Materials and Hardware: Use of Light Alloys in Aircraft Construction - Essay Example

?Materials & Hardware 503791) Introduction The principal criterion in the design of an aircraft is ways to provide an efficient structural design at the same time reducing weight. The design differs from that of civil engineering because apart from being robust in construction the aircraft members must provide aerodynamic lift and overcome dynamic drag forces. Due to the large stiffness to weights ratio and strength to weight ratio aluminium and titanium have been the used predominantly for the past several years.( Aitchison Leslie, 1924) However of late, composite materials have found increased acceptability since this causes a saving of 30 to 40 percent in weight compared to aluminium and titanium. Aircrafts can be of fixed wing construction or rotary wing construction. The fixed wing construction consists of the Fuselage, wings, stabilizers, flight control mechanisms and the landing gears while the rotary wing aircraft consists of a main rotor assembly, tail rotor assembly apart from the fuselage and landing gear. (Sun.C.T, 2006) The main properties that are relevant to the maintenance cost and the performance of the aircraft are 1. Density of the material used 2. Stiffness (Young’s Modulus) of the material. 3. Strength (Ultimate and Yield strength) of the material. 4. Fatigue strength of parts which is the ability of a structural member to absorb sustained loads. 5. Toughness to resist fracture and prevent crack propagation. 6. Resistance to corrosion. Use of light alloys in aircraft construction. The different parts of an aircraft that are critical to its functioning include fuselage and the wings, landing systems and stabilizing equipment that form part of the aerospace system. Source: Quilter Adam, Composites in Aerospace Applications, Viewed on 28th February 2011. Fuselage is the body of the aircraft and is the space which houses the cargo shipment and all human personnel. This usually employs the monocoque or semi-moncoque construction and uses frames and bulkheads to define the shape of the fuselage. It is however the skin that would bear the entire load of primary stress. (Sun.C.T, 2006) Steel alloys, Aluminium alloys and Titanium alloys are generally used in aircraft construction. Steel alloys have the largest densities and are used generally where high strength and yield strength are of importance. Landing gear units especially employ steel alloys of grade 300M. This has strength of 27000psi and yield stress of 220000 psi. (Sun.C.T, 2006) Alumunium alloys have excellent mechanical properties with low weight to volume ratio. The commonly used aluminium alloys include 2024 and 7075 alloys. Of the 2024 alloys, 2024-T3, T42 have superior fracture toughness. These alloys are also resistant to fatigue failure with a slow propagation of crack rate. T3 and T42 indicate the heat treatment process that has been used. These are generally used in the construction of aircraft skins due to its shiny and excellent finish characteristics. Ultimate strength of 2024-T3 is around 62000psi with an allowable shearing stress of 40000psi. (Experimental Aircraft Info , 2006) 6061-T6 has good welding characteristics and can be fabricated with the commonly used manufacturing methods. Source: Fuselage of Boeing 777 under construction, Boeing Company, Viewed 28th Feb, 2011 These have an ultimate strength of 45000 psi with an allowable shearing stress of 30000psi and are typically used in aircraft landing mats. 7075-T6, T651 on the other hand have greater strength but has low resistance to fracture. (Engineering studies, 1999) Different aluminium alloys are used in different locations on the aircraft. Since the upper part of the wing is exposed to compressive stress these parts are made of 7075-T6 whiles the fuselage and lower wing sections that have tendencies to fail by fatigue due to the cyclic nature of the stress involved, are made of 2024-T3. (Sun.C.T, 2006) 7075 alloys typically have an ultimate strength of 33000 psi and an allowable shearing stress of 22000 psi. 5052-H32 aluminium alloys are not used for structural parts. These offer excellent corrosion resistance and are typically used in the construction of fuel tanks. With an ultimate strength of 21000 psi and an allowable shearing stress of 14000psi 3003-H14 is used in the construction of baffle plates. (Experimental Aircraft Info , 2006) This has manganese added to aluminium and can be spun and brazed. Aluminium alloys have also been used in the construction of rockets. The first satellite that was developed by the Soviet Union had a casing constructed of aluminium. American rockets ‘Avantgarde’ and ‘Titan’ have also many key structural parts like chassis, brackets and casings made of aluminium alloys. Aluminium alloys 2219 are also used in cryogenic conditions with an increase in performance. This is because alloys in contact with liquid oxygen, hydrogen and helium experience a sort of cryogenic reinforcement that adds to its strength and ductility with lowering of temperatures. (AL, n.d) Recent developments include the production of aluminium lithium alloys that are more efficient than conventional aluminium alloys. Source: Corus Aluminium Walzprodukte web site , Viewed on 28th February 2011 Titanium alloys have also been getting increased acceptance in military aircraft. Alloys such as Ti-6Al-4V have a density of 4.5 g/cm^3 which is lighter than steel but heavier than aluminium both of which have densities of 7.8 g/cm^3 and 2.7 g/cm^3 respectively. The ultimate stress and the yield stress of these alloys are twice that of aluminium 7075-T6. This also has increased corrosion resistance characteristics and also can be used for areas up to 1000?C as compared to aluminium which can only be used till 350?C. (Sun.C.T, 2006) The use of composites in