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JUL 21, 2012

Aircraft design - The aircraft location reference system, FuselageStations, Buttock Lines and Water Lines

When you are performing

maintenance or repairs/modifications

to aircraft´s you need to correctly

locate places inside the aircraft. You

must easily find a damage or

equipment inside the aircraft, how we

can find something in the aircraft?

Because of this requirement the

aircraft has is on coordinate system.

But what type of coordinate system is

this? Lets learn more about the

coordinate system of the aircraft.

The first thing to know is locate the Right (RH) and Left (LH) of the aircraft, in order to

identify the LH and RH you need to be seated on the pilot seat, LH side of seat is the LH

side of aircraft. RH side of seat is the RH side of aircraft. If you are outside the aircraft you

must be located in the aircraft rear looking to the aircraft, your LH is the aircraft LH, your

RH is the aircraft RH. Never use the forward looking of the aircraft, to locate the LH and

RH.

After you know the LH and RH of the

aircraft you must be capable to locate

something in the aircraft. How we do

that? Using a specific coordinate

system where a point on the aircraft

can be easily located.

The aircraft location coordinate

system is a x, y and z system, where the x is called FS - Fuselage Stations, the y is

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Aircraft damage (3)

Aircraft design (5)

Aircraft hardware (7)

Aircraft materials (3)

Aircraft protection and finish (2)

Aircraft requirements (3)

Aircraft stress (1)

Aircraft structures (3)

Aircraft weight (1)

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called BL - Buttock Line and the z is called the WL- Water Line.

The FS - Fuselage Station is the reference axis that begins at aircraft nose and finish at

aircraft tail, the axis define the length of the aircraft. The FS cut the aircraft fuselage in

splices and each splice is from a predetermined distance from the aircraft nose. The

code location FS253 indicates that damage/modification/equipment is located at fuselage

section located at 253 inches from the nose of the aircraft, the distance can be in inches

or millimeters depending of the aircraft manufacturer, in Europe use millimeters, in US

use inches. Remember, letters FS indicate fuselage stations and the number indicates

the distance from the aircraft nose.

The y axis is identified by the letters

BL meaning Buttock Line. The

beginning of the stations is located in

the center of the aircraft fuselage in

the y axis direction , just in the middle

of the fuselage diameter. If you go to

the LH of the aircraft this is BL left if

you go to the RH of the aircraft this is

BL right.Now is just missing the

distance from the reference line,

distance is giving in inches or

millimeters just has in the Fuselage

Station definition. Now locate yourself inside the aircraft at FS300 looking forward at the

middle of the fuselage ring. You are at BL 0, if you move 10 inches to your left , you will be

located at BL 10 LH, this is what you must use to locate something in the aircraft. In this

case you will be located at FS300, BL10LH.if you move 10 inches to your right , you will

be located at BL 10 RH, in this case you will be located at FS300, BL10RH.

There is missing the last axis, the z axis, the height of the aircraft. This axis is defined as

WL water line, this line cuts the aircraft in splices in the vertical direction. You can locate

the aircraft height using WL. If you have WL200 that means that you are located 200

inches above reference WL0, this reference line can be the ground or a imaginary

manufacturer line.

Knowing the 3 references axis

designation Fuselage Station for x

axis, Buttock Line for y axis and

Water line for z axis, we can now find

and define each point on the aircraft.

A damage point or equipment

location is defined by the three

reference axis as example FS300,

BL60LH, WL200, this three

references indicate that damage is

located at 300 inches from the nose of the aircraft, 60 inches to the Left from the center

of fuselage ring and 200 inches in height from the ground reference line. The point of

damage is located in the three-dimensional system by the FS, BL and WL code.

The FS, BL and WL code can be used to locate anything in the aircraft but sometimes is

more easy to use other references according to the main structure that we are working

with. And because of this, the wing, stabilizers, nacelles and movable aircraft surfaces

can have they on coordinate reference system. The wing as Wing Stations defined as

WS where the initial reference line is located in the middle of the total wing. Generally

WS0 is located at BL0 the center of fuselage ring that coincide with the center of wing

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box. Wing station WS must have the distance from the WS0 defined by a number that

defines a distance in inches or millimeters, after that we must define if the wing is in the

left or right then RH defines right wing and LH left wing. The distance from the nose of

fuselage is FS and the height is defined has WL. Then a damage located in the wing is

defined with the coordinate system WS, FS and WL. Typical definition of location is

WS100LH, FS300 and WL280. This coordinates defines that damage is located at 100

inches from the wing center of symmetry , left wing at 300 inch from the fuselage nose

and 280 inches from the ground reference.

