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Many of the material could be used for the hull of the hovercraft based on the purpose of its application.In the project ,the main concern is to detect the landmines which is certainly over land usage.Hull is the main structure which the hovercraft floats over water or land and It supports the whole weight of the craft.The parts include craft floor,side panel forward,and craft panel untill the skirt line attachment.Five materials were listed among suitable materials for the landmines detection application and from these five materials,the best material will be selected for the hull.
- Polyvinyl Chloride (PVC) foam sandwich(Carbon skin Panel)
-Acrylonitrile butadiene styrene(ABS)
-Balsa sandwich(Glass Skin Panel)
The materials were listed by the commercially produced hull for boats and hovercraft.By far, hull for ships and hovercraft highlight the same mechanical and physical features and it covers the interior part of body.Hull needs exceptionally high strength-to-weight,thus suitability among 5 main materials must be accounted singly.Since the hovercraft is unmanned and navigated by remote control system,the human weight problem could be ignored and the lightness of hovercraft could be optimized as far as possible and concurrently able to carry crutial load like mine detector and engine efficiently.
In this scope selection,balsa wood core sandwich with the glass fibre skin panel is chosen as the best material that can be used to make hull.
Polyvinyl Chloride (PVC) foam sandwich(Carbon skin Panel)
Acrylonitrile butadiene styrene(ABS)
Balsa Wood sandwich(Glass Skin Panel)
Fracture Toughness(MPa · m1/2)
Table 1:Hull Material Selection Datasheet With Highlighted Chosen Material
Chosen Material Description
From the 5 choice of materials,balsa wood sandwich with glass fibre skin said to be the best compared to other materials.The density of balsa wood is considerably low among 5 materials after PVC foam core. HDPE and ABS found to have too much different in weight compared to core material like PVC foam and ABS while aluminium alloy seems to have highest in weight.As the target is to find the light and strong material,eventhough PVC foam is lighter than Balsa wood,if there was a repeated impact,some of PVC foams could be brittle compared to Balsa wood while in this hovercraft design,the hull is the major part that would always making high-stress contact with the land obstrucion like rocks and impact vibration of engines .With the aim is to detect a landmines,it is must be ensured that pressure contact with the land has to be as low as possible because if any exceeding pressure could lead to detonation of landmines.Therefore,Balsa wood said to be lightest wood and able to withstand continuos impact as well as practically used in craft modelling due to its less loft force to stay in air and excellent floatation.In terms of the reinforced skin layer,Balsa wood surface generally found to be easier to bond the laminate skin because of nature of its wood where its capiliary action helps draw the resin into the balsa and produce strong bond.By chosing Balsa wood,it is stronger in shear and compression compared to PVC foam.In addition,the Young Modulus indicates that balsa wood has low reaasonably low stiffness which is necessary for elastic bending whenever loadss are imposed.Aluminium seems to have very high Young Modulus which is very stiff whereas PVC is too low and not practically good for deformation.Balsa has very good UV resistant and the reason not to choose ABS ,HDPE and PVC is because,they are thermoplastic materials which are weaken over high temperature and tense UV light,that can result in serious deterioration of physical, chemical, and mechanical properties .High level of heat can boost up the embrittlement of material eventually cracking and since we want long operational life,the hull must be avoided from any degradation in material properties level that can reduce the reliability of hovercraft.Aluminium happens to be unsuitable in this hovercraft design because,the metallic element on aluminium may interrupt the signal and feedback of metal detector where high level of interference can lead to undetected landmines and this reduces effectivity of of the main purpose of hovercraft.That is why the hull design should possibly isolate any metallic element by using balsa wood core.Furthermore,balsa wood can be easily worked with simple cutting tools without affecting the strength which is necessary for instance hole drilling at line attachment to attach the skirt.Balsa wood core is light and able to withstand high pressure load and this makes the best choice to sustain engine load and air thrust.The good acoustic absorption gives the balsa wood ability to reduce the noise level of hovering.Generally,the reason balsa wood is chosen rather than other materials is due to its availability where it can be easily obtained in Asia and eventhough it is commercially produced in South America(Ecuador),but now commonly found in asia like Indonesia and Thailand.Although HDPE,ABS and aluminium are somewhat cheap,since they are unapplicable in this hovercraft hull design,the selection were shifted to PVC foam and Balsa wood and based on these two materials,Balsa wood is cheaper with most desired properties surpass the PVC foam.Cost should be minimised but since this is not a mass production which is in small volume,a bit high cost on material selection could be tolerated for the sake of high quality hull design.
Figure 1:Shear stress distribution on solid laminate and cored laminate based on applied load
Core Laminated Fibre.
