Landing Gear (Autosaved)
Content
1. Introduction
1.1Background theory
Otto Lilienthal was one of the first men who flew above the earth in 1891. There were only two wings and one stabilizer in Lilienthal‟s airplane. He flew from the hill to fly and landed on his feet. The Wright Brothers made the first powered flight after Lilienthal. The landing gear of the Wright Flyer I was made of skis. The air plane was further developed there comes the landing gear after the Wright Brothers had flown.
In landing gear there are few systems involved. Strut; Shock absorber; Extraction/Retraction mechanism; Brakes; Wheel; Tire.
In small air planes shock absorber and extraction/retraction may not present. The landing gear is the join of airplane to ground, so that all the ground loads are transmitted by it to the aircraft structure. On the local structure there is then a high influence of the landing gear, which must be taken into account since the initial design stage.
The main sizing conditions for the system and its surrounding structure is the landing. Braking is which determines both vertical and horizontal loads that influence structural sizing. Steering and taxi stability is taxi control.
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1.2 Basic landing gear principles
During the time, different types of landing gear started to be designed by people. Generally, thinking a new design would fulfill its purpose more efficient with fewer disadvantages than the landing gear configuration which were designed before. Moreover, all landing gears have the same purpose despite for the various configurations.
There are different types of landing gear due to the relation between weight and size of aircraft. The landing is being stored after concerning the aerodynamic performance of the aircraft. The right use of the material is important in designing landing gears due to its complex structure.
1.3Purpose
The title of the project is to design and fabricate a landing gear system using pneumatic system. Basically, to enable aircraft movement on the ground is why landing gear is designed. Therefore, at least one tire must be able to steer and rotate. It also enable the aircraft to take-off and land. The landing gear absorbs the landing shock and converts kinetic energy into heat while landing. To slow down the aircraft there is brake on the landing gear which increases drag. This helps the aircraft to control on the ground. Furthermore, for loading and unloading also the landing gear can be used. It enablesthe aircraft to change the height between the aircraft and the ground.
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1.4General aspects
In last few years a lot of research has improved the landing gear configuration. Different types of landing gear resulted from difference between the weight and size of aircraft. A fixed landing gear causes a lot of resistance during flight. Retractable landing gear is used to avoid that the aerodynamic performance of the aircraft to be affected. Therefore, to store the landing gears during flight are provided.
2.0 Functional Analysis and Design Requirements
The landing gear is the aircraft major component which is designed in design procedure. Moreover, all major components such as wing, tail, fuselage, and propulsion system must be designed prior to the design of landing gear. The landing gear design may drive the aircraft designer to change the aircraft configuration to satisfy landing gear design requirements.
The primary functions of a landing gear are as follows: 1. To keep the aircraft stable on the ground and during loading, unloading, and taxi. 2. To allow the aircraft to freely move and maneuver during taxing. 3. To provide a safe distance between other aircraft components such as wing and fuselage while the aircraft is on the ground position to prevent any damage by the ground contact. 4. To absorb the landing shocks during landing operation. 5. To facilitate take-off by allowing aircraft acceleration and rotation with the lowest friction.
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Figure 2: Landing gear design flowchart
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While take-off and landing the aircraft landing gear is a crucial component, during airborne flight operation it is a dead weight. For this reason, it is recommended to retract the landing gear inside aircraft to reduce aircraft drag and improve aircraft performance. Figure 1 illustrates landing gear design flowchart including design feedbacks. The landing gear design is an iterative process and the number of iterations depends on the nature of design requirements as well designer‟s skills. Moreover, the design of mechanical subsystems and parameters is grouped in one box and should be performed by the mechanical design group. The landing gear design we initiate by defining landing gear design requirements and the process is ended by the optimization. More research and studies were done to get the details of the landing gear design.
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3.0Motivation Study
3.1 Idea Generation
The motivation of study is to identify which is better to use between pneumatic and hydraulic landing gear system. As we compare with hydraulic system, pneumatic system is substantially safer and more reliable. A pneumatic system is substantially safer and more reliable than a hydraulic system. For years, hydraulic landing gear system is used in all planes. Due to occasional failures, a pneumatic “back up system” is used when hydraulic system failed. However, recently hydraulic system has been eliminated and using pneumatic as landing system became common. Furthermore, a pneumatic system is more responsive than a hydraulic system because of its lower viscosity of air which can pump more rapidly than oil. As we compare with the responsive time, pneumatic system is fully 40% faster than the hydraulic system.
3.2 Ease of Installation
As we compare with hydraulic system, pneumatic system easier to install and takes up less space. There are no electrical components involved in pneumatic system, so one major source is eliminated in the installation procedure. In addition to that, in pneumatic system the components are significantly smaller, thus it is easily installed within the compartment.
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3.3Safety and the break-away fin
The pneumatic system is safer than a hydraulic system because leaks can occur in hydraulic. Hydraulic uses oil which is flammable while pneumatic uses air which is not flammable. From the standpoint of fire an air-powered system is safer.
4.0 Objective
The main objective of the project is to build and distribute an aircraft landing gear system using pneumatic pump that I have designed for my final year project. It is also designed to study the importance of landing gear in an aircraft and also researchers have done to come with an efficient landing gear system. Besides that, it is also to identify the best landing gear system either using hydraulic or pneumatic system. To build a product that won‟t fail while functioning.
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5.0 Significant Contribution of Study
In designing this landing gear a lot of researches have been done. The pneumatic landing gear system is substantially safer and reliable. Therefore, pneumatic landing gear system has been designed. Since there are very much hydraulic landing gear system failure, replacing with pneumatic will be a better choice. It is more responsive than hydraulic system and occupies less space in an aircraft. The concept of this project is used as back-up system in planes when hydraulic system fails.
6.0 Project Selection Process
Among various landing gear system arrangements that we have studied. We have chosen Nose Landing gear system. The selection process is done to select and to satisfy design requirements. A landing gear chosen is based on a number of factors. The selection of landing gear system is based on several design requirements which affect the decision. These include: cost, weight, performance, take-off run, landing run, ground static stability, ground taxi stability, and maintainability. A comparison Table6.0 was used to undergo this selection process for further development in this project.The landing system which gains the highest point is often the most appropriate landing gear for the aircraft. Hence based on aircraft mission and design requirements, one arrangement is usually the best alternative. Number 1: worst and 10: best
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Criteria Cost Aircraft weight Manufacturability Take-off/landing run Stability on the ground Stability during taxi Total
Single main 9 3 3 3 1 2 21
Bicycle 7 4 4 4 2 3 24
Tail gear 6 6 5 6 7 1 31
Nose gear 4 7 7 10 9 8 45
Quadr icycle 2 9 7 5 8 9 40
Multi bogey 1 10 1 8 8 9 37
Human Leg 10 1 10 2 5 28
Table 6.0: Comparison among various landing gear configuration The results shown in table shows the nose gear gets the highest points. This criteria is important in designing.
