Abstract .uk



Final ReportECM3101/ECM3102Title: Redesign and stress analysis of simplified landing gear Date of submission: 01/05/2014Student Name: Junghoon SuhProgramme: BEng Mechanical EngineeringStudent number: 600060234Candidate number: 030412Supervisor: Philippe G YoungAbstract This project was carried out with the aim of redesigning the tricycle main landing gear based on the reference data of a Boeing 777 aircraft under the landing conditions. In particular, the redesign of a safe and durable target landing gear considering a Boeing 777 aircraft should withstand the maximum landing loading of 1016196.52N and ensure the completion of 10,000 lifecycles. To achieve this, an in depth understanding of the landing gear system, theoretical design and 3D visualisation and stress analysis have been performed. In addition, the main part of the landing gear, shock strut, originally incorporated an oil-pneumatic type shock absorber, but was replaced by a spring shock absorber to verify its feasibility as an alternative solution. Comparisons between the two types of shock absorbers were made by using vibration equation to theoretically compare the differences in terms of landing gear weights and sizes with different shock struts. However, the usage of a spring shock absorber resulted in an increase of shock absorber dimensions and weight. The height increased from 4.27m to 12m and the weight increased from 4,536kg to 39,000kg, with much of the weight increase caused by the spring. The design of the spring shock absorber was carried out based on three different materials: alloy spring steels, stainless spring steels and copper-base spring alloys, and alloy spring steel. These materials were all selected to be used in the shock absorber as fatigue rate of spring was found to be the slowest with the lowest concentration of stress. Although the weight increased with varying levels, durability of the landing gear was verified from stress analysis which showed the results of maximum concentration of 733MPa out of yield strength of 745MPa of landing gear material of titanium 5553, and 10,000 times of life cycle was assured.Keywords: landing gear, main strut, shock absorber, deflection, spring, SolidWorks, ABAQUS, Stress analysis, materialsAcknowledgment I am fully appreciated to my supervisor Philippe Young who enabled me to successfully accomplish completion of the project with a number of supports and advices under all the conditions, and to deepen the level of analysis performed during project duration. Moreover, I dedicate this project to my family and friends who always back up and encourage me to carry on my project. Table of Contents TOC \o "1-3" \h \z \u AbstractAcknowledgment1. Introduction PAGEREF _Toc386633606 \h 12. Background Theory PAGEREF _Toc386633607 \h 22.1 Research methodology PAGEREF _Toc386633608 \h 22.2 Structural analysis PAGEREF _Toc386633609 \h 22.2.1 Nose landing gear PAGEREF _Toc386633610 \h 22.2.2 Main landing gear PAGEREF _Toc386633611 \h 32.2.3 Troubleshooting of present landing gear PAGEREF _Toc386633612 \h 52.2.4 Safety PAGEREF _Toc386633613 \h 52.3 Mechanical analysis PAGEREF _Toc386633614 \h 52.3.1 Boeing 777 reference data PAGEREF _Toc386633615 \h 62.3.2 Static load balance PAGEREF _Toc386633616 \h 62.4 Design of shock absorber PAGEREF _Toc386633617 \h 112.4.1 Spring shock absorber PAGEREF _Toc386633618 \h 113. Methodology PAGEREF _Toc386633619 \h 174. CAD Design PAGEREF _Toc386633620 \h 184.1 Shock absorber PAGEREF _Toc386633621 \h 184.1.1 Spring design PAGEREF _Toc386633622 \h 184.1.2 Shock absorber PAGEREF _Toc386633623 \h 184.2 landing gear PAGEREF _Toc386633624 \h 195. Results and discussion PAGEREF _Toc386633625 \h 205.1 ABAQUS analysis PAGEREF _Toc386633626 \h 205.1.1 Spring shock absorber PAGEREF _Toc386633627 \h 215.1.2 Landing gear PAGEREF _Toc386633628 \h 266. Design and quality of research and innovation in research process PAGEREF _Toc386633629 \h 287. Sustainability PAGEREF _Toc386633630 \h 298. Conclusions and recommendations PAGEREF _Toc386633631 \h 30References PAGEREF _Toc386633632 \h 31Appendix PAGEREF _Toc386633633 \h 33Appendix A Vibration data sheet PAGEREF _Toc386633634 \h 33Appendix B ABAQUS analysis figures PAGEREF _Toc386633635 \h 34Appendix C Spring material variation PAGEREF _Toc386633636 \h 35Appendix D Project management PAGEREF _Toc386633637 \h 36Appendix E Health and Safety risk assessment PAGEREF _Toc386633638 \h 37Appendix F Preliminary report PAGEREF _Toc386633639 \h 381. Introduction This project was conducted with the aim of investigating the applied mechanics of the current landing gear (Boeing 777). Throughout the analysis, the ensured safety of the landing gear is the priority whilst seeking the feasibility of design modifications to obtain higher levels of durability, and green sustainability during the landing process under assumptions considered in the simplified design model. The landing gear is one of the major parts critical to the safety of passengers, as it should fully support the aircraft weight and absorb impact generated during landing. Before commencing the analysis and design modification, the target landing gear, Boeing 777, was extensively investigated. The Boeing 777 incorporates a tricycle landing gear, which consists of a single nose gear positioned at the front and two six wheel bogie type main landing gears positioned at the rear side. [4] Six wheel bogie type main landing gears are built in the Boeing 777 and successfully enable it to sustain higher level of loads from the aircraft body, as landing loads of 201,840kg are distributed over six wheels. This report contains an investigation of the landing gear system, analysis, calculation, 3D design and virtual testing for durability of landing gear redesigns. In the first section, the landing gear system in the Boeing 777 aircraft is described, design guidelines for components sizing and positioning, and justification of target landing gear choices are made with a brief statement of target landing gear malfunctions and problems identified. During the analysis section, assumptions and limits of the project are explained, followed by an explanation of the required formulas used for the redesign of the landing gear. Furthermore, a deeper level of troubleshooting studies for defects of target landing gear has been performed. The design section includes the 3D target landing gear model, which was designed based on the information and calculated with approximations taken for the few parts where information was not available. The redesigned landing gear had a target of 10,000 life cycles and not to weigh more than five times the original landing gear. These are then compared and component modifications are explained with 3D design illustrations. Finally, a case study has been conducted in order to compare the cost variance and environmental factors of materials of landing gear components, both of which are major factors when selecting the optimum redesign model among prototypes. 