Others specific references are used

to locate something in structures as

the horizontal stabilizer, vertical

stabilizer, nacelles, flaps, ailerons,

elevator or rudder. The horizontal

stabilizer locations is like the wing

changing the Wing reference to

horizontal stabilizer reference. The

horizontal stabilizer is divided in HSS

- Horizontal Stabilizer Stations this

stations replace the BL - Buttock line

reference and begin in the line of symmetry of the horizontal stabilizer. Locations on the

horizontal stabilizer are defined as HSS, FS and WL. The vertical stabilizer is divided in

VSS - vertical stabilizer stations that replace the WL - water lines axis and that begins in

the vertical stabilizer root. The same happens with the Flaps defined has Flap Stations

FS, Nacelle Station - NS, Elevator Station - ES, Rudder Station - RS, etc.. Each movable

surface of the wing or stabilizers have they on station definition.

Now with all aircraft references

defined and understood it is easy to

find or define the location of anything

in the aircraft. If we define that

damage is located at FS200,

BL40RH and WL220 you will, with no

difficulty enter inside the aircraft and

locate the damage. If with define the

location as WS200LH, FS260 and

WL200 you will go to the left wing and

locate the damage very easy, with no difficulty. Remember this coordinate systems,

because if you work with aircrafts one day this will appear in front of you and you will need

this information. All aircraft reference lines are defined in the Aircraft Maintenance Manual

or Aircraft Structural Repair Manual, find it or go after it, if you have access to it. Hope this

can help in your work. See the Ads.

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APR 26, 2012

Aircraft design - The Margin of Safety and Reserve Factor definition anddescription. The parameters of the stress analysis.

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When you are developing a stress

analysis to check if the structure as

adequate strength to resist the

applied load, you must have a

parameter that gives you with a

certain degree of safety the

confidence of the analysis. All over

the world depending of the

companies or countries you can find

two different parameters that makes

this evaluation. The parameters are:

The Margin of Safety = MS

The Reserve Factor = RF

This two parameters are different and must not be confused. This parameters evaluate

the safety of your analysis, they compare the applied load with the allowable load of the

structure. The difference

between them is the reference number for failure or no failure. The MS as the reference

number zero, if the calculated number is higher than zero then the structure will resist the

applied load with safety. If the calculated number is inferior to zero then the structure will

fail by that particular load. The RF work differently, the reference number is one, if the

calculated number is higher than one then the structure will resist the applied load. If the

calculated number is lower than one and higher than zero, the structure will fail.

Remember:

MS>0, no failure safe design

MS<0, failure occur

RF>1, no failure, safe design

RF<1, failure occur

How we calculate the MS or RF? What is the formulation to find this value?

The MS formula is given by allowable stress/load divided by the applied stress/load minus

one, MS=(Allowable stress/Applied Stress)-1. The results obtained for safe design will be

0.25, 0.10, 0.5, 2, 3, etc... The results obtained for component failure will be -0.25, -0.1,

-0.5, -2, -3, etc...

The RF formula is given by allowable

stress/load divided by the applied

stress/load, RF= Allowable

stress/Applied Stress. The results

obtained for safe design will be

1.25,1.1,2,3,4 etc... The results

obtained for component failure will be

0.9, 0.8, 0.1, 0.0002, etc...

When you are developing the analysis please use only one of this parameters. Do not mix

the two in the same analysis, this will be confusing and cause mistakes in the

interpretation of the analysis. I personally recommend the use of the MS parameter. More

easy to understand, positive number= safe design, negative number=Failure. Of course

that some time, additional safety factors must be added to the formula. For example the

fitting factor, casting factor, seat attachment factor, when safety factor are used you must

multiply the applied load by the safety factor in the formulation of the Margin of Safety and

the formula becomes: MS=(allowable load/(safety factor x Applied load))-1.

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Posted by AeroTeaching at 2:45 PM No comments:

Remember the following points:

1. Use the same safety

parameter all over the analysis,

do not mix them.

2. MS= positive number = safe

design

3. MS= negative number = failure

4. RF> 1 = safe design

5. 0<RF< 1 = failure

6. Add the safety factors to your analysis, multiplied to the applied load.

Make good analysis with MS always higher than zero...