7.0 Existing Technology
In existing technology, pneumatic landing gear system is a better system which can be replaced with hydraulic system. There are planes which are using pneumatic system as back-up system in case of failure of hydraulic system.
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8.0 Methodology
8.1 Landing Gear Mechanical Structure
The scope of this project mainly concerns the engineering aspects of landing gear design which contain parameters such as landing gear configuration, fixed or retractable, landing gear tire size, wheel and brakes, shock absorber principles, braking system and strut mechanism. The design of the landing gear parameters such as retraction system, shock absorber, tire sizing, braking subsystem, and strut sizing are reviewed in brief in this section
8.2 Tire Size, Wheel and Brakes
Technically, the term „wheel‟ refers to a circular metal/ plastic object around which the rubber “tire” is mounted. The brake system is mounted inside the wheel to slow the aircraft during landing. However, in majority of cases, the entire wheel, tire and brake system is also referred to as the wheel. The primary materials of modern tires are synthetic or natural rubber, fabric and wire along with other compound chemicals. Nowadays, majority of tires is generally pneumatic, inflatable and includes a doughnut-shaped body of cords and wires encased in rubber consist of a tread and a body as in Figure 8.1. Tires perform four important functions with the air contained within them which are:
1. Tires support the aircraft structure off the ground 2. Absorb shocks from the runway surface 3. Help transmitting acceleration and braking forces to the runway surface
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4. Help to change and maintain the direction of motion
Figure 8.1: Tire geometry
The choice of the main wheel tires is made on the basis of the static loading case. The total main gear load (Fm) is calculated assuming that the aircraft is taxiing at low speed without braking as shown in Figure 8.2
Where: W is the weight of the aircraft and lm and ln are the distance measured from the aircraft cg to the main and nose gear, respectively.
The design condition occurs with the aircraft cg at its limit. For single axle configurations, the total load on the strut is divided equally over the tires, whereas in tandem configurations, the load per wheel depends on the location of the pivot point; to reduce overloading of the front wheels during braking, the pivot is usually positioned such that the distance between it and the front and rear wheel axles is about 55 and 45 percent of the truck beam, respectively. The choice of the nose wheel tires is based on the nose wheel load (Fn) during braking at maximum effort, i.e., the steady braked load. Using the symbols shown in Fig. 8.2, the total nose gear load under constant deceleration is calculated using:
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(
)
(
)
Where L is the lift, D is the drag, T is the thrust, and hcg is the height of aircraft cg from the static ground line. Typical values for ax/g on dry concrete vary from 0.35 for a simple brake system to 0.45 for an automatic brake pressure control system. As both D and L are positive, the maximum nose gear load occurs at low speed. Reverse thrust decreases the nose gear load and hence the condition T = 0 results in the maximum value:
( )
Figure 8.2 Forces acting on the aircraft during a braked roll To ensure that the rated loads will not be exceeded in the static and braking conditions, safety factor is used in the calculation of the applied loads. In addition, to avoid costly redesign as the aircraft weight fluctuates during the design phase, and to accommodate future weight increases due to anticipated aircraft growth, the calculated loads are factored prior to tire selection.
8.3 Wheel Design
The design of the wheel is influenced primarily by its requirement to accommodate the selected tire, to be large enough to house the brake, and to accomplish the above tasks with minimum weight and maximum life. As shown in Figure 8.3, two basic configurations of wheel design are currently available in the industry which is A-frame and bowl-type.
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a) A-frame
b) Bowl-type
Figure 8.3 Basic configuration of wheel design Continued heavy dependence on forged aluminum alloy wheels is foreseen by industry, whereas steel and magnesium alloy wheels are no longer taken into consideration due to weight and corrosion problems, respectively. Although practicable, titanium wheels are still quite expensive. Most of the premium for titanium wheels results from the expense for the forging process, which could be 10 to 11 times those of aluminum alloy. In addition, current titanium forging tolerances have yet to reach the precision obtainable for aluminum material, thus machining of all surfaces is required to control weight and obtain the desired form.Based on statistical data, the wheel assembly weight is determined as a function of the rated per wheel static load (F) and average tire outer diameter (D) f w FD/1000
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8.4 Brakes
Most airplanes are equipped with disc brakes, with a functioning principle similar to that of the automotive systems, but based on different sizing principles. Drum brakes are almost outdated. The main components of a disc brake, which is usually powered by the hydraulic system, are as following: • Pressure plate • Stator discs • Rotor discs • Back plate
The complete equipment is housed inside the wheel, then occupying a large part of the room between the axle and the wheel. The stator discs are keyed to the axle, or anyway constrained in such a way to be only free to move along its axis. They bring, on the two flat surfaces, lining blocks, or pads, made of a mixture of metallic and ceramic materials. The rotor discs are keyed to the wheel, rotating then with it and being free to move along its axis. They are alternated to the stator discs, so that the assembly results a sandwich of rotor discs and stator discs packed together. The stator disc at one extremity, or back plate, is fully constrained to the axle and brings lining blocks on one side only.
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Figure 8.4: Disc brake
The stator disc at the opposite extremity, or pressure plate, brings lining blocks on one side only and, during braking, is pushed against the first rotor disc of the assembly by a series of hydraulic pistons. This action compresses the entire disc package, because rotor and stator parts are all free to move along the wheel axis, with exception of the back plate at one extremity, which contrasts the pressure.
Since the rotor and stator discs are in relative rotation, the contact between the lining blocks and the rotor discs will generate a tangential friction, responsible for braking. The higher the hydraulic pressure, the higher the normal contact force and then the friction force. When pressure is reduced, the discs are released by a series of springs.
