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Saturday, January 3, 2015

Materials in Aerospace Engineering - 5

Composites in Choppers

Rotors

Composite materials have been used for helicopter rotors for some time now and have gained virtually 100% acceptance as the material of choice. The use of fibrous composites offers improvements in helicopter rotors due to improved aerodynamic geometry, improved aerodynamic tuning, good damage tolerance and potential low cost. Strength properties are very desirable for the long, narrow foils. Additionally, a cored structure has the provision to incorporate the required balance weight at the leading edge. The favorable structural properties of the mostly fiberglass foils allow for increased lift and speed. Fatigue characteristics of the composite blade are considerably better than their aluminium counterparts with the aluminium failing near 40,000 cycles and the composite blade exceeding 500,000 cycles without failure. Vibratory strain in this same testing program was 4 times better for composite as compared to aluminium.

Sikorsky Aircraft of United Aircraft Corporation has proposed a Cross Beam Rotor (XBR)TM, which is a simplified, lightweight system that makes extensive use of composites. The low torsional stiffness of a unidirectional composite spar allows pitch change motion to be accommodated by elastic deformation, whereas sufficient bending stiffness prevents aeroelastic instability.

Structure and Components
 
The extreme vibratory environment that helicopters operate in makes composites look attractive for other elements. In an experimental program that Boeing undertook, 11,000 metal parts were replaced by 1,500 composite ones, thus eliminating 90% of the vehicle's fasteners. Producibility and maintenance considerations improved along with overall structural reliability.

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