An Overview of the Aircraft Cabin Interiors Industry and Stress Engineering

By Surya Batchu

Senior Stress Engineer

Founder of STRESS EBOOK LLC.

http://www.stressebook.com

©2014- Current Stress Ebook, LLC. All Rights Reserved

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Finite Element Analysis TheoryAircraft Cabin Interiors Industry OverviewTypical Cabin Interiors Manufacturer ORG StructureThe Role of a Stress EngineerEducational QualificationsIncome ExpectationsEmployment and Business OpportunitiesStress Ebook LLC. CoursesThe future of the aviation industryCost of Similar TrainingTake the Next Step

What is Finite Element Analysis?

Basic FEA Direct Formulation Theory with Matrix Method, using a 1D Element

What is Finite Element Analysis?• In a nut shell, Finite Element Analysis is the process of determining an approximate solution by

discretizing or splitting the actual global structure into ‘finite’ elements, these elements and the model closely represent the actual structural stiffness, loading and constraints

• In simple terms it can be summarized as (kind of a spring equation):

𝑭 = [𝑲] ∗ {𝒙}Where:

• {F} is the Applied Load Matrix (known)• {K} is the Global Stiffness Matrix (known)• {x} is the Displacements Matrix (unknown)

• Let us consider a simple truss example made from three 1D elements (1D element examples are bars or beams or rods)

• Assume all the links are made from metal

F_total

Link ‘l’

• Now let us isolate the link ‘l’ shown in the previous slide as a balanced free body with its portion of the total load, say ‘F’

• It has two nodes, 1 and 2

• It has material properties such as a Young’s Modulus ‘E’, Poisson’s ratio ‘’ and has an initial length ‘L’, an area of cross section ‘A’

FF

FF

21

21

L

• We can see the analogous system of a simple spring that stretches by L• Therefore Strain = L/L and Stress = F/A

• Young’s Modulus E = / = ൗF/AL/L

• Rearranging the terms in the equation for E, we get:

𝐸𝐴

𝐿*{L} = {F} Similar to a spring equation {F} = [K]{x}

FF

FF

21

21

L

• From the previous slide we have stiffness K = 𝐸𝐴

𝐿

• At the end, in the assembled matrix form, [K] will be the global stiffness matrix. It will be assembled

from the material properties and geometry of all the finite elements in the model

• So let us look at the matrix method

MATRIX METHOD

F2F1

21 K

u2u1

F2F1

21 K

u2u1• We have the same element here, except we will assume a force and displacement at each node

• Assume the sign convention as shown above is positive. Don’t worry at the end we will end up with

the right signs, for now let us just go with this

• So we have our single element with stiffness K, end forces F1 and F2 and nodal displacements u1 and

u2

u2

F1

21 K

u1Equation for node 1:

• F1 = K*(u1-u2) = AE/L*(u1-u2)

• Let us keep it consistent by using u1 first and then u2 for node 1

• This way, if u2 is zero then we get a positive F1 = K*u1, thus matching the sign convention we are

using. So F1 is positive if (u1-u2) is positive, K is always positive

• Unknowns are F1, u1 and u2

u2

F2

21 K

u1Equation for node 2:

• F2 = K*(u2-u1) = AE/L*(u2-u1)

• Let us keep it consistent by using u2 first and then u1 for node 2

• This way, if u1 is zero then we get a positive F2 = K*u2, thus matching the sign convention we are

using. So F2 is positive if (u2-u1) is positive, K is always positive

• Unknowns are F2, u1 and u2

Load Vector, Element Stiffness Matrix and Displacement Vector:

• So we know that F1 = AE/L*(u1-u2)• And we also know that F2= AE/L*(u2-u1) = AE/L*(-u1+u2)• Thus, in matrix form, we now have the governing equations for the 1D bar element:

𝑭𝟏𝑭𝟐

=𝑨𝑬/𝑳 −𝑨𝑬/𝑳−𝑨𝑬/𝑳 𝑨𝑬/𝑳

𝒖𝟏𝒖𝟐

• Here, the element stiffness matrix is [K] for the single element• Similarly, the stiffness matrix, applied loads, unknown reactions and displacements for the entire

system is “assembled” by the FEM solver, resulting in a single large stiffness matrix for all elements and a whole bunch of equations to solve

• Alright, let us now look at an example problem to make it easier to understand

{F} [K] {x}

AE/L = 100

Sample Problem of 1D Bar Elements:

• We have two bars with stiffness AE/L = 100 (bar 1) and AE/L = 1000 (bar 2)

• A load F=10 is applied at the mid point of the first bar

• So we split the structure into 3 elements (1,2,3) and 4 nodes (1,2,3,4)