2. Background Theory2.1 Research methodology As this project was focused on analysis and visualization of landing gear through the simulation utilising CAD tools by hand works and computers, any experiments in university workshop were not required. However, an immense amount of research and support were vital for forming the successful redesign of the landing gear. Research and supporting background work was carried out in the following forms: Research using online sources due to the ease of data collection and immense quantity of information available. Through the Exeter e-library, Google scholar and Journal of aircraft provided dissertations and literature that served as a framework for this project. Alternatively, offline research, such as paper books and journals, sourced primarily from the library which provided an excellent source of technical information. 2.2 Structural analysis The landing gear in a Boeing 777 has a highly sophisticated system to ensure steering and stabilisation during take-off, landing and taxing. Tricycle type landing gear in a Boeing 777 has two types of gears. First, the nose gear positioned at the front, and secondly, two main gears positioned on the rear side of the aircraft.2.2.1 Nose landing gearFigure 1: nose landing gear structure [22]050165The nose landing gear is located at the front of the body of the plane and provides steering and shock absorption. It consists of a shock strut, drag strut assembly, lock link assembly, torsion links and tow fitting. [18] Starting with the shock strut, it utilises a forged steel air-oil shock absorber at the same time as the landing gear, but has relatively smaller amount of forces to withstand in comparison to the shock absorber of the main landing gear. Torsion links are connected to the shock strut to prevent spins of outer and inner cylinders which cover the shock absorber inside. [22] Drag strut assembly is located above the fitting of the shock strut and its function is to maintain the shock strut in a retracted or extended position. Lock link assembly of front and after links fixes the nose landing in both the retracted and stretched positions alongside the drag strut assembly, and is connected by lock springs that maintain the lock links in locked position, and hinge. 2.2.2 Main landing gearFigure 2: Main landing gear structure [6]-3238543180 The main landing gear is located at the rear side of the aircraft. As each main landing gear is connected to the wing, each time the aircraft takes off it retracts into side parts of fuselage and comes out from fuselage during the landing. The main gear also contains the main strut which consists of a shock strut, drag strut and wheels. [19] The shock strut is vital part of the landing gear as its primary function is to support the aircraft from loadings. It contains oil oleo pneumatic shock absorber which is filled by oil and nitrogen. In comparison to light type of jets, large transporting aircraft incorporate pneumatic shock absorber as it offers great shock absorption and higher rates of efficiency. Hence, it also offers immense size and weight reductions in contrast to the light weight jets. In case of drag, the strut functions as a stabiliser when the aircraft is landing. The main strut is connected by reaction links to the side fuselage. Torsion links play a considerable role in preventing internal rotations of outer and inner cylinder and holding each cylinder. Axle assembly from truck beams normally attaches wheels and brakes. 2.2.2.1 Main shock strutFigure 3: oil-pneumatic shock absorber [14]48463202103755-1016071755 The Boeing 777 incorporates a steel air-oil pneumatic shock absorber. When the aircraft lands, landing loads are absorbed into shock absorber and transmitted to the end of truck beam to the wheels. It is consisted of inner cylinder, which is expanded and retracted inside the outer cylinder. [23] Torsion links connect both the outer and inner shock absorber in order to prevent rotation between two cylinders under the landing loading and to support them in horizontal line. Torsion links which nose landing gear is rotated by, are supported by the actuator. In terms of the principle of shock absorption, both compressed nitrogen gas which is located in the upper part of main strut and oil which is located in the lower part, flows into the shock strut that is located in the middle of main strut. Figure 4: section view of spring shock absorber [21] In most cases, the aircraft shock strut incorporates two types of absorption types, spring and pneumatic absorption. Light weight aircrafts use shock spring absorber rather than oil pneumatic as a suspension system, although pneumatic suspension system bring higher rates of efficiency. Therefore, as part of the project, spring shock absorb was designed to replace pneumatic absorption system in the target landing gear, and evaluation will be conducted between original landing gear with oil-pneumatic shock absorber and redesigned landing gear with spring shock absorber in terms of sizes and weights to check whether usage of pneumatic type is much efficient as shown in figure 5. Comparisons between the results of the model in terms of weight, sizes and prices were made. In addition, three of different spring materials was compared and tested to be chosen for redesign.Graph 1: Comparison of efficiency of several type of shock absorption [28]2.2.2.2 Drag strut During landing as the main strut moves downwards, it becomes extended, with the drag strut supporting main strut to prevent it from becoming excessively stretched. The upper part and lower link, hinged in centre, are components of the drag strut. There is a connection between upper drag strut and nose wheel side, whereas the outer cylinder is connected to the lower link of drag strut. Retraction of the main strut is allowed by the drag strut, which will fold during retraction and extension. [22]2.2.2.3 Torsion linkTorsion links prevent internal rotation between the outer cylinder and inner cylinder of the main strut. During steering, force is applied with the torsion link allowing the struts to be rotated. Normally, torsion links allow vertical movements to occur and are located in the outer cylinder and inner cylinder of the main strut. [22]2.2.2.4 Brake system The brake system is one of the essential parts in the landing system, as it should stop the airplane in certain distances from landing against the exerted drag. Hydraulically powered brake systems are used for landing gears. [3]2.2.3 Troubleshooting of present landing gearThe main issue of the current landing gear is the overhaul of the landing gear. In particular, the squat switch problem is believed to be the major associated problem with the landing gear. [11] Therefore, following several repair and maintenance operations should occur to ensure the safety of the landing:No sign of landing gear warning The operation of the horn should be checked under the extracted