The rotor discs usually have radial slots, to minimize disc deformation during heat up. The discs can be made of steel but, when affordable, carbon discs allow lower weight. For a short period in the 60‟s they were made of beryllium, but its manufacturing costs and difficulties excluded it from standard use. The stator disc is usually made of steel. The lining is fragmented into sector blocks because it is made of a brittle compressed mixture of metals and ceramics, that may be broken if the pressure is not distributed uniformly on the entire surface.
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Brake sizing is based on heating during a single landing, considering that ventilation has a limited effect and neglecting the contribution of possible thrust reverse, flaps and spoilers. This means that a high part of the kinetic energy at landing will be converted into brakes heating; the part of brakes that is involved is often referred to as heat sink. This event can be expressed by a simple formula of energy balance:
Where: k = fraction of energy converted to brake heat; M = aircraft mass; v = landing velocity; m = total heat sink mass; cv= heat sink specific heat; ΔT = temperature increment during braking. Considering that anyway there is a drag contribution to braking, k may often be approximated to 0.8. For sizing, M should be the max landing mass, v the max landing velocity and ΔT the difference between the allowable disc material temperature and the highest possible initial temperature. Materials with high specific heat and high operating temperature are of course preferred, because they allow a reduced total disc mass.
Usually brakes are located in all the main landing gear wheels: the above-mentioned mass m is then related to all the brakes located in the wheels. Large aircraft may have braking nose wheels. A multiple disc brake is used whenever a high braking power is necessary, because the lining friction surface increases with the number of discs. Moreover a thick disc would not allow a suitable temperature distribution, but would be rather cold at the core and overheat in periphery. On the other hand, discs must be sized in such a way to withstand the high tangential stress that
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is generated by friction.
Figure 8.5: Evaluation of the braking intensity The braking intensity is a function of the brake geometry and, in a minor extent, of the aircraft geometry. From fig. 8.5 the braking torque C can be easily evaluated as a function of the hydraulic pressure and discs geometry, as follows:
B
Where: p = hydraulic pressure A = total lining friction area μ = disc friction coefficient, normally around 0.3 RB = radius of the lining block centroids The braking force TM will be then given by
R
Where RR is the rolling radius (that depends on the normal reaction NM, tyre pressure and tyre geometry).
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Of course the braking force cannot increase indefinitely with pressure, but is limited by the tire grip; this is defined by a factor μG that ranges from 0.9 for dry runway and tire and asphalt in good conditions to 0.5 in case of wet runway and down to 0.1 for iced runway. Then T M is limited by: ( )
G.NM
Where, NM is the normal reaction of the main landing gear. In static conditions (airplane at rest) NM will be equal more or less to 90% of the aircraft weight, due to the conventional position of the aircraft centre of gravity. In braking conditions NM is lower and the inertia forces overload the nose landing gear. Depending on the longitudinal and vertical position of the centre of gravity, a simple algebraic system in the two unknowns NM and T can be set up and solved if any drag contribution is neglected.
In determining the ground loads on nose wheels and tail wheel, and affected supporting structures, it must be assumed that the shock absorbers and tires are in their static positions. When a solid spring is chosen, the main parameter for the design is the geometry and the cross section of the beam. In case that a hydraulic shock absorber is selected for the landing gear, the typical parameters which must be determined include stroke, orifice, outer and inner diameter, and internal spring sizing.
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8.6 Shock absorber
The main role of the shock absorber is to zero the vertical component of the airplane velocity during landing, with no rebound and limited load transfer to the vehicle structure (and occupants). Its secondary requirement is to allow a comfortable taxiing. Different types of shock absorbers are available, but when costs and dimensions allow, a hydraulic system is commonly used. The hydraulic solution is anyway the mostly adopted one, and fig. 8.5 shows some of the many possible versions. Substantially the system structure is made of a movable piston that, when loaded, compresses a gas (nitrogen) in a cylinder and causes an oil flow through orifices. The system elasticity is due to the gas transformation and the damping effect to the liquid pressure losses. The complexity of the system increases with the requirements.
8.7 Shock absorber functioning principles
In its most basic configuration, the shock absorber can be depicted as a hydraulic cylinder linked to an accumulator. When the piston moves, the oil from the cylinder passes through an orifice and compresses the gas in the accumulator. According to this approach the reaction R of the shock absorber is function of the piston position and its time derivative, as follows:
(
)
(
)
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Where: p = pressure on piston; A = piston area; pA= accumulator pressure; k = orifice pressure loss coefficient; Q = oil flow rate; p0 = initial accumulator pressure; V0 = initial accumulator gas volume; γ = polytrophic exponent; x = piston stroke (x=0 for all extended shock absorber)
8.8 Strut Sizing
The wheel strut must be sized, in that the cross section and its area need to be determined. The cross section is primarily a function of aircraft mass, load per wheel, landing gear height, safety factor, strut deflection, strut material, and “g” load during touch-down. Two typical strut cross sections are circular and rectangular. If the landing gear is a non-retractable one, it is recommended to use fairing for struts; such that the cross sectional area resembles a symmetric airfoil. This technique will considerably reduce the strut drag. Most aircraft are designed to be able to safely land while there is a crosswind. One of the techniques in such condition referred to as crabbed landing. An impact of crabbed landing is on the landing gear design, due to the lateral-force on touch-down. As the crab angle is increased, the banding moment on the struts of main gear is increased.
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8.9 Extraction and retraction
A retractable landing gear is installed whenever a drag improvement is worthy. This means in all aircraft with exception of agricultural and small general aviation airplanes, where the installation of a movable landing gear would increase the costs beyond the requirements of the aircraft category. Landing gear extraction is a primary operation and always its actuation has high redundancy. There are different solutions for the mechanism to obtain suitable landing gear movement. Some are schematically shown in fig. 8.6.
Many solutions are based on the four bar linkage (cases A to C), where one bar is represented by the aircraft frame. In other solutions (case D) one bar end can slide along a slot. More complex kinematics includes three-dimensional motion and the deflection of the bogie that for the main landing gear of large airplanes is made of double tandem wheels. Actuators, normally of the hydraulic type, control the extraction/retraction operation.
In general the mechanism should be designed in such a way that gravity and aerodynamic drag favor extraction; if the conditions on gravity and drag are satisfied, the extraction is possible with no power from the hydraulic system. In both extracted and retracted configurations, the mechanism must be blocked (down lock and up lock respectively). A kinematic lock at extraction can be obtained by making the four bar linkage to reach its dead centre at full extraction. In any case a down lock based on a hydraulic or electric device is activated to prevent any movement of the strut when the aircraft is taxiing. An up lock is also activated when the landing gear is fully retracted, to prevent non-intentional.
