The landing gear system plays a vital role in an aircraft, and often it is configured in the aircraft structure due to its impact on the structural configuration of the aircraft. The need to come up with a new design that enhances minimum weight and volume, lower maintenance costs, reduced development cycle possess various challenges to the designer or aircraft engineers.therefore, these challenges have been countered by the employment of improved technologies, methods of analysis, and materials ((NASA), 2007, p. 535).
The principal function of the landing gear in the aircraft is to offer a suspension system during the taking off of an aircraft and also during the landing. Basically it is designed in such a way that it absorbs the landing impact energy in the form of kinetic energy and then later dissipate it and thus helps in minimizing the impacts that are transmitted to the airframe by the loads .besides, the landing gear enables the braking of the aircraft by the help of a wheel braking system thereby providing a directional regulation of the aircraft when on the ground.
In most cases, the landing gear is made retractable for purposes of minimizing the aerodynamic drag on the aircraft when it is flying .the design of the landing gear should ensure that strength, stability, control of the ground clearance and unconditional aircraft damping is all taken into account. Moreover, it should be sizable enough but not occupy a big space and also have a minimum weight (Conway, 2004, p. 564).
The aircraft landing gears essentially provide support to the aircraft engine during the period of landing and also the ground operations. There are various types of gears, and each gear depends on the design of the aircraft and the purpose the aircraft is designed for. For example, the aircraft that operate on the seas and snowy regions may be designed to have landing gears having skis. Similarly, the aircraft that maneuvers on the surface of the water may have float type of the landing gears. The landing gear comprises of various parts that enable it to perform its primary role of the supporting the engine of the aircraft. There exists a variety of landing gear configurations. For instance, there is also amphibious aircraft that are designed with the landing gears that makes the aircraft possible to land both in water and on dry land .the skis configurations allow the aircraft to land in both slippery and dry runways (Currey, 2009, p. 265).
They are the primary arrangements that are normally used for the landing gears. They include
The conventional landing gear, the tricycle landing gear and finally the tandem landing gear.
Conventional landing gears
This type of landing gear was, mainly used for the early aircraft and it has the main gear located at the forward position of the center of gravity. This makes the tail to necessitate for an additional wheel assembly. This position resulted into increased forward fuselage thereby an advantage during the operation of paved runways.
This type of landing gear is not common in most of the aircraft as a result of its tail gear and the main gear arrangement. They are arranged on the longitudinal axis of the aircraft. These type of landing gears are commonly found only in the military aircraft.
This is the most frequently applied landing gear type in the current aircraft systems because of the benefits that it comes along with. These benefits include
The airplane can also be grouped as fixed or retractable landing gear. Whereas as in most cases with the large aircraft, the small aircraft often have the fixed landing gear. For the fixed landing gear, the gear is linked to the airframe, and then it is left exposed to the slipstream, when the aircraft speed increases, then the parasite drag also is increased. This parasite drag that is caused by the retractable landing gear can be utilized to reduce the aircraft speed. For the retractable landing gear, the gear stowed in the fuselage, and once they get to the wheel, they do not result in the parasite drag hence. They are to a closely attached to a fitting panel.
For easy control of the landing aircraft, the forces generated that acts on the aircraft must be regulated by the landing gear. And this can only be achieved in two ways. I.e. by alternating of the shock power and being transmitted through the aircraft frame at a changed speed and time, after that this shock gets absorbed by the conversion of the mechanical energy into heat energy.
The landing gear system is one of the most significant systems in an aircraft as a result of the role that it plays during the operation of the aircraft. Its design should take into account the minimization of weight, volume while at the same time having an increased performance. Furthermore, it is essential to minimize the design of the landing gear system while taking into account the regulatory and the safety requirements .this prepare provides an overview of the existing landing gear systems and how its system and structural components integrate to provide for safe landing and taking off of the aircraft. Further, a new proposed design with fluid circuit diagrams of the landing gear system is s presented which is more advantageous in that it will help to provide better control in the named areas: flow control, pressure control and directional control using sufficient minimum controlling devices and valves (Roskam, 2005, p. 994).
Additionally, this part focuses on the safety and the maintenance practices that should be made in the landing gear system so as to ensure smooth operation of the aircraft and in turn preventing both the failure of the landing gear system and the accidents experience during taking off and during landing. When the practices are ensured to the dot, then the life span of the landing gear system is also increased. Finally, the conclusion and recommendation are made on the proposed design of the aircraft whereby it needs to be simulated in software so as to ensure its effectiveness and reliability.