and extended positions of landing gear. Moreover, connecting wires from landing gear to the control monitor in pilot control system room should be checked to verify whether it is broken from fatigue. Malfunction of throttle and squat switches should also be inspected. [27]2.2.4 SafetySafety is the first priority when designing a landing gear, as it is integral to a flight that a safe landing and planned stop of the aircraft occurs. There are several safety systems present in order to prevent landing gear from failure:1. Alternate extension system: Is an emergency function when the absence of hydraulic pressure in the brake or vertical extension does not occur. This system is activated by an electric motor which allows extra hydraulic fluids to flow into the brake system. Furthermore, unlocking pin is overridden by this motor to extend the landing gear from extracted position. [11]2. Ground locks: When the landing gear is on the ground, ground locks assist the landing gear with stability, preventing the landing gear from collapsing. A Pin or spring is installed in the landing gear to support landing structure from the occurrence of falling back. [12]2.3 Mechanical analysis In this section, general explanation of exerted forces during landing were carried out to help understand the functions and systems of the landing gear of the Boeing 777 alongside the calculations for the transmitted forces and energy in order to generate the redesign of the landing gear. Reference data, mainly taken from Boeing technical information page, is introduced to show the range of values to be used during the investigation. After that, the results are narrowed down from weight distribution of entire aircraft to specific components, mainly considering the main strut and tire.2.3.1 Boeing 777 reference dataFigure 1 below shows the obtained reference data available from the technical information page of Boeing website that is utilized for the usage of analysis. In the cases where values and conditions were not available, several assumptions were made for the redesign of the landing gear.NameSymbolValueUnitsMaximum landing weight [5]Mmaz landing201,840kgApproach speed [25]Vapproach136kiasThrustFthrust77,000lb*2Table 1: Table of reference value [4] Furthermore, several assumptions have been set for the factors that cannot be identified, and for the simplification of calculations: Landing process occurs in an extreme situation, so, maximum landing mass is used, hence the maximum vertical velocity that the aircraft should sustain, is calculated.Centre of gravity is located in the middle of the aircraft. Landing angle is assumed to be 3 degree nose up. Therefore, forces of drag, lift and thrust shall be 3 degree tilted during landing. [11]2.3.2 Static load balance2.3.2.1 Force distribution on two landing gears Working on the weight exerted on the landing gear offers a good start to estimating the size for the components, such as the dimension of main strut and wheel sizes. Figure 2 shows the vertical force distribution of the aircraft.052705 During landing, the force that is exerted from maximum landing mass is applied downward. As single nose and two main landing gears sustain the aircraft mass, the equation for the forces around the aircraft can be made underneath.Fy = F(max landing weight) - 2F1 - F2 - (1)Figure 5: Distribution of forces over landing gears Level of loads, applied to the nose gear (F1) is normally 14% and 86% of landing weight applied to 2 main landing gears, where each main landing gear accounts for 43%. Therefore, the equation can be rewritten for simplifying the calculation for finding gravitational force:F (max landing weight) = 50/7 F2F2 (nose landing gear) = 7/50 FF1 (single main landing gear) = 43/100 FMass proportion on M1 = 7/50 M, therefore, mass on nose gear 28257.6kgMass on each main landing gear 86791.2kg In order to work out the forces during the landing, iteration works are performed to find the displacement of landing and the acceleration from vibrating parts. From part 3, it was found that required time to the maximum compression is 1.3182 seconds. In addition to this, the amount of force that each main strut in the main landing gear should sustain under the maximum landing mass is calculated to be 1016196.52N. Therefore, viscous shock absorber which is the spring shock absorber is designed to sustain 1016196.52N of force.2.3.2.2 Landing gear positioning Before designing the landing gear, the location of landing gears should be demonstrated in order to assure static balance and proper stress transmissions. [20] Figure below shows the side view of the aircraft. The distance of A between the tip of head and nose gear is approximated to be 4m, and whole length of airplane is 63m. [2] The distance between two type of landing gears were found to be located the main landing gear at the position that could sustain transmitted force from the landing impact.Figure 6: Side view of the aircraftSum of FyWtotal = 0.14W + 0.86WSum of MomentM = -0.14WE +0.86WCThe distance of E can be found as centre of gravity locates in the centre of the airplane and A is 4m.= -0.14*(1016196.52N)*(31.5m-4m) + 0.86(1016196.52N)*C= -3.85m +0.86*C=> C= 2.99 m = 3m=> B = E + C = 31.5m + 3m = 35mTherefore, the distance of E, between the center of gravity and nose gear, is 27.5m, distance of C, between the center of gravity and main landing gear, is 3m, and the distance between nose and main gear was found to be 35m.2.3.2.3 Specification of forces on landing gear The landing gear when landing should be conducted under the static balance of the aircraft. Therefore, an investigation was conducted into 4 different types of forces around the landing gear during landing, drag, lift, thrust and gravitational forces. [3] Among different kinds of forces, the values for thrust and maximum landing mass were found. - Thrust: Boeing 777 has 2 engines which are capable of generating the maximum thrust of 154000 lbs (=77,000 lbs *2), which is equivalent to 69,853.2kg. The amount of thrust is 685,259.892N. The safe landing cannot be done with maximum thrust. The percentage of thrust for the landing is 40% of the thrust (40% N1), as stated in the manual, which leads to the necessary thrust of 274,103.9568N. [9]Maximum landing mass: Landing operation can be safely conducted when the landing mass is equal or less than maximum landing mass. For the analysis, maximum design landing mass is used for the calculation of static balance, which value is 223,198kg. From vibration equation, the gravitational for during landing was found to be 1016196.52N. (3)Figure 7: 3 Degree nose-up landing viewFrom the (2.3.1), 3 degree of nose-up is shifted during landing was set, hence, figure below shows the directions of forces acting upon 3 degree shifted-up plane. Actual landing occurs with 3 degree of nose up as depicted above, so calculations for forces were derived as below:Sum of FyFlift*cos(3°) + Fdrag*cos(87°) + Fthrust*cos(87°) - Fgravitational =0Sum of FxFthrust *cos(3°) + Flift *cos(87°) - Fdrag *cos(3°) = 0 For the ease of calculation, landing angle is assumed to be tilted 3 degree above to reduce trigonometry calculations. Therefore, equations can be newly derived as below:Fgravitational : 1016196.52NFthrust : 274,103.9568NSum of FyFlift + Fthrust *sin(6°) - Fgravitational *cos(3°) = 0Flift + 274,103.9568 * sin(6°) - 1016196.52 *cos(3°) = 0Flift = 986152.2NSum of FxFthrust * cos(6°) + Fgravitational *sin(3°) - Fdrag = 0274,103.9568 * cos(6°) + 1016196.52 * sin(3°) - Fdrag = 0Fdrag = 325786N2.3.2.4 Calculation of vertical velocity during landingSince it is assumed that aircraft touches down with 3 degree nose up and reference data of approach speed of 136 KIAS (=69.69m/s). The vertical speed during landing can be found as follows:Vvertical = Vapproach * sin (3°)= 69.96m/s * sin (3°) = 3.66m/s2.4 Design of shock absorber2.4.1 Spring shock absorberFigure 8: 2D Modelling of spring shock absorber and free body diagram of forces Design of the landing gear should consider the maximum deflection that shock strut needs to be compressed and maximum landing force shock strut should withstand. From applying vibration equation, maximum deflection and maximum force under the vertical speeds and maximum landing mass of the aircraft. In addition, the following equations were constrained to deflection of shock strut without deflection from the tire. These vibration equations were used in order to find exerted force in the maximum compression and requiring stiffness of spring and damping constant for steel spring shock absorber. [10]Equation from free body diagram F = ma-kx- cxdx = mxdx2 Damping ratio is assumed to be 1 which implies that the aircraft landing with the least oscillation from the landing gear, it leads to the definition of critical damping. Therefore, initial conditions need to be set to find the maximum displacement of landing gear with displacement equations below:xt=C1+C2*te-Wnt - (1)xt= C2e-Wnt-WnC1+C2te-Wnt - (2)Initial conditions are defined in order to solve equations by using spring-force equation, to be used in calculations:K*x=m*gx= mgk Using this, initial conditions for first equations can be set (when fully stretched before compression):x0= -(mgk) - (3)x(0) = V vertical (max displacement occurs when V=0) - (4)Apply initial conditions of (3) and (4) into displacement equations of (1) and (2) to find out C1 and C2.x0=C1+0e-Wn*0=> C1 = -mgkx0= C2-Wn-mgk=> C2=x0=Vvertical+ Wn-mgkNow, apply known C1 and C2 into displacement equations again to obtain appropriate equations.Therefore, two equations can be derived:xt=-mgk+Vvert+Wn-mgkte-Wntxt= Vvert+Wn-mgke-Wnt-Wn-mgk+Vvert+Wn-mgkte-WntSimplifying x/dt (t)xt = 1-WntAe-Wnt+Wnmgke-WntPutting Vvert+Wn-mgk back into Axt=1-WntVvert+Wn-mgk e-Wnt+Wnmgke-WntBy replacing Wn-mgk to g Wn, this can be rewritten as:xt=gWne-Wnt+V-gWne-Wnt-V-gWnWnte-WntAs maximum displacement can be determined once xt = 0Equation of xt can be again simplified to find maximum time to find maximum displacement:0 = gWne-Wnt+V-gWne-Wnt-V-gWnWnte-WntThis can be derived in terms of t:=> t= 1Wn1+gWn*1v-gWnUsing the equation of t when xt = 0, maximum time can be calculated following by maximum displacement by varying stiffness of k. Stiffness value was set as 700000N/m, and following equations were used to find maximum deflection and forces:t= 1Wn1+gWn*1v-gWn=17000003.66ms-11/2 1+9.817000003.66ms-11/2*13.66ms-1-9.817000003.66ms-11/2 = 1.318seconds.Maximum deflection was calculated by substituting t of 1.318 seconds:xt=-mgk+Vvert+Wn-mgkte-Wnt = -86791kg*9.81700000Nm+3.66ms+7000003.66ms-112*-86791kg*9.81700000Nm*1.318se-7000003.66ms-112*1.318s = 1.411m Value of stiffness was iteratively performed in order to obtain the target range of maximum time between 1 second to 1.5 seconds and maximum deflection between 1.2 meters and 1.5 meters. Consequently, using stiffness of 700000N/m, maximum compression time of 1.318 seconds and maximum deflection of 1.411 meters were obtained. Detailed table of values and methodology of using spreadsheet can be found in appendix A.Graph 2: Changes of original lengths over time Checking the validity of the equations and calculations of previous works, the graph of compression against time was plotted. As time passes, it can be clearly noted that the compression exponentially decreases over time. In other words, deflection increases over time. Also, from the point where time passes 1.4 seconds, the compressions slows down and significant level of deflections would not be expected to occur in comparison to the period of first 1.4 seconds. This again proves that usage of maximum time of 1.318 would be appropriate. Graph 3: Changes of damping and spring forces over time This graph shows the changes of the level of two different forces, damping force and spring force. As observed, while the level of spring force increases, damping force falls. This could be interpreted that as the immense amount of deflection occurs, spring force increases with the increase of deflection, but damping forces decreased due to the fact the level of oscillation caused by landing impact falls over the same period. In addition, due to the setting of damping ratio of 1, damping force line was plotted smoothly without any fluctuation. Therefore, this graph demonstrates that the calculations were appropriate and results were successful. For the maximum force, it was calculated to be 1016196.519N where the maximum deflection occurs.2.4.1.2 Spring design Spring design is an important phase of modeling the landing as the spring is a major part which absorbs the impact of landing. As the shock absorber in the main landing gear is a spring, which stores the exerted impact from the landing, the properties and dimensions of the spring should be identified. Spring material was used with Chromium Vanadium (ASTM A231), as it is most commonly used for spring shock absorber. For the design criteria, the spring was designed to withstand the maximum deflection of 1411mm and ultimate loads of 1321055.476N, which was multiplied by 1.3 of safety factor from maximum landing force of 1016196.519, factor of safety for spring of 2, and spring index of 9 were assumed to be used in the equation. [24]Figure 9: Section views of spring [24]It is already given that τy=690MPa G=79340MPaDiameter of wireShear stress τ=τy2=690MPa2=345MPaWahl’s stress factor k=4c-14c-4+0.615c=4*8-14*8-4+0.6158= 1.1621Spring Index c=Dd=9 => D =9dUsing equation of shear stress, standard diameter of wire was calculated.