Aircraft Notes

7/27/2019 Aircraft Notes

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AI RCRAFT - B AS I CS

What is aeronautics?

Aeronautics is typically defined as the art or science of flight, or the science of

operating aircraft. This includes a branch of aeronautics called aerodynamics.

Aerodynamics deals with the motion of air and the way it interacts with objects in

motion, such as an aircraft. Both of these branches are a part of the tree of

physical science. Aviation, however, refers to the operation of heavier-than-air

craft.

How did aeronautics begin?

The theoretical basis for these branches stems from the work of Sir Isaac

Newton in the 1600s. Newton developed laws that defined the effects of forces

acting on objects in motion or at rest. He also developed the concept of viscosity,

or fluid friction, which is the resistance of air or any other fluid to flow. Daniel

Bernoulli, in the 1700s, developed the principle that the speed of a fluid is directly

related to pressure. That is, the faster the flow of a fluid, the lower the pressure

that is exerted on the surface it is flowing over. For example, if air is flowing

faster over the top of a surface than under a surface, the pressure on the top of


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the surface will be less than that underneath. Understanding of these concepts

was necessary to the development of flight. Without understanding the

aerodynamic principles of flight, humans would simply be mimicking the actions

of birds. It was demonstrated through many spectacular yet often disastrous

attempts, that pure imitation would not enable humans to fly.

How did aeronautics evolve past the imitation of birds?

The science of aeronautics really began to evolve in the late 18th and early 19th

centuries. Philosophers and early scientists began to look closely at physical

phenomena such as gravity and motion. As paths of communication were

established between distant cultures, the understanding of flight began to

coalesce. With their wealth of understanding of kites, rockets and fireworks, the

Asian cultures defined and harnessed propulsion. The Europeans with their

penchant for analysis, definition and precision, began to piece together the

concept of force. This growth in knowledge and communication continued

throughout the 19th century. By the very late 19th and early 20th centuries, this

knowledge had evolved to the point where people sought to put it to practical

use. As space is the frontier of today, flight was a frontier of that time.

Heavier than air flying machines were attempted by Cayley (1843), Lillenthal

(1896), Prof. Langley (1903) before the Wright brothers succeeded in late 1903

(actually 17th Dec, 1903). The military potential of the airplane was quickly

recognized and airplanes used for surveillance very early. Airplanes equipped

with machine guns fought each other and developed into modern fighters. Larger

airplanes to carry bombs and troops rapidly developed and are the forerunners of

the bombers and military transports of the current time. After World War 2 the

pure fighter was replaced by a multi role combat aircraft combining some

features of fighters and bombers. Civil transports also developed during the time

and a large variety of civil aircraft at the current time. They cover a wide range

starting with 2 seater weighing less than a ton to large airplanes going up to 600

tons for A380.


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The classification and definition of forces involved with flight were developed. We

know them today as lift, drag, weight and thrust. Scientists began to understand

how they worked together to enable an object heavier than air to fly. Once these

concepts were well understood, it was only a matter of time before humans

figured out how to not only fly, but to control their flight. Balloons, which by this

time were old news, enabled people to fly, but aeronauts remained at the mercy

of the wind to determine where they went. With the invention of the airplane

people could fly when, how and where they wanted. Another frontier had been

conquered. Within a few short years, airplane designers refined the shape of

wings and overall construction to improve airplane performance and safety.

Further improvements in airplane design allowed flight to become accessible to

everyone.

What is an airplane?

What is the difference between aircraft and airplane? Aircraft is the more general

term, and refers to any heavier-than-air craft that is supported by its own

buoyancy or by the action of air on its structures. An airplane is a heavier-than-air

craft that is propelled by an engine and uses fixed aerodynamic surfaces (i.e.

wings) to generate lift. So, every airplane is an aircraft, but not every aircraft is an

airplane! Gliders are aircraft that are not airplanes. The Space Shuttle is

definitely an aircraft, but it is not an airplane. It does not carry engines for

propulsion. Helicopters are also aircraft that are not airplanes because their

aerodynamic surfaces are not fixed - they rotate.

Why are there so many different types of airplanes?

