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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.

 
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