The subject of helicopter rotor blade repair is sometimes a scary one. As aircraft mechanics, we routinely use various types of adhesives in our day-to-day maintenance activities. Go to any hangar and you'll find a host of different adhesives used to "glue" a myriad of different components in place, seals, baffles, carpet, Velcro, the list goes on and on.
When it comes to adhesively bonding a repair to a helicopter rotor blade though, you'll find many mechanics become very apprehensive, and rightfully so. The thought process is nearly automatic. If a customer's door seal you glued on last week comes loose, they'll be annoyed. If the rotor blade comes off in flight, they'll be considerably more than just annoyed.
Rotor blade repairs must be strong, durable, and they must be aerodynamically smooth. They must not add excessive weight to the blade, nor can they make the blade overly stiff in one location. And when it comes to the quality of your adhesive bond, nowhere in aviation are the stakes higher.
The primary factors that will influence the quality of your repair are surface preparation and proper handling of adhesives, and bond-line control.
Initial preparation and cleaning
First, a clean repair area is required, free of contaminants both on the surface and in the core material (if applicable). Composite skins and cores that are contaminated with anything other than water can not be cleaned adequately to ensure a good adhesive bond. Attempts to leach oil-soaked cores or skins with solvent in an effort to clean them usually result in a seemingly good bond that fails a short time after it's been returned to service. The best policy is to remove any oil-contaminated skin and core as "damaged" material and replace it during the repair.
Simple water contamination can be removed from most blades, metal or composite, using a vacuum bag. Under vacuum, water becomes a vapor. As such, it is drawn slowly out of the damaged blade toward the vacuum pump. Some repair manuals suggest using heat to accelerate the drying process, but extreme caution must be exercised when using this technique. While some heat is beneficial, aiding the water to flash to a vapor, too much heat creates two potentially serious problems. First, too much heat may begin to soften the adhesive that bonds the skin to the core. Second, the water vapor, or steam, that forms in the core cells expands in proportion to the amount of heat being applied. If heated without proper controls, this can easily generate enough pressure to lift the skin away from the core well outside the repair area and may destroy an otherwise repairable blade.
While other aspects should not be discounted, surface preparation is by far the most important. Whether or not the patch stays bonded to the blade and carries loads as intended is contingent upon a properly prepared surface. The primary goal of surface preparation is to create an optimal surface on both the blade and the patch that will yield a well-bonded, durable repair that we can expect to do its job and conduct flight loads for the life of the blade. However, how we go about creating such a surface depends on whether we're repairing a metal blade or a composite blade.
Surface preparation of composite materials is relatively simple. Abrasion is the most effective means of preparing surfaces for bonding, but it's essential to understand why we are abrading and what to avoid in the process.
When we abrade a composite surface we are shearing away the relatively dirty, weathered, low surface energy surface at an almost microscopic level revealing a fresh, "high surface energy" surface. High-energy surfaces promote efficient flow of the adhesive and wetting-out the surface. This wetting of the surface is a critical factor in developing strong, durable bonds. High surface tension liquids like resins and adhesives tend to flow and wet out high surface energy solids.
Airframe Technology Advanced Composites & Helicopters By Greg Mellema September 2004 It seems as though advanced composite materials are found on more and more...