The characteristic that most readily identifies the type, performance and purpose

of an airplane is the shape of its wings. There are four basic wing types: straight

wings, sweep wings (forward-sweep/sweepback), delta wings and the swing-

wing (or variable sweep wing). Each shape allows for premium performance at

different altitudes and at different speeds.


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Another important discriminator between airplanes is speed. Airplanes fly at

subsonic, transonic, supersonic, and hypersonic speeds. These speed

classifications are called the "regimes" of flight. The suffix -sonic refers to the

speed of sound, which is dependent on altitude and atmospheric conditions

(nominally 340 meters per second). "Mach" is a term used to specify how many

times the speed of sound an aircraft is traveling. Mach 1 is one times the speed

of sound. Mach 2 is twice the speed of sound, and so on. Mach numbers less

than 1 are speeds less than the speed of sound. Subsonic refers to all

speeds less than Mach 1. Transonic refers to all speeds from

approximately Mach .9 to Mach 1.5 -that is, the speeds at which an aircraft is

going through the speed of sound or "breaking the sound barrier". Supersonic

refers to all speeds greater than the speed of sound, which is the same as saying

all speeds above Mach 1. Hypersonic refers to all speeds greater than Mach 5.

Note that an aircraft flying at hypersonic speeds can also be said to be flying at

supersonic speeds During flight the four forces acting on an

aircraft are Lift, Drag, Weight and Thrust


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Lift is the upward force created by the airflow as it passes over the wings. This

force is the key aerodynamic force, and is opposite the weight force. In straight-

and-level, unaccelerated flight, the aircraft is in a state of equilibrium. The lifting

force is equal to the weight of the aircraft, therefore the aircraft does not climb or

dive. If the lifting force were greater than the weight, then the aircraft would climb.

If the aircraft were to loose some of it's lift, it would continue to climb unless the

weight of the aircraft was more than the lifting force. In this instance, the aircraft

would begin to descend back to earth. Of course these observations are very

simplified. In the true world of aerodynamics, all the forces are heavily dependent

upon each other.

Lift is generated by what is known as Bernoulli Principle. It's the basic principle of

pressure differential. The discoverer, Daniel Bernoulli, simply stated "as the

velocity of a fluid increases, its internal pressure decreases." Air is considered a

fluid and therefore falls within the Bernoulli Principle. Now, let's break down this

analysis to figure out how lift is generated:

A flow that is traveling faster will have a smaller pressure, according to Bernoulli.

Airplane wings are shaped to take advantage of this principle. The curvature on

top of the wing causes the airflow on top of the wing to accelerate. This

acceleration leads to a higher velocity air on top of the wing than on bottom,


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hence a lower pressure area on top of the wing than on bottom. The resulting

pressure differential between the two wing surfaces causes an upward force we

call lift.

One thing to keep in mind is that not all the lift of an aircraft is generated by this

principle (although most of it is). Newton's Third Law of Motion also helps to

generate a small portion of the lift. The Law states "for every action there is an

equal and opposite reaction." So, the air that is deflected downward by the

bottom surface of the wing produces an upward, or lifting, force. This law of

motion is the principle reason water skiiers can stay on top of the water. The skis

deflect the water downward, producing the lifting force necessary to sustain the

skiier above the water.

Drag is the retarding force (backwards force) that limits the aircraft's speed. It is causedby the production of lift. Anytime the aircraft is producing lift, it is also producing drag.

Any deflection or interference with a smooth airflow around the airplane will cause drag.

Reducing drag is one of the main concerns of aeronautical engineers whendesigning aircraft. Drag can stress different parts of an aircraft which can lead to

structural failure during certain maneuvers. Further, reduction of drag has a"domino" effect on other important aspects of flight. The less drag an aircraft has,the faster an aircraft can go, or the less power is needed from the engine. Lesspowerful engines are generally lighter (that is, have less weight) and need lessfuel (that is, cost less to fly). A lighter aircraft means that less lift is needed to flyand the airplane can be more maneuverable. If less lift is needed, a smaller wing


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is required which decreases weight and drag. All of this, taken together, reducesthe cost of building and flying the airplane.

Thrust is the forward force that "propels" the aircraft through the air. Usually, it isprovided by an engine that turns a propeller. Each propeller blade is similar to a

wing on an aircraft. The shape and angle-of-attack of the blades produces a low

pressure region in front of the propeller and increased pressure behind it. Going

back to the Bernoulli Principle and Newton's Third Law of Motion, the aircraft has

a great tendency to move forward.

