like a wheel. The

three axes intersect at the center of gravity and each one

is perpendicular to the other two.

Longitudinal Axis: The imaginary line that extends

lengthwise through the fuselage, from nose to tail, is the

longitudinal axis. Motion about the longitudinal axis is

roll and is produced by movement of the ailerons located

at the trailing edges of the wings.

Lateral Axis: The imaginary line which extends crosswise,

wing tip to wing tip, is the lateral axis. Motion about the

lateral axis is pitch and is produced by movement of theelevators at the rear of the horizontal tail assembly.

Vertical Axis: The imaginary line which passes vertically

through the center of gravity is the vertical axis. Motion

about the vertical axis is yaw and is produced by movement

of the rudder located at the rear of the vertical tail

assembly.

AIRFOILS AND LIFT

The angle of incidence is measured by the angle at which the wing is

attached to the fuselage.

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An airfoil is a device which gets a useful reaction from air

moving over its surface. When an airfoil is moved through

the air, it is capable of producing lift. Wings, horizontal

tail surfaces, vertical tails surfaces, and propellers are

all examples of airfoils.

Generally the wing of small aircraft will look like the

cross-section of the figure above. The forward part of an

airfoil is rounded and is called the leading edge. The aft

part is narrow and tapered and is called the trailing edge.

A reference line often used in discussing airfoils is the

chord, an imaginary straight line joining the extremities of

the leading and trailing edges.

Angle of Incidence:The angle of incidence is the angle

formed by the longitudinal axis of the airplane and the

chord of the wing. The longitudinal axis is an imaginary line

that extends lengthwise through the fuselage from nose

to tail. The angle of incidence is measured by the angle at

which the wing is attached to the fuselage. The angle of

incidence is fixed --it normally cannot be changed by thepilot.

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Bernoulli's Principle: To understand how lift is produced, we

must examine a phenomenon discovered many years ago by

the scientist Bernoulli and later called Bernoulli's

Principle: The pressure of a fluid (liquid or gas) decreases

at points where the speed of the fluid increases. In other

words, Bernoulli found that within the same fluid, in this

case air, high speed flow is associated with low pressure,

and low speed flow with high pressure. This principle was

first used to explain changes in the pressure of fluid

flowing within a pipe whose cross-sectional area varied. In

the wide section of the gradually narrowing pipe, the fluid

moves at low speed, producing high pressure. As the pipe

narrows it must contain the same amount of fluid. In this

narrow section, the fluid moves at high speed, producing

low pressure.

An important application of this phenomenon is made in

giving lift to the wing of an airplane, an airfoil. The airfoil

is designed to increase the velocity of the airflow above

its surface, thereby decreasing pressure above the airfoil.

Simultaneously, the impact of the air on the lower surface

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of the airfoil increases the pressure below. This

combination of pressure decrease above and increase below

produces lift.

Lift: Probably you have held your flattened hand out of the

window of a moving automobile. As you inclined your hand

to the wind, the force of air pushed against it forcing your

hand to rise. The airfoil (in this case, your hand) was

deflecting the wind which, in turn, created an equal and

opposite dynamic pressure on the lower surface of the

airfoil, forcing it up and back. The upward component of

this force is lift; the backward component is drag .

Pressure is reduced is due to the smaller space the air has above the

wing than below. Air cannot go through the wing, so it must push 

around it. The surface air molecules push between the wing and outer 

layers of air. Due to the bump of the airfoil, the space is smaller and 

the molecules must go faster. According to Bernoulli's Law, faster air 

has lower air pressure, and thus the high pressure beneath the wing 

pushes up to cause lift.

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Parts Of An Airplane - Level 1

Control surfaces are the moveable outer surfaces of an

airplane. These surfaces control the flow of air over the

various sections of the aircraft causing it to move in

different ways. Inside the airplane, pilots control the

movement of the surfaces with their hands or feet by

pushing, pulling or turning the controls to make the

airplane move in the proper manner.

By learning the names and functions of the various

surfaces, you will appreciate the construction, design, and

aerodynamics of the airplane.

AIRPLANE An airplane is a vehicle heavier than air,

powered by an engine, which travels through

the air by the reaction of air passing over its

wings.

FUSELAGE The fuselage is the central body portion of an

airplane which accommodates the crew and

passengers or cargo.

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COCKPIT In general aviation airplanes, the cockpit is

usually the space in the fuselage for the pilot

and the passengers: in some aircrafts it is just

the pilot's compartment.

LANDING GEAR The landing gear, located underneath the

airplane, supports it while on the ground.

WINGS Wings are the parts of airplanes which provide

lift and support the entire weight of the

aircraft and its contents while in flight

PROPELLER A propeller is a rotating blade located on the

front of the airplane. The engine turns thepropeller which most often pulls the airplane

through the air.

FLAPS Flaps are the movable sections of an airplane's

wings closest to the fuselage. They are moved

in the same direction (down) and enable the

airplane to fly more slowly.

AILERONS Ailerons are the outward movable sections ofan airplane's wings which move in opposite

directions (one up, one down). They are used in

making turns.

RUDDER The rudder is the movable vertical section of

the tail which controls lateral movement.

HORIZONTAL

STABILIZER

The horizontal stabilizer is the horizontal

surface of the aft part of the fuselage used

to balance the airplane.

ELEVATOR The elevator is the movable horizontal section

of the tail which causes the plane to move up

and down.

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