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.
While the actual physics behind the surface energy of solids is a bit more complicated, put simply: If the molecules of the liquid (adhesive) have a stronger attraction to the molecules of the solid surface (blade and patch) than to each other, wetting of the surface occurs. Conversely, if the liquid molecules are more strongly attracted to each other than the molecules of the solid surface (low energy), the liquid beads-up and does not wet the surfaces to be bonded. This results in a substandard adhesive bond.
Caution must be exercised when abrading a composite structure. Ideally, we would only abrade the resin at the very surface of the laminate. To go any deeper would mean cutting fibers in the composite. We must select abrasives that are less likely to damage the blade skins. Abrasive pads like Scotchbrite are an excellent choice. Due to their soft, pliable nature they are capable of getting down into shallow surface irregularities and ensure a consistent high-energy surface.
Preparation of metal surfaces for adhesive bonding is substantially trickier than with composite materials. When bonding composites together there are two interfaces — one between the adhesive and the adherends on either side. When bonding two metals together there are six interfaces.
The difficulty in preparing metal surfaces (particularly aluminum) stems from the fact that the aluminum oxide layer that naturally forms at the surface of the aluminum is poorly bonded to the aluminum itself. So, it doesn't really matter how well the adhesive is stuck to the primer, or how well the primer is stuck to the oxide layer. Like any chain it is only as strong as its weakest link and is most likely to fail at the boundary between the oxide layer and the aluminum. This risk of failure increases dramatically should the bond-line become exposed to moisture.
A relatively new means of preparing aluminum and titanium surfaces for bonding is AC-130 Sol-Gel. Developed by Boeing's Phantom Works and manufactured by AC Tech, AC-130 is a liquid that is applied by brush to a freshly sanded aluminum surface. Its chemistry alters, or evolves the aluminum oxide surface into a surface that bonds readily to primers and adhesives while resisting the deleterious effects of moisture.
The final step in metal surface preparation is priming. Most primers used for adhesive bonding are formulated differently than those used for painting. While they usually share an epoxy chemistry, corrosion inhibiting adhesive primers (CIAP) for adhesive bonding are designed to be applied at approximately 1/10th the thickness of paint primers. Typical cured thickness of these systems is usually around .0001 to .0004 inch.
Each manufacturer calls out particular epoxy structural adhesives for any given repair. The adhesives all have the correct physical properties (strength, durability, etc.) to perform well. However, their properties can be reduced by ignoring the shelf-life, or failing to mix the adhesive properly.
A common shelf-life is 12 months if kept at or below 77 F. This means that the lids must be tight, and the materials must be maintained at or below 77 F, not at an "average" of 77 F in some locker in the hangar.
In order for any resin system to develop its full strength after it is cured it must first be mixed properly. The amount of hardener added to a resin system is usually measured by weight, not volume and is expressed as a ratio, e.g. 100:30.
Assuming our unit of measurement is grams, this means to 100 grams of resin, we add 30 grams of hardener for a total of 130 grams of mixed material.
The importance of understanding mix ratios cannot be stressed enough. Most high performance structural adhesives will tolerate mix ratio errors up to 3 percent. Errors beyond 3 percent may dramatically reduce an adhesive's ability to perform properly in service.
Bond line control
Any adhesive performs best when applied in a certain thickness. Deviation from this can result in a premature failure of the adhesive bond. There are several methods available to keep this thickness constant. Micro beads can be mixed with the adhesive in small quantities to maintain the optimal distance between adherends.
Another method is scrim cloth or carriers which have a uniform cross-sectional thickness, but also feature significant distance between the fibers to allow adhesive to flow to both sides readily. Non-woven carriers have similar cross-sectional features, but the fibers have a random, tissue-like appearance. Both types are available in polyester or fiberglass.
You can do it with confidence
Once you master the subtleties of adhesive bonding, questions about the quality of the repair begin to fade away. Following NDI inspection according to the maintenance manual, or according to your local procedures should confirm the repair's integrity, raising your confidence further.
Greg Mellma is an instructor at Abaris Training. Abaris is introducing a course in helicopter rotor blade repair that will focus on OEM repair techniques, surface prep, and bonding of both composite and metal rotor blades. For more information call (800) 638-8441.