• We use a node (2) at the middle of the first bar so we can apply the load there

• We have 4 degrees of freedom (DOF) in this model, one at each node – u1, u2, u3, u4

AE/L = 1000

F=10

1 2 3 4

1 2 3

Actual

FEMx

u

For Element 1:

𝐹1𝐹2

=𝐴𝐸/𝐿 −𝐴𝐸/𝐿−𝐴𝐸/𝐿 𝐴𝐸/𝐿

𝑢1𝑢2

𝐹1𝐹2

=100 −100−100 100

𝑢1𝑢2

1 2 3 4

1 2 3FEM

xu

1 2

1E1

For Element 2:

𝐹2𝐹3

=𝐴𝐸/𝐿 −𝐴𝐸/𝐿−𝐴𝐸/𝐿 𝐴𝐸/𝐿

𝑢2𝑢3

𝐹2𝐹3

=100 −100−100 100

𝑢2𝑢3

2 3

2E2

For Element 3:

𝐹3𝐹4

=𝐴𝐸/𝐿 −𝐴𝐸/𝐿−𝐴𝐸/𝐿 𝐴𝐸/𝐿

𝑢3𝑢4

𝐹3𝐹4

=1000 −1000−1000 1000

𝑢3𝑢4

2 3

3E3

Global Element Stiffness Matrix Assembly

F1

F2

F3

F4

=

100

-100

0

0

-100

100+100

-100

0

0

-100

100+1000

-1000

0

0

-1000

1000

1

2

3

u1

u2

u3

u4

Sample Problem of 1D Bar Elements:

• The elements are red numbers (1 2 3)

• We can see how at the common nodes, the stiffness terms are assembled by addition

• Next we apply the boundary conditions

Boundary Conditions and Loads

F1

F2

F3

F4

=

100

-100

0

0

-100

100+100

-100

0

0

-100

100+1000

-1000

0

0

-1000

1000

1

2

3

u1

u2

u3

u4

F=10Actual

Applied = 0

Applied = 10

1 2 3 4

0

0

1 2 3

Two Equations and Two Unknown Displacements

F1

10

0

F4

=

100

-100

0

0

-100

100+100

-100

0

0

-100

100+1000

-1000

0

0

-1000

1000

1

2

3

0

u2

u3

0

Solve using matrix method for u2 and u3:

• We will focus on first finding the displacements using matrix equations• The FEM solver would use other techniques of Gaussian elimination or mathematical search

techniques to solve all the equations in the solution algorithms

Stiffness matrix reduction as u1 and u4 were 0

10

0=

100+100

-100

-100

100+1000

u2

u3

Solve using matrix method for u2 and u3:

• So let us eliminate the first and last rows and columns in the stiffness matrix, then we have:• 10 = 100(0)+200(u2)-100(u3)+0(0)• 0 = 0(0)-100(u2)+1100(u3)-1000(0)• Solving the equations we get u2 = 5.2381E-2 and u3 = 4.7619E-3

Next solve for the unknown forces

Solve using matrix method for F1 and F4:

• F1 = 100(0)-100(5.2381E-2)+0(4.7619E-3)+0(0)

• F4 = 0(0)+0(5.2381E-2)-1000(4.7619E-3)+0(0)

• Solving the equations we get F1 = -5.2381 and F4 = -4.7619

• Therefore F1+F4 = -10lb. Reaction forces balance the applied load of 10lb

F1

10

0

F4

=

100

-100

0

0

-100

100+100

-100

0

0

-100

100+1000

-1000

0

0

-1000

1000

1

2

3

0

u2

u3

0

So in summary:

• First we define all the FE entities in the FEA pre processor based on the actual structure to mimic the

structures stiffness and boundary conditions as closely as possible

• In the FEA solver, the geometry and material properties of the model are used to generate and

assemble the global stiffness matrix

• Then the boundary conditions kick in to reduce the global stiffness matrix

• In the FEA solver, numerical techniques are used to solve for unknown displacements first

• Following that, reaction loads strains stresses and other derivative quantities are calculated and

stored in different files based on what is requested in the output control

What are aircraft cabin interiors? Everything you can see and touch inside the pressurized passenger cabin or the

cargo areas of the aircraft Examples: Galleys, seats, overhead bins, lavatories, closets, floor panels, class

dividers, bulkheads, cargo floor panels, cargo nets, crew rest modules, among other structures

Typical interiors structures are built from very lightweight honeycomb sandwich panels and aluminum materials

Cabin structures go through similar certification processes as primary aircraft structure except the requirements and regulations are different

The primary emphasis is always on passenger safety and safe egress in a given situation, such as a rapid decompression event, crash or ultimate landing conditions, in flight turbulence etc.

