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Composite Materials and Helicopter Rotor Blades
French Gendarmerie rescue helicopter taking off on the Massif du Sancy mountains, France*
Since the very earliest concepts of rotor-powered aircraft the design and manufacture of helicopters has been greatly developed and refined. In addition to the increased knowledge of the aerodynamics of flight and the avionics of aircraft, one extremely important factor in the advancement of the design, production and performance of helicopters is the use of composite materials. With their great versatility and desirable properties such materials can be found in numerous layers of a helicopter.
From the seats and the engine bay door to the fuselage and the tailplane, composites form an integral part of helicopters and their design. However, the component whose performance and service-life has perhaps benefited most significantly from the use of these materials is the rotor blade. A typical cross-section of this component is illustrated in Figure 1 below.
Helicopter rotor blades were originally constructed of laminated wood and fabric; this design was retained until the 1960s, when the first steel and aluminium structures were introduced. These metal blades were a huge improvement on previous designs, amongst whose problems was mass alteration due to moisture absorption. However, despite this and other benefits, such as cheapness and ease of manufacture, steel and aluminium blades suffered from various design and structural problems.
The most critical of these were poor fatigue resistance, and low strength-to-density ratios. These problems, together with many other design drawbacks were hugely reduced by the use of composite materials for rotor blade construction.
Radical advancement in rotor blade design was made possible due to the structure and basic “ingredients” of composite materials, for example, glass fibre reinforced plastics (GFRP). These consist of glass fibres dispersed within a polymeric matrix, both of which determine the properties and characteristics of the resulting material. The matrix has several functions, the first being to bind the fibres together, allowing any external stresses to be conveyed and distributed to them. In addition, being ductile, relatively soft and with quite a high plasticity, the matrix is able to play its second role to prevent crack propagation between fibres.
The fibres themselves have their own characteristics. They are produced by means of drawing continuous fibres, and are readily available at low cost. Their strength and chemical inertness also make them highly desirable for use in rotor blades.
Thus, composite materials such as GFRPs, offer many advantages over metals, including lightness, ease of manufacture, relative cheapness and strength. GFRPs do, however, have one major drawback; they lack stiffness, a vital property required of helicopter rotor blades. The solution to this problem lies in another variety of composite material called carbon fibre reinforced plastic (CFRP). The high strength constituent fibres used in these materials are manufactured from polyacrylonitrile (PAN), pitch and rayon, and as a result have the highest specific modulus1 of all reinforcing fibre materials.
However, as is the case with GFRPs, these properties are dependent on fibre direction, since such sheets are anisotropic2. To overcome this, sheets of fibre reinforced material are sandwiched together alternately at right-angles, as shown in figure 2.
Thus, such composite materials can be tailored in such a way as to display desired properties in specific directions and areas.
1. Specific Modulus gives an indication of a material’s stiffness for a given mass or density of the material. A high stiffness/low density is very desirable in aerospace applications, for weight-saving.
2. Anisotropy implies a material’s physical properties are directionally dependent.
Rotor Blade Case Study
Rotor blades are subjected to extremely harsh conditions, both operational and environmental. Rotational tip velocities of approximately 200 m/s (~480mph), and "flapping" during flight, are coupled with extremes in both humidity and temperature. The latter can vary from -40°C to +90°C. So, a number of specific material properties are required for efficient and effective rotor blades. Composites can be made that fulfil these property requirements.
Figure 3 (click the picture to enlarge)
The manufacture of rotor blades begins with the ultrasonic profiling of partially cured fibre reinforced plastics known as pre-pregs, which allows the production of advanced shaped and sectioned blades. Such components are virtually impossible to fabricate economically from metal. The contoured pre-pregs are then positioned, using a specific 'lay-up' pattern, within a mould. This is then closed, crushing the material into the desired shape and form, and an external hydraulic pressure is applied. Curing is completed by means of a computer-controlled process, during which the pressure is maintained and the temperature slowly increased to 125ºC. Finally, the blade construction is finished with the simple adhesion of the honeycomb core between the two constituent blade layers, which are illustrated in figure 3.
Many other desirable properties and characteristics are achieved by the use of composites, including good strength-to-density ratios, which are four to six times greater than those of steel or aluminium. The specific modulus of certain composites is also far greater than those of steel and aluminium, leading to composite blades that are up to 45% lighter than their metal equivalents. In addition, complex blades are much easier to process and manufacture, are joined with adhesives, negating the need for riveting and simplifying assembly and can be produced using much cheaper tooling than for metals.
Developments in composite materials such as carbon fibre reinforced plastic have allowed the creation of rotor blades that far surpass their predecessors in every way, and continued research into new areas of Materials Science will no doubt improve on these blades in the future.
This article is based on a case study developed by Emma L. Williams under the supervision of Dr. Irene Turner of the University of Bath.