Redesign and stress analysis of simplified landing gear Junghoon Suh
Final Report
ECM3101/ECM3102
Title: Redesign and stress analysis of simplified landing gear
Date of submission: 01/05/2014
Student Name: Junghoon Suh
Programme: BEng Mechanical Engineering
Student number: 600060234
Candidate number: 030412
Supervisor: Philippe G Young
Abstract
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, theredesign of a safe and durable target landing gear consideringa Boeing 777 aircraft should withstand the maximum landing loading of 1016196.52N and ensurethe completion of 10,000 lifecycles. To achieve this, an in depthunderstanding 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 interms 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 muchof the weight increasecaused 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, materials
Acknowledgment
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
Abstract......
Acknowledgment......
1. Introduction
2. Background Theory
2.1 Research methodology
2.2 Structural analysis
2.2.1 Nose landing gear
2.2.2 Main landing gear
2.2.3 Troubleshooting of present landing gear
2.2.4 Safety
2.3 Mechanical analysis
2.3.1 Boeing 777 reference data
2.3.2 Static load balance
2.4 Design of shock absorber
2.4.1 Spring shock absorber
3. Methodology
4. CAD Design
4.1 Shock absorber
4.1.1 Spring design
4.1.2 Shock absorber
4.2 landing gear
5. Results and discussion
5.1 ABAQUS analysis
5.1.1 Spring shock absorber
5.1.2 Landing gear
6. Design and quality of research and innovation in research process
7. Sustainability
8. Conclusions and recommendations
References
Appendix
Appendix A Vibration data sheet
Appendix B ABAQUS analysis figures
Appendix C Spring material variation
Appendix D Project management
Appendix E Health and Safety risk assessment
Appendix F Preliminary report
Redesign and stress analysis of simplified landing gear Junghoon Suh
1. Introduction
This project was conducted with the aim of investigatingthe 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 feasibilityof 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 partscritical 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 incorporatesa tricycle landing gear, which consists ofa single nose gear positioned at the front and two six wheel bogie type main landing gearspositioned at the rear side. [4] Six wheel bogie type main landing gears arebuilt in the Boeing 777 and successfully enableit 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 approximationstakenfor the few parts where information was not available. The redesigned landing gear had a target of 10,000 life cycles and not to weighmore 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 whenselecting the optimum redesign model among prototypes.
2. Background Theory
2.1 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 easeof data collection and immense quantity of information available.
- Through the Exeter e-library, Google scholar and Journal of aircraftprovided 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 providedan 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 gearspositioned on the rear side of the aircraft.
2.2.1 Nose landing gear
The nose landing gear is located atthe 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 utilisesa forged steel air-oil shock absorber atthe 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 maintainthe lock links in locked position, and hinge.
2.2.2 Main landing gear
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 higherrates 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 whenthe 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 strut
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.
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.
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 link
Torsion links prevent internal rotation between the outer cylinder and inner cylinder of the main strut. During steering, force is applied with the torsion link allowingthe 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 gear
The 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 Safety
Safety 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 systemspresent 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 data
Figure 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 forthe redesign of the landing gear.
Name / Symbol / Value / UnitsMaximum landing weight[5] / Mmaz landing / 201,840 / kg
Approach speed[25] / Vapproach / 136 / kias
Thrust / Fthrust / 77,000 / lb*2
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 balance
2.3.2.1 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.
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)
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 F2
F2 (nose landing gear) = 7/50 F
F1 (single main landing gear) = 43/100 F
Mass proportion on M1 = 7/50 M, therefore, mass on nose gear 28257.6kg
Mass 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.