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Q1 Plans Chapter 3 Page 3-4

UNI cloth has 95% of the glass volume woven parallel to the selvage giving exceptional strength in that direction and very little at right angles to it.

BID is generally used as pieces which are cut at a 45—degree angle to the selvage and are laid into contours with very little effort. BID is often applied at 45-degree orientation to obtain a desired torsional or sheer stiffness. UND is used in areas where the primary loads are in one direction, and maximum efficiency is required, such as the wing skins and spar caps.

Multiple layers of glass cloth are laminated together to form the aircratt structure. Each layer of cloth is called a ply and this term will be used throughout the plans.

Marking and cutting the plies of glass cloth is a job that you will repeat at often in the construction of your Quickie. Glass cloth should be stored, marked, and cut in a clean area with clean hands and clean tools. Glass contaminated with dirt, grease, or epoxy should not be used. A clean, smooth surface is needed for marking and cutting. The area used for storing and cutting glass cloth should be separated from the aircraft assembly area because it will be exposed to foam dust, epoxy, and other things which can contaminate the cloth. You will need a good sharp pair of scissors, a felt—tipped marker, a fairly straight board, and a tape measure for marking and cutting. The small amount of ink from marking and numbering plies has no detrimental effects on the glass cloth.

In each step the size, type, and fiber orientation of each ply is given. Take the list to your glass cutting table, roll out a length of the appropriate cloth, straighten the selvage, mark all of the plies, and cut them.

Now is a good time to stop reading long enough to go and cut a square ply of BID and see how easy it is to change its ahape by pulling and pushing on the edges as shown in the sketches. Cut a square with the fibers running at 45° and pull on the edges to shape the piece.


Shaping a BID Square

It helps if you make fairly straight cuts but don’t worry if your cut is within 1/2 inch of your mark. As you cut BID it may change shape, just as the square ply that you are experimenting with does when you pull on one edge. Plies that distort when cut are easily put back into shape by pulling on an edge. Rolling or folding cut plies will help keep them clean and make it easier to maintain their shape. If several plies are called for, it may help to number them before cutting. Save your clean scraps and make an effort to use them for smaller plies. If the cloth is spotted with epoxy, throw it away.

When cutting long strips or large pieces of 45° BID, always roll or fold it so it keeps its shape when handled. When it’s applied it can be set on one end of the part and rolled onto it. If you pick up each end, it will distort and not fit the part properly.

The fiber orientation called for in each materials list is important and shouldn’t be ignored. UNI is characterized by the major fiber bundles running parallel to the selvage and being much larger than the small cross fibers which run at right angles to the selvage. In BID the cross fibers are the same size as those running parallel to the selvage, giving BID an even “checker board” appearance. BID is commonly used for piles cut at 45° to the selvage. Your tailor would call this a "bias" cut. The 45° cut makes it easy to work wrinkles out of a ply locally, without having to chase it to the far edge. The 45° cut also makes it possible to make a ply slightly longer than originally cut by pulling on the ends or wider by puiling the sides. The 45° orientation isn’t critical; you don’t need to measure it. Your eyeball of a rough diagonal (45° ± 10 ) is adequate when either cutting or laying up the cloth.

Epoxy

In recent years the term “epoxy” has become a household word. Unfortunately “epoxy” is a general term for a vast number of specialized resin/hardener systems, the same as “aluminum” is a general term for a whole family of specialized metal alloys. Just as the “aluminum” in the spar of a high performance aircraft is vastly different from the “aluminum” pots and pans in your kitchen, the “epoxy” in your Quickie is vastly different from the hardware store variety.

Epoxy is the adhesive matrix that keeps the plies of load-carrying glass cloth together. Epoxy alone is weak and heavy. It is important to use it properly so that the full benefits of its adhesive capability are obtained without unnecessary weight. A large portion of your education in composite structural work will be spent learning how to get the full strength of an epoxy/glass mixture with the minimum weight. This section will discuss the terminology and techniques for working with epoxy resin and its hardener.

An “epoxy system” is made up of a resin and a hardener tailored to produce a variety of physical and working properties, The mixing of resin with its hardener causes a chemical reaction called curing, which changes the two liquids into a solid. Different epoxy systems produce a wide variety of solids ranging from extremely hard to very flexible, Epoxy systems also vary greatly in their working properties, some are very thick, slow pouring liquids and others are like water. Some epoxy systems allow hours of working time and others harden almost as fast as they are mixed. A single type of resin is sometimes used with a variety of hardeners to obtain a number of different characteristics. In short, there is no universal epoxy system; each has its own specfic purpose and while it may be the best for one application, it could be the worst possible in another use.

The RAE-type epoxy systems used in the construction of your Quickie are tailored for the best combination of workability and strength, as well as to protect the foam core from heat damage and solvent attack. These systems are also low in toxicity (SPI-2) to minimize epoxy rash. These epoxies are not similar to the common types normally marketed for fiberglass laminating. Three different systems are used in the Quickie, for three different types of work: a slow curing system, a fast curing system, and a 5-minute system. The very fast curing (5-min) system is used much like clecos are used in sheet metal construction (or clamps in woodwork) for temporary positioning. Five-minute is also used in some areas where high strength is not required, but where a fast cure will aid assembly.