The scope of this report is to study, analyze and assess the existing conventional landing gear system of one aircraft while considering information available in the web. After that, coming up with a new design of a landing gear system or modify the existing design that has better performance than the current landing gears system.
Below were the objectives of the report
To present an Introduction to fluid power system to aircraft industries targeting landing gears
To provide a Literature review on the Conventional Landing gear system along with Tandem Landing gears and Tricycle Type Landing gears
To present an overview of the Working principles of existing landing gear systems
Moreover, the focus of the study also captures the Development Of a proposed design and analysis of a new or modified landing system which will give a better control: flow control, pressure control and directional control using sufficient minimum controlling devices and valves with fluid circuit diagrams.
Besides, the study also aims at Safety and healthy check of both the pipes and pressure cylinders using (thick-walled or thin walled analysis), and noting the required maintenance practices
The research was done based on information from online sources and books. The main aspects that were covered are the working principle of the current landing gear system and the designing of an improved design that takes care of a better control
The operation of the landing gear system is based on various parts that work together to ensure safe landing of the aircraft. It consists of both the structural and system components. The structural components are the main fitting, the shock absorber, the trailing arm, axle, torque link, drags, the traction actuator, the lock mechanism, the wheel, and the tire .below is a brief description of how these components integrate to enable the function of the landing gears system
During the taking off and landing aircraft tires are normally exposed to extreme dynamic and static loads. They function primarily to provide a sufficient amount of shock absorption to the forces that they experience. Thus, they are also responsible for reducing the shear stresses in the rubber for uniform load distributing. They are further constructed with extra steel belts that offer a larger footprint area and also can run in the radial direction ((NASA), 2007, p. 564).
The wheels that are selected should allow for the housing of the brakes and the fitting of the tires while acknowledging that the design should have a minimized weight. Normally there are two types of wheels; the `A` type and the bowl type but the latter has the limitation that it lacks enough space for accommodating the brakes. The system components are the brake unit, the antiskid system and finally the retraction system. Aircraft wheels are a vital part of a landing gear system since they provide support to the entire weight of the aircraft during taxi, takeoff, and landing. A distinctive aircraft wheel should be light in weight, robust/durable, and made of an alloy of aluminum. Early aircraft wheels were of single piece construction
Essentially the purpose of the brakes is to aid in the stopping, the speed control and keeping of the aircraft in a stable position. They also ensure the turning of the can.
The current actuation systems that are available for the brake sis the hydraulic brake system,
Kinematics refers to the design and the analysis of the landing gear parts .in accordance with the gears retraction and extension. The extension and retraction should be possible within the space that is available. The retraction technique comprises a retractor actuator, a locking mechanism and a folding brace which together Ensures that the gear can automatically lock in cases of hydraulic failure. The wings on the gears are also designed in such a way that they get retracted inwardly while restoring the fuselage and the boogie.
The absorption of the shocks is essentially done by the tires and the loads .the shock absorbers are constructed from either rubber springs, steel springs or fluid springs. They absorb the shocks by pushing oil through an opening into a chamber containing dry air. Below is a diagram of the conventional landing gear system (Currey, 2009, p. 453).
Below shows the steps or the stages that need to be taken during the preliminary design of the landing gear system. The preliminary design begins with the aircraft requirements. Besides, there is concept proposal and the concept of the formulation phase whereby the other concepts are also formulated and analyzed .this helps to determine and improve the final weight and range of the aircraft. Performing a review on the runway floatation and weight, availability and the compatibility of the airframe structure helps to determine the number and size of the wheels needed.
The loads on the landing gear and the gear position can also be determined. After that, the dimensions, clearances, braking and the materials and weight are evaluated.
The design of the system is initiated using AutoCAD which is composed of a set of initializing tools and applies minimum top level requirements for the making of the aircraft baseline design. It is of great significance since it reduces the complexity and the computation time of the design processes of the aircraft. Moreover, it produces Inputs which provides better analysis of the design (Conway, 2004, p. 564).
In addition to the AutoCAD, software is proposed for implementation of the design which is Simulink and ADAMS which will help to realize the functionality of the proposed landing gear system.
New landing gear design capabilities are added to the Initiator, and a new landing gear analysis tool is added. The overall structure is shown as a design optimization loop, which is the ?nal goal of the greater aircraft design framework.
The various components of the landing gear help to essentially achieve the proposed design. They will ensure that they are compliable with the standard regulations of an aircraft landing system.
