AEROSPACE STRUCTURES Prof. Alessandro Airoldi INTRODUCTION TO THE COURSE

AEROSPACE STRUCTURESProf. Alessandro Airoldi

INTRODUCTION TO THE COURSE

Space Structures - Prof. Alessandro Airoldi 2

Introduction to the course

Objectives of the course

Stressed skin constructions in aircraft structures

Peculiar aspects of helicopter structures

Examples of space structures

Contents and organisation of the course



Approaches to the analysis of structures in aerospace

constructions




Aerospace constructions Aerospace structures

THRUST

INE

RT

IAL

LO

AD

AERODYNAMIC LOADS AERODYNAMIC

LOADS

INERTIAL LOAD

Structure works to transfer the applied

loads

Force equilibrium (D’Alembert principle)

Such considerations apply to all type of structures (not only aerospace structure)




Aerospace constructions Aerospace structures

Structure works to transfer the applied

loads

Force equilibrium (D’Alembert principle)

Such considerations apply to all type of structures (not only aerospace structure)


ROTOR THRUSTTHRUST AERODYNAMIC

LOADS

INERTIAL LOAD



Stiffness

THRUST

INE

RT

IAL

LO

AD

AERODYNAMIC LOADS AERODYNAMIC

LOADS

INERTIAL LOAD


Requirements:Limitation to the relative displacements due to functional requirements (e.g. aerodynamics)

Avoid frequency coupling (resonance)

StrengthAvoid permanent deformation and the collapse of the structures under operative load

Shape ConstraintsAERODYNAMICS, INTERNAL VOLUMES FOR PAYLOADS

OBJECTIVE: perform structural functions, fullfilling requirements, respect constraints with

MINIMUM WEIGHT




Analysis of existing structures helps understanding the functions of structural elements, critical issues in design, the available solution for design (synthesis)

Design is an iterative process, which involve analysis of design hypothesis at different level of detail

Why analysis ?

1

2




Enhance the capability to apply the approaches of structural mechanics to the structural types that are employed in aerospace structure. Given the applied loads:

-methods for the evaluation of internal stress and strain states

-methods for the evaluation of stiffness, displacements, natural frequencies

Learn the main features of aerospace structures: comprehension of structural roles, capability to critically analyse a structure

Achieve the bases for a proper use of structural calculation software (Finite Element Method): knowledge of principles, technologies, limitations










Stressed skin constructions in aircraft structuresTruss Structures were used at the beginning of flight history

They consist of slender members connected at the ends

They are a very efficient structural concept, which is still widely used in aerospace engineering

Engine Mounts Integrated Truss Structure Section in ISS

Vickers Wellington (1943)

PA-18 welded tubes structure



Stressed skin constructions in aircraft structuresHowever, the peculiar and severe requirements for aircraft structures led to the development of another very effective structural typology:

Thin load bearing skin (stressed skin), reinforced by longitudinal stringers and internal frames

SEMI-MONOCOQUE STRUCTURES

They still represent the basic structural concept in all aerospace structures (toghether with truss structures)




STRESS SKIN STRUCTURES

Thin load bearing skin

Longitudinal stringers

Transverse Frames

Airbus A340




Motivations for the development of stressed skin constructions can be traced to the beginning of flight

TRUSS STRUCTURE

INTERNAL FRAMES (RIBS) AND LONGITUDINAL REINFORCEMENT (SPARS)

FABRIC COVER

Biplane (1916)




2

2

1VCS

Qp

High pitch angle

Low pitch angle

WING TORSION

Torsional stiffness: critical issue in wing design




Torsional stiffness of biplane wings



Stressed skin constructions in aircraft structuresHurricane had originally a fabric cover (1935)

All metal stressed skin provided in 1939

Load bearing skin provides a closed high-stiffness path for shear stress, contribute to bending stress and stiffness




Semi-monocoque structure (1943)

Thin load bearing skin (stressed skin), reinforced by longitudinal stringers and internal frames




Modern airliner structure

COMPOSITE VERTICAL TAIL WITH SEMI-MONOCOQUE MORPHOLOGY

CLOSELY SPACED FRAME AND RIBS (internal diaphragm in fuselage and wing



Stressed skin constructions in aircraft structuresWing box and ribs

Integrally stiffened composite skin

REAR SPAR FWD SPAR




Type of diaphragms and

instability

TRUSS STRUCTURES

C – SHAPED BEAMS WITH VARIABLE SECTION AND CUTOUTS

CLOSED LOOP OF BEAMS WITH L, C, Z or other shape SECTIONS




Tail structure and bulkheads

AIRBUS A 300 TAILBULKHEAD WORKING UNDER PRESSURE LOADS

A380 COMPOSITE BULKHEAD




Supersonic fighters

WINGS BECOME VERY THIN AND STRINGERS ARE MERGED IN A SERIES OF SPARS

FUSELAGE FRAMES ARE MORE SPACED DUE TO NEED OF LARGE CUT-OUTS (Cockpit, cut-out inspections of engines, air inlets)









Helicopter anatomy

Different structural parts can be distinguished for the static design:

fuselage;

tail boom;

tail planes;

rotor blades.