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Fig.8.6–Some kinematic patterns
9.0 Prototype Analysis
The prototype of this project has been designed to understand the principle and working mechanism of the landing gear. A real landing gear needs a lot of components and parts that need to fit together such as absorbers, strut, and brake and so on where in building a prototype it‟s difficult to install the parts due to financial constrain and hard to get material for it. Therefore, it‟s done base on the pneumatic cylinder and controller for understanding purpose. The description of the project and the parts will be discussed here.
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9.1 Materials
A 5/2 way control valves (0 - 8 kgf/cm2) , 2meter x 6mm tubing for the pneumatic cylinder (0 1.0Mpa), (200 x 40)mm pneumatic cylinder, 15meter mild steel plate, one trolley tire and few bolts, nuts. One unit of 1 hp air compressor also needed.
Whenever we press the valve, it will control the pump as it will extract the tire from the datum line and as the valve was pressed again it will retract the tire where we can see the landing gear mechanism and working principles. Besides that, the timing taken to extract and retract the tire was recorded in a table according to the pressure range setting in an air compressor. From here, it can be clearly seen that the four bar linkage system working principles.The pictures of the pump, valves and the complete prototype were shown in the appendix.
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9.2 Data and Analysis
9.2.1 Extraction
Pressure (Bar) 0.8 1 1.5 1.7 1.9 2 2.2 2.5 3 Time(S) 3.8 3 1.5 1.4 1.1 0.9 0.9 0.8 0.8
9.2.2 Retraction
Pressure (Bar) 0.8 1 1.5 1.7 1.9 2 2.2 2.5 3 Time(S) 2.9 2.0 1.5 1.4 1.1 1.0 1.0 1.0 0.9
Based on the table above, it can clearly see that the time taken for a tire to extract is longer than the time taken to retract. This may due to the weight and the arrangements of the pivot points joints in the 4 linkage bar system. The optimum pressure setting for both extraction and retraction will be around 2.0 bar as stated in the table it only needs about around 1.0 seconds for this pneumatic pump at approximate 0.9kg of the weight. The real calculation and analysis may varies due to the air pressure during flight take-off and landing time in the air, therefore this type of system is recommended to use an alternate system for the normal type of hydraulic system.
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9.3 Cost Analysis
Item Pneumatic Cylinder Control Valve(5/2 way) with Fittings Metal plates Tube Bolts and nuts Trolley 1pcs x 15m 5m x 6mm 8 1 Total 25 12 4.50 7.60 249.10 Quantity(pcs) 1 1 Cost (RM) 120 80
Based on the above analysis, it shows that the costly item will be the pneumatic pump and its controller in this project. Therefore, due to the financial constrain, the project is simplified for the prototype part where more into understanding on the working principle of it only. Otherwise, it needs braking system, suspension and so on to build a complete prototype. The difficult part is to find the controller valves and it‟s expensive. Therefore, to maintain the budget level of our group, we tried to cover the cost around RM300.00.
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9.4 The air flow description
In this project there were two 3/2 way valves and one 5/2 way valve. This is to make the air flow from the compressor to the 5/2 way valve first to trap the air and separate the flow accordingly for both 3/2 valves. Then the air flows to the pneumatic pump for extract purpose and the other valve for retract purpose. This would have also ensured the pump won‟t retract by itself whenever the compressor air valve was off. It‟s a kind of locking system otherwise if the compressor even stops continuously supplying the air but it will still in extract mode until the retract valve was pressed.
But unfortunately it became difficult to buy the three valves due to expensive and financial constrain. The concept was initially same as this but due to financial constrain, the other 5/2 way controller valve was replaced for this valves to attain the same type of operation in one control system. Pictures of the valves and the complete assembly shown in the appendix.
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10. Conclusions
The landing gear system for plane is very important part which determines the successful takeoff and landing of a plane. There are a lot of factors that have been discussed in this part to design a landing gear. The top down and bottom up approach is needed in such aviation parts designs otherwise it may lead to failures which will results several injuries even deaths.
Therefore, as a mechanical engineer the landing gear design analysis must be done deeply from the hydraulic system to the pneumatic system. Although the hydraulic system is widely in the usage but there is still improvements needed in order to avoid failures mainly during landing of a flight. The pneumatic actuated landing gears as per this design is also another best type of system according to its fast response due to the air pressure but the hydraulic is still considered the best due to its strength and capability during landing of a flight, the high impact force when touching the ground. So, to avoid such a conflict, this type of pneumatic system can be used as an alternative method for hydraulic pump.
In future, the landing gear system will be more advance level which will use jet fuel or arcreactor technology such as in the Iron-Man English movie. Due to the deep analysis, the failures of the landing gear are reduced recently but a deep consideration is always needed as such alternative method in the case of hydraulic pump failure, the pneumatic will help to actuate in fast response
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DETAILS DRAWINGS
FRONT VIEW
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LEFT END VIEW
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3 DIMENSIONAL VIEW
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TOP VIEW
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11. References
1. Canada Aviation and Space Museum (undated). "Messerschmitt Me 163B-1a Komet". Retrieved 13 May 2012.
2. "Arado Ar 234 - The Flight Deck - The Hangar - WW II Bomers". http://flightdeck.forumotion.com. Retrieved January 8,2013
3. "Europa XS Monowheel Overview". Europa Aircraft Ltd. 2011. Retrieved 13 May 2012.
4. "Landing gear integration in aircraft conceptual design" Sonny T. Chai and William H. Mason, September 1996. Retrieved: 26 January 2012.\
5. Goodyear Tire & Rubber Co., Retrieved: 26 January 2012.
6. The Office of the NASA Aviation Safety Reporting System (January 2004). "Gear Up Checkup". Call Back ASRS (NASA) (292). Retrieved 1 April 2012.
7. Scislowska, Monika (3 November 2011). "Warsaw airport back to work after plane emergency". MSNBC. Retrieved 13 January 2012.
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8. Wegg, John (1990). General Dynamics Aircraft and their Predecessors. London: Putnam. ISBN 0-85177-833-X.