aircraft construction. Composites have begun getting increased acceptance in the aerospace industry due to its strength and excellent physical and forming characteristics. This also offers good strength to density ratios. A composite material consists of strong fibres that are laid in a resin mix. The commonly used fibres are CFRP (carbon fibre reinforced plastic) and GFRP (glass fibre reinforced plastic). (Quilter Adam, 2011) Obtaining a composite thus generates an all purpose material that loses all the inherent weaknesses of the individual components. Other composites like MMC (metal matrix composites) are manufactured by mixing non metallic particles in a metallic matrix. A common example would be that of silicon carbide particles in aluminium alloy. Source: Dr. Cairns Douglas, Composite materials for aircraft Structures, Viewed on 28 February 2011 Fibrous composites when compared to metallic materials are anisotropic i.e the properties of stiffness and strength vary with the direction in which the load is applied. To overcome this problem in aerospace applications, the layers of laminates are arranged in a particular sequence to overcome the applied loads. (Quilter Adam, 2011) Source: Dr. Cairns Douglas, Composite materials for aircraft Structures,Viewed on 28 February 2011 The workability characteristic of the composite also helps in the formation of complex shapes which reduce the requirement of additional parts. This reduces the number of bolts and hence bolts holes thus reducing the tendency for the formation of localized stress concentrations. The time for assembling this product is also reduced considerably. A composite matrix is produced by stacking of individual layers in a definite sequence and then subjecting this stacked assembly to different pressures and temperatures. This process is called curing. After the process has been completed the product is checked for dimensional accuracy and also to ensure that no bubbles are entrapped in between layers. The percentage of use of composites in aerospace applications has grown over the years. Use of composites in military aircraft has risen considerably with the F22 using up to 24 percent. (Quilter Adam, 2011) The rudder of the A300 and A310 series of Airbus playing during 1983 was built using composites. This resulted in reducing component parts from 2000 to 100. The A320 airbus on the other hand had 28% weight of composites and was specifically used in the construction of spoilers, wheel doors and fuselage belly skins. The Boeing 777 utilizes around 20 percent of composites, these being used for flaps, spoilers and edges of the wings. A saving of approximately 1500 pounds was obtained in using these composites. (Quilter Adam, 2011) The matrix materials that are usually used for composites are polymers, ceramics and metals. Apart from the MMC which has been stated earlier polymer matrix composites (PMC’s) and ceramic matrix composites (CMC’s) are also other variants. Polymer matrix composites are used for temperatures less than 300? F while ceramic composites can be used in temperatures of 1500?F in hot environments that exist like the engine. Another example of a composite would be thin graphite epoxy layers that are bonded to an aluminium honeycomb structure. (Dr. Cairns Douglas, 2009) The material specifications and Process specifications decide the requirements of a composite to be used in aircraft structures. The material specification includes the qualification and trustworthiness of the supplier, the nature of fibre including volume, chemical properties of resin, whether the fibre is in tape form or fabric form and other manufacturing parameters. The Process specifications include the nature of storage facilities, the cure cycle, pre and post process quality control and the acceptable limits of non compliance. Comparing light alloys and composites (Dr. Cairns Douglas, 2009) Some of the most visible advantages of using composites instead of light alloys include 1. An approximate reduction of 20-50% in the weight of the component. 2. Increased corrosion resistance 3. Increased Fatigue resistance. 4. Composites are readily workable hence the number of moving/ attachment parts can be reduced. The disadvantages include 1. The recurring and non recurring costs are high. 2. The material required for the composites are costly with the visibility of any kind of impact damage, being minimal to be identified. 3. The procedure of repairs is bit tedious when compared to light alloys. 4. The adjacent aluminium parts can also experience galvanic corrosion. Aircraft spares and safety Maintenance of all systems concerned with the flight of an aircraft is important to ensure the aircraft and the travelling personnel are safe. Keeping a proper maintenance log and carrying out periodic planned and routine maintenance is a key activity to ensure that all parts are working properly and in good condition. Defective parts if any should be serviced if possible or the entire part should be replaced in situ. Care should be taken to replace the spare with that of the original. A number of accidents including the crash involving Northwest flight 520 were detected due to the use of shoddy, cheap spares that were conveniently used.