intro course aerospace wajdi
TRANSCRIPT
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Introduction course
Aerospace engineering
February 2009
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Program of course
1. Topology of airplanes Top level
Names and location of airplanes parts/structure
Barrels / Typical section names Airbus fuselages
2. Engineering life cycle
DevelopmentDesign and Stress
Certification
Sustaining
3. Failure modes
What is basically our work at GT?: Report smallest Reserve Factor (RF) Applied loadsKnow your structure by knowing your loads
Allowed loads
RF = Allowed load/ Applied load
Types of failure modes with explanations, pictures and references (handbooks, authority requirements,Issy etc.)
4. Detailed description of fuselage engineering process
Skin geometry, loads, relevant failure modes, material, stress state
Frames geometry, loads, relevant failure modes, material, stress state
Stringers geometry, loads, relevant failure modes, material, stress state
NOTE:This is a rough setup
of the course. More chapterswill be added and contentcan be modified!
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Topology of airplanes Top level
Names and location of airplanes parts/structure:There are many aspects of design of aircraft structure. Generally,the main components of an aircraft are :
Fuselage
Empennage
Wings
The next figure shows a detailed structural design of acommercial aircraft.
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Topology of airplanes Top level
Structural Design of commercial aircraft
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Topology of airplanes Top level
Typical section name
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Movements of an airplane
Yawing: Rotating around itsvertical axis (Z- axis)
Rolling: Rotating around itslongitudinal axis (X-axis)
Pitching: Rotating around itstransverse axis (Y-axis)
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Empennage
Structurally, the empennage
consists of the entire tailassembly.
Its main purpose is to givestability to the aircraft.
The fixed parts are the verticaland horizontal stabilizer
The elevator is a movable airfoilthat controls changes in pitch,the up-and-down motion of the
aircraft's nose.
The rudder is a movable airfoilthat is used to turn the aircraft incombination with the ailerons
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Typical arrangement of the transport tail
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Wings
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It conventionally takes form of:
Spars
Ribs
Covering skin
Stringers
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Spars
Webs - resist shear loads and stabilise skin (i.e. increasebuckling resistance).
Flanges - resist compressive loads caused by wing bending.
Stringers
Further increase skin buckling resistance.
Take some of the bending load
Ribs
Maintain aerodynamic shape.
Provide anchorage points for landing gear, weapons, etc.
Skin Resist shear torsion loads ( box shapes of combined skin/web)
React axial bending loads
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Barrels / Typical section names Airbusfuselages
The fuselage is a stiffened shell commonly referred to as semi-
monocoque construction
The different sections of an aircraft fuselage are :
Forward section
Mid section
Aft section
Afterbody
In order to support the skin, its necessary to provide stiffening
members, frames, bulkheads, stringers and longerons .
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Typical section names Airbus fuselages
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Fuselage structures:shells
Fuselages are too big to be built in one piece. So, instead, they are builtas shells that are later assembled.
C for frame(cadre)
P for stringer
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The fuselage as a beam
contains:
Longitudinal elements :
- Longerons
- Stringers
Transverse elements :
- Frames
- Bulkheads
External skin
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Engineering life cycle
Development-Design
The modern aeronautical engineering of aircraft design has beenan evolutionary process accelerated in recent times from thedemanding requirements for safety and the pressures ofcompetitive economics in structural design.
The primary objective of the structural designer is :
- To achieve the maximum possible safety margin
- To achieve a reasonable lifetime of the aircraft structure
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Development testing of a transport airplane
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Engineering life cycle
Phases of airframe structural design:
Specification of function and design criteria
Determination of basic external applied loads
Calculation of internal element loads
Determination of allowable element strengths and margins ofsafety
Experimental demonstration or substantiation test programs
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Airplane design, development and certification
Design Specification
Design Criteria
Basic Loads
Airplane Design
Certification TestProgram
Approved typeCertificate
LaboratoryDevelopment
Test Data
Flight TestData
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Engineering life cycle
The dotted arrows indicate feed-back where experimental data is
utilized to modify the design as necessary
The laboratory development test is an important feature of anynew vehicle program:
To develop design data on materials and shapes
To substantiate any new theory or structural configuration
The certification test program will demonstrate success withoutdegenerating into more and expensive development work
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Engineering life cycle
Planning and structural weight
A good design is the result of proper planning and scheduling
Every aircraft engineer in a company is concerned about weight.
Finite Element Modeling (FEM)
It is the most versatile tool in structural analysis NASTRAN is one of the earliest FEM programs developed by
NASA in the mid-1960s to handle the analysis of missiles andaircraft structures
NASTRAN is one of the most used program in the aeronautic
field
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Engineering life cycle
Entire airframe finite element model
D il d d i i f f l i i
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Detailed description of fuselage engineeringprocess
Skin geometry, loads, relevant failure modes, material, stress
state:
The largest single item of the fuselage structure is the skin andits stiffeners
It is the most critical structure since it carries all of the primaryloads due to fuselage bending, shear, torsion and cabin pressure
The fuselage skin carries the shear from the applied externaltransverse and torsional forces and cabin pressure
The skin thickness required on a fuselage is thinner than onwing
External pressure loads are much lower on the fuselage than on
the wing
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Skin most important load carrying part of the fuselage.
Carries the cabin pressure load (Dp).