τ=8FDkπd3By substituting D=9d and known values into equation, equation can be newly derivedd=72Fkπτ=72*1321055.476N*1.1621π*345Mpa= 336.623mmD=9*d=3029.613mmStandard diameter of wire is calculated to be 336.623mm.Diameter of coilMean diameter of 3029.613mmOuter diameter of coil is 3366.237mm (3029.613mm + 336.623mm)Inner diameter of coil is 2692.989mm (3029.613mm - 336.623mm)Number of turnsUsing the equation of deflection, number of coils in use can be clarified.y=8FD3iGd4This can be rearranged in terms of active coil, i.i=Gd4y8FD3=79340MPa*336.6234mm*1411mm8*1321055.476N*3029.6132mm=4.873Therefore, active coil turn number is 5.Free lengthFree length of the entire spring could be calculated using the equation of free length of helical springl≥i+nd+y+a Clearance, a, was assumed to be 25% of maximum deflection. In terms of n, ends of the spring, it was not added as this spring was modeled to withstand great level of maximum landing force, and fittings to maintain spring compression in vertical line were added in CAD designs. Putting all the values into equation:l≥5+0336.623mm+1411mm+(0.25*1411mm)l≥3340.249mmPitchp=l-2di=3340.249mm-2*336.623mm5=553.400mmMass[13]M=Volume*Density=3.14*D*i*3.14*d24*7.861kgm3*10-6=33,260kg As a result, the spring to withstand the ultimate force of loading, 1321055.476N, was designed theoretically, and will be represented visually with aid of CAD, Solidworks. The design is based on the following dimensions:Weight of spring = 33,260kgStandard diameter of wire: 336.623mm.Mean diameter of coil: 3029.613mmNumber of turn: 5Free length: 3340mmPitch: 553.400mm3. Methodology Redesign of target landing gear proceeded from looking at the functional requirement from research performed, with forces and stresses analysis around the aircraft, mainly focused on main landing gear from handworks and CAD tools. From the investigation of structural analysis, construction of nose and main landing gear was acknowledged in order to analyze realistic forces and stresses around the landing gear during landing process. After understanding of the landing gear system, exerted forces in association with landing and stress proportions in each component in landing gear were theoretically calculated by far through the researches and hand works. CAD modeling and FEA analysis were performed in order to validate and optimize to the final model of the landing gear obtained. In the CAD modeling, SolidWorks were used to visualize the redesign from theoretical calculations. Apart from spring shock absorber, all the other parts were sized randomly as first modeling, but were then able to achieve final modeling through the optimization process of checking in competency of the model under the loading and stress analysis. FEA analysis by ABAQUS enabled the investigation of the location of the maximum stress concentration where it is anticipated to deteriorate relatively quickly, and the level of deflection of the model. Variation of the dimensions and materials were made in order to compare different designs of landing gears and generate the final model. 4. CAD Design Using SolidWorks, landing gear was modeled in a logical manner. Starting from the spring design, the spring shock absorber was created, and then the landing gear which incorporates outer and inner cylinder, torsion links, and wheel truck were built.4.1 Shock absorber4.1.1 Spring design6858066675The spring was designed in a helical shape in order to incorporate it into the cylindrical shape of the landing gear main strut. It has a good advantage in reducing the nominal compression under the load in comparison to the wire shaped spring for shock absorption. It has dimensions of height of 3440mm, width of 3366mm, and pitch is 553mm. In the shock absorber, fitting for the spring shall be modeled in order to ensure the nominal compression apart from deflections in other directions.Figure 10: Spring design4.1.2 Shock absorber0243205Spring shock absorber was then designed based on the dimensions of the spring. The damper was safely designed for the unexpected over compression above 1.4m, it empties the height of 2 meters. Furthermore, it is designed to function as fitting for the spring that width of upper part of damper is 3100mm in order to encourage the spring to be compressed in one direction. The shock absorber has dimensions of 8 meters height and 3.4 meters of width. Two mounts were designed to be connected with each outer and inner cylinder of main strut.Figure 11: Spring shock absorber design3195955434975-1193804394204.2 landing gearFigure 13: Section view of landing gearFigure 12: Landing gear redesignThe landing gear was modeled successfully after the designs of spring and spring shock absorber were created. The outer cylinder was designed to protect shock absorber from external hazards and environmental danger. Additionally, torsions links are connected to the inner shell in order to prevent rotations of the landing gear under the landing impacts. In case of torsions links, they were designed to enable outer cylinder move downwards for 2 meters in comparison to maximum feasible compression of 1.4 meters from spring in order to prevent unnecessary collision with inner cylinder. The outer cylinder is robust and fixed with the shock absorber by linkage axle on the top of shock absorber. The inner cylinder is fixed with bottom parts of shock absorber and truck parts for wheels. With these models, stress analysises were conducted to iteratively optimise the model in an efficient and economical way. After the stress analysis, optimisation process of solidworks modelling was to generate the final model of the landing gear. 5. Results and discussion5.1 ABAQUS analysis Based on the SolidWorks models of the original spring shock absorber and landing gear, FEA analysis of model was conducted by ABAQUS. Through the FEA analysis, the concentration of maximum stress and maximum deflections were identified, enabling the redesign of the original landing gear with various materials and dimensions. The analysis was divided into two sections, the analysis of spring shock absorber, and the landing gear. Both analyses were set up by 1.4 seconds of time periods when maximum force is exerted and with a damping ratio of 1. All parts were assigned separately and boundary conditions were assigned on the bottom of each spring shock absorber and landing gear as in real landing situations. Consistency of units was in Pa, N and m. First, spring shock absorber was analysed in order to ensure the maximum force of 1321055.476N, which was utilised to theoretically design the spring and damper. After that, different materials were separately incorporated to determine the best materials in terms of weight and cost in order to maximise the economy and sustainability. Upper parts and lower parts used aluminium and spring materials, which were varied from the original alloy spring steel, stainless spring steels and copper-based spring alloys. Second, the original landing gear was tested in two stages, basic stress analysis and fatigue testing to check redundant fittings and excessive material usages, and ensure the durability of the landing gear under the maximum landing mass. Table 2 shows the types of materials used in testing and each property.TypesAlloy Spring Steels[17]Stainless Spring SteelsCopper-Base Spring Alloys [15]TitaniumUsageSpring testSpring testSpring testlanding gearNameChromium