9. O'Leary, Michael (November 2003). "Dayton-Wright RB-1". Air Classics.
10. Taylor, Michael J. H. (1989). Jane's Encyclopedia of Aviation. London: Studio Editions. p. 305.
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12.0 Appendix
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1.1Background theory
Otto Lilienthal was one of the first men who flew above the earth in 1891. There were only two wings and one stabilizer in Lilienthal‟s airplane. He flew from the hill to fly and landed on his feet. The Wright Brothers made the first powered flight after Lilienthal. The landing gear of the Wright Flyer I was made of skis. The air plane was further developed there comes the landing gear after the Wright Brothers had flown.
In landing gear there are few systems involved. Strut; Shock absorber; Extraction/Retraction mechanism; Brakes; Wheel; Tire.
In small air planes shock absorber and extraction/retraction may not present. The landing gear is the join of airplane to ground, so that all the ground loads are transmitted by it to the aircraft structure. On the local structure there is then a high influence of the landing gear, which must be taken into account since the initial design stage.
The main sizing conditions for the system and its surrounding structure is the landing. Braking is which determines both vertical and horizontal loads that influence structural sizing. Steering and taxi stability is taxi control.
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1.2 Basic landing gear principles
During the time, different types of landing gear started to be designed by people. Generally, thinking a new design would fulfill its purpose more efficient with fewer disadvantages than the landing gear configuration which were designed before. Moreover, all landing gears have the same purpose despite for the various configurations.
There are different types of landing gear due to the relation between weight and size of aircraft. The landing is being stored after concerning the aerodynamic performance of the aircraft. The right use of the material is important in designing landing gears due to its complex structure.
1.3Purpose
The title of the project is to design and fabricate a landing gear system using pneumatic system. Basically, to enable aircraft movement on the ground is why landing gear is designed. Therefore, at least one tire must be able to steer and rotate. It also enable the aircraft to take-off and land. The landing gear absorbs the landing shock and converts kinetic energy into heat while landing. To slow down the aircraft there is brake on the landing gear which increases drag. This helps the aircraft to control on the ground. Furthermore, for loading and unloading also the landing gear can be used. It enablesthe aircraft to change the height between the aircraft and the ground.
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1.4General aspects
In last few years a lot of research has improved the landing gear configuration. Different types of landing gear resulted from difference between the weight and size of aircraft. A fixed landing gear causes a lot of resistance during flight. Retractable landing gear is used to avoid that the aerodynamic performance of the aircraft to be affected. Therefore, to store the landing gears during flight are provided.
2.0 Functional Analysis and Design Requirements
The landing gear is the aircraft major component which is designed in design procedure. Moreover, all major components such as wing, tail, fuselage, and propulsion system must be designed prior to the design of landing gear. The landing gear design may drive the aircraft designer to change the aircraft configuration to satisfy landing gear design requirements.
The primary functions of a landing gear are as follows: 1. To keep the aircraft stable on the ground and during loading, unloading, and taxi. 2. To allow the aircraft to freely move and maneuver during taxing. 3. To provide a safe distance between other aircraft components such as wing and fuselage while the aircraft is on the ground position to prevent any damage by the ground contact. 4. To absorb the landing shocks during landing operation. 5. To facilitate take-off by allowing aircraft acceleration and rotation with the lowest friction.
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Figure 2: Landing gear design flowchart
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While take-off and landing the aircraft landing gear is a crucial component, during airborne flight operation it is a dead weight. For this reason, it is recommended to retract the landing gear inside aircraft to reduce aircraft drag and improve aircraft performance. Figure 1 illustrates landing gear design flowchart including design feedbacks. The landing gear design is an iterative process and the number of iterations depends on the nature of design requirements as well designer‟s skills. Moreover, the design of mechanical subsystems and parameters is grouped in one box and should be performed by the mechanical design group. The landing gear design we initiate by defining landing gear design requirements and the process is ended by the optimization. More research and studies were done to get the details of the landing gear design.
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3.0Motivation Study
3.1 Idea Generation
The motivation of study is to identify which is better to use between pneumatic and hydraulic landing gear system. As we compare with hydraulic system, pneumatic system is substantially safer and more reliable. A pneumatic system is substantially safer and more reliable than a hydraulic system. For years, hydraulic landing gear system is used in all planes. Due to occasional failures, a pneumatic “back up system” is used when hydraulic system failed. However, recently hydraulic system has been eliminated and using pneumatic as landing system became common. Furthermore, a pneumatic system is more responsive than a hydraulic system because of its lower viscosity of air which can pump more rapidly than oil. As we compare with the responsive time, pneumatic system is fully 40% faster than the hydraulic system.
3.2 Ease of Installation
As we compare with hydraulic system, pneumatic system easier to install and takes up less space. There are no electrical components involved in pneumatic system, so one major source is eliminated in the installation procedure. In addition to that, in pneumatic system the components are significantly smaller, thus it is easily installed within the compartment.
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3.3Safety and the break-away fin
The pneumatic system is safer than a hydraulic system because leaks can occur in hydraulic. Hydraulic uses oil which is flammable while pneumatic uses air which is not flammable. From the standpoint of fire an air-powered system is safer.
4.0 Objective
The main objective of the project is to build and distribute an aircraft landing gear system using pneumatic pump that I have designed for my final year project. It is also designed to study the importance of landing gear in an aircraft and also researchers have done to come with an efficient landing gear system. Besides that, it is also to identify the best landing gear system either using hydraulic or pneumatic system. To build a product that won‟t fail while functioning.
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5.0 Significant Contribution of Study
In designing this landing gear a lot of researches have been done. The pneumatic landing gear system is substantially safer and reliable. Therefore, pneumatic landing gear system has been designed. Since there are very much hydraulic landing gear system failure, replacing with pneumatic will be a better choice. It is more responsive than hydraulic system and occupies less space in an aircraft. The concept of this project is used as back-up system in planes when hydraulic system fails.
6.0 Project Selection Process
Among various landing gear system arrangements that we have studied. We have chosen Nose Landing gear system. The selection process is done to select and to satisfy design requirements. A landing gear chosen is based on a number of factors. The selection of landing gear system is based on several design requirements which affect the decision. These include: cost, weight, performance, take-off run, landing run, ground static stability, ground taxi stability, and maintainability. A comparison Table6.0 was used to undergo this selection process for further development in this project.The landing system which gains the highest point is often the most appropriate landing gear for the aircraft. Hence based on aircraft mission and design requirements, one arrangement is usually the best alternative. Number 1: worst and 10: best
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Criteria Cost Aircraft weight Manufacturability Take-off/landing run Stability on the ground Stability during taxi Total
Single main 9 3 3 3 1 2 21
Bicycle 7 4 4 4 2 3 24
Tail gear 6 6 5 6 7 1 31
Nose gear 4 7 7 10 9 8 45
Quadr icycle 2 9 7 5 8 9 40
Multi bogey 1 10 1 8 8 9 37
Human Leg 10 1 10 2 5 28
Table 6.0: Comparison among various landing gear configuration The results shown in table shows the nose gear gets the highest points. This criteria is important in designing.