During the taking off and landing aircraft all landing tires are normally exposed to extreme dynamic and static loads. They function primarily to provide a sufficient amount of shock absorption to the forces that they experience. Thus, for the proposed design, the new tires should be designed approximately to the maximum static load. The best type of tire for the proposed design are the radial tires since they have the additional benefit of reduced weight and longer lifespan. As compared to the current landing gear design, they are 20 % lighter, and they will reduce the shear stresses in the rubber thus equally distributing the loads .they are further constructed with extra steel belts that offer a larger footprint area and also can run in the radial direction. Another response to the selection of the radial tires is that they can withstand both inflation circumstances and higher overload stresses. Besides, the wear is easily noticeable in the radial tires (Roskam, 2005, p. 674).
The wheels that rare selected should allow for the housing of the brakes and the fittings of the tires while acknowledging that the design should have a minimized weight. The best choice that is preferred for the proposed design is the bowl type wheels. Normally there are two types of wheels; the type and the bowl type but the latter has the limitation that it lacks enough space for accommodating the brakes. The bowl wheels are made from forged aluminum which promotes minimized aircraft weight
Essentially, the purpose of the brakes is to aid in the stopping, the speed control and keeping of the aircraft in a stable position. They also ensure the turning of the aircraft. The material for making the braking of the proposed design is carbon as a result of high specific heat and thermal conductivity. These properties help to provide a uniform heat distribution. Moreover, they have a low maintenance cost and long lifespan.
To determine the size of the brake and consequently the brake weight, some specifications are considered, 250 halts at the landing weight, five stops at the landing weight, and a single stop 6 square feet for deceleration at maximum weight.
The current actuation systems that are available of the brake is the hydraulic brake system, with the advancement, an electric brake system has been developed hence this will be the best actuation system for the proposed design. The reason lies in the following reasons
Kinematics refers to the design and the analysis of the landing gear parts .in accordance with the gears retraction and extension. The extraction and retraction should be possible within the space that is available. To ensure this, there will be a need for limiting g the interference between the gear, and it’s surrounding in addition to sufficient support to the landing gear system.
The retraction technique comprises a retractor actuator, a locking mechanisms, and a folding brace. For the proposed design, the retraction mechanism is positioned in the forward direction to ensure that the gear can automatically lock in cases of hydraulic failure. The wings on the gears are also designed in such a way that they get retracted inwardly while restoring the fuselage and the boogie (Conway, 2004, p. 825).
Moreover, an algorithm is required for which will assist in determining the gear stowed position helpful in obtaining the fuselage interferences. Also, the positioning of the retractor actuator is made a bit higher than one -half the extended length. Besides, the extension time for the proposed design is limited to 15 seconds at temperatures above -30 degree Celsius and 30 seconds at temperatures ranges between -55 and -29 degrees Celsius.
The absorption of the shocks is essentially done by the tires, and the loads .the shock absorbers are constructed from either rubber springs, steel springs or fluid springs .the preferred choice for the proposed design is the pneumatic fluid spring shock absorber since it has relatively reduced weight and high gear efficiency. They absorb the shocks by pushing oil through an opening into a chamber containing dry air (Roskam, 2005, p. 480).
Take off stability
During the taking off of the automated landing gear aircraft, its pitch angle will be determined by the below equation
?LOF=Σ lOF+ d ?/dt {2l1/VLOF+ (l2CLOF/gCLσ)}
During taxiing at high speed may result to tipping of the aircraft by its side. Thus the new aircraft will have a turnover angle less than 63 degrees as given by the equation
Tan ψ= hcg /ln sin σ
Where σ is equal to
Tan σ=t/2(lm +ln)
Landing stability
The landing stability will be determined by the hcg which not will be exceeded. i.e.
lm> (hcg +es) tan ?
The gear position
The position of the gear concerning the center of gravity is very crucial to the stability of the aircraft. Thus below is a diagram showing the position of both the main and the nose gear (Pullinger, 2014, p. 340).
The maximum load gear is obtained by the below formula
Fm max = {ln/ (lm +ln)} * W
Whereas the minimum load = {lm/(lm +ln )}* W
The implementation of the design will require the estimation of both the weight and the position of the landing gear .the weight estimation will be done using a class 2 landing gear weight estimation process. Below are some of the assumptions that will be made during the implementation.