VanadiumStainless Type 302Phosphor Bronzetitanium type 5553Modulus of elasticity (MPa)207000193000128000103000 [6]Poisson's ratio0.310.30.20.31[16]Table 2: Table of mechanical properties of materials5.1.1 Spring shock absorber In the original design, an Alloy spring steel (Chromium Vanadium, ASTM A231) was used for the spring for the shock absorber. On the other hand, several materials were selected as candidates which could replace the original alloy spring for the better durability of spring shock absorber, which are Stainless Spring Steels and Copper-Base Spring Alloys.Figure 14: Pressure areas6921525400 For the analysis of shock absorber, equivalent pressures to the ultimate landing load of 1321055.476N were calculated to be applied in the model. The linkage part of top was excluded to assist with calculations for pressures. It is not anticipated that significant differences in accuracy will occur as result of this exclusion, as the transmitted forces from landing evenly propagates throughout the landing gear. Total pressures were calculated in a simple way, which is shown below.P=FA1321055.476N/ (1.22m2)*3.14= 292165.4892Pa1321055.476N/( (1.72-1.32m2)*3.14)= 350598.587Pa1321055.476N/ ( (1.32-1.22m2)*3.14)= 1682873.218Pa5.1.1.1 Mesh convergence Graph 4: Mesh convergence Since the model was designed in 3D, a number of elements should be multiplied by eight in order to double the resolution. For processing mesh refinement, due to the size and complexity of the model, it was not possible to achieve resolution-doubled meshes, but meshes with doubled number of elements were acquired from controlling global sizes and elements sizes. The figure below shows the finer meshes, 1691, 3162, 6140 and 11235 elements, but finer meshes than 20000 nodes were not feasible because of the restriction of student licensed product. As a result, the finest mesh of 11235 with stress of 6.077MPa succeeded obtaining the highest stress value, but it was presumed that higher stress value could be achieved at the mesh elements around 4000. Eventually, 6.794MPa of stress was found by analysing 4073, and it was used to conduct stress analysis and material replacement.028575 With the 4073 elements, meshing was done with tetrahedron mesh element shape and shows good dimensions. As helical in spring has complicated issues, this part requires additional mesh elements compared to the upper and lower parts of shock absorber. These were set differently to assign different number of meshes. Appendix B.1 showed the shape changes of the mesh models depending on the number of mesh elements.Figure 15: Meshed model5.1.1.2 Stress analysis -3238552070 Using von-mises stress analysis, an investigation of stress was carried out. The location of the highest stress point was at damping pins. [8] This is an expected phenomenon in which the damping forces over deflection increases require higher level of force to stabilise growing oscillation. With regard to the high concentration of stresses over the damping bars, the thickness of it could be increased to expand the pressure area in order to slow down future fatigue around this highly stress-concentrated area.Figure 16: Concentration of stressesGraph 5: Stress against strain graph Having performed the stress analysis, a graph which shows the table of stress changes over the strain, was plotted from the data of analysis. Alloy spring steel was tested under the two conditions, elastic property and elasto-plastic property to identify any permanent deflection occurrences until the time reaches when maximum landing loads have been exerted. As a result, the alloy spring steel was proved to be recoverable under the landing loads. Furthermore, it could be demonstrated that the maximum stress of 6.794MPa does not reach average yield stress of 320MPa of shock absorber, this model could be used for landing. However, judging from the significant differences between maximum stress and average stress, it could be anticipated that unnecessary materials could be reduced so as to optimise the process in sizing. 5.1.1.3 Deflection analysis-2401570163195Figure 17: Concentration of deflection094615 As pressure is applied to the top of the shock absorber, the shock absorber also displays shape change in terms of deflection. In z axis, vertical axis, pressures were applied as landing loads acting in same direction, and it could be clearly seen that the shock absorber deforms with gradual deflection of the spring over the period of time. More accurate phases of deflection changes can be found in appendix B.2. Thus, this model could now be validated to be used for a landing gear as the spring compresses with consistent rate of deflection changes. As can be seen from the figure 17, location of the highest deflection was found to be in the centre of the string, and taking account of forces which exerted from x and y axis, failure of the centre of spring could be suspected. Therefore, optimisation was done in a way that landing could be applied to only single direction z axis and minimising the effects from x and y direction as much little as possible. This will lead to prevent deterioration of spring in long term perspective, moreover, downsizing of the volume of top parts and lower parts of shock absorber that has allowable deflection could result in reduction in cost from decreasing thickness of the part.5.1.1.4 Material replacementGraph 6: Changes of deflection against force increase The spring used in the shock absorber was replaced with three different materials that can be incorporated for shock absorption, in order to select the best material. With regards to general results, it can be seen that most of materials have similar values of deflection. Among materials, stainless spring steels shows the increase of high deflection changes over the landing force increases, whereas alloy spring steels showed the least deflection.The table of Graph 5 and 6 were derived from the spreadsheet (Appendix B.3) that organised and divided the values according to the materials and type of values. Graph 7: Changes of stresses over time increase In case of stress comparison, copper-base spring alloy showed the highest rise in stresses among three materials, whereas current spring material, alloy spring steels, resulted in the second highest stress increases with only small differences from stainless spring steels, which demonstrates the reason alloy steel has been widely preferred as a spring absorption material. This also means fatigue at the point where maximum stresses occur will have the slowest rate in comparison to springs with alloy spring steel or copper-base alloys. Summing up the results from both analyses of material replacements, it could be said that the three materials demonstrate similar results, particularly considering the alloy spring ranked the best in material in terms of the lowest deflection where replacement of material does not seem to be vital. This reason could be backed up by the