Weight is the opposing force to lift. It is caused by the downward pull of gravity

on the aircraft's mass. The weight of an airplane is not constant. It varies with the

cargo on board, the different type of equipment, passengers, and most

importantly, the fuel. As an airplane flies along, it is getting lighter because it is

burning of fuel. Crop dusters, military cargo planes, and sky diving planes also

decrease their weight during flight by either loosing their cargo or some

passengers.


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One important note to point out is that of all four of the forces, only the weightacts in a constant direction (towards the center of the earth). The other three can

vary in direction depending upon the orientation of the aircraft in the air and the

current maneuver.

The Four Forces in Balance


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Let us look more closely at the interplay between the four forces. Recall that in

our model, the four forces work in oppositional pairs: lift versus weight and thrust

versus drag.

When forces are in balance, that is their magnitudes are the same and their

directions are opposite, the speed and direction of the object will not change.

Imagine an airplane, flying along at its cruising speed and its cruising altitude.

The wings are creating enough lift to counteract the weight of the aircraft and

keep it at its cruising altitude. The engines are creating enough thrust to

counteract the drag of the aircraft and keep it at its cruising speed.

Let's say that the lift force is increased. Now there is an imbalance between the

lift force and the weight force and the airplane will ascend. Conversely if the lift

force is decreased, or the weight of the aircraft is decreased (it's using up fuel, for

instance) the lift force and the weight force will not be balanced and the

airplanewilldescend. In the same way, if the thrust force is increased, an

imbalance is created, and the airplane will increase its speed in the direction the

thrust is directed. Similarly, if the thrust is decreased, or the drag increased (say

the flaps on the wings are extended), the airplane's speed will decrease.

Thus, the task of a pilot is to manage the balance between these four forces .


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The modern aircraft has five basic structural components: fuselage, wings, empennage

(tail structures), power plant (propulsion system) and the undercarriage.

Fuselage

The main body structure is the fuselage to which all other components areattached. The fuselage contains the cockpit or flight deck, passenger

compartment and cargo compartment. While wings produce most of the lift, thefuselage also produces a little lift. A bulky fuselage can also produce a lot of

drag. For this reason, a fuselage is streamlined to decrease the drag. We usuallythink of a streamlined car as being sleek and compact - it does not present abulky obstacle to the oncoming wind. A streamlined fuselage has the same

attributes. It has a sharp or rounded nose with sleek, tapered body so that the aircan flow smoothly around it.

Unlike the wing which is subjected to large distributed air loads, the

fuselage is subjected to relatively small air loads. The primary loads on the

fuselage include large concentrated forces from wing reactions, landing gear

reactions and pay loads. For airplanes carrying passengers, the fuselage must


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also withstand internal pressures. Because of internal pressures, the fuselage

often has an efficient circular cross-section. The fuselage structure is a

semimonocoque construction consisting of a thin shell stiffened by longitudinal

axial elements (stringers and longerons) supported by many traverse frames are

rings (Bulkheads) along the length. The fuselage skin carries the shear stresses

produced by torques and transverse forces. It also bears the hoop stresses

produced by internal pressures. The stringers carry bending moments and axial

forces. They also stabilize the thin fuselage skin.

Fuselage frames often take the form of a ring. They are used to maintain the shape

of the fuselage and to shorten the span of the stringers between supports in order to

increase the buckling strength of the stinger. The loads on the frames are usually smalland self equilibrated. Consequently their constructions are light. To distribute large

concentrated forces such as those from the wing structure, heavy bulkheads are needed.

A transverse partition or a closed frame in a structure separating one

portion from another is called a Bulkhead. Also used to designate solid, webbed

or trussed members to dissipate concentrated loads into monocoque or semi-

monocoque structure especially a fuselage. Members approximately parallel

to the longitudinal axis of a beam or semi-monocoque structure are called

Longitudinal stiffeners. They are designed to stiffen the skin and assist in

resisting shear and bending loads. A stiffener is a member used to reinforce thin

sheets. Some times they are called stringers. Stringers are longitudinal members

in the fuselage to support the skin and to hold the frames in position. It is used to carry

direct load in the direction of its length. Longerons are main structural

members of the fuselage. It is generally used when there is a big cutout to be

provided. Ex: cockpit. Fairing: an auxiliary member or structure whose

primary function is to reduce the drag of the part to which it is fitted.


