Rotor blade materials

The main rotor blades are a vital part of a helicopter, because they are responsible for supporting the entire weight of a helicopter. The forces on the rotor blades can increase even more when performing maneuvers, for example: pulling up at 1.5G means that the rotor needs to support 1.5 times the weight of the helicopter.

The first helicopter rotor blades were constructed out of laminated wood and fabric. One of the major drawbacks of using wood to construct the rotor blades is that wood absorbs moisture, which changes the mass of the rotor blade.

Wooden rotor blades were used up until the 1960s, until they were replaced by steel and aluminium. Advantages of steel and aluminium rotor blades is that they're cheaper and easier to produce, and that they do not suffer from moisture absorption. However, disadvantages include a low strength to density ratio and a poor resistance to fatigue.

Major improvements were made to the rotor blades by using composite materials. Composite materials are made by combining two different materials together. For example, glass fiber and plastic can be combined to form a composite material. The plastic binds the fibers together, and distributes the forces among them. The plastic also helps prevent the propagation of cracks. Composite materials are anisotropic, the material's properties depend on the direction of the fibers. Because of this, multiple layers are put on top of each other at 90° angles. Of course, glass fiber isn't the only material used in composite materials, carbon fibres, and many others are also used, depending on the specific requirements of the rotor blades.

Modern rotor blades start out with a core, made out of Nomex (a brand of aramid), or honeycomb aluminium, which is cut to size. Then, precisely cut pieces of composite materials are placed inside a mold, and are partially cured. The core is then placed within the mold, and is crushed into shape by a hydraulic press. The composite material is then cured using pressurized steam, and excessive material is trimmed off.

Rotor blades constructed out of composite materials can be up to 45% lighter than their metal equivalents, and they can be more easily manufactured in complex shapes.

Sources:
http://www.whystudymaterials.ac.uk/casestudies/helicopter.asp
http://www.madehow.com/Volume-1/Helicopter.html
http://www.advancedtechnologiesinc.com/rotor_blade_development.asp