Typical Cabin Interiors Manufacturer ORG Structure

CEO

VP of Engineering

Director of Engineering

Design

Tech Pubs

Test Labs

Document Control

Director of Stress

Stress Managers

Stress Engineers

Static Test Lab

Director of Materials and Processes

CFO

Accounting and Finance Staff

VP of Operations

General Managers

QualityShipping & Receiving

Production Mock Up Purchasing PlanningProgram

Management

Contracts

Project Management

Customer Interface

VP of HR

HR Staff

VP of Marketing

Marketing Staff

The Role of a Stress Engineer:

The stress engineer is mainly responsible for demonstrating and documenting compliance of any structure or product installed on a type certified aircraft to the FAA airworthiness regulations applicable to that product or structure (mainly 14 CFR, Part 25, Subparts C&D).

The stress engineer is responsible for detailed engineering calculations and suggesting any design changes necessary to comply with the FAA regulations, sometimes stress analysis is sufficient and sometimes testing is required

The stress engineer develops structure models, interface loads into the aircraft structure, documents strength and deflectioncapabilities of the structure under different loading conditions. For certification by test develops static test plans and conducts static tests per the applicable FAA regulations. He or she then documents the results in approved stress report formats whichare finally submitted for approval to the FAA by the integrator via ODA

Only then a particular structure or product is allowed to be installed and to fly on that aircraft to ensure passenger safetyduring the flight and emergency landing or crash situations

A good stress engineer has at least a decent understanding of end customer expectations, engineering design constraints, tolerances, manufacturing, supply chain issues (such as time required and cost to get parts and materials etc.). On the management side he or she needs to know things such as schedules, budgets, time and people management

In this industry, knowledge is typically gained with time and experience under other experienced engineers. This is where www.stressebook.com tries to drastically cut down on the time required for you to learn these advanced engineering skills to succeed in this career

Educational Qualifications:

For an entry level stress engineer positions you need a Bachelors Degree in either aerospace, mechanical, civil or general engineering that includes engineering mechanics, strength of materials, material science, and maybe a basic level of FEM (finite element method or modeling) exposure

Then the training happens mostly on the job. However, to gain the skills to become leaders or managers, it will take you many years of on the job learning and hard work and a keen sense of detail

In order to get into lead positions, you typically need a Masters Degree plus 3 to 5 years of work experience, or a Bachelors Degree plus 5 to 10 years of work experience

Whether you are just trying to get into aerospace stress analysis or looking for a lateral move, www.stressebook.com helps you learn these skills much more quickly in a compact and easy to understand format, thus giving you and edge over your competition

Following a few years of working as a stress engineer, you can try to venture out on your own as a contractor

Contracting requires you to be on top of your game with the cutting edge skills needed to do a quality job and hit the road running. Its more rewarding financially but also more risky, and www.stressebook.com tries to help you with clear instructional videos to gain the skills and achieve success

Rough Income Expectations:Salaried Employees: Entry level out of college: $55K to $65K per year, depending on the college and

employer/location this bracket could shift to the left or right by $5K - $10K per year

Experienced 2 to 5 yrs: Between $65K/yr to $75K/yr Experienced between 5 and 10 yrs: $75K/yr to $100K/yrMore than 10 yrs: 100K/yr – 130K/yr depending on level, employer, location and

role

Contractors (experienced)*: 5 yrs: $40 to $50 per hour 5 to 10 yrs: $50 to $70 per hourMore than 10 yrs: $70 to $85 per hour or probably more depending on the role

* Note: These are general ranges solely based on my own experience, not hard numbers, and will vary based on market conditions, job location, employer/client company and your skill level

With experience and knowledge, opportunities open up:Possibly create your own company – Ambitious goal, but not impossibleOffer engineering and consulting servicesOffer highly specialized training services - Either in person or online, like what I am doingWorking as independent contractorsBecoming ODA Unit Members -This typically requires a company (that has an ODA) and their DERs to sponsor and endorse you as their Unit Member (UM)Working as a DER - There are DER exams and qualifications, this is more independent, but also more challenging to maintain activityLateral shift or multitask with Design and Stress skillsLateral shift from Tech Pubs or Flammability or other departments to StressOr simply go up the ladder of your current company into leadership roles, or management rolesIt is possible, www.stressebook.com is here to help!!