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WINGS

The amount of lift produced by an airfoil depends upon many factors:

angle of attack

the lift devices used (like flaps)

the density of the air

the area of the wing

the shape of the wing the speed at which the wing is traveling

The wings are the most important lift-producing part of the aircraft. Wings vary indesign 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 areattached to the fuselage. This upward angle is called the dihedral and helps keepthe airplane from rolling unexpectedly during flight. Wings also carry the fuel for

the airplane.


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The shape of a wing greatly influences the performance of an airplane. The speed

of an airplane, its maneuverability, its handling qualities, all are very dependent onthe shape of the wings. There are four basic wing shapes that are used onmodern airplanes: straight, sweep (forward and back), delta and swing-wing.

The straight wing is found mostly on small, low-speed airplanes. GeneralAviation airplanes often have straight wings. These wings provide good lift at lowspeeds, but are not suited to high speeds. Since the wing is perpendicular to theairflow it has a tendency to create appreciable drag. However, the straight wingprovides good, stable flight. It is cheaper and can be made lighter, too.

The sweepback wing is the wing of choice for most high-speed airplanes made

today. Sweep wings create less drag, but are somewhat more unstable at low

speeds. The high-sweep wing delays the formation of shock waves on the

airplane as it nears the speed of sound. The amount of sweep of the wing


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The swing-wing design attempts to exploit the high lift characteristics of a primarily

straight wing with the ability of the sweepback wing to enable high speeds. Duringlanding and takeoff, the wing swings into an almost straight position. During cruise, the

wing swings into a sweepback position. There is a price to pay with this design, however,

and that is weight. The hinges that enable the wings to swing are very heavy

High Lift Devices


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The trailing edge of the wing is equipped with flaps which move backward and

downward. These are not to be confused with ailerons, which are also located on

the trailing edge of the wing, but have an entirely different purpose. The flaps

increase the area of the wing, and the camber of the airfoil. With this increase in

area, the airflow has farther to travel which spreads the pressure difference

between the top and bottom of the wing over a larger area. An equation for the lift

force is

lift = pressure x area

Given this equation, if the area increases, the lift increases also. Conversely, if

the area decreases, so will the lift.

Slats are located on the leading edge of the wings. They slide forward and also

have the effect of increasing the area of the wing, and camber of the airfoil.

Flaps and slats are used during takeoff and landing. They enable the airplane to

get off the ground more quickly and to land more slowly. Some airplanes have

such large flaps and slats that the wing looks like it's coming apart when they are

fully extended!


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Spoilers are devices that are located on top of the wings. Spoilers have the

opposite effect from flaps and slats. They reduce lift by disrupting the airflow over

the top of the wing. Spoilers are deployed after the airplane has landed and lift is

no longer needed. They also substantially increase the drag which helps the

airplane to slow down sooner.

The wing cross-section takes the shape of an airfoil, which is designed

based on aerodynamic considerations.The wing as a whole performs the

combined function of a beam and the torsion member. It consists of axial

members in stringers, bending members in spars and shear panels in the cover

skin and webs of spars. The spar is a heavy beam running spanwise to take

transverse shear loads and spanwise bending. It is usually composed of a thin

shear panel (the web) the heavy cap or flange at the top and bottom to take

bending. Wing ribs are planer structures capable of carrying in-plane loads.

They are placed chordwise along the wingspan. It decides serving as load

redistributers, ribs also hold the skin stringer to the designed counter shape. Ribs

reduce the effective buckling length of the stringers (or the stringer-skin system)

and thus increase their compressive load capability.


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The cover skin of the wing together with the spar webs form an efficient torsion member.

For subsonic airplanes, the skin is relatively thin and may be design to undergo post

buckling. Thus, the thin skin can be assumed to make no contribution to bending of thewing box, and spars and stringers take the bending moment. Supersonic airfoils are

relatively thin compared with subsonic airfoils. To with stand high surface air loads and

to provide additional bending capability of the wing box structure, thicker skins are oftennecessary. In addition, to increase structural efficiency, stiffeners can be manufactured

(either by forging or machining) as integral parts of the skin.