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MAR 29, 2012

Aircraft design - Cabin safety Compliance Verification Engineer EASAinterview topics

If your company

want to be a EASA Design Organization Approval (DOA) to develop modifications in the

aircraft cabin, then DOA must be certified to cabin interiors and the company is required

to certify a Compliance Verification Engineer(CVE) for cabin safety.

If you want to be a CVE for cabin safety, please prepare yourself with some technical

experience and study. Because EASA will send a Cabin safety expert engineer to make

the interview and check, if you have the capability and knowledge to be a CVE. The

interview is not easy, you will spend more that two hours responding question to the

EASA expert, hopping that you answers are the correct ones. Because if you are not

approved to be a CVE the company will not be approved to aircraft cabin modifications.

Then the interview is a big issues for you, because the approval for the company

depends of you. If you fail, the company fails and this is not very easy to deal.

If you want to be a Compliance Verification Engineer for Cabin Safety you must be well

prepared for the EASA interview and evaluation. In this post I will highlight you about some

important points and aspects, covered by the interview.

1. Remember that the TCDS of the aircraft specifies the airworthiness requirements of

aircraft certification, you must access to EASA website and download the TCDS for the

A320, B737, E170 to check what was the certification basis used in the aircraft

certification. The CS25 is a recent specification only new aircrafts are been developed

using this requirement. Older aircrafts have been certified by JAR25 amendment, you

must use the correct airworthiness requirements to develop the aircraft cabin

modification.

2.The number of the requirement is an important aspect to kwon. Learn the requirement

number for the main aspects of cabin safety, for example: cabin evacuation test, fire

protection, fire test, evacuation exits. You don´t need to know all the requirement

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numbers but you need to know the most important. EASA expert will asks you about it.

3.Learn that the aircraft minimum number of passenger specified to perform the

evacuation test. Requirement CS25.803 defines that less that 44 passengers no

evacuation test is required.

4.Remenber that the conditions for evacuation test is defined in the appendix of CS25,

where you can find that the test must have man and woman, simulated children, must

escape from one side (LH or RH) evacuation doors only, etc...more generic aspect of the

evacuation. This will be focus in the interview.

5.The requirement of the width of aisle, nº CS25.815, remember that this requirement

must be maintained in take-off, landing and flight. In flight, because in case of fire in the

rear fuselage, the cabin crew must have free aisle available to reach the fire the soon as

possible.

6.If the aircraft as an in-flight accessible cargo compartment, this compartment must

have a fire detection system because the fire is not visible by the crew. At the door of the

compartment a mask and fire extinguisher must exist, accessible to the crew to fire

extinguisher. In the case of no accessible cargo compartment, a fire detection and

extinguisher system must be installed.

7.What are the requirements for the installation of a new galley on the aircraft: Fire

resistant material, escape route (emergency exits, width of aisle), structural loads,

placards and signs, emergency exit signs, emergency lights (can be cover by the

monument), ...

8.After modification if the aircraft as free areas, not occupied by seat, galleys or other

monument . This free areas must be labeled with placard to avoid the use of this space

for baggage and free items. Free items in the cabin can be a risk in case of emergency

landing. Receiving a bag with 20kg in the head at 9g acceleration can kill a person.

9. Also if you have big free areas the passengers when they leave the seats must have

something to grab in case of aircraft turbulence. Then you must have hand points in free

areas to make possible to the passenger to grab a fixation points when they leave the

seat.

10. You must know the distance from the width of aisle requirement, 15in and 20 in.

These are some of the questions that the EASA cabin safety expert will asks you in the

interview, please remember this points. In the end of the evaluation a cabin safety

inspection can be requested. EASA will request to you to perform a cabin safety

inspection walk around, to check if you have the knowledge and capability to evaluate the

aircraft cabin safety.

Good luck for the interview.






MAR 26, 2012

Aircraft design - The top ten books to read as Structural DesignEngineer



If you want to be an aircraft structural engineer I recommend that you read and learn with

some of the standard and best books of the aircraft industry. In this post I will recommend

you the top ten books that you need to read before begin your journey in the aircraft

industry.

