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AIRCRAFT STRUCTURE-I

(ASEG 331)

Vijay Kumar PatidarAssistant Professor

College of Engineering

UPES, Dehradun


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Course Plan

LEVEL OF KNOWLEDGE REQUIRED:

Elementary concepts of Strength of Materials andAppliedMechanics.

SYLLABUS:

Unit 1- Basic Concepts of Structural Analysis

Unit 2- Bending, Shear and Torsion of open and closed thin walled tubesUnit 3- Stress Analysis of Aircraft components

Unit 4- Introduction of Matrix method in Structural analysis

Unit 5-Introduction to Finite Element Method in Structural Analysis

EVALUATION CRITERIA: Assignments + Class tests : 30%

Mid Term Examination : 20%

Final Term Examination : 50%


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Contd.

INTERNAL ASSESSMENT:WEIGHTAGE30%

Internal Assessment shall be done based on the following:

Internal Assessment Record Sheet (including Mid TermExamination marks) will be displayed on LMS at the end of

semester i.e. last week of regular classroom teaching.

Sl. No. Description % of Weightage out of 30%

1 Class Tests (2)/Quizzes(2) 12%

2 Assignments(5-6)(Problems/Presentations)

12%

3 General Discipline 6%


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Contd.

CLASS TESTS/QUIZZES:

Two Class Tests based on descriptive type theoretical &numerical questions.

Two Quizzes based on objective type questions will be held.

One class test and one quiz atleast ten days before the Mid

Term Examination and second class test and second quizatleast ten days before the End Term Examination.

Those who do not appear in Class test and quiz examinations

shall lose their marks.

ASSIGNMENTS: After completion of each unit or in the mid of the unit, there

will be home assignments based on theory and numerical

problems. Those who fail to submit the assignments by the

due date shall lose their marks.


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Contd.

DETAILED SESSION PLANOUT

TOPICS SESSIONS(No.)

READINGS Assignment

Unit-1: BASIC CONCEPTS OF STRUCTURAL

ANALYSIS

Stress, Strain, Stress-Strain andThermal relationship in 3D and 2D.

Equations of equilibrium,

Compatibility, Static and Kinematics

Indeterminacy.

Energy concepts, Virtual Work.

Loads on Aircraft Structural

Components, Functions of Different

Structural Components.

V-n Diagram.

10

Aircraft

Structures for

Engineering

Students

By

T.H.G.

MEGSON

Assignments 1


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Contd.

TOPICS SESSIONS

(No.)

READINGS Assignment

Unit-2: BENDING , SHEAR AND

TORSION OF OPEN AND CLOSED

THIN WALLED TUBES

Bending, Shear and Torsion of openand closed Thin-walled Beam.

General Stress, Strain and

Displacement Relationship for open

and single cell closed section.

Structural Idealization, Effect ofIdealization on the Analysis of open

and closed Section Beams.

12

Aircraft

Structures for

Engineering

Students

By

T.H.G. MEGSON

Analysis and

Design of Flight

vehiclesStructures

By

E.F. Bruhn.

Assignments 2

Class test-1


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Contd.

TOPICS SESSIONS

(No.)

READINGS Assignment

Unit-3: STRESS ANALYSIS OF AIRCRAFT

COMPONENTS

Tapered Beams, Wing, Fuselage frame

and Wing Ribs. Cutouts in Wings and

Fuselage. Landing Gear.

8

Same used

in Unit-2 Assignments 1Class test-1

Unit-4: INTRODUCTION OF MATRIX

METHOD IN STRUCTURAL ANALYSIS

Introduction of Flexible and Stiffness

Methods, Choice of Method Stiffness

Matrix for an Elastic Spring.

Analysis of Pin Jointed Framework,

Matrix Analysis f Space Frames, Stiffness

Matrix for Uniform Beams.

5

Aircraft

Structures

for

Engineering

Students

By

T.H.G.

MEGSON

Assignments 1


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Contd.

TOPICS SESSIONS

(No.)

READINGS Assignment

Unit-5: INTRODUCTION TO FINITE

ELEMENT METHOD IN

STRUCTURAL ANALYSIS

Introduction, Mathematical

Idealization of Structure, Elementof Discreatization, Application of

Finite Element Method, Stiffness

Method Concept, Formulation,

Formulation Procedures for

Element Structural Relationship,

Element Shape Function fromElement to System Formulation,

Simple Problem

5

Introduction to

finite

Element Analysis

byJ.N. Reddy,

Fundamentals

Of Finite Element

ByD.V. Hutton.

Assignments 1


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Contd.

SUGGESTED READINGS:

TEXT BOOK: Aircraft Structures for Engineering Students, Fourth Edition,

T.H.G. MEGSON,

REFERRENCE BOOKS:

Ref. 1. Analysis and Design of Flight vehiclesStructures, E.F. Bruhn.

Ref. 2. Aircraft structures, D. J. Perry

Ref. 3. Analysis of Aircraft Structures, B. K.Dolnaldson.