Some terminologies with respect to wing are listed below.

Airfoil: any surface such as airplane wing, aileron or rudder designed to obtain

reaction from the air through which it moves.

Area: the area of the wing is the area of the projection of the actual out line on a

plane of the cord (in plan). It is usually denoted by the symbol S.

Ground angle: the acute angle between the wing cord and the horizontal when

the airplane is resting on the level ground in its normal position. It is also called

landing angle.

Angle of attack: the acute angle between the reference line in a body and the

line of the relative wind direction projected on a plane containing the reference

line and parallel in the plane of symmetry.

Angle of wing setting: the acute angle between the plane of the wing chord

and the longitudinal axis of the airplane. The angle is positive when the leading

edge is higher than the trailing edge.


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Chord line: chord line is line joining the centers of curvature of leading and

trailing edge.

Chord length: the chord length is the distance between the leading and trailing

edges measured along the chord line.

Mean Aerodynamic Chord (MAC) : the chord of an imaginary airfoil which

would have force vectors through out the flight range identical with those of an

actual wing or wings.

Camber: the curvature at the top and bottom surfaces called the camber affects

the lift and the drag significantly. Increase in camber increases lift significantly

but also drag. Camber causes the air that flows over the top of the airfoil to move

faster than the air that flows beneath it. In the 1700s, Daniel Bernoulli showed

that a fluid that flows faster over a surface will create less pressure on the

surface than fluid that flows more slowly. This concept later became known as

Bernoulli's Principle. Further, since air is a fluid, air follows Bernoulli's Principle.

Thus, we have a situation where there is less air pressure on the top of an airfoil

than underneath. This difference in pressure will cause the wing to move. That is,

the difference in pressure will generate a force. The force that is generated iscalled "lift". Bernoulli's Principle applies only to subsonic flight.

Aspect ratio: Aspect Ratio which is defined as Span / Chord or Span Square /

Area is an important feature of the plan form. With high aspect ration the induced

drag is less. A high aspect ratio (8 to 10) is often adopted for transport aircraft.

For fighter it is not practicable since long spar wing would not be stiff at very high

speeds. Other aerodynamic considerations also dictate the choice of a low

aspect ratio (2 to 4) for high-speed aircraft.


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Spar: spar is a primary beam, which extends to the full length of the wng. It is a

principle span-wise member of the wing structure of an aircraft.

Span: it is the distance measured from wing tip to the other wing tip in the plan.

Rib: a light structure conforming to the shape of the airfoil over which the skin is

attached and which transfers the air load to the spars.

Nose rib: rib between front spar and the leading edge of the airfoil.


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Inter-Spar rib: Rib between the adjacent spars.

Aileron: A hinged or movable portion of the airplane wing. The primary function

of which is to impress a rolling motion on the airplane. It is usually a part of the

trailing edge of the wing.

Differential Aileron: It is a rigging arrangement of aileron linkages such that the

deflection of the up-going aileron is approximately half that of the down-going

aileron. This is done to have drag at the wing tip of each wing has the same

value thereby eliminating the yawing moment.

Aileron angle: The angular displacement of an aileron from its neutral position. It is

positive when the trailing edge is below the neutral position.

The Empennage

The empennage is most commonly referred to as the tail of the aircraft. It

consists of two primary structures, the vertical stabilizer and the horizontal

stabilizer. Both of these stabilizers help the aircraft maintain a straight path

through the air as it flies. Both are also stationary (fixed) to the aircraft. In

essence they act like the feathers on an arrow.

o Vertical Stabilizer- This stabilizer is as its name suggest. It is the

vertical "fin" you see on an aircraft. The vertical stabilizer is home to

another control surface of the aircraft: the rudder. The rudder looks

just like the vertical stabilizer but is hinged on the trailing edge of

the stabilizer and can deflect to the left or right. This control surface

yaws the aircraft.o Horizontal Stabilizer - The horizontal stabilizer is home to the

control surface known as the elevator. The elevator is attached to

the horizontal stabilizer in much the same way the rudder is to the

vertical stabilizer. The elevators pitch the aircraft. More specifically,


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they point the nose of the aircraft either up or down in the desired

direction.