Carried most of the bending loads (e.g. aircraft mass)
Work like membranes (plane stress)
sx
sy
txy
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Frames geometry, loads, relevant failure modes, material,
stress state: It serves to maintain the shape of the fuselage and to reduce the
column length of the stringers to prevent general instability ofthe structure
Frames are generally of light construction
Frame load are generally small and often tend to balance eachother
Fuselage frames are equivalent in function to wing rib
The design of fuselage frames may be influenced by loadsresulting from equipment mounted in the fuselage
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Frames provide stability to the skin in circumferential direction
Work like beams (carry axial, shear, and bending loads)
F
skin alone can not
carry shear load
F
frames have bending stiffness, distribute
the shear load
deformedshape
F F
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Typical frame designs (1)
Normal frame with clip
Clip
Frame outer flange
Stringer
Skin
Frame inner flange
Frame web
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Typical frame designs (2)
Integral frame (skin connection is integrated in frame profile)
Cleat
Frame outer flange
Stringer
Skin
Frame inner flange
Frame web
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Typical frame designs (3)
Z-Section can be replaced by C-section profiles
Z-Frame + clip and skin Integral Z-frame and skin
Clip
Frame
Stringer (z-shape)
Continuous under frame
Skin
Inner flange
Stringer (z-shape)
Continuous under frame
Skin
Outer flange
Web
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Stringers geometry, loads, relevant failure modes, material,
stress state:
Further increase skin buckling resistance.
Provide stiffness in axial direction
sy in the skinforces in stringersin axial direction
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F l L
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Fuselage structures : Loads
To understand a structure, you must:
Understand the loads
Make abstraction / find analogies (e.g. fuselage looks like a
beam)
Visualize the deformation:
deformations lead to stresses
stresses lead to reaction forces
reaction forces lead to equilibrium
Important term: Load case.This is applied loads!
The applied loads lead to internalreaction loads, to give equilibrium.
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Mixture of:
Hoop stress
Shear
Longitudinal tension
Typical dominating internal loads in fuselage skin
ShearHoop stress
Longitudinal tension stress
Compression load Compression load
Fuselage weight
Applied fuselagebending moment
Typical dominating load case for a fuselage structure:
symmetric down bending + internal pressure Dp
Wing upload and torsion moment
Horizontal tail plane download
Dp
Applied fuselagecabin pressure
Reactions inthe fuselage
R F t (RF)
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Reserve Factor (RF)
A measure of strength frequently used in Europe is the Reserve
Factor (RF) with the allowed loads and applied loads expressedin the same units .
The Reserve Factor is defined as :
LoadsApplied
LoadsAllowedRF
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The field of aerospace engineering uses generally lower design
factors because the costs associated with structural weight arehigh.
This low design factor is why aerospace parts and materials aresubject to more stringent quality control
The usually applied safety factor is 1.5, but for pressurizedfuselage it is 2.0 and for landing gear structures it is 1.25
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Limit Loads are the maximum loads expected in service
At limit load, the structure may not fail neither have permanentdeformation of the structure.
Before ultimate load, no failure is allowed but permanent
deformation is allowed.
At ultimate load (usually the limit load multiplied with the safetyfactor), the aircraft structure is allowed to fail.
Materials (i e A350)
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Materials (i.e. A350)
Explanation of Failure Mode Types (SAMOD Users
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Explanation of Failure Mode Types(SAMOD User sManual)
B : Failure due to excessive bearing stress
BF : Initial buckling of skin panel at fatigue load cases (FAT..).Activated with SAMOD option sasel Ah: initial buckling of skin atlimit load of flight load cases (Information only).
BL : Lateral Stability (Buckling) of frame
BN : Tension Blunt Notch in GLARE skins
BU : Buckling of structural part, e.g. skin or web CR : Crippling acc. HSB 53211; Check for sufficient support
from a free flange (20%-rule)
Dn : Geometric check for middle flange stiffness accordingDIN4114 for different load types n
FK : Compressive strength analysis acc. to Fokker, see:SAMOD Theoretical Manual
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D1 : Compressive strength analysis of skin acc. to MBB-UT
(Erdmann), see: SAMOD Theoretical Manual D3 : Compressive strength analysis of skin acc. to modified HSB
method (Meier), see: SAMOD Theoretical Manual
DT : Damage tolerance
GB : Global Buckling
FC : Failure due to diagonal folds on the skin panel (forcedcrippling), see: SAMOD Theoretical Manual
FT : Fatigue failure
HS : Allowable stress values acc. to HSB Manual
WM : Allowable compressive forces for web modulations as
described in PROPER Theoretical Manual
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JE : Buckling according to Johnson/Euler, see: SAMOD
Theoretical Manual JM : Web buckling analysis
LS : Lateral Stability analysis of cross-beams
MT : Allowable stress values based on material values
R : Rivet failure
RC : Riveting circumferential (analysis of circumferential joints)
RF : Riveting frame (analysis of frame riveting - clip/shear web)
RL : Riveting longitudinal (analysis of longitudinal joints)
RS : Riveting skin (analysis of skin riveting on the frame)
SH : Shear
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UD : Allowable user-defined stress values Explanation of
Location 1 (LOC1) WI : Windenburg; Geometric check for sufficient support from
free flange
1-8 : Rivet row for reserve factors for riveted joints
Failure due to shear load
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Failure due to shear load
Skin panel failure due to shear :
Failure in the upper critical range differ from those in the lowercritical range
Excessive buckling concentrations occur in panel zones withlarge deformation caused by diagonal tension
This causes a reduction of the skin panel load capacity
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Forced crippling of stringer
Local failure of the compressively loaded stiffener elements (e.g.stringers ) takes places caused by deformation in the diagonaltension field.
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Stringer column buckling failure :
The column buckling is due to compressive stress in thestiffener caused by the effect of diagonal tension in the skin
Crippling of stringer sections
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Crippling of stringer sections
Crippling failure modes :
Compressive strength of stiffened shells
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Compressive strength of stiffened shells
Instability modes :
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