The top ten books that a true structural design engineer knows:

1. Airframe Structural Design - Practical design information and

data on aircraft structures, Michael C.Y.Niu

This is really a good book, with images, explanations, examples,

exercises, etc... This is the first book to read if you want to be a

structural design engineer. You can find several useful chapters

with excellent information regarding aircraft structural design:

aircraft loads, aircraft structures, aircraft materials, aircraft

fatigue and damage tolerance, aircraft weight and structural

design guidelines. The best book to buy and read, from the first

page to the last. With very useful and detail information about

the design of structures.

2. Airframe stress analysis and sizing, Michael C.Y.Niu

This is also a Michael Niu book.This book is dedicated

to stress analysis, you can find more detail explanation

about structures stress analysis. The book presents the

formulas, graphs, examples, failure criteria, failure

modes of aircraft structural elements.Very useful as

reference when you are developing a analytical stress

analysis. He presents also several structural repair

options, design and development. Very useful for

designers that develop repair in aircrafts. I personally

use this book as reference for repair together with the aircraft structural repair manual.

You must buy this book, mandatory for stress analysis.

3. Practical Stress Analysis for design engineers, Jean-Claude

Flabel

This is a very practical book, very simple to read and understand,

please read this book before Michael Niu stress book. This is about

pure stress calculation, easy understanding, lots of example,

practical examples with real aircraft structures, very good book.Only

applicable to static stress analysis, not fatigue or damage tolerance

analysis.

4. Roark´s Formulas for stress and strain, Warren C.Young, Richard G. Budynas

This book is the Bible of formulas, he present all formulas and parameters calculation for

hundred of physical and mechanical situations. He presents all formulas for beams in

several type of constraints and loads, the same applies for plates, shells, pressure

vessels, etc...Please don't need to develop formulation, use roark's formulas. Compare

analytical formulation from Roark's formulas with the finite element analysis that you

develop to check for mistakes or check an estimation of the FEM results.



5. The practical use of fracture mechanics, David Broek

This is the fatigue and damage tolerance book of reference. Is a

very complete, easy and simple understand book. He explain very

well the Phenomena of fatigue and what is the damage tolerance

definition. The fracture mechanics theory is clear and well defined.

The best book to understand fatigue and damage

tolerance.Please use this as reference, its a very good reference.

6. Peterson´s Stress Concentration Factors, Walter D. Pilkey

Peterson's is a book that defines and explains what is stress

concentration factors. You can find in this book the graphs to

determine the stress concentration factor for a particular

geometry. For example a rivet hole, rivet installed, a ellipse, a

specific crack, a hole between to stringer, a crack between two

stringer, etc...If you don't have finite element analysis is the best

way to calculate the stress concentration factor that you must

use in the fatigue calculation.

7. Mechanical Engineering design, Joseph E. Shigley, Charles

R. Mischke, Richard G. Budynas

This book shows you the all that you need to know about

mechanical science. Is not a direct aircraft book, but you can

find several mechanical formulation that you will need in

aircraft stress analysis. You can find several material failure

criteria, bolt calculations, rivet calculations, welding

calculations, connections calculations, springs calculations

and several others useful calculation used in the aircraft

industry.

8. Aircraft structures for engineering students, T.H.G. Megson

This is a pure academic book, used in the university, he shows

you the basics of stress analysis, the formulas development until

the final formula that must be applied. As design engineers we

must know the final formula and how to use it. We really don't

need to know how the formula appears. But some times to

understand what is happening, mainly if you are working with

finite elements, is very useful to know the basics to understand

the FEM results and errors. If you work with FEM please read this

book. The book also shows examples and calculations about

aircraft structures but in a generic way. Preliminary calculation of a wing box, fuselage




section, but is missing the details, the load transfer, bolts, rivets, geometries etc...

9. Analysis and design of flight vehicles structures,

Bruhn

This book is essential to read, is the first reference book

for structural design. Is an old book, a little confuse

because of the organization of the book. Missing some

pictures to better understand the formulation. But the

book as great information about stress analysis, very

simple to see and understand with the simple examples

that we can find all over the book.

A require book to read and understand for the aircraft

structural engineer.

10. MMPDS - metallic materials properties development and

standardization, FAA

This is also a fantastic book. This book is where we can find all

the allowable for metallic materials approved by the

autority.This book gives you all mechanical properties for

aluminium alloys, steel alloys, titanium alloys and several other

materials that we can use in our aircrafts. Additionally we have

also allowable for rivets, bolts and special fasteners that we

can use in the stress analysis of the joint.We can find the

requirements to develop a material certification tests to add

and approved new material to the aircraft industry. Use this book for reference for the

materials allowable.