Some aircraft have "trim tabs". Most have a trim tab only on the elevator, but

some have them on the rudder and ailerons. They relieve the pressure a pilot

must exert on the control yoke to maintain the aircraft orientation he/she desires.

It is common for pilots to trim an aircraft for steady, level flight so that their hands

are free to do other things.

Figure below shows different tail configurations

Controlling Motion

An airplane has three control surfaces: ailerons, elevators and a rudder. Within

the cockpit, two controls operate the control surfaces. The control stick controls

the ailerons and elevators. The rudder pedals control the rudders


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Once in flight, an airplane can have six motions along and around the three axes.

Mathematically speaking, all possible movements that an airplane can make can

be defined in terms of these six directions. This is the basis of the mathematical

modeling of airplanes and flight. Three of the movements are linear: front and

back along the longitudinal axis; side to side along the lateral axis; up and down

along the vertical axis. The other three movements are rotational: movement

around the longitudinal axis, called roll; movement around the lateral axis, called

pitch; movement around the vertical axis, called yaw.

An airplane, can roll, pitch and yaw through the use of its control surfaces. It can

move forward and backward by using its engines. However, unlike a car, it can

move side to side or up and down by using the three rotational motions. We usestabilizers to keep the airplane flying primarily longitudinally.

The ailerons are flap-like structures on the trailing edge of the wings -one oneach side. When the pilot moves the control stick to the right, the right aileron will

tilt up and the left aileron will tilt down. This will cause the airplane to roll to theright. When the pilot moves the control stick to the left, the left aileron tilts up, theright aileron tilts down and the airplane rolls to the left. This happens because asthe aileron tilts downward (effectively increasing camber) more lift is created and

the wing rises. As it tilts upward, less lift will be created and the wing willdescend. If the wing on one side of the airplane rises and the other descends,

the airplane will roll towards the side of the decrease in lift.


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The elevators are also flap-like structures that are mounted on each side of thehorizontal stabilizer. When the pilot pushes the control stick forward, the

elevators tilt downward. This causes the tail of the airplane to rise and thefuselage to tilt down - this is called pitching down. When the pilot pulls the controlstick back, the elevators tilt upward, the tail goes down and the fuselage pitchesnose-up. When the elevator tilts downward more lift is created (like the ailerons)and the tail rises. When the elevator tilts upward, less lift is created and the tail

descends.

The two rudder pedals are located at the pilot's feet. When the pilot pushes on

the right rudder pedal, the rudder tilts to the right and the airplane yaws nose-

right. When the pilot pushes on the left rudder pedal, the rudder tilts to the left

and the airplane yaws nose-left. Again this is due to lift. However, the direction of

this lift force is different than the lift force that causes the airplane to rise. When

the rudder tilts to the right, more lift is created on the right, which "lifts" or pushes

the vertical stabilizer to the left. This, in turn, causes the airplane to yaw nose-

right. The opposite motion occurs when the rudder tilts to the left.

In trying to figure out all of this tilting right and left, remember that if the rudder is

extended so that it obstructs the airflow, then the airflow is going to push hard on

that rudder. An imbalance will be created between the side where the rudder is


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obstructing the airflow and the side where it isn't. This will cause the airplane to

move away from the side where the rudder is extended. That is why when a pilot

pushes the right rudder pedal, the rudder tilts to the right - the air will push harder

on the right side of the tail causing the tail to swing left, which will cause the nose

to swing right.

The power plant is simply the propulsion system and consists of the engines.

The sole purpose of the engines is to provide thrust for the airplane. There are

many different types of aircraft engines including: piston, turboprop, turbojet and

turbofan. Turbojet and turbofan are jet engines.

The undercarriage or landing gearconsists of struts, wheels and brakes. The

landing gear can be fixed in place or retractable. Many small airplanes have fixed

landing gear which increases drag, but keeps the airplane lightweight. Larger,

faster and more complex aircraft have retractable landing gear that can

accommodate the increased weight. The advantage to retractable landing gear is

that the drag is greatly reduced when the gear is retracted.