7.0 Existing Technology
In existing technology, pneumatic landing gear system is a better system which can be replaced with hydraulic system. There are planes which are using pneumatic system as back-up system in case of failure of hydraulic system.
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8.0 Methodology
8.1 Landing Gear Mechanical Structure
The scope of this project mainly concerns the engineering aspects of landing gear design which contain parameters such as landing gear configuration, fixed or retractable, landing gear tire size, wheel and brakes, shock absorber principles, braking system and strut mechanism. The design of the landing gear parameters such as retraction system, shock absorber, tire sizing, braking subsystem, and strut sizing are reviewed in brief in this section
8.2 Tire Size, Wheel and Brakes
Technically, the term „wheel‟ refers to a circular metal/ plastic object around which the rubber “tire” is mounted. The brake system is mounted inside the wheel to slow the aircraft during landing. However, in majority of cases, the entire wheel, tire and brake system is also referred to as the wheel. The primary materials of modern tires are synthetic or natural rubber, fabric and wire along with other compound chemicals. Nowadays, majority of tires is generally pneumatic, inflatable and includes a doughnut-shaped body of cords and wires encased in rubber consist of a tread and a body as in Figure 8.1. Tires perform four important functions with the air contained within them which are:
1. Tires support the aircraft structure off the ground 2. Absorb shocks from the runway surface 3. Help transmitting acceleration and braking forces to the runway surface
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4. Help to change and maintain the direction of motion
Figure 8.1: Tire geometry
The choice of the main wheel tires is made on the basis of the static loading case. The total main gear load (Fm) is calculated assuming that the aircraft is taxiing at low speed without braking as shown in Figure 8.2
Where: W is the weight of the aircraft and lm and ln are the distance measured from the aircraft cg to the main and nose gear, respectively.
The design condition occurs with the aircraft cg at its limit. For single axle configurations, the total load on the strut is divided equally over the tires, whereas in tandem configurations, the load per wheel depends on the location of the pivot point; to reduce overloading of the front wheels during braking, the pivot is usually positioned such that the distance between it and the front and rear wheel axles is about 55 and 45 percent of the truck beam, respectively. The choice of the nose wheel tires is based on the nose wheel load (Fn) during braking at maximum effort, i.e., the steady braked load. Using the symbols shown in Fig. 8.2, the total nose gear load under constant deceleration is calculated using:
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(
)
(
)
Where L is the lift, D is the drag, T is the thrust, and hcg is the height of aircraft cg from the static ground line. Typical values for ax/g on dry concrete vary from 0.35 for a simple brake system to 0.45 for an automatic brake pressure control system. As both D and L are positive, the maximum nose gear load occurs at low speed. Reverse thrust decreases the nose gear load and hence the condition T = 0 results in the maximum value:
( )
Figure 8.2 Forces acting on the aircraft during a braked roll To ensure that the rated loads will not be exceeded in the static and braking conditions, safety factor is used in the calculation of the applied loads. In addition, to avoid costly redesign as the aircraft weight fluctuates during the design phase, and to accommodate future weight increases due to anticipated aircraft growth, the calculated loads are factored prior to tire selection.
8.3 Wheel Design
The design of the wheel is influenced primarily by its requirement to accommodate the selected tire, to be large enough to house the brake, and to accomplish the above tasks with minimum weight and maximum life. As shown in Figure 8.3, two basic configurations of wheel design are currently available in the industry which is A-frame and bowl-type.
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a) A-frame
b) Bowl-type
Figure 8.3 Basic configuration of wheel design Continued heavy dependence on forged aluminum alloy wheels is foreseen by industry, whereas steel and magnesium alloy wheels are no longer taken into consideration due to weight and corrosion problems, respectively. Although practicable, titanium wheels are still quite expensive. Most of the premium for titanium wheels results from the expense for the forging process, which could be 10 to 11 times those of aluminum alloy. In addition, current titanium forging tolerances have yet to reach the precision obtainable for aluminum material, thus machining of all surfaces is required to control weight and obtain the desired form.Based on statistical data, the wheel assembly weight is determined as a function of the rated per wheel static load (F) and average tire outer diameter (D) f w FD/1000
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8.4 Brakes
Most airplanes are equipped with disc brakes, with a functioning principle similar to that of the automotive systems, but based on different sizing principles. Drum brakes are almost outdated. The main components of a disc brake, which is usually powered by the hydraulic system, are as following: • Pressure plate • Stator discs • Rotor discs • Back plate
The complete equipment is housed inside the wheel, then occupying a large part of the room between the axle and the wheel. The stator discs are keyed to the axle, or anyway constrained in such a way to be only free to move along its axis. They bring, on the two flat surfaces, lining blocks, or pads, made of a mixture of metallic and ceramic materials. The rotor discs are keyed to the wheel, rotating then with it and being free to move along its axis. They are alternated to the stator discs, so that the assembly results a sandwich of rotor discs and stator discs packed together. The stator disc at one extremity, or back plate, is fully constrained to the axle and brings lining blocks on one side only.
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Figure 8.4: Disc brake
The stator disc at the opposite extremity, or pressure plate, brings lining blocks on one side only and, during braking, is pushed against the first rotor disc of the assembly by a series of hydraulic pistons. This action compresses the entire disc package, because rotor and stator parts are all free to move along the wheel axis, with exception of the back plate at one extremity, which contrasts the pressure.
Since the rotor and stator discs are in relative rotation, the contact between the lining blocks and the rotor discs will generate a tangential friction, responsible for braking. The higher the hydraulic pressure, the higher the normal contact force and then the friction force. When pressure is reduced, the discs are released by a series of springs.
The rotor discs usually have radial slots, to minimize disc deformation during heat up. The discs can be made of steel but, when affordable, carbon discs allow lower weight. For a short period in the 60‟s they were made of beryllium, but its manufacturing costs and difficulties excluded it from standard use. The stator disc is usually made of steel. The lining is fragmented into sector blocks because it is made of a brittle compressed mixture of metals and ceramics, that may be broken if the pressure is not distributed uniformly on the entire surface.