mechanical properties of each material. Alloy spring, Chromium Vanadium, has relatively higher resistances to the impact loading and fatigues of the parts, whereas rest of materials also have property of good tensile strength but even in comparisons of expected weights of spring, alloy spring steel showed the lowest weight at 33,000kg. [15] (Appendix C).5.1.2 Landing gear5.1.2.1 Stress analysisFigure 18: Concentration of stress over landing gear under the loading Stress analysis on the landing gear was performed throughout the model. The titanium 5553 type was assigned to be material of the landing gear. The maximum von mises stress value was 730MP in comparison to the yield strength of 745MP the titanium 5553. It showed that present model of landing gear would not break or fail due to the loads. Relatively higher stress concentrations were found around the areas where vertical motions occur from outer cylinder and inner cylinder. Weight of the model was 44560kg, and in comparison to original weight of target landing gear of 6,636kg, it resulted in a significantly higher weight. It is expected result that oleo pneumatic offers high shock absorption than spring shock absorption in terms of weight and sizes, but this redesign of landing gear incorporated the spring as suspension within the gear, leading to a large increase of the overall general weight. 5.1.2.3 Final designFrom the optimisation process of different sizes of landing gear, it was possible to utilize a spring material, alloy spring, which has a small shock absorber volume and could still sustain the landing load. Additionally, the dimension of the landing gear could be downsized as well. In the final model, the volume of the truck beam was shrunk by a factor of 0.8 and upper and lower parts of shock absorber became smaller as maximum stresses did not reach the overall yield strength of the shock absorber. After all, maximum stress over the landing gear was found to be 733MPa, and model weight decreased to the 39000kg. -56515326390Figure 19: Concentration of stress over final landing gearFigure 20: Final model of landing gear-42862526492206. Design and quality of research and innovation in research process Research of the specification and materials used in the landing gear design were based on aircraft related websites. Core information, such as dimensions and landing mass information, related to the target landing gears were extracted from the Boeing technical information section of the company website. Furthermore, the large amount of research in literatures and technical documents enabled an in depth analysis and the generation of the final model of the landing gear. The design process proceeded in a logical manner with the understanding of landing gear structure, theoretical designs of landing gear, visualization, analysis and generation of final model. The understanding of the structure of the landing gear occurred prior to the modelling of the landing gear, hence related papers and books from library offered a variety of opportunities to become familiar with the landing gear system. In particular, unanswered questions or unclear information from literatures were again personally clarified from aviation forum and email correspondences from Boeing Company. The theoretical design of the landing gear was assisted by the lecture notes from Solid Mechanics lecture and materials lectures. As a spring shock absorber located in a real landing gear can be designed from principles of vibration study, this allowed for the design of the spring and damper, with the lecture slides from university making a great contribution to achieve theoretical design of the landing gear. Regular supervisor meetings were of great benefit, particularly as the supervisor was the lecturer of solid mechanics module meaning that the theoretical design of landing gear was accurate and reliable. Material lecture notes provided a great resource with the selection of materials for the spring for spring shock absorber and information from the notes assisted with the analysis of the fatigue and failure of landing gear. Visualization of landing gear was largely helped by Solidworks forums and guidebook from the web, as level of CAD model was advanced than the curriculums, used in lectures. Finally, the analysis of the landing gear using ABAQUS and SolidWorks tools were assisted by a research fellow, who is very competent with ABAQUS. 7. Sustainability0288925 In terms of life cycle, 100,000 times of life cycles of the model was achieved with respect to the objective of 10,000 cycles. This brought down the fact that landing gear could sustain for maximum landing load of 1016197N. Figure 21: Life cycle of landing gear Sustainability should be another consideration in designing the aircraft and landing gear. Materials should be either reusable or manufacturable with less energy consumption. For example, two major leading airplane manufacturing companies, Boeing and Airbus, have developed lighter and stronger carbon-fibre composites parts in comparison with titanium which has been used more frequently, resulting in a significant reduction in carbon emissions. Boeing shows the statistical results that increased energy discipline and increased usages of carbon-fibre composites into aircraft parts, has enabled an achievement of carbon emissions of 24.4 percent from 2002 to 2008.[4] Moreover, EU ETS (EU emission trading system) strengthened the level of emission law, and anticipated of saving of 176 million tonnes of carbon dioxide emission by 2015, which will boost aircraft manufacturers develop energy-efficient parts to reduce amount of required fuels for flight.[26] 8. Conclusions and recommendations Investigation and analysis of the landing gear was finalized with the generation of the final model which incorporates the spring shock absorber. Structural analysis of the landing gear was carried out with significant amounts of research to boost 3D visualization of the landing gear. After that, vibration equation successfully enabled us to evaluate the target maximum deflection of 1.411m, and from optimization of work to find appropriate spring stiffness, the stiffness value was found to be 700000N/m. Based on theoretical redesign, CAD tools, SolidWorks was used to design a 3D model of landing gear, able to withstand the landing loads. Stress analysis was flawlessly proceeded to check the fatigue of redesign and as final model showed the 730MPa of maximum stress in comparison to yield strength of titanium 5553 of 747MPa, durability of the redesigned landing gear was assured. Through the sustainability investigation, methods and actions of reduction in carbon emission were explored to manufacture landing gear in much greener ways. With regard to the future continuation of work, modelling of landing gear was performed entirely by using the principles of solid mechanics for sizing and analysis. Moreover, several parts of the landing gear were intentionally