If the number of wheels is greater than 12, it is assumed that the number of main gear struts is 4
The nose gear is assumed to consist of 2 wheels and placed at 25 percent of the prescribed nose gear stowage space
Speci?c rear or forwards per locations are not taken into account;
The maximum take-off scrape angle is assumed 12 degrees
Propeller clearances are not taken into account
It is very costly to repair a damaged landing gear system hence proper measures should be taken so as to ensure that the landing gear is not damaged.
Ensure that there is regular lubrication. -This is because insufficient lubrication during the normal operations results into unnecessary friction and consequently wear-and-tear in most of the constituents of the landing gear. Lubrication is very significant more so to the movement of all the components of the landing gear system and joints. Proper lubrication will guarantee the smooth operation of the landing gear, reduced friction and wear, and consequently, will result in the suitable transmission of forces all over the landing gear system during operation. Also, Lubrication also helps to prevent water and other substances that causes corrosion is one of the major challenges that results into the failure of the landing gear system.
Before the lubrication of the landing gear system, check the aircraft’s handbook for you to determine the elements which you should avoid and the ones that you are supposed to use since using the wrong lubricant can be very detrimental to the operation of the landing gears system apart from providing insufficient lubrication.
Then there is the protection of the landing gear system from paint, and other destructive substances since the reaction between hydrogen and the high tensile steel are corrosive. When acidic substances contaminate the gear, the hydrogen cracks thereby exposing the landing gear to cracks which are very expensive to repair or fix. In most cases where cracks have been realized, their action taken is usually to replace the component.
Moreover, evading influences of any kind is another best maintenance practice for landing gear system. This is because the shock-absorbing struts are mostly susceptible to dents and any alteration in the physical outlook of the struts or just any other landing gear support structure results in stress to the landing gear and hence making it more susceptible to fatigue failure(Pullinger, 2014, p. 908).
Additionally, coincidences of failure in the landing gear should be documented and specified as which type of failure including the visual damages that can be observed normally, the best action to take is to replace the component immediately and take its nondestructive testing and repair together with an account of what transpired to it. This will enable the landing gear MROs to perform the repair operation more precisely in advance when the damages haven’t become more serious hence more costly to fix.
Regular adding of grease to the landing gear system every six months -During conditions of low temperature, either anhydrous calcium or lithium should be used and also it is advisable to fill the gearbox at approximately 75% with grease to help in minimizing the accumulation of moisture that can result in freezing and the consequent cracking of the gear.
During the lubrication process, first, the gear should be fully retracted and then with the aid of high gear, lowered to 3 legs and then proceed to lubricate the lift screw assembly with a fitting of grease. Then using a grease fitting B, lubricate the gearbox and then proceed to lubricate the bevel gear using a grease fitting C. after that distribute the lubrication on the other legs.
Do a proper observation on the inner main gear doors – the main gear doors should be tightly closed with a minimal tension hence after the aircraft arrives for inspection it should be checked to see whether it is operating normally .the electrical system or the retract motor should be checked since any default may indicate that the door is operational yet it is faulty which can result in cracking and failure of the gear box
The checking of the condition of the inflation tires should be done – any sign of wear or tear in the tires should be checked for, and when any is found not to be in good condition, it should be replaced.
The main gear and the nose gear should be regularly checked for any signs of leakages or inflations. The leakages are checked around the strut component(Roskam, 2005, p. 309).
Ensure that the main gear outboard doors are disconnected – this ensures that there is minimal confusion during the processes of inspection and maintenance.
Normally pull the circuit breakers that is located in the landing gear motor by turning the hand crank in the anticlockwise direction. The pulling of the circuit breaker helps to prevent the aircraft from any incidences of collapse and locking of the gear box.
Do not disconnect the squat switches at the same time; this ensures that ay incidences of retraction are prevented.
Before flying the aircraft, check that the nose gear is working plus the main gear too. This is done by grasping the nose wheel and wiggling it then observe for any play. If there is the presence of play, then it is operational.
The retraction of the landing gear should be done electrically but not manually or by any mechanical means.
Moreover, the gear warning horn should also be checked for proper functioning before taking off. This is electrically enabled by applying power and reducing the throttle for it to idle.
During the cleaning operations of the landing gear components, it is recommended to use only solutions that are neutral in their pH since other solutions may corrode the parts of the landing gear and reduce its life span and affectivity(Currey, 2009, p. 217).