In the design type,the cored strucured material gives the best design to make hull of hovercraft.The reasons is because a cored laminate is lighter and stiffer than a solid fiberglass panel for instance of the same weight and area. The core material is very effectively acting as a stress web between the two skins of laminated bonded. The principle is the same with the inner perpendicular section of I-beam, which acts as a stress web for two outer section of the I-beam. Essentially, in order cored laminate to bend, it need to be compressed on the skin on the impact side and stretch the skin on the opposite side and by implying the core, it greatly turned out the single layer with tension on one side and compression on the other side through the force that being transmitted to the core material.The core would absorb the shear and impact imposed in addition to the additional structural value ,cored laminate are actually commonly used. Through a sandwich,strength of main core material is optimised effectively. Generally, for core such as balsa or foam, the laminated layer is in constant contact with the entire core and hence during impact, the force is distributed over a wider area. In a sandwich panel(skin) also, the compression strength of the core helps prevent the structure from delaminating, wrinkling or buckling and many other similar failure.With Balsa wood being the best material selected,the layer which is reinforced glass fibre enhances the strength of core.Eventhough Carbon fibre is noted to be better than glass fibre,glass fibre is least expensive of fibre compared to carbon fibre and widely used in simple craft design due to good translucent effect.Other than that,glass fibre is very good electrical insulator and it is important in this design for the hull not to have conductivity so that the metal detector which is highly electical configurated to function without problems like erratic pulse due to charge interruption on materials.
Skirt Material Selection
Skirt of hovercraft is where the lowest part of craft body that closest to the ground level and must come in flexible sheet type of materials.It is said to be critically important part that governs the functionality of hovercraft alone.The air need to be contained so that the pressurized cushion can be produced in suspension.By controlling the sufficient pressure differential of air cushion,the energy required to lift the hovercraft can be minimized thus reducing power consumption as well as increasing other performance level.Degree of stability performance must be conformed by the design by considering the correct type of material chosen for skirt and its configuration.In the design,below are 5 commonly used materials for skirt.
According to the type of skirt design like bag skirt,finger skirt(segmented) ,angled jupe skirt or combination of any those 3,the materials that be used has to find its suitabiity and match to the purpose of the design.Since the skirt must be the type of sheet and flexible,all the main materials listed for screening are elastomer.It is unpractical to apply other material types especially metal and the flexibility is very important to pledge the smoothness of working process.The materials selection must based on performance requirement like high strength-to weight ratio ,acceptable tensile and tear strength,cyclic tensile strength and compression, air tight and resistance to abrasion.
In this scope of design, polyurethane is regarded as most suitable to be used as skirt material on hovercraft.
Fracture Toughness(MPa · m1/2)
Table 2:Skirt Material Selection Datasheet with Highlighted Chosen Material
Chosen Material Description
Polyurethane(PU) is the best material chosen because when compared to other materials like Neoprene,PE and EVA the weight is reasonably good which is crutial for hovercraft weight where the point is to optimise the minimum bareable weight.Eventhough natural rubber and PE seem good alternative for the skirt weight material but when considering elasticity,the application of skirt must have the elasticity but high tensile and compressive load strength.PU exploits the important parts in material requirement. With too low young modulus could lead to easily deform and the skirt eventually quick fail due to simple loading for example continuos air thrust.The force of air inside the interior skirt always vary with the thrust power and sometimes high and low.Due to this reason,the high young modulus needed for material to withstand this condition.In addition,PU is used as it is the most demanded film and sheet application due to long-term flexibility at very low temperature and amazing shelf life.This is very important although it is quite expensive compared to other material,the long term reliable life should be primary target rather than cheap material but unsustainable and easy to fail.Besides,in term of extreme environment,PU can be depended since it is good for high abrasion resitant and high-wear application and these give PU ideal choice for skirt.With high durability,it has good UV resistant without yellowing and finally embrittlement of material which is essential in skirt since the skirt must not experience such behavior to provide an effective air cushion without leaking in air pressure for instance tear due to brittle.If the air pressure leaking happened,the stability of hovercraft is affected thereby air pressure cannot be maintained on the ground where it could contribute to detonation of landmines.With good chracteristic of softy and strecthy PU is very easily fabricated without affecting the strength.It is said to be the most strongest elastomer.Produced with good translucent effect,PU has advantage of beautiful skin for aesthetic value for hovercraft.PU could be found most in variety application in very wide range and due to this, it is chosen because its availibility to obtained just in common market in every country .
Rudder Material Selection
Rudder is used for controlling the hovercraft in turning and through the air flows at the back generated,the turning moment can be directed by steering the rudder .It is an outside part that is fastened normally by pivoted outside the hull .For the steering process to effectively works,the airfoil shape of rudder must be symmetrical about the vertical center plane.There are soo many material could be used for the rudder of hovercraft based on the speed and maneuverability type within the scope of hovercraft purpose .In the design,the main concern of rudder is to control the hovercraft movement at a low speed as in this design the hovercraft just only to detect the landmines which are over land and not for high speed action.The hovercraft must be stable with a low speed yawn so that the detection of landmines can be improvised since fast speed not advisable for effective detection signals.