excluded due to the irrelevancy or low level of correlation to the core of project. However, realization of redesigned landing gear needs more specific steps until the manufacturing phase, as stated below:Thermodynamics – During the landing, as outer cylinder slides downward over the inner cylinder, heat is generated. Due to the heat, corrosion of the joint part is expected to be boosted, but since this project was focusing on solid mechanics, forces and stresses, this area could not be covered. However, thermodynamics is also another part should be considered for landing gear design.Oil-pneumatic shock absorber – The actual target landing gear uses the oil-pneumatic shock absorber, but designing of this type required the much of time due to the complexity of the model apart from its aesthetic. It needs to concern about landing force that shock absorb should withstand, but the flows of both nitrogen and oil should be appropriately designed. Therefore, from the plenty of researches from solid mechanics and thermofluid, the deeper understanding and appropriate application of knowledge would be expecting to generate more appropriate comparison between spring and oil –pneumatic types. Missing landing gear parts – In this project, size of landing gear was begun sizing with approximation and optimized. However, since most of the parts were size with approximation, the appropriate amount of materials for each part could not be specified as well. Moreover, this part was to design the simplified landing gear under the landing conditions, redesigned landing gear only incorporated few parts which only needed to absorb impact. As continuous work, what requires to be continued are designing missing parts, such as drag strut and lock links, which requires for steering or taxiing apart from landing and more precisely calculation of the dimensions of each part of landing gear.References[1] ACE. (2009).?Shock Absorber Function.?Available: . Last accessed 10th Apr 2014.[2] Airliners net. (2013).?The Boeing 777-200.?Available: . Last accessed 8th April 2014.[3] Badal et al. (2008).?Project Landing gear- Boeing 737-800.?Available: . Last accessed 20th Apr 2014.[4] Boeing. (2013).?Technical charateristics Boeing 777-200LR.?Available: . Last accessed 20th Mar 2014.[5] Boeing B. (2009).?777-200 performance summary.?Available: . Last accessed 12nd Feb 2014.[6] Boyer, R. (2010).?Attributes, Characteristics, and Applications of Titanium and Its Alloys.?Available: . Last accessed 9th Mar 2014.[7] COMPREHENSIVE SPRING DESIGN. (2009).?COMPREHENSIVE SPRING DESIGN.?Available: . Last accessed 27th Mar 2014.[8] COMSOL. (2013).?Stresses and Heat Generation in Landing Gear.?Available: . Last accessed 10th April 2014.[9] Delta virtual airlines . (2014).?Boeing 777-200ER aircraft operations manual.?Available: ht0tp://library/B777%20Manual.pdf. Last accessed 22nd Feb 2014.[10] D.J.Dunn. (2012).?TUTORIAL – DAMPED VIBRATIONS.?Available: . Last accessed 24th Mar 2014.[11] Effendi et al. (2013).?Landing Gear Boeing 777.?Available: . Last accessed 20th Mar 2014.[12] Landing Gear Safety Devices. ().?Landing Gear Safety Devices.?Available: . Last accessed 28th Apr 2014.[13] Lawrence R. (2000).?Equivalent Mass of a Coil Spring.?Available: . Last accessed 2nd Apr 2014.[14] llaslco. (2008).?OLEO INTERNATIONAL ENERGY ABSORPTION SYSTEMS..Available: . Last accessed 26th Feb 2104.[15] Nong lam univeristy. (2010).?STRENGTH OF MATERIALS.?Available: . Last accessed 23rd April 2014.[16] OptimumSpring Solutions. (2014).?Spring Materials.?Available: . Last accessed 4th Apr 2014.[17] Philip D. Harvey. (1982).?Engineering Properties of Steels.?Available: . Last accessed 10th April 2014.[18] POLITECNICO DI MILANO. (2009).?Landing gear system .?Available: . Last accessed 1st Apr 2014.[19] RASER et al. (2013).?CONCEPT DESIGN OF AIRCRAFT LANDING GEAR.Available: . Last accessed 23rd April 2014.[20] Seeckt, K. (2008).?Aircraft Preliminary Sizing with PreSTo.?Available: . Last accessed 15th Mar 2014.[21] Shock absorber specifications . (2011).?shock absorber specifications .Available: . Last accessed 18th Apr 2014.[22] Smith, B. (2003).?The Boeing 777.?Available: . Last accessed 10th Feb 2014.[23] VirginiaTech. (2010).?Chapter 5 Shock Absorber Design.?Available: . Last accessed 29th Mar 2014.[24] Visvesvaraya Technological University. (2011).?DESIGN OF SPRINGS. Available: . Last accessed 10th April 2014.[25] Yourkowski, S. (2003).?777 TOUR FUN FACTS & DATA.?Available: . Last accessed 14th Mar 2014.[26] EU ETS (2014), Inclusion of aviation in the EU ETS. Available: [27] American Bonanza Society Air Safety Foundation. (2009).?Landing Gear Repair Guide .Available: . Last accessed 27th Apr 2014.[28] Retallack et al (2010), LANDING GEAR SHOCK ABSORBER. Available: A Vibration data sheetAppendix B ABAQUS analysis figures288607536766543910253683001381125368300-666753676651. Mesh convergences (1691, 3162, 6140 and 11235 numbers of mesh elements from left to the right)2 Animation of deflection changes ( 0 second to the 1.4 seconds with increment of 0.2 seconds)3. Table of values from analysis (shock absorber)Appendix C Spring material variation-13709651677035-14605001728470Appendix D Project managementAppendix E Health and Safety risk assessmentIDRisk itemEffectCauseLikelihoodSeverityImportanceAction to minimise risk1Insufficiency of mesh elements in ABAQUSStress analysis will not be able to work for the best and most accurate resultStudent license of softwareHighMLRequest of professional version or seeking alternative ways for reliable analysis2Calculation errorCAD tools are unable to be used for analysisComplexity of applied mechanics in landing gearLowMLAsk for postgraduate in research area of aerodynamics or solid mechanics and ask for the help to professor3Inaccurate design of landing gearFEA analysis cannot be used to unreliable solidworks designOccurrence of design error in SolidworksMediumLHRedesign of landing gear or get support from solidworks supervisor4Simulation error in solidworksFatigue testing cannot be doneImprecise entities or overlapping of surfacesHMHABAQUS was simultaneously used for the error in solidworks5Exact weight of redesign landing gear could be impossibleComparison with weight of original landing gear could not be possible.Downgrade of quality of evaluation of new landing gear.MLLSolidWorks has feature of measuring sizes and weights of the part.6Original dimensions of landing gear cannot be achievedComparison in dimensions cannot be doneClassification of landing gear values from companyHMMApproximation of sizing was done and optimisation was proceeded.7CAD programmes did not work on campus Analysis and Design of landing gear could not be carried out for a weekLicense of each programme and my university account had crashed each otherLHHWent to meet technician to fix the problem. Spent extra time for revision of theoretical resultsAppendix F Preliminary report ................
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