Originally by Kurt Krempetz for AMA Glider
When building gliders, one item that is almost always used is sandpaper. The process of removing material using sandpaper is called sanding. Sanding is something that almost everybody has done, but few have been given lessons or instructions on how to sand. The inexperienced builder sometimes considers sanding easy and thinks that there is not much to it, but the experienced builder realizes that sanding is an art that takes practice and years of experience before done correctly. The typical instructions for building a glider just tells one to sand the balsa wood to shape. This document will explain my techniques for sanding/building gliders.
Sanding
Sanding is the process of removing material by taking a rough/harder surface and moving it across the piece that you want material removed from. The rough/harder surface is typically referred to as sandpaper and small “stones” on the paper grooves/cuts out material.
For the construction of gliders, sanding is typically used to shape the elevator, rudder and airfoil of the wing. These shapes need to be precisely made (to within a few thousands of an inch; .002”) if you want a glider to perform well.
Sanding vs. Planing
Many builders plane or carve the wood before they start sanding it to shape, especially if there is a lot of wood to remove. The advantage of planning/carving wood is that it removes wood quickly and does not create a lot of dust. It also typically does not crush the wood as much as other techniques that remove/shape wood.
A hand plane is a typical tool that every glider builder has. See Photo 1. Hand planes are generally the combination of a cutting edge, such as a sharpened metal plate, attached to a firm body, that when moved over a wood surface, take up relatively uniform shavings, by nature of the body riding on the 'high spots' in the wood.
Photo 2: Collection of many different sizes and grit balsa sanding blocks
The next tool that you need is a hard, flat surface to sand on. I use glass plate. If you watch the garbage others throw out, you can typically find pieces of glass at the curb. The best is when someone replaces their windows because you can just pick up the window frame with the glass inside. After removing the frame, you will have your nice building surface. Marble is also good to work off of, but more expensive and harder to find. See Photo 3.
Photo 4: SandBlaster sandpaper in a variety of grits
Basically, sandpaper is made from three components. One component is the paper or backing surface