So what kind of training does www.stressebook.com offer?Bronze Level:– The first step towards becoming a stress engineer is to get comfortable with

Finite Element Analysis– The Bronze level covers a ton of different aspects of finite element modeling and

analysis such as geometry, meshing, materials and properties, creating models, sample models, downloadable files, detailed step by step video tutorials and other tips and tricks (click here more details)

– In addition to general FEA stuff, some topics specific to aircraft cabin interiors are also touched on such as defining 2D orthotropic panel materials (PCOMP)

– Once you get through this course, it will give you the foundation you need to navigate through the silver level and more advanced level courses

Silver level and beyond:– The higher level courses go deeper into the finite element modeling and stress

analysis of actual cabin structures such as partitions, galleys, wardrobes etc. Here we use the knowledge from the FEA course, and more advanced modeling techniques with real world material data. You will learn:• Aircraft structure/stiffness modeling• Geometry processing and mid surfacing• Load case and loads definition and modeling• Unit mass and inertia properties and modeling• Panel joint modeling (e.g. panel pins)• Panel material and property definitions• Aircraft attachment (tie rods, floor fittings) modeling• Rapid decompression loading cases and criteria• Running models and extracting results, free body loads etc.

Continued:• Extracting and organizing results data in the form of spread sheets• Stress analysis methods• Margin of safety calculations• Panel pin margins• Attachment margins• Panel margins• Restraint device margins• Load and moment equilibrium checks• Unit interaction and contact• Reviewing and reading engineering drawings• Document control procedures• Drawing revisions and changes• Stress analysis certification reports• Finally Static Test Plans, Static Test Results, Test Fixture and equipment, Load Cells, Test Reports and much more

– See the next page for some structure examples

Some Structure Examples:– Shown here are just a few example structures,

the top left image shows an aircraft with an empty floor

– We can see a galley, some wardrobes, a class divider and some full views of interiors

Structure FEM Examples– Shown here are a few model structures– We can see the installation FEM on the left– This structure includes a Boeing 747-8 VIP

aircraft partition with a sliding door cutout, FWD facing wardrobes, and a cabinet on the aft side

– You can see the tie rods at the top and panel pins in the model on the left

– We can see the 9.0G FWD deflection plot of the entire structure on the right

– The entire load path needs to be certified either by stress analysis or static test per FAA regulations

Contn’d:

– No matter what type of structure it is, the concepts covered in the courses will enable you to model them in detail with confidence

– We will look at various documents that give us the information we need such as panel properties, allowable data for margin calculations, and much more

– For continuing members, fresh content and monthly updates will be included at no additional cost

– If it helps improve the quality of the courses, member feedback and requests will be reviewed in detail and new lessons will be added at no extra cost for continuing members

– Paid members will also be able to access the exclusive member forum which is another valuable benefit of ongoing monthly membership, the member community

– You can ask questions and get them answered by other active members in the forum community or myself

The future of the aviation industry:– The aviation industry is here to stay– The industry is coming out of the recession, airline profitability is up– Significant research and advancements are happening on sustainable and alternative jet fuels,

airframes (the Boeing 787, Airbus A320 NEO etc.) and engine technologies (the P&W GTF engine)– There is no viable alternative at this point or in the near future for international travel, aviation is it– Aircraft design and analysis technology is evolving rapidly with the use of ever lighter and stronger

materials such as composites– Every commercial or VIP/business aircraft will need interiors, so there are many opportunities for those

who have the will power and interest to take them on and succeed– Interiors covers the pressurized cabin, the cargo area and the cargo loading technologies– Other areas could be equipment racks, overhead support structures, seats etc.– The number of capable and qualified aerospace stress engineers in the entire world is probably around

100 to 200 thousand engineers. A majority of them are employed by the primary structures, systems and engine/nacelle manufacturers

– So there are a very limited number of engineers who are willing to remain and are available in the cabin interiors industry, and you could be one of them…

Costs, why www.stressebook.com?– If you research stress engineering or FEA training courses, most cover a fraction of what is covered on

www.stressebook.com, they can cost thousands of US dollars for a few days or sometimes a week of training

– Although not all, most training is typically limited to a few topics or a narrow subject matter, and more theoretical than practical

– You may have to take time off or vacation or get your employer to pay you for attending away from work

– The course offering location may be far away, you may also have to pay for travel, boarding and lodging– So what to do? No worries. For a fraction of that cost, you could get many times more industry specific

and valuable information at www.stressebook.com– You can learn at your own pace, anywhere, anytime, on any device in the comfort of your own home or

on the road, all online– And most importantly, I am excited to share my knowledge with you. If I can help you get ahead in your

career, I’d think that I did my job…

OK, so now that you have a basic understanding of the industry and the possibilities with www.stressebook.com, I hope that you will act on this knowledge and gain even moreDo not put it off for another day, because once this day passes, it isn't coming back!Get that valuable edge over your competition in the job marketHead on over to www.stressebook.com, start learning!

Thank you!!