AIRFRAME
at least the same number of plies at the same orientations as the original laminate. This being the case, you might be asked to taper, sand or step the edge of the repair to expose underlying plies. Exposing these plies allows us to apply a patch in which each ply of the patch is bonded to its counterpart within the laminate being repaired (Figure 2). In many cases, however, the repair manual will not call for a scarf or a step and simply require a scab patch. Even with scab patches, you will find that the number of plies and their orientations specified by the repair procedure closely mimic those found in the original laminate. It is also common with both methods to see one or more additional repair plies called out in the procedures. This is due to the fact that the processing methods available to us for field repairs are usually less sophisticated than those used to manufacture the part originally. This results in a repair that might have less strength than the surrounding laminate. By adding extra plies to many repairs, engineers ensure a positive margin of safety.
Mixing and Applying Resins
The vast majority of field-level repairs for helicopters are wet lay-up repairs. This means we start with dry fabric (glass, carbon or aramid) and we impregnate it using the correct amount of properly-mixed epoxy resin (Figure 3). There are two key elements here: what constitutes “properly mixed” and what is the “correct amount.”
Remember all that basic math you didn’t think you’d need after high school? It’s about to come back to haunt you. Epoxy resins are furnished as two-part systems, Part A (the base resin) and Part B (the hardener). Known as the mix ratio, the amount of Part B added to Part A is critical. If the instructions for mixing and curing are not followed carefully, the cured resin in the repair might not have sufficient properties to be airworthy and catastrophic failure could result. A corollary to sheet metal repairs would be to use untempered aluminum (O-material) in a patch. We simply wouldn’t do that because we know it would fail. Similar consideration should be given to the epoxies we use in composite repair. Understanding the mix ratio as it’s presented on the container or instructions is key to mixing your resins and adhesives properly. Let’s consider a mix ratio of 100:42. When posed as a mathematical word problem, a mix ratio of 100:42 would read something like this: “To 100 parts-by-weight of Part A, add 42 parts-by-weight of Part B.” A simple, easy-to- remember method for converting a ratio to a percentage is found below:
Ratio as seen in instructions
Change the colon to a “plus” sign Add
100 : 42 100 + 42
100 + 42 = 142 Divide the addends by the sum = 0.7, or 70% = 0.3,or 30%
Figure 3 October | November 2015
HelicopterMaintenanceMagazine.com 31
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44