-Balsa Wood (CFR skin panel)
-CFR (epoxy matrix)
Based on the most craft application either aircraft or marine,if aluminium hulls used,the rudder should be choosen as aluminium rudder and if somehow GRP or composite hulls,rudder much more suitable to be chosen between bronze, stainless steel or brass. As the design is targeted on metalless element, CFR(epoxy matrix) rudder likely to be the most suitable material.The rudder is supposingly made of wood just like the hulls.Historically,stainless steel has been preferred rudder material and the reason why material has been used as it was strong and non corrosive.For high performance applications nowadays complete carbon fibre rudder blade are commonly used.
In this scope of selection,solid CFR with exposy matrix is chosen as the best material for rudder that can be incorporated on hovercraft.
Balsa wood(CFR laminate)
CFR(carbon reinforce plastic-epoxy)
Fracture Toughness(MPa · m1/2)
Table 3:Rudder Material Selection Datasheet with Highlighted Chosen Material
Chosen Material Description
The basic crutial consideration for the material selection on the rudder are sensible weight,strength and stiffness.In the design process, the particular weight is regarded importantly.From 5 material accounted,Balsa wood(CFR laminate) shown the high reduction in weight compared to other material.To highlight the application of rudder,the weight of rudder must be not too heavy and too low where very low weight contributes easy vibration on the attachement and highly sensitive movement where rudder movement should be smooth according to air thrust flow in order to have ease of steering .Heavy rudder obviously undesirable because the weight on hovercraft must significantly low for effective lifting.CFR comes after Balsa wood where it s the second lowest in weight based on the list and from comparison,this huge different amount of weight between stainless steel,bronze and brass making CFR better to be integrated in the design.The best reason why to chose CFR is because if compared to glass fibre,carbon fiber which weighs less per given volume and is stronger both by weight and volume. For any metals behaviour,whenever they highly are stressed, they begin to deform but still provide strength,unlike CFR,produced with good strength,it will take loads nearly to 100 percent of strength without permanent deformation.This is why the in aircraft engineering,CFR is an ideal material for component that is contantly exposed to high load like air force thrust. It is called resilient material and easy to form into complex shapes plus does not suffer from metal fatigue or corrosion .The same concept must be applied to rudder where high ductility and toughness are imperative porperties.To have highest tenssile strength and young modulus among 5 materials listed in the design,with obvious difference CFR proves that, it has high toughness level and could absorb much energy upon high loading like force of air.Besides carbon fiber also has better fatigue life than most metals, and the resins typically epoxy used to bond the fibers which offers extremely good vibration damping.It is an advantage of CFR where the finishing surface is excellent when sanded carefully with smoothness and clear quality thereby improves aerodynamic by reducing the drag of air flows. One good chracteristic of CFR has is that magnetic shielding property where it does not interrupt but providing path for the magnetic field which is necessary for metal detector to work without false signal.Based on price,the CFR is seemed to be highest yet due to the weight and other advantages,it is exceptionally important for rudder material to put on hovercraft as it is worth the extra cost. The low maintenance required for CFR compared to other material is another advantage that endorse this choice where initial high manufacture cost is better than high maintenance cost.CFR is widely known for its popularity to be used in many application with limited production volume since the process to produce it is complicated.Nevertheless,it is familiar and easily obtained in many countries including Malaysia.
Figure 2:Carbon Fibre Reinforced with matrix epoxy
Fibre Special Review
Carbon fiber reinforced composite has several highly unique traits that can be benefit in the design of advanced materials and systems especially in hovercraft. The most common uses for carbon fibre are in applications are primarily due to high strength-to-weight and high stiffness-to-weight are desirable. Utilization of it include in plantation,aeorspace, robotics, wind turbines, military structures ,manufacturing component, and many others. The toughness can be enhanced when collated or combined with other materials.In particular uses,many extreme purpose make use of carbon fibre in terms of its durability, as well as reliability in the case of specialized carbon fiber. Furthermore, in basic mechanical properties, carbon fibre yield a unique and beautiful surface finish for final products.The fibre that been reinforced which is embedded actually can be modified in terms of fineness of fibre and orientation on combination in order to get many arrangement on the composites which can produce different level of properties and characteristic based on certain application.For example,some parts of fibre composites can be very stiff and the other parts of fibre composite can be so much flexible.All of those obtained by adjustment of fibre which is very interesting in producing variety of special equipment.Dealing with carbon fibre required a high skill level and many complicated processes to produce high structured materials for instance, solid carbon sheet, sandwich laminate and tube.