Composite Rotor Blade Inspection and Repair
Helicopter composite rotor blades offer an exceptional challenge to those wishing to make repairs because of the critical role that the rotor plays in flight. Most technicians today choose not to make any type of repairs to the rotor, in fear that an incorrect repair might result in disaster. While these fears are understandable, taking the time to know composites and learn basic preventive maintenance and minor repairs can actually result in the prevention of catastrophic failures.
Practice makes perfect
Prior to attempting any types of repairs on any composite component, it is imperative that two things be done. First, there must be a thorough understanding of the type of material that is going to be repaired or inspected. And second, the technicians skill level when working with the material should be developed by practicing the repair on practice parts. It is highly advisable to attend a composite school where practical hands-on experience can be gained and repair technique, that can contribute to an airworthy repair, can be learned.
What's a composite?
A composite is a combination of two materials: a mass of fibers, the principal load-carrying material, and a matrix (commonly epoxy), which bonds the fibers together and gives them lateral support. The combination of the two leads to a very high strength-to-weight ratio material. Today's advanced composites are also referred to as "fiber-reinforced plastics."
Know the materials
Fiberglass is one of today's most commonly used composite materials. Fiberglass is actually made of molten silica and, for the purpose of aviation, is divided into two categories: Electrical grade, which is referred to as Type "E," and structural grade, which is referred to as Type "S." Fiberglass has been used in aviation since the 1950's. Kevlar® is a lighter (about 40 percent), more durable material than fiberglass. It is an organic aromatic polyamide that is derived from nylon. One of its more distinguishable features is its yellow color. Kevlar 49 is the only grade that is approved for use on aircraft structures. It has superior tensile strength and toughness but is inferior to carbon in compressive strength. Carbon also referred to as graphite, is strong but rather brittle. It is black in color, stiffer than titanium, and 40 percent lighter than aluminum. Boron, according to FlightSafety, is made by depositing boron gas vapor onto a thin filament of tungsten or carbon. The resulting fibers are approximately 0.004 inch in diameter, have excellent compressive strength and stiffness, and are very hard. Boron is black in color and highly toxic.
Ceramic fibers such as Nextel®, made by 3M™, are used in high temperature applications of around 2,200 degrees F and its fibers are white.
Matrix refers to the resin system that is use to bond the reinforcing material together. Epoxy is the most common matrix material used today. Core Material is the central material that is bonded between two surface skins. The core material, usually foam or a honeycomb structure, provides a rigid light weight component that supports the two external skins. According to FlightSafety, Honeycomb has a greater strength-to-weight ratio, but foam is more durable and if damaged, has a memory and will return to about 80 percent of its original strength.
Many sophisticated methods of inspection are available, but the most valuable and economical inspection is still done with no special tools. The eyes, ears, and hands can give you valuable indications that there are defects in the rotor. It is important, however, to refer to manufacturers recommendations for proper inspection techniques.
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