Share Report Abuse Next Blog» Create Blog Sign In Aeroteaching: May 2012 http://aeroteaching.blogspot.com/2012_05_01_archive.html 1 of 7 7/27/2012 11:44 AM Home The objective About the Author Contacts Aircraft Structural Design MAY 25, 2012 Aircraft requirements -The CS25.305 structural design requirement, what is the structural failure of an aircraft . When you are developing an aircraft structure you need to perfo rm the stress analysis regarding the loads vs the static resistance of the material used. Yo u need to check if the materi al and geometry used in the design as the adequate strength for the job. But to develop the analysis you need to know when the structure as fail or what is considered a failure of the aircraft structure. Aircraft structure failure is specified in the airworthiness requirements, check in the CS25 or FAR 25 for large aircraft's the CS25.305 or FAR25.305 requirement. What do we find in this requirement? The CS25.305/FAR25.305 specify the criteria used to define the fai lure for aircraft structural components. But, what is failure? When can we say that my aircraft structure as fail? The requirement CS25.305/FAR25.305 shows us several different types of failures that can occurs in the aircraft structure. The first is of course, the simple one! If the aircraft structure breaks, thi s is failure. This is the obvious mode of failure, the loss of structural efficiency by the rupture or crack of my structure. The structure will be no longer capable to maintain the applied l oads causing the aircraft crash. This type of failure is not permitted up to Ultimate Load. What is Ultimate Load? Ultimate Load is the Umit Load multiplied by the factor of safety. According to airworthiness code CS25/FAR25 factor of safety for large aircraft's is 1.5, in some specific structures or detai ls additional factors of safety are added to the general1 .5 value. Limit Load is the maximum load appli ed to the aircraft during service life. Therefore, if we have maximum load appli ed to the Wing structure = 1000 lbs, Ultimate Load will be = 1.5 x 1000 lbs = 1500 lbs. The material/geometry used must be capable to resist failure up to 1500 lbs applied on the wing as a static strength point of view. According to the requirements the margin of safety must be higher than CHAPTERS Aircraft damage (3) Aircraft design (5) Aircraft hardware (6) Aircraft materials (3) Aircraft protection and finish ( 2) Aircraft requirements (3) Aircraft stress (1) Aircraft structures (3) Aircraft weight (1) BLOG ARCHIVE T 2012 (27) .,.. July (3) .,.. June (3) T May (3) Aircraft requirements - The CS25.305 structural de.. . Aircraft hardware - The hi-lok fastener system. th ... Aircraft materials - Selecting metallic material a... .,.. April (8) .,.. March ( 10 ) t-:OLLOW I i:.ACHINu ON 1-A<.;EBOOK AI RCRAFT ENGINEERING WEB SITES TO VISIT Aircraft design software CATIA VS Aircraft structural hardware Aeroteaching Google site FOLLOWERS
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Aeroteaching: May 2012 http://aeroteaching.blogspot.com/2012_05_01_archive.html
1 of 7 7/27/2012 11:44 AM
Home The objective About the Author Contacts Aircraft Structural Design
MAY 25, 2012
Aircraft requirements -The CS25.305 structural design requirement, what is the structural failure of an aircraft .
When you are developing an aircraft
structure you need to perform the
stress analysis regarding the loads vs
the static resistance of the material
used. You need to check if the
material and geometry used in the design as the adequate strength for
the job. But to develop the analysis
you need to know when the structure
as fail or what is considered a failure
of the aircraft structure. A ircraft
structure failure is specified in the
airworthiness requirements, check in the CS25 or FAR 25 for large aircraft's the CS25.305
or FAR25.305 requirement. What do we find in this requirement? The
CS25.305/FAR25.305 specify the criteria used to define the failure for aircraft structural
components. But, what is failure? When can we say that my aircraft structure as fa il?
The requirement CS25.305/FAR25.305 shows us several different types of fa ilures that
can occurs in the aircraft structure.
The first is of course, the simple one!