Stressed-skin and helicopter structure

EH 101 structure

-FEW SPACED FUSELAGE FRAMES - LARGE CUT-OUTS

-TAIL BOOM AND PLANES FOLLOW A MORE CONVENTIONAL STRESSED-SKIN CONSTRUCTION SCHEME



Fuselage and tail boom

Apache structure



Application of beam schemes

A 109 structure

BEAM AXES



Fuselage, floor and subfloor


FUSELAGE STRUCTURE WITH LARGE CUTOUTS

FORWARD FRAME

REAR FRAME

CUTOUT FOR DOORS

ROTOR THRUST

INERTIAL LOADFLOOR AND SUBFLOOR


Importance of crashworthiness


Controlled failure of the tail boom to reduce the mass

to be decelerated

Fuel tank response Subfloor

Fuselage structure must

withstand to avoid

occupants injuries

Crashworthy seats

Roof response

Landing gears

Crashworthy seats

SOIL

OCCUPANTSSeatsSubfloor

Landing Gear Fuselage

Energy Absrober

Structural part the influences occupant survivability


Rotor Blades

-ORIGINAL DESIGN FOLLOWED THE PRINCIPLES OF STRESSED SKIN CONSTRUCTION

APPLICATION OF COMPOSITES:

D SPAR (UD FIBRE REINFORCEMENT)

TORSIONAL STIFFNESS PROVIDED BY +/-45 BOXES

FOAM AND HONEYCOMB FILLERS


VERY LARGE CENTRIFUGAL LOADS


Damage tolerance and helicopter composite components

D-spar: 0° UD bulk core wrapped by +/- 45° anti-torsion box

Carbon +/-45° trailing edge skin

Metallic wear strap Nomex honeycomb

Rotor Blades: section


Damage tolerance and helicopter composite components

Blade root

Centrifugal load is reacted at two lugs connecting the

blade to the hub

UD material of D-spar arranged in ribbons wrapped

around lug bushes

Rotor Blades: root










Examples of space structures: SPACE SHUTTLE

SRB External tank

Orbiter

Unique combination of semi-monocoque, pressure vessels, truss structures structural concepts

Many different materials used:

-Aluminium alloy

- high strength steel

-Titanium

-Boron/aluminium composite

-Carbon/epoxy composites

-Fibreglass

-Ceramics



Solid bustersMAIN STRUCTURE:

SEGMENTED STRUCTURE (11 SEGMENT)HIGH STRENGTH STEEL 13 mm THICK

JOINED BY STEEL PINS

JUNCTIONS WRAPPED BY FIBERGLASS

SEALED WITH RUBBER BANDS

SUCH MAIN STRUCTURE IS CLOSED BY THE FWD AND AFT SEGMENT DOMES

IT IS THE EXTERNAL STRUCTURE BETWEEN THE FORWARD AND THE AFT SKIRT

EXTERNAL COVER, SUCH AS NOSE CAP AND SKIRTS ARE MADE OF WELDED ALUMINUN




PRE-FORMED ALUMINUM ELEMENTS (PANELS, MACHINED THICK ELEMENTS)

PRESENCE OF INTEGRALLY MACHINED STRINGERS AND RING FRAMES

RING FRAMES STABILIZE THE TANK AT HIGH COMPRESSIVE LOADS

External TankTWO TANKS: OXYGEN AND HYDROGEN

INTERTANK STRUCTURE IS A MORE CONVENTIONAL SEMIMONOCOQUE STRUCTURE (PANELS-SKIN-FRAMES MECHANICALLY JOINTED)




Orbiter


BASED ON SEMIMONOCOQUE PRINCIPLES

LARGE PARTS MADE OF ALUMINUM ALLOY

PECULIAR ASPECTS

CONVENTIONAL FORWARD FUSELAGE STRUCTURE HOSTS WELDED PRESSURISED CREW MODULE

CENTRAL SECTION FRAMES MADE OF BORON/ALUMINUM TRUSS STRUCTURE

THRUST BEARING TRUSS STRUCTURE WITH BORON/EPOXY REINFORCEMENTS

WINGS WITH HONEYCOMB SKIN COVER



Orbiter


FORWARD FUSELAGE: EXTERNAL SHELL STRUCTURE (SEMIMONOCOQUE CONCEPT)

INTERNAL PRESSURIZED VESSEL




Orbiter

CENTRAL SECTION

LONGHERON CARRY BENDING LOADS

HIGH STIFFNESS- STRENGTH REQUIRMENT FOR FRAMES: TRUSS WITH BORON/ALUMINUM TUBES

CONCEPTS OF STRESSED SKIN CONSTRUCTION LARGELY EMPLOYED




Orbiter

GRAPHITE\EPOXY PAYLOAD BAY DOORS

REINFORCED BY FRAMES AND END TORQUE BOXES

HIGH STRENGTH 3D TRUSS STRUCTURE TO SUSTAIN THE THRUST LOAD OF MAIN ENGINES




Orbiter

CONVENTIONAL ALUMINUM STRUCTURE WITH MULTI SPAR AND RIB ARRANGEMENT

HONEYCOMB SKIN REINFORCED BY ALUMINUM HAT-SHAPED STRINGERS



SEMI-MONOCOQUE AL 7075 INTERTANKS AND SKIRTS

AL 2219 – T87 TANK WITH ANTI-SLOSH BAFFLES (diaphragms that reduces fuel movements)

