Aero Landing Gear Simulation.pdf

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International Journal of Mechanical and Production Engineering, ISSN: 2320-2092, Volume- 1, Issue- 4, Oct-2013

Design and Analysis of Main Landing Gear Structure of a Transport Aircraft and Fatigue Life Estimation For The Critical Lug

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DESIGN AND ANALYSIS OF MAIN LANDING GEAR STRUCTURE

OF A TRANSPORT AIRCRAFT AND FATIGUE LIFE ESTIMATION

FOR THE CRITICAL LUG

1R RAVI KUMAR, 2P. K DASH, 3S R BASAVARADDI

1Product Design and Mfg, Visvesvaraya Technological University, Belgaum, India2Chairman, Bangalore Aircraft Industries Pvt. Ltd., Bangalore

Mechanical Engineering, KLE College of Engg., Belgaum, India

Email: [email protected]

Abstract: The current work includes the design and analysis of a medium size transport aircraft landing gear unit. A typicallanding load case will be assumed for which structural analysis will be carried out.During landing, there will be three different types of loads

i.

Vertical load

ii.

Drag loadiii.

Side loadEach of these loads will cause axial compression, bending and torsion on the strut of the landing gear. As a first

approximation the landing gear space structure will be idealized as a statically determinate structure and a stress analysis willbe performed using strength of Material approach. The stresses developed because of all three types of loading on thestructural members of the landing gear will be calculated.A finite element model of the landing gear structure will be developed and analyzed. The FEA stress and deformation resultswill be compared with that obtained from SOM approach.

These stresses and internal loads can then be used for the design of various structural members of the landing gear unit.

Fatigue life to crack initiation will be estimated for a critical lug of the landing gear unit by considering the constantamplitude landing cycles

I. INTRODUCTION

Landing gear is one of the primary structural

components of the airframe. Landing gear enables the

airplane to take off and land on ground. Its designconsiderations are significantly different .A variety of

landing gear configurations and types are in usetoday. The most common type being tri-cyclearrangement with a nose landing gear and a mainlanding gear[2]. Impact loads during landing are themain design loads for the landing gear design.

Landing gears should also be checked for variousother ground handling loads as specified in theregulatory requirements [1]. The landing gearwithstands the ground impact load and absorbs the

impact energy and diffuses the load to thesurroundings attachment.

II. METHODOLOGY

Fig A : Methodology

Conceptual ModelConceptual model of main landing gear is as shown

in figure B

Fig B : landing gear conceptual model

Landing Gear Analysis Problem

Fig c : landing loads and dimensions


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The landing loads shown in fig C acting at the wheelaxle will be transferred to the supporting structure atpoints A, B, C through the landing gear structure. The

vertical and drag components of the unknownreactions are shown at point A, B and C. Calculationof these reactions is required to carry out the stress

analysis of the landing gear. These calculation aredone and reaction acting on each members are shown

in the fig D .

Fig D: reactions of the structure

Design of the landing gear structure

The forces obtained on the members are taken and

design for the members are done in my previouspaper [5]Material Used: Al7075T6

Youngs modulus=73GPaYield strength=503MPaFOS=1.5

Calculated Designed dimensions [5] taken for

analysis

For, Strut G d=57.41mmStrut F d=55.42mm

Strut C d=42.9 mmOleo strut OE d=80 mmMember AB d=112.5mm

III.

FINITE ELEMENT ANALYSIS OF THELANDING GEAR

Stages of FEA

Steps used for analysis of FEM model

The important steps are done from creation ofgeometry model to until get results for FEM model.The geometry is modeled for given dimensions by

using MSC PATRAN.The geometric structure is discretized usingCBAR2 elements as shown in fig E

After meshing, have to check things likeequivalence, duplicates and boundary and after that

we have to check the quality of mesh.Important criteria considered here to check thequality of mesh, like aspect ratio, normal offset,wrap, and Jacobean ratio.

Type of material, elastic modules and Poisson ratioare input given for analysis.Material properties are given for different

subgroups.Loads/Boundary conditions given to meshed model.In Analysis we give output requests fordisplacements,stresses, grid point force balance,

elemental forces etc..,From analysis after applying the above requests weget BDF file, its containing all input data given to themodel.To solve the problem of analysis model

MSC/NASTRAN is used as solver.After solving the problem from MSC/NASTRAN,We get the XDB file, it Contains output results.