Ref. 4. Introduction to finite Element Analysis ByJ.N. Reddy

Ref.5. Fundamentals Of Finite Element, D.V. Hutton


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Main structural Parts and Their Functions

Conventional aircraft usually consist of fuselage, wingsand tail plane. The

basic functions of an aircraft's structure are to transmit and resist the

applied loads; to provide an aerodynamic shape and to protect

passengers, payload, systems, etc. from the environmental conditions

encountered in flight.

Wing:

Spars

Stringers

Ribs

Skin


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Contd.

SPAR:

Longitudinal member in the wing.

Generally wing having Two spars called Front spar (located at35% of wing chord from leading edge) and Rear spar (locatedat 65% of wing chord from the leading edge).

Generally Spar having I cross-section, because I section having

maximum moment of inertia, hence Highest strength, for thesame weight.

Spar webs takes Torsional load

(i.e. shear stresses) and

spar flanges takes bending

loads (i.e. bending stresses).


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Contd.

Stringer: Used for Bending loads.

Generally having Z, L, T, channal and small wings having rectangular cross-

sections because of easy attachment to the skin and space and weight

advantage.


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Contd.

RIBS: The dimensions of ribs are governed by their span-wise location in the

wing (i.e. Airfoil shape) and by the loads they are required to support.

Used for maintain the Airfoil shape through out the wing section.

They also act with the skin in resisting the distributed aerodynamic

pressure loads.

They distribute concentration loads (e.g. undercarriage and additional

wing store loads) into the structure.


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Contd.

Skin: The outer cover of the wing structure is skin.

The primary function of the wing skin is to form an

impermeable surface for supporting the aerodynamic

pressure distribution from which the lifting capacity of the

wing is desired.

Skin is efficient for resisting shear and tensile loads.

Skin buckles under comparatively low compressive loads.

Stringers are attached to the skin and ribs thereby dividing the

skin into panels and increasing the buckling stresses.


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FUSELAGE

The fuselage of any aircraft has TWO main functions:1. Carries the payload: passenger & cargo.

2. It forms the main structural links in the complete assembly

that is the aircraft. The fuselage often carries the engines and

undercarriage. It also responsible for providing a safeenvironment so that the crew and passenger can survive.

The fuselage is considered to be made in three sections:

The nose section.

The centre section. The aft section.

The three sections carries different loads depending on the role

of the aircraft.


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There are mainly two types of fuselage structures:

1. Monocoque structure: it is possible to make a skin strong

enough to carry all the loads without the need for anysupporting framework.

Consists of-

Skin.

Formers.

Bulkheads.


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Contd.

2. Simi monocoque structure:

In this fuselage structure the skin is used to avoid buckling, itis common for the stress skin to carry about half of the total

load carried by the skin and longerons together.

the typical fuselage structure consists of series of hoops, or

frames at intervals along the skin, which gives the fuselageits cross-sectional shape, connected by longerons that run

the length of the fuselage.

mainly consists of-

Skin

Bulkheads/ Formers (frames)

Longerons:


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TAIL PLANESThe tail-plane provides stability in pitch & roll.

Large Aircraft having

cross-section same

as wing structure.

Small Aircraft having

solid section.


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Importance of structural weight

The structure of an airplane must withstand the appliedaerodynamic load and interior loads not only for the normal

flight but also for extreme conditions may be encounteredvery rarely: High velocity vertical gust.

The essential character of an aircraft structure is light weight,because weight plays such an important role in theperformance and economics of an airplane.

The importance of empty weight should be clear from thelimitations placed on maximum takeoff weight by theavailable runway.

A pound more weight of structural weight is a pound less of

payload. The specific range is inversely proportional to the airplane

weight, so in increase in structural weight raises the fuelconsumption and the fuel cost.


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Contd. The first cost of the airplane is generally found to be

proportional to the empty weight. If the payload and range

cannot be reduced, a higher structural weight requires alarger engine to meet the takeoff and landing requirement,

thereby raising the structural weight even further.

For all these reason, the aircraft structural design has always

sought to meet the load requirements with a least possible

weight.

The potentially effect of an aircraft structural failure means

that the structure must be designed for long life either with

safe life or with fail safe design.

Safe life: safe life means that the stresses in a components are so

low that fatigue failure is not possible over the life of the

airplane.


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Contd.

Fail safe:- fail safe means that the structure has alternateloads paths so that no single failure will be effected to the

aircraft. This can be achieved by designing so that no one

component carries a large part of the load. Therefore, if one

part fails, the reminder of the structure can still carry most of

the maximum load.


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General loads on Aircraft

Before the structural design of an airplane can be made, theexternal loads acting on the airplane in flight, landing and

takeoff conditions must be known.

Limit load: limit loads are the maximum loads anticipated on the

airplane during its life time.The airplane structure shall be capable of supporting the limit

loads without suffering detrimental permanent deformations.

Ultimate or design loads: these two terms used in general to

mean the same thing. Ultimate or design loads are equal tothe limit load multiplied by a factor of safety. In general the

overall factor of safety is 1.5.


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Contd.