All this books are required to the design engineer work. It is essential that the engineer

understand the structures and the problems involved in there solutions, and for that this

books are the reference of kownledge.Please enjoy the books.






MAR 17, 2012

Aircraft design - What does an Aircraft design engineer?

The aircraft structural design is a complex task, where different types of knowledge are

required to develop the best aircraft of the world. The design engineer must develop the

structure with safe design, cheap, reparable, maintainable, durable, minimum weight

etc...this is the challenge for the engineer.

The aircraft structure is the main component of the aircraft, the performance, cost,

durability, comfort, and maintenance depends of the structure. Therefore the aircraft

developer spent lot of the development time increasing the structure efficiency. Always



trying to obtain the best comfort, performance,

cost, maintenance program and durability.

The main innovation in modern

commercial aircraft is the structure.

Boeing 787 and Airbus A350 are the

aircrafts of the future, why? Because they

innovate in the main component of aircraft

: the structure. This two aircrafts are

changing the typical design philosophies

of aircraft structures. They use composite

materials for wings and fuselage,

structures where the aluminium where

invincible and untouchable. Advantage of composite structures: lower weight, density is

lower than aluminium and strength can double aluminium alloys. No corrosion problems

in composites materials, less critical to fatigue problems, lower maintenance costs,

longer life. But the composite is not only good stuff, he also as problems, cost of

materials, higher production cost, hidden damages caused by impact, all this points are a

disadvantage to composites materials. The future of aircraft structures will be

composites materials this is certain.The success of this aircrafts will be determined by

there behavior in operation, only 20 years from now, the 787 and A350 project will

demonstrates that the composite structure is safer, cheaper and better than the

traditional aluminium.

If one day in the future YOU want to be

an aircraft design engineer my

suggestion is to do the following.

First: learn composite materials, work

with composites, understand

composites, study the failure modes,

the stress analysis, failure criteria,

design of composites parts, because

this will be the material of futures aircrafts and this will be your future.Learn about aircraft

structures, understand the applied loads, the typical assembly of parts, the existing

simple parts, the hardware, the finish materials and techniques, the sealant, the paints,

etc... All this knowledge is required to begin the development of structures. After you

understand what you need to build a structure, then you can begin.

Second: learn to work with a 3D modeling software, I use CATIA V5, generally this is the

software that the aircrafts manufacturers use to develop there aircrafts.3D modeling is

the easy way to develop our structures. We can check interference, see the parts,

simplify the work of the manufacturing people, optimize to minimum weight, obtain

illustrations to maintenance manuals, optimize the assembly by delivering pictures or

movies of the assembly sequence. All this points are advantages of the 3D software,

please learn to use the CATIA V5 software, the aircraft software.

Third: learn to use Finite Element Modeling software, I use PATRAN/NASTRAN. The FEM

software will help you in the structural stress analysis, reducing time and money,

optimizing the thickness of parts and reducing the weight of the aircraft. The FEM can

also avoid detailed unsafe conditions or expensive structural tests. FEM analysis is well

accepted by the authority! And if supported by similar tests, FEM is indispensable. Please

learn finite elements and stress analysis of structures.



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Posted by AeroTeaching at 1:52 AM No comments:

Four: learn fatigue and damage

tolerance analysis, this type of

analysis is special, well different of

the traditional static analysis.

Understand the phenomenon, the

formulas and the objective, all this

information is fundamental to the

design enginner.Wings and fuselages are governed by fatigue, and the damage tolerance

philosophies is applicable to this main structural components.

If you reach to learn all this points to a

moderate level, then you will be an excellent

aircraft design engineer, and you will have a

brilliant professional future.

Good luck for your future...






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Bharath wrote...Thanks a lot.. U have explained everything very clearly..Each article here contains a simple method for easy understanding and also great images.. Thanks formaking me understand things better..

AeroTeaching wrote...At ultimate load, when the doubler and skin have the maximum load, load transfer depends of stiffness of doubler and stiffness of rivets. Doubler as the capability ofpick-up maximum load of 1000...Continue >>

Anonymous wrote...Hi,I understand that using the ratio of axial stiffness to estimate the percentage of load being transferred into the doubler is common in static strengthcheck.However, this seems more applicable...Continue >>

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