If the aircraft structure breaks, this is
failure. This is the obvious mode of
failure, the loss of structural efficiency
by the rupture or crack of my
structure. The structure will be no
longer capable to maintain the
applied loads causing the aircraft
crash. This type of failure is not
permitted up to Ultimate Load. What
is Ultimate Load? Ultimate Load is
the Umit Load multiplied by the factor
of safety. According to airworthiness
code CS25/FAR25 factor of safety
for large aircraft's is 1.5, in some specific structures or details additional factors of safety
are added to the general1 .5 value. Limit Load is the maximum load applied to the aircraft
during service life. Therefore, if we have maximum load applied to the Wing structure = 1000 lbs, Ultimate Load will be = 1.5 x 1000 lbs = 1500 lbs. The material/geometry used
must be capable to resist failure up to 1500 lbs applied on the wing as a static strength
point of view. According to the requirements the margin of safety must be higher than
CHAPTERS
Aircraft damage (3)
Aircraft design (5)
Aircraft hardware (6)
Aircraft materials (3)
Aircraft protection and finish (2)
Aircraft requirements (3)
Aircraft stress (1)
Aircraft structures (3)
Aircraft weight (1)
BLOG ARCHIVE
T 2012 (27)
.,.. July (3)
.,.. June (3)
T May (3)
Aircraft requirements - The CS25.305 structural de .. .
Aircraft hardware - The hi-lok fastener system. th ...
Aircraft materials - Selecting metallic material a ...
.,.. April (8)
.,.. March (10)
t-:OLLOW AB~O I i:.ACHINu ON 1-A<.;EBOOK
AIRCRAFT ENGINEERING WEB SITES TO VISIT
Aircraft design software CATIA VS
Aircraft structural hardware
Aeroteaching Google site
FOLLOWERS
Aeroteaching: May 2012 http://aeroteaching.blogspot.com/2012_05_01_archive.html
2 of 7 7/27/2012 11:44 AM
zero, because the structure must be capable to resist u~imate load application for 3
seconds. If the authority as doubts about it, they can request a structural test to prove the
1.5 factor of safety and the 3 seconds rule. All aircraft wings do this test for certification
As stress engineer you must find the manufactured part properties from the material. Now,
what type of statistical basis I must use? The table present the A basis and B basis. The A
basis defines that 99% of the samples fail at superior strength values with 95% of
confidence. The B basis defines that 95% of the samples fa il at superior strength values
with 95% of confidence. If you are developing a one loading path structural part (landing
gear) the A basis must be used. If you are developing a redundant structure (two or more
loading paths, stringers as example) the B basis can be used. If weight is not a problem ,
use the conservative value of the table: the lower value.
t sr dor~ctaon Other parameter that we can f ind in
the table is the applicable
LT d .roctt.on
grain direction. The L, LT and ST
designation define the properties of
the part according to the
manufacturing grain direction of the
material. If you check, you can find
that the grain direction can influence significantly the material properties. For yield stress,
the influence can go up to 5 Ksi of stress resistance. For that reason if you are not
controlling the grain direction in the installation process, be conservative and use the lower
value of all directions.
As presented is the table the direction
of the material grain will present
higher mechanical properties the L
direction. The perpendicular direction
of the grain as lower mechanical
properties, the LT direction. The
ST direction is the vertical axis of the
grain direction and for sheets and
plates this propertie are not required.
Remember selecting material allowable:
1. Go to MMPDS
LTd~reclloni
2. Select material designation chapter (2024)
·-LT direction
3. Select table for the material specification, form and temper
(AMS-00-A-250/4, Sheet, T3)
4. Select the thickness of the used (0.063 in)
5. Select the A orB column according to the airworthiness
STdirection -t ST d irection
E.lrtruslon Angle
Aeroteaching: May 2012 http://aeroteaching.blogspot.com/2012_05_01_archive.html
7 of 7 7/27/2012 11:44 AM
requirements (use conservative, A basis)
6. Select the proper grain direction properties (use conservative,
lower of the two)
7. Obtain the mechanical properties of the material.
Hope this help to obtain approved metallic material data to your aircraft structural design. If
you work on a manufacturer design company just go to your company design manual and
select the materials allowable from there. The same way that from MM PDS handbook. Use
always approved design data for your analysis, otherwise EASA or FAA will reprove the
modification or repair design.
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