As an epoxy system cures, it generates heat and in some areas, the heat buildup of a medium or fast curing epoxy system is unacceptable. Where this is a potential problem, a slow curing system is used. Slow cure epoxy is always used with styrofoam, where heat can melt the foam away and ruin the joint. In other

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Q1 Plans Chapter 3 Page 3-5

areas where heat buildup isn’t a problem and a faster cure is desirable, a fast curing system is used. Both the fast and slow cure epoxies will cure to a firm structure at room temperature within one day. Complete cure takes 14 days.

The RAF epoxy systems are called RAES (for slow curing), RAEF (fast curing), and 5-MIN. Both the RAES and RAEF hardener use the same RAE resin.


Mixing Epoxy Resin

Note: If working conditions are hot (85° to 90°F), RAES can be used where RAEF is called out. While this practice is allowable, the best results are obtained working between 70 and 80°F with the specified epoxy.

The working and strength characteristics of an epoxy system are dependent on the resin, the hardener, and on the amount of each in a given mixture. Epoxy systems are engineered for a specific ratio of resin and hardener. It is quite important that the proper mixture be obtained. An accurate balance or ratio pump must be used to accomplish this. A drawing of an inexpensive ratio balance is included in these plans. The mix ratio accuracy is particularly important with RAEF & RAES. The 5-MIN can be adequately ratioed by merely pouring a blob of part A in a cup and adding a blob of part B that looks the same volume before mixing. Never eyeball estimate RAEF or RAES, always carefully use the balance or pump.

Epoxy resin and hardener are mixed in small batches, usually 6 ounces or less, even in the largest layup. The reason for small batches is that, in large batches, as the hardening reaction progresses, heat is generated which speeds the reaction, which causes even more heat, which speeds up in a fast reaction called an exotherm. An exotherm will cause the cup of epoxy to get hot and begin to thicken rapidly. If this occurs, throw it away and mix a new batch. The small volume batch avoids the exotherm. For a large layup, you will mix many small batches rather than a few large ones. With this method you can spend many hours on a large lay-up using epoxy that has a working life of only a few minutes. If the epoxy is spread thin as in a layup its curing heat will quickly dissipate and it will remain only a few degrees above room temperature. However, in a thick buildup or cup, the low surface area to mass ratio will cause the epoxy to retain its heat, increasing its temperature. This results in a faster cure causing more heat. This unstable reaction is called an exotherm. Exotherm tesperatures can easily exceed the maximum allowable for foam (200°F) and damage the foam-to-glass bond. This is why the slow epoxy system is used in joining foam cores as exotherm is impossible as long as epoxy joint thickness is less than 1/8”.

Unwaxed paper cups are used for mixing and ratioing resin and hardener. Convenient 8-oz cups for resin are available from RAF distributors. The hardener cups are the 3-oz unwaxed bathroom paper cups available at any grocery store. Don’t use waxed cups; the wax will contaminate your epoxy.

If you are using the homebuilt balance, follow this procedure. Place the resin (8 oz) cup on the right cradle. The resin cup can be either a new clean cup, one with a little uncured epoxy left in the bottom, or a clean cup from a previous layup with hard epoxy in the bottom (smooth, nor lumpy). Now, take a clean 3-oz hardener cup - pour a splash of hardener into it then scrape the hardener back into the container. This gives the hardener a wet surface, so its remaining hardener will not be counted in the balancing. Now, place the wet hardener cup on the scale, check that it swings freely and balance it perfectly by moving the small weight. Epoxy is then poured into the 8 oz cup (6 oz or less). Hardener is then poured into the 3—oz cup at the other end of the balance until the arm is level. When resdy to mix, pour the hardener into the resin cup and mix completely. If you have the ratio pump, you simply put one cup under the spout, pump out the amount that you want and mix.

Mixing is done by stirring with a stick, being careful not to spill any. If you spill part of an unmixed cup, the ratio of resin and hardener may be inaccurate and it shouldn’t be used. Mix each cup for at least two minutes. You should spend 80% of your mixing time stirring the cup and 20% scraping the sides to assure complete mixing. Do not mix with a brush. The bristles can soak up the hardener, changing the ratio. Use a tongue depressor or wood stick.

The working temperature has a substantial effect on the pot life and cure time. Very hot conditions will cause the cure to speed up. In cold working conditions the cure will be delayed and if it is cold enough, epoxy may not cure at all. Working temperatures must be between 60° and 90°F. A range of 70 to 75°F is best. Be sure to get a wall thermometer (approx. $1.50 at any general store) to check the temperature of your work area. At 75°F, 5-mim must be used within four minutes, RAEF must be used within 20 minutes and RAES must be used within 50 minutes.

Cold epoxy results in increased time required to do a layup, since it takes longer to “wet” and to squeegee the cloth. A layup at 65° may take almost twice the time as st 75°F. On most layups (except for joining foam cores) its best to have 70 to 80°F room temperature and 80 to 90°F epoxy. Resin and hardener can be kept warmer than room temperature by keeping it in a cabinent with a small light bulb on. DO NOT store your resin or hardener on a cold floor if you plan to use it within the next several hours, if you let your shop get cold between

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