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Brake sizing is based on heating during a single landing, considering that ventilation has a limited effect and neglecting the contribution of possible thrust reverse, flaps and spoilers. This means that a high part of the kinetic energy at landing will be converted into brakes heating; the part of brakes that is involved is often referred to as heat sink. This event can be expressed by a simple formula of energy balance:
Where: k = fraction of energy converted to brake heat; M = aircraft mass; v = landing velocity; m = total heat sink mass; cv= heat sink specific heat; ΔT = temperature increment during braking. Considering that anyway there is a drag contribution to braking, k may often be approximated to 0.8. For sizing, M should be the max landing mass, v the max landing velocity and ΔT the difference between the allowable disc material temperature and the highest possible initial temperature. Materials with high specific heat and high operating temperature are of course preferred, because they allow a reduced total disc mass.
Usually brakes are located in all the main landing gear wheels: the above-mentioned mass m is then related to all the brakes located in the wheels. Large aircraft may have braking nose wheels. A multiple disc brake is used whenever a high braking power is necessary, because the lining friction surface increases with the number of discs. Moreover a thick disc would not allow a suitable temperature distribution, but would be rather cold at the core and overheat in periphery. On the other hand, discs must be sized in such a way to withstand the high tangential stress that
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is generated by friction.
Figure 8.5: Evaluation of the braking intensity The braking intensity is a function of the brake geometry and, in a minor extent, of the aircraft geometry. From fig. 8.5 the braking torque C can be easily evaluated as a function of the hydraulic pressure and discs geometry, as follows:
B
Where: p = hydraulic pressure A = total lining friction area μ = disc friction coefficient, normally around 0.3 RB = radius of the lining block centroids The braking force TM will be then given by
R
Where RR is the rolling radius (that depends on the normal reaction NM, tyre pressure and tyre geometry).
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Of course the braking force cannot increase indefinitely with pressure, but is limited by the tire grip; this is defined by a factor μG that ranges from 0.9 for dry runway and tire and asphalt in good conditions to 0.5 in case of wet runway and down to 0.1 for iced runway. Then T M is limited by: ( )
G.NM
Where, NM is the normal reaction of the main landing gear. In static conditions (airplane at rest) NM will be equal more or less to 90% of the aircraft weight, due to the conventional position of the aircraft centre of gravity. In braking conditions NM is lower and the inertia forces overload the nose landing gear. Depending on the longitudinal and vertical position of the centre of gravity, a simple algebraic system in the two unknowns NM and T can be set up and solved if any drag contribution is neglected.
In determining the ground loads on nose wheels and tail wheel, and affected supporting structures, it must be assumed that the shock absorbers and tires are in their static positions. When a solid spring is chosen, the main parameter for the design is the geometry and the cross section of the beam. In case that a hydraulic shock absorber is selected for the landing gear, the typical parameters which must be determined include stroke, orifice, outer and inner diameter, and internal spring sizing.
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8.6 Shock absorber
The main role of the shock absorber is to zero the vertical component of the airplane velocity during landing, with no rebound and limited load transfer to the vehicle structure (and occupants). Its secondary requirement is to allow a comfortable taxiing. Different types of shock absorbers are available, but when costs and dimensions allow, a hydraulic system is commonly used. The hydraulic solution is anyway the mostly adopted one, and fig. 8.5 shows some of the many possible versions. Substantially the system structure is made of a movable piston that, when loaded, compresses a gas (nitrogen) in a cylinder and causes an oil flow through orifices. The system elasticity is due to the gas transformation and the damping effect to the liquid pressure losses. The complexity of the system increases with the requirements.
8.7 Shock absorber functioning principles
In its most basic configuration, the shock absorber can be depicted as a hydraulic cylinder linked to an accumulator. When the piston moves, the oil from the cylinder passes through an orifice and compresses the gas in the accumulator. According to this approach the reaction R of the shock absorber is function of the piston position and its time derivative, as follows:
(
)
(
)
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Where: p = pressure on piston; A = piston area; pA= accumulator pressure; k = orifice pressure loss coefficient; Q = oil flow rate; p0 = initial accumulator pressure; V0 = initial accumulator gas volume; γ = polytrophic exponent; x = piston stroke (x=0 for all extended shock absorber)
8.8 Strut Sizing
The wheel strut must be sized, in that the cross section and its area need to be determined. The cross section is primarily a function of aircraft mass, load per wheel, landing gear height, safety factor, strut deflection, strut material, and “g” load during touch-down. Two typical strut cross sections are circular and rectangular. If the landing gear is a non-retractable one, it is recommended to use fairing for struts; such that the cross sectional area resembles a symmetric airfoil. This technique will considerably reduce the strut drag. Most aircraft are designed to be able to safely land while there is a crosswind. One of the techniques in such condition referred to as crabbed landing. An impact of crabbed landing is on the landing gear design, due to the lateral-force on touch-down. As the crab angle is increased, the banding moment on the struts of main gear is increased.
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8.9 Extraction and retraction
A retractable landing gear is installed whenever a drag improvement is worthy. This means in all aircraft with exception of agricultural and small general aviation airplanes, where the installation of a movable landing gear would increase the costs beyond the requirements of the aircraft category. Landing gear extraction is a primary operation and always its actuation has high redundancy. There are different solutions for the mechanism to obtain suitable landing gear movement. Some are schematically shown in fig. 8.6.
Many solutions are based on the four bar linkage (cases A to C), where one bar is represented by the aircraft frame. In other solutions (case D) one bar end can slide along a slot. More complex kinematics includes three-dimensional motion and the deflection of the bogie that for the main landing gear of large airplanes is made of double tandem wheels. Actuators, normally of the hydraulic type, control the extraction/retraction operation.
In general the mechanism should be designed in such a way that gravity and aerodynamic drag favor extraction; if the conditions on gravity and drag are satisfied, the extraction is possible with no power from the hydraulic system. In both extracted and retracted configurations, the mechanism must be blocked (down lock and up lock respectively). A kinematic lock at extraction can be obtained by making the four bar linkage to reach its dead centre at full extraction. In any case a down lock based on a hydraulic or electric device is activated to prevent any movement of the strut when the aircraft is taxiing. An up lock is also activated when the landing gear is fully retracted, to prevent non-intentional.