The board general category of external loads on conventional

aircraft can be broken down into such classifications as

follows:

Air loads:

Due to Airplane Maneuvers (under the control of the pilot)

Due to air gust (not under the control of pilot).

Landing loads:

Landing on land (friction on tyre)

Landing on water.

Power plant loads:

Thrust.

Torque.

Weight and Inertia Forces:


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Contd.

Weight:

The term weight is that constant force, proportional to itsmass. Which tends to draw every physical body towardsthe centre of the earth.

Inertia Forces:

Inertia Forces for motion of pure translation of rigid body

If the unbalanced forces acting on a rigid body causeonly a change in the magnitude of the velocity of thebody, but not in the direction, the motion is calledtranslation and from the basic physics:

Accelerating force F = M a

From the basic physics


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Inertia forces on rotating rigid bodies:

A common airplane maneuver is a motion along a

curved path in a plane parallel to the XZ plane of theairplane, and generally referred to the pitching plane.

A pull up from steady flight or a pull out from a dive

causes an airplane to follow a curved path.


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If the velocity of the airplane along the path is

constant then at= 0 and thus the inertia force Ft= 0,

leaving only the normal inertia force Fn.If the angular acceleration is constant the following

relationships hold:


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Forces on Airplane in Flight

Figure shows in general the main forces on the airplane in an

accelerated flight conditions:


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Contd.

T= engine thrust.

L = Total wing lift.D = Total airplane drag.

Ma = moment of L and D at Aerodynamic Centre.

W = weight of the airplane.

IL = inertia force normal to flight path.

ID = inertia force parallel to flight path.

Im = rotation inertia force.

E = tail load normal to flight path.

For horizontal constant velocity flight conditions, the inertia

force IL, ID and Im would be zero.


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Equation of equilibrium in steady flight:

Equation of equilibrium in accelerated flight:


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Load factors: the term load factor normally given the symbol

n can be defined as the numerical multiplying factor by which

the forces equivalent to the dynamic force system acting

during the acceleration of the airplane.

For steady flight L = W. Now assume that airplane is

accelerated upward, shows the additional inertia force acting

in downwards, or opposite to the direction of acceleration.

Thus the total airplane lift L for the un-accelerated conditionmust be multiplied by a factor nzto produce static equilibrium

in the z-direction.

Since L = W, then


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Contd.

An airplane can be accelerated along the x-axis as well as the

z-axis.


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Problem

Figure shows an airplane landing on a navy aircraft are being

arrested by a cable pull T on the airplane arresting hook. If the

airplane weight is 12000 lbs, and the airplane is given a

constant acceleration of 3.5g, find the hook pull T, wheel

reaction R, and the distance (d) between the line of action of

the hook pull and the airplane c.g. if the landing velocity is 60

MPH.


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Contd.

On contact of the airplane with the arresting cable the

airplane is decelerated to the right the motion is purely

translation horizontally. The inertia force is:

The inertia force acts opposite to the direction of motion,

hence to the left.

The unknowns T and R can now be solved for by using the

static equations of equilibrium.

To find the distance d, take moment about the airplane c.g.


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Problem

Assume that the transport aircraft as shown, has just

touchdown in landing and that a breaking force of 35000 lb,on the rear wheel is being applied to bring the airplane to

rest. The landing horizontal velocity is 85 MPH. neglecting air

forces on the airplane and assuming the propeller forces are

zero, what are the ground reactions R1 and R2. what is the

landing run distance with the constant breaking force.


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Contd.

The airplane being accelerated horizontally hence the inertia

force through the airplane c.g. acts towards the front of the

airplane. From the equilibrium equations:

Landing run:


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Contd.

To find R2, take moment about point A:


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V-n Diagram (Velocity load factor Diagram) The load Factor:

Hence

At higher speeds, nmax is limited by the structural

design of the airplane. These considerations are best

understood by examining by diagram showing load

factor versus velocity for a given airplane- the V-n

diagram.


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Consider an airplane is flying at velocity V1, Assume

that the airplane is at an angle of attack such that

CL< CLmax. This flight condition is represented bypoint 1.

Now assume that the angle of attack is increased to

that to obtaining CLmax, keeping the velocity constant

at V1. The lift increases to its maximum value for the

given V1, and hence the load factor n=L/W reaches

its maximum value of nmax for the given velocity is

given by point 2. If the angle of attack is increased further, the wing

stalls and the load factor drops. Therefore, point 3 is

stall region of the V-n diagram.


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Now as V1 is increased to a value V4, then the

maximum possible load factor nmaxalso increases, as

given by point 4.

However nmax cannot be allowed to increases

indefinitely. Beyond a certain value of load value,

defined as the limit load factor as shown by the

horizontal line BC. Structural damage may occur tothe aircraft.

The right hand side of the V-n diagram, line CD, is

high speed limit. At velocities greater than this, the

dynamic pressure becomes so large that again

structural damage may occur to the airplane.


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Finally, the bottom part of the V-n diagram, given by

curves AE and ED, corresponds to negative absolute

angles of attack, that is, negative loads factor. CurveAE defines the stall limit.

Line ED gives the negative limit load factor, beyond

which structural damage will occur.

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