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Saturday, January 3, 2015

Materials in Aerospace Engineering - 4

Composites used in Military Aircraft

Advanced Tactical Fighter (ATF)

Advanced composites enable the ATF to meet improved performance requirements such as reduced drag, low radar observability and increased resistance to temperatures generated at high speeds. The ATF will be approximately 50% composites by weight using DuPont's Avimid K polyamide for the first prototype.
 
B-2 Spirit Bomber
 
The B-2 derives much of its stealth qualities from the material properties of composites and their ability to be moulded into complex shapes. Each B-2 contains an estimated 40,000 to 50,000 pounds of advanced composite materials. According to Northrop, nearly 900 new materials and processes were developed for the plane.

McDonnell Douglas AV-8B Harrier II
 
This vertical take-off and landing (VTOL) aircraft is very sensitive to overall weight. As a result, 26% of the vehicle is fabricated of composite material. Much of the substructure is composite, including the entire wing. Bismaleimides (BMI's) are used on the aircraft's underside and wing trailing edges to withstand the high temperatures generated during take-off and landing.

F-18 Navy Fighter
 
The wing skins of the F-18A represented the first widespread use of graphite/epoxy in a production aircraft. The skins vary in thickness up to one inch, serving as primary as well as secondary load carrying members. It is interesting to note that the graphite skins are separated from the aluminium framing with a fiberglass barrier to prevent corrosion. The carrier-based environment that Navy aircraft are subjected to has presented unique problems to the aerospace designer. Corrosion from salt water surroundings is exacerbated by the sulphur emission from the ship's exhaust stacks.

V-22 Osprey
 
The tilt-rotor V-22 is also a weight sensitive craft that is developed by Boeing and Bell Helicopter. Up to 40% of the airframe consists of composites, mostly AS-4 and IM-6 graphite fibers in 3501-6 epoxy. New uses of composites are being exploited on this vehicle, such as shafting and thick, heavily loaded components. Consequently, higher design strain values are being utilized.

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This topic will continue in Part - 5

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