Aircraft materials

Traditional metallic materials used in aircraft structures are Aluminium,

Titanium and steel alloys. In the past three decades applications of advanced

fiber composites have rapidly gained momentum. To date, some modern military

jet fighters already contain composite materials up to 50% of their structural

weight. Selection of aircraft materials depends on any considerations,


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which can in general be categorized as cost and structural performance. Cost

includes initial material cost, manufacturing cost and maintenance cost. The key

material properties that are pertinent to maintenance cost and structural

performance are

Density (weight)

Stiffness (youngs modulus)

Strength (ultimate and yield strengths)

Durability (fatigue)

Damage tolerance (fracture toughness and crack growth)

Corrosion

Seldom is a single material able to deliver all desired properties in all

components of the aircraft structure. A combination of various materials is often

necessary. Table 1.1 lists the basic mechanical properties of some metallic

aircraft structural materials.

STEEL ALLOYS


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Among the three metallic materials, steel alloys have highest densities, and are

used only where high strength, high yield stress are critical. Examples include landinggear units and highly loaded fittings. The high strength steel alloy 300 M is commonly

used for landing gear components.

Besides being heavy, steel alloys are generally poor in corrosion

resistance. Components made of these alloys must be plated for corrosion

protection.

ALUMINUM ALLOYS

Aluminum alloys have played a dominant role in aircraft structures for

many decades. They offer good mechanical properties with low weight. Among

the aluminum alloys, the 2024 and 7075 alloys are perhaps the most used. The

2024 alloys (2024-T3,T42)have excellent fracture toughness and slow crack

growth rate as well as good fatigue life. The code number following T for each

aluminum alloy indicates the heat treatment process. The 7075 alloys(7075-T6,

T6510 have higher strength than the 2024 but lower fracture toughness. The2024-T3 is used in the fuselage and lower wing skins, which are prone to fatigue

due to applications of cyclic tensile stresses. For the upper wing skins, which are

subjected to compressive streses, fatigue is less of a problem, and 7075-T6 is

used.

The recently developed Aluminum Lithium alloys offer improved properties

over conventional aluminum alloys. They are about 10% stiffer and 10% lighter and have

superior fatigue performance.

TITANIUM ALLOYS


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Titanium such as Ti-6Al-4V (the number indicates the weight percentage

of the alloying element) with a density of 4.5 g/cm3 is lighter than steel (7.8 g/cm3)

but heavier than aluminum (2.7 g/cm3). Its ultimate and yield stresses are almost

double those of aluminum 7075-T6. Its corrosion resistance in general is superior

to both steel and aluminum alloys. While aluminum is usually not for applications

above 350o F, titanium, on the other hand, can be used continuously up to 1000 o

F. Titanium is difficult to machine, and thus the cost of machining titanium

parts is high. Near net shape forming is an economic way to manufacture

titanium parts. Despite its high cost, titanium has found increasing use in military

aircraft. For instance, the F-15 contains 26% (structural weight) titanium.

FIBER-REINFORCED COMPOSITES

Materials made in to fiber forms can achieve significantly better

mechanical properties than their bulk counterparts. A notable example is glass

fiber v/s bulk glass. The tensile strength of glass fiber can be 2 orders of

magnitude higher than that of bulk glass. Listed in table 1.2 are the mechanical

properties of some high performance man made fibers.

Fibers alone are not suitable for structural applications. To utilize the superior

properties of fibers, they are embedded in a matrix material that holds the fibers


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together to form a solid body capable of carrying complex loads. Matrix materials

that are currently used for forming composites include 3 major categories:

polymers, metals, and ceramics. The resulting composites are usually referred to

as polymer matrix composites (PMC), metal matrix composites (MMC), and

ceramic matrix composites(CMC). Table 1.3 presents properties of a list of

composites. Its matrix material often determines the range of service temperature

of a composite. Polymer matrix composites are usually for lower

temperature( 1500 o F) environments such as jet engines.

Fiber composites are stiff, strong, and light and are thus most suitable for aircraft

structures. They are often used in the form of laminates that consists of a number of

unidirectional laminae with different fiber orientations to provide multidirectional load

capability. Composite laminates have excellent fatigues life, damage tolerance, and

corrosion resistance. Laminate constructions offer the possibility of tailoring fiber

orientations to achieve optimal structural performance of the composite structure.

Aircraft Notes

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