First Stage

Examples of space structures: SATURN V

Separate serial tanks within semimonocque structure



INTEGRALLY STIFFENED TANKS MADE OF DIFFUSION-WELDED AL 2014 PARTS

Second Stage


COMMON BULKHEAD: AL 2014 SHEET + FIBERGLASS/ PHENOLIC HONEYCOMB CORE

Integral serial tanks with common bulkhead

SKIRTS, INTERSTAGES, THRUST STRUCTURE: AL7075 SEMIMONOCOQUE



Third Stage

Serial tanks with common bulkhead

VERY SIMILAR TO 2° STAGE STRUCTURE


INTEGRALLY STIFFENED TANKS AND SEMIMONOCOQUE STRUCTURES



Examples of space structures: SPACECRAFTS

PRIMARY STRUCTURES

IN SPACECRAFTS THE DISTINCTION BETWEEN PRIMARY AND SECONDARY STRUCTURES IS IMPORTANT:

Experimental spacecraft designed at John Hopkins University: multisensor platform including a Spatial Infrared Telescope

TRANSMIT LOADS TO THE BASE OF THE SATELLITE THROUGH SPECIFICALLY DESIGN COMPONENTS (CENTRAL TUBE, HONEYCOMB PLATFORM, BAR TRUSS, ETC.).

PROVIDE THE ATTACHEMENT POINTS FOR THE PAYLOAD AND THE ASSOCIATED EQUIPMENTS.

FAILURE OF THE PRIMARY STRUCTURE LEADS TO COLLAPSE OF SATELLITE

SECONDARY STRUCTURES

BAFFLE, THERMAL BLANKET SUPPORT AND SOLAR PANELS MUST ONLY SUPPORT THEMSELVES AND ARE ATTACHED TO THE PRIMARY STRUCTURE WHICH GUARANTEE THE OVERALL STRUCTURAL INTEGRITY.




SEVERAL DIFFERENT STRUCTURAL TYPES:-TRUSS (ALSO IN HIGH STIFFNESS/STRENGTH COMPOSITE MATERIAL)- HONEYCOMB PANELS (OFTEN USED FOR ELECTRONIC SUPPORT AND SOLAR CELL SUPPORT)-MACHINED BEAMS AND PLATES




MANNED SPACECRAF INCLUDES -TRUSS STRUCTURES-SEMI-MONOCOQUE CONCEPTS (THIN WALLED STRUCTURES WITH STIFFENERS AND FRAMES)-STIFFENED PRESSURE VESSELS


• Central role of two different but very effective structural types: semi-monocoque and truss structures

Beam models can be applied at level of the vehicle structure, for the analyses of truss systems, for the analyses of ribs and frames

Plate theory is required to understand the behavior of panels and covers

What have we learned ?Structural concepts: truss and semi-monocoque

Basic structural elements: beams and plates

Different materials: metals, fiber reinforced composites, sandwich plates


Composite materials are used to increase structural efficiency: lower weight, higher stiffness (and strength ?)

Composite structures requires additional tools for analysis and design


Introductory lessons

1. CONTINUUM MECHANICS

2. BEAM MODELS AND BEAM SYSTEMS

3. SEMI-MONOCOQUE STRUCTURES

4. DISPLACEMENT BASED APPROACHES

5. PLATES AND COMPOSITES

6. INSTABILITY

7. FE METHOD

Course Content and Organisation:Lectures (theory)



Course Content and Organisation:

Course Material & Textbooks

Slides of the lectures will be provided during the course

MALVERN, MECHANICS OF CONTINUOUS MEDIUM

T.H. MEGSON, AIRCRAFT STRUCTURES FOR ENGINEERING STUDENTS, BUTTERWORTH-HEINEMANN, 1972

J.N. REDDY, ENERGY PRINCIPLES AND VARIATIONAL METHODS IN APPLIED MECHANICS, WILEY 2002

K.J. BATHE, FINITE ELEMENT PROCEDURES, PRENTICE HALL 1982

Continuum mechanics, general principles

Semi-monocoque structures, force and displacement approach to beam systems

V. GIAVOTTO, STRUTTURE AERONATICHE CITTA’ STUDI

Energy methods, Ritz Method, Plate Theory

Finite elements

Covers several parts of the course








6. INSTABILITY

7. FE METHOD


Exercise Classes (for written test)




Computer Labs






6. INSTABILITY

7. FE METHOD



Course Content and Organisation:Written Examination

Based on the same type of exercises that have been presented, solved and discussed during classes

Capability to critically apply concepts as well as to organize and carry out calculations

Admission to oral examination is possible only if the written text will obtain a positive mark

Oral Examination

Comprehension of structural concepts, analytical and numerical approach

Will include proofs of main theorems and formulation development

Documents

AEROSPACE STRUCTURES Prof. Alessandro Airoldi INTRODUCTION TO THE COURSE