The pressure cylinders and pipes are regarded as thin-walled vessels in the hydraulic design process, and their designing should consider some factors such as the principal stress and strains that occur in these vessels. These variables are not measured directly but by the aid of a strain gauges which determines the pressure on the surface. An FL 130 experimental device tries to do this by investigating the internal pressure generated by a hydraulic pump in this way; the vessel containing the hydraulic fluid is closed with a lid at one end and on the other end using a movable piston. A hand wheel is then used to displace the piston by the help of a threaded spindle. The readings of the internal pressure and strains will be indicated by a manometer and strain gauges respectively, after which these values are then amplified by an FL 152 and display them. Finally, the measured values are taken to software for further evaluations. To obtain the principal strains and strain conversion graphically, a Mohr’s stress cycle is used. Using the law of elasticity, the principal stress can then be calculated from the principle strains
For a thick cylinder that is filled with oil, their design includes an allowance for non-uniform distribution through the wall. They undergo various forces including circumferential, radial, hoop and axial stress hence an FL 140 is used to obtain these stresses. The measuring amplifier that is used is FL 152 that displays the noted signals and the taken to software for evaluation. The graphical representation is done using the Mohr’s circle for stress and eventually the using the law of elasticity, the direct strains and stresses are calculated (Pullinger, 2014, p. 684).
There are a variety of recommendations made for the proposed design of the landing gear system which is an improvement. Furthermore, the addition of some systems such as structural component interfaces will help to improve the landing gear analysis model systems which in turn will help to optimize the weight to the aircraft. The confidence of the proposed landing gear design will also be enhanced when there is an addition a ?nite element analysis of the structure since it will enable the optimization of the shape of landing gear components hence reducing the overall weight of the aircraft (Roskam, 2005, p. 541).
Further, a simulation of ground handling can also help in the analysis of the aerodynamic drag and interferences. For the Delft Initiator, it can be recommended that the center of gravity range optimization be improved. Centre of gravity range optimization is not only vital for the landing gear positioning, stability and control of the aircraft during ?ight but it also highly dependent on the CG range. Added support for evaluation of nonconventional aircraft concepts will then add the capability to perform the design process within a multidisciplinary optimization loop.
Conclusion
The scope of the of the study was to expand the theoretical knowledge of the existing landing gears system and thereby come up with a Design and analysis of a new or modified landing system which provides a better control in the following sectors : flow control, pressure control and directional control using sufficient minimum controlling devices and valves.
The automated landing gear design comprises of an analysis of various structures, kinematics, runway ?otation and weight which helps to achieve the scopes of the study. During the determination of the main and nose landing gear positions, reversal factors pose a challenge. The factors include the takeoff stability requirement which takes into account the appropriate aircraft pitching. For the aircraft to make a safer landing, it should have a pitch down tendency while at the same time allowing a crosswind landing without touching the ground. Moreover, for sideways safe landing, there is a requirement for additional ground clearance (Conway, 2004, p. 165).
The Loads on the nose gear should be high enough to keep the ability of proper steering. The main gear also should keep a high enough load to make sure that the brakes can provide enough braking power. Ground operational requirements also require that the aircraft should be able to make an 180-degree turn on a runway. All these limitations result into different positions of the landing gear such as lateral, longitudinal and height. Based on these positions, the landing gear struts are evaluated and also the selection of the wheels stopping power dictates the needed size of the brakes. The design and analysis of the retraction and extension of the gear (kinematics) result in joint and actuator positions. When considering different kinematic schemes, the least complex scheme is preferred, on the safety and maintenance costs.
When hard landings result into shocks and by taxiing over rough surfaces they are absorbed ef?ciently by oleo-pneumatic shock absorbers and tires. The shock absorbers need to be able to cope with different load cases provided by certi?cation speci?cations. To show compliance with these speci?cations the piston stroke, the internal pressures and volumes are designed. The oleo-pneumatic shock absorber forces are then modeled accordingly. Reduced complexity will also keep landing gear weight minimal (Conway, 2004, p. 623).
References
(NASA), N. A. a. S. A., 2007. An Overview of Landing Gear Dynamics. 2nd ed. Minnesota: NASA Langley Research Center.
Conway, H. G., 2004. Landing gear design. 1st ed. Chicago: Chapman & Hall.
Currey, N. S., 2009. Aircraft Landing Gear Design: Principles and Practices. 3rd ed. new York: AIAA,
Pullinger, K., 2014. Landing Gear. 2nd ed. Carlisle: Kate Pullinger Books.
Roskam, J., 2005. Airplane Design: Layout of landing gear and systems. 3rd ed. Westminster: DARcorporation.
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