- Category: Quickie Plans
- Published: Sunday, 16 November 2008 00:00
- Written by Dan Yager
- Hits: 8732
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.
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.
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
CONTINUED ON NEXT PAGE