( (Vale john P De, 1999) The national Transportation Safety Board had identified the reason that the flight crew had not checked whether the flaps and slats were in correct open position required for takeoff. It was also hampered by the fact that there was no electrical signal warning the crew of such a eventuality. It was later found out that input power to the CAWS unit was interrupted due to the failure of the P-40 circuit breaker. (Vale john P De, 1999) This might have been caused by improper replacement of the circuit breaker during maintenance. The Spanair flight 5022which crashed in Madrid was also due to the use of incorrect spares used in the warning system. Case study (Incidents and accidents, 2001) Flight: 5X-UVA Addis Ababa , 18th April 1972 5X-UVA seen at Paris Orly Airport on 7 June 1969 Photo A.J. Altevogt Source: Incidents and accidents web site, Viewed on 28th February 2011 Date: 18.04.1972 Time: 09.39 Type: Vickers VC10-1154 Operator: East African Airways Corp. - EAAC Registration: 5X-UVA C/n: 881 Year built: 1966 Total airframe hrs: 18586 hours Crew: 8 fatalities / 11 on board Passengers: 35 fatalities / 96 on board Total: 43 fatalities / 107 on board Location: Addis Ababa-Bole IAP (Ethiopia) Phase: Take-off Nature: Scheduled Passenger Flight: Addis Ababa-Bole IAP - Roma (Flightnumber 720) Abstract This flight before takeoff developed a puncture on the nose gear tyre and the take off was subsequently aborted. The momentum of the flight however did not allow it to stop on the runway and subsequently it fell onto a lower ground and exploded. Incorrect spares being used in the braking system was the reason that was associated with this accident. Findings The 5X-UVA was one of the five aircraft that were used by the East African Airways. The route that it chartered was between East and Central Africa and also covered Europe and a Asia. When this mishap occurred the aircraft had already flown a total of 18586 hours. In 1972 a problem was detected in the left hand main gear and the component was changed several times. There was unfortunately no system to check whether the new component was affecting the performance of the aircraft in its entirety even though the individually the spare part was good enough. Later it was found out that one of the anti skid units had a seal that was obstructing several ports thus making the entire unit inefficient. This new seal was not part of the original design of the component and was never identified till after the accident. The flight was to have taken off from the Haile Selassie 1st International airport from a runway that was newly laid with asphalt and increased in length to 3700metres. To allow for the expansion the ends of the runway were raised causing a drop of 10.6 metres at the end of the runway. There was also a steel lattice tower at a distance of 24 metres from the runway end. Before the take off a small hydraulic leak was detected in the left hand main gear but was confirmed to be within acceptable limits. A decision was taken to have it rectified in London, the destination to which the flight was headed. After traversing about half the length of the runway the jacking pad develops a puncture on account of a piece of metal scrap that was left behind by a Cessna aircraft that had flown earlier in the day. A decision was taken to abort the flight with the engines being reversed to obtain maximum braking. Thereafter the crew experience a second bang as the rear tire bursts due to useless anti-skid unit that had been rendered non- functioning due to the non compliant seal. The misplaced seals that were blocking the ports were preventing the aircraft form developing the braking power. Unfortunately, this further lead to no.2 and no.4 brakes developing serious damages. With the aircraft still plying at 60 knots it inevitably became airborne for a brief moment of time before it dropped down to the slope of depth 10 metres. The wings o the aircraft subsequently hit the steel approach lighting tower rupturing the fuel tank. A major fire broke out causing a huge explosion and splitting the aircraft into three pieces. The doors being jammed on one side and the other side being inaccessible due to the heavy fire that was raging caused 43 persons to die due to burns and suffocation. Luckily 15 people survived the ordeal. This case study shows that even though the flight personnel were true in sensing the nature of the problem and was correct in calling it to be aborted; they were hampered by an inefficient braking system due to the replacement with an incorrect spare. The aircraft was thus able to get only 70% of the braking energy which ultimately catapulted it over the runway. Conclusion As the cost of aviation fuel increases, it has but become inevitable for the airline industry to reduce the weight of the aircraft by introducing more lightweight components thus making the aircraft more fuel efficient. The chosen materials however would also need to satisfy all the requirements of design and aerodynamics. In this context more research is currently being undertaken in the use of new and better aluminium alloys and also in the use of composites structures in different other parts of the aircraft. This engineering design would however become redundant if incorrect spares are used and proper maintenance of key systems is not carried out. Reference Lists 1. Quilter Adam, 2011, ‘Composites in Aerospace Applications’, an IHS White Paper. 2. Aitchison Leslie, 1924, Materials in Aircraft Construction. 3. Sun.C.T, 2006, Mechanics of Aircraft Structures. 4. Dr. Cairns Douglas, 2009, “Composite materials for aircraft Structures”, Department of Mechanical and Industrial Engineering, Montana State University. 5. Aluminium in aircraft construction, n.d, AL, Available at:http://www.aluminiumleader.com [Accessed 28nd February 2011] 6. Aluminium Alloys in Aviation, 2006, Experimental Aircraft Info, Available at: http://www.experimentalaircraft.info [Accessed 28nd February 2011] 7. Aluminium and its alloys used in aircraft, 1999, Engineering studies, Available at:http://www.hsc.csu.edu.au/engineering_studies/aero_eng [Accessed 28nd February 2011] 8. Vale john P De, 1999, Shoddy spares, Customer Circumvention, Carnegie Mellon University, Available at:http://www.ece.cmu.edu [Accessed 28nd February 2011] 9. 5X-UVA Addis Ababa 1972, 2001, Incidents and accidents, Available at:http://www.vc10.net/History/incidents_and_accidents [Accessed 28nd February 2011] Read More