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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.