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” myriad 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, you’ll find many mechanics become very apprehensive, and rightfully so. The thought process is nearly automatic in an A&P’s mind — if a customer’s door seal that you glued on last week comes loose, they’ll be annoyed. If the rotor blade repair that you glued on last week comes off in flight, they’ll be considerably more than just annoyed.
Rotor blade repairs must be strong and 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.
Nowhere in aviation are the stakes higher than when it comes to the quality of your adhesive bond.
Though each manufacturer is different, blade repairs covered in maintenance manuals usually fall into two categories. The first category is the replacement of leading edge components such as nickel, titanium, or sometimes elastomeric erosion strips. The second category involves patching by means of adhesive bonding of the blade skins. This sort of repair might or might not include the blade’s core material, and often involves the use of a pre-cured patch material available from the original equipmet manufacturer (OEM). In either case, the primary factors that will influence the quality of your repair are surface preparation, 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). This is particularly important with composite blades with Nomex honeycomb or foam cores.
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 to aid 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 (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.
Surface preparation
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.
Composite blades
Surface preparation of composite materials is relatively simple in principle. Abrasion is considered the most effective means of preparing composite surfaces for bonding, but it is essential to understand why we are abrading and what to avoid during 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 to reveal 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 it works like this: 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 will result in a substandard adhesive bond.
Caution must be exercised when abrading a composite structure for bonding. 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.
Since the fibers are the principle load-carriers in any composite laminate, 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.
Metal blades
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. Historically, when we bond 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. Since this is the case, 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 one that bonds readily to primers and adhesives while resisting the deleterious effects of moisture.
Aluminum bonds prepared using Sol-Gel perform at roughly 90 to 95 percent of those prepared using phosphoric acid anodization (PAA) treatments, but are often much easier to apply in a repair scenario.
The final step in metal surface preparation is priming. Most primers used for adhesive bonding are formulated somewhat differently than those used for painting. While they usually share an epoxy chemistry, corrosion inhibiting adhesive primers (CIAP) used for adhesive bonding are designed to be applied at approximately 1/10 the thickness of paint primers. Typical cured thickness of these systems is usually around .0001 inch to .0004 inch, and they can often be co-cured with the structural adhesive.
Adhesives
Each manufacturer calls out particular epoxy structural adhesives for any given repair. Theoretically, these adhesives all have the correct physical properties (strength, durability, etc.) to perform well in a repair.
However, their properties can be drastically reduced by ignoring the material’s shelf-life or by failing to mix the adhesive properly.
The shelf-life of structural adhesives varies from one manufacturer to another, though a common figure is 12 months if kept at or below 77 F. This means that the lids on the cans must be tight and that materials must be maintained at or below 77 F, not at an “average” of 77 F in some locker out on the hangar floor.
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 that should be 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 we add 30 grams of hardener to 100 grams of resin 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. Significant 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 the use of scrim cloth or carriers. Scrim cloth and woven carriers 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 and raise your confidence further.
If some techniques aren’t currently in your maintenance manuals, seek to gain approval through a DER or the OEM. AMT
Greg Mellema is from Abaris Training. For more information call him at (800) 638-8441.