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Fig.8.6–Some kinematic patterns
9.0 Prototype Analysis
The prototype of this project has been designed to understand the principle and working mechanism of the landing gear. A real landing gear needs a lot of components and parts that need to fit together such as absorbers, strut, and brake and so on where in building a prototype it‟s difficult to install the parts due to financial constrain and hard to get material for it. Therefore, it‟s done base on the pneumatic cylinder and controller for understanding purpose. The description of the project and the parts will be discussed here.
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9.1 Materials
A 5/2 way control valves (0 - 8 kgf/cm2) , 2meter x 6mm tubing for the pneumatic cylinder (0 1.0Mpa), (200 x 40)mm pneumatic cylinder, 15meter mild steel plate, one trolley tire and few bolts, nuts. One unit of 1 hp air compressor also needed.
Whenever we press the valve, it will control the pump as it will extract the tire from the datum line and as the valve was pressed again it will retract the tire where we can see the landing gear mechanism and working principles. Besides that, the timing taken to extract and retract the tire was recorded in a table according to the pressure range setting in an air compressor. From here, it can be clearly seen that the four bar linkage system working principles.The pictures of the pump, valves and the complete prototype were shown in the appendix.
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9.2 Data and Analysis
9.2.1 Extraction
Pressure (Bar) 0.8 1 1.5 1.7 1.9 2 2.2 2.5 3 Time(S) 3.8 3 1.5 1.4 1.1 0.9 0.9 0.8 0.8
9.2.2 Retraction
Pressure (Bar) 0.8 1 1.5 1.7 1.9 2 2.2 2.5 3 Time(S) 2.9 2.0 1.5 1.4 1.1 1.0 1.0 1.0 0.9
Based on the table above, it can clearly see that the time taken for a tire to extract is longer than the time taken to retract. This may due to the weight and the arrangements of the pivot points joints in the 4 linkage bar system. The optimum pressure setting for both extraction and retraction will be around 2.0 bar as stated in the table it only needs about around 1.0 seconds for this pneumatic pump at approximate 0.9kg of the weight. The real calculation and analysis may varies due to the air pressure during flight take-off and landing time in the air, therefore this type of system is recommended to use an alternate system for the normal type of hydraulic system.
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9.3 Cost Analysis
Item Pneumatic Cylinder Control Valve(5/2 way) with Fittings Metal plates Tube Bolts and nuts Trolley 1pcs x 15m 5m x 6mm 8 1 Total 25 12 4.50 7.60 249.10 Quantity(pcs) 1 1 Cost (RM) 120 80
Based on the above analysis, it shows that the costly item will be the pneumatic pump and its controller in this project. Therefore, due to the financial constrain, the project is simplified for the prototype part where more into understanding on the working principle of it only. Otherwise, it needs braking system, suspension and so on to build a complete prototype. The difficult part is to find the controller valves and it‟s expensive. Therefore, to maintain the budget level of our group, we tried to cover the cost around RM300.00.
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9.4 The air flow description
In this project there were two 3/2 way valves and one 5/2 way valve. This is to make the air flow from the compressor to the 5/2 way valve first to trap the air and separate the flow accordingly for both 3/2 valves. Then the air flows to the pneumatic pump for extract purpose and the other valve for retract purpose. This would have also ensured the pump won‟t retract by itself whenever the compressor air valve was off. It‟s a kind of locking system otherwise if the compressor even stops continuously supplying the air but it will still in extract mode until the retract valve was pressed.
But unfortunately it became difficult to buy the three valves due to expensive and financial constrain. The concept was initially same as this but due to financial constrain, the other 5/2 way controller valve was replaced for this valves to attain the same type of operation in one control system. Pictures of the valves and the complete assembly shown in the appendix.
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10. Conclusions
The landing gear system for plane is very important part which determines the successful takeoff and landing of a plane. There are a lot of factors that have been discussed in this part to design a landing gear. The top down and bottom up approach is needed in such aviation parts designs otherwise it may lead to failures which will results several injuries even deaths.
Therefore, as a mechanical engineer the landing gear design analysis must be done deeply from the hydraulic system to the pneumatic system. Although the hydraulic system is widely in the usage but there is still improvements needed in order to avoid failures mainly during landing of a flight. The pneumatic actuated landing gears as per this design is also another best type of system according to its fast response due to the air pressure but the hydraulic is still considered the best due to its strength and capability during landing of a flight, the high impact force when touching the ground. So, to avoid such a conflict, this type of pneumatic system can be used as an alternative method for hydraulic pump.
In future, the landing gear system will be more advance level which will use jet fuel or arcreactor technology such as in the Iron-Man English movie. Due to the deep analysis, the failures of the landing gear are reduced recently but a deep consideration is always needed as such alternative method in the case of hydraulic pump failure, the pneumatic will help to actuate in fast response
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DETAILS DRAWINGS
FRONT VIEW
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LEFT END VIEW
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3 DIMENSIONAL VIEW
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TOP VIEW
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11. References
1. Canada Aviation and Space Museum (undated). "Messerschmitt Me 163B-1a Komet". Retrieved 13 May 2012.
2. "Arado Ar 234 - The Flight Deck - The Hangar - WW II Bomers". http://flightdeck.forumotion.com. Retrieved January 8,2013
3. "Europa XS Monowheel Overview". Europa Aircraft Ltd. 2011. Retrieved 13 May 2012.
4. "Landing gear integration in aircraft conceptual design" Sonny T. Chai and William H. Mason, September 1996. Retrieved: 26 January 2012.\
5. Goodyear Tire & Rubber Co., Retrieved: 26 January 2012.
6. The Office of the NASA Aviation Safety Reporting System (January 2004). "Gear Up Checkup". Call Back ASRS (NASA) (292). Retrieved 1 April 2012.
7. Scislowska, Monika (3 November 2011). "Warsaw airport back to work after plane emergency". MSNBC. Retrieved 13 January 2012.
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8. Wegg, John (1990). General Dynamics Aircraft and their Predecessors. London: Putnam. ISBN 0-85177-833-X.
9. O'Leary, Michael (November 2003). "Dayton-Wright RB-1". Air Classics.
10. Taylor, Michael J. H. (1989). Jane's Encyclopedia of Aviation. London: Studio Editions. p. 305.
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12.0 Appendix
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35