Finally the post processing results are checkedthrough MSC/PATRAN.Main quantities observed in result sheet are vonmisses stress, stress components, elemental stresses,free body loads

Fig E : Meshed model of the landing gear

IV. RESULTS

Free body diagrams calculated theoretically andobtained through software does matches as shown infig F tells us that the calculations are correct


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Fig F :FBD of the landing gear structure

The maximum stress obtained in the member is 278MPashown in fig G which is less than the yieldstrength of the structure taken 324 MPa. Thus states

that the design is safe

Fig G: Stress distribution of the landing gear structure

Design and analysis of Critical lug attachment of

landing gear Conceptual modelA lug is a member which is a connecting memberbetween wing or fuselage and the landing gear.The conceptual model of the attachment lug is shown

in fig

Design of lug

The material used for the members are Al7075T6,where, E=73GPa, Yield strength=503MPa, FOS=1.5

Calculation of the diameter of the pinTo calculate the diameter of the pin, we takeallowable shear stress

We know that

Shear stress allowable =0.5*Yield stressHere we know the yield stress of the material,Shear stress allowable = 162 N/mm2Thus by knowing the shear stress we know that if the

material subjected to shear stress is taken asShear stress allowable= P/2A

Here we can calculate the area as we know by fig D

the maximum load which can be taken by thematerial is 472821N which is a critical load which isacting near strut G.

Thus diameter of the pin is 43.105mm

Once designed pin diameter we design the lug thedimensions and relation is as shown in fig H

Fig H: Dimensions of lug

To find thickness of the member we can find it byusing normal stress formula

Stress=load/pin areaHere to take stress value we take nomial stress for thedesign

Wkt,Stress Intensity Factor = stressmax/stress nominalKt = max / nom

For loaded hole , stress intensity factor is taken as 4by referring the stress intensity chartThus

nom = max / Kti.e nom=324/4 =81we know the maximum load acts on member is

472821 N . we know the area subjected here for loadis (129.315-43.105)*t

thus thickness (t) =75 mm

Analysis of lugThe dimensions obtained is modeled in Msc

PATRAN and meshed as shown in fig I.


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Fig I : meshed model of lug

Later, Boundary conditions are given and the modelis sent for processor to solve, the solver used is MSc

Nastran. The solver solves the problem and theresults are obtained from MSc PatranThe results obtained is shown in fig J

.Fig J : stress plot of the critical lug

V. FATIGUE LIFE PREDICTION

Fatigue is a phenomenon caused by repetitive loads

on a structure. It depends on the magnitude andfrequency of these loads in combination with theapplied materials and structural shape. Structural

members are frequently subjected to repetitiveloading over a long period of time. Here the lug jointis subjected for the repetitive loads while landing,

taxing, takeoff, etc..., so these joints are tested for thefatigue condition of failure.Steps used in the fatigue design is that first we takeproper SN curve of the material of the lug jointshown in fig K. Later we calculate the stressamplitude and stress ratio by using

amp =

R=

Later we have plotted a table as shown in the fig l

Fig K : S-N curve for material AL7075T6

The damage accumulated in the member is calculatedfor 1 flight, where it can be given byD=ni/Ni,The total damage accumulated wasdi= 2.389*10-4 per flight

The block load spectrum T when structure is failure

can be expressed as [25];

The total number of load blocks by Eqn. gives

4185.85blocks.which says that the crack initiation inthe structure takes plays after 4185.85 flight hoursand suggested that to replace for maintenance

department of aircraft


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CONCLUSION

The landing gear space structure idealized as astatically determinate structure its reactions,

forces acting on each member are determinedby using simple numerical approach.

From the dimensions obtained, Analysis of thelanding gear structure is done

The free body diagram of the landing gearmember which we got shows us that the

reactions calculated for the members throughanalytical approach and the software validationis done

The main landing gear structure which carriesload is connected to the wing through lug joint

The design of the maximum load carrying lug

is done and analyzed the same

As the lug joint is subjected to repetitive loads

the fatigue life estimation is done

It can be concluded that lug joint is a failsafe

design and recommended to change every timeafter 4185.85 hours of flight.

REFERENCES

[1] E. F. Bruhn, Analysis and design of flight structure, 1973[2] Michel-chun-yungniu, Aircraft structural design, 1995

[3] Aerospace engineering / March 1996 landing gear structural

integrity

[4] Dr.R.K. Bansal Strength of materials 4thedition

[5] Ravi kumar R ,Nithin Kumar , S R Basavaraddi Design of a

landing gear structure of a transport aircraft ICETE,2013

[6] J. C. Newman, Jr, Advances in fatigue and fracture

mechanics analyses for aircraftstructures, Mechanics and

Durability Branch, NASA Langley Research Center,

Hampton,VA, USA.

[7] Grigory I. Nesterenko, Service life of airplane structures,

Central AerohydrodynamicInstitute (TsAGI), Russia, 2002.

[8] C.M. Sonsino, Course of SN-curves especially in the high-

cycle fatigue regimewith regard to component design and

safety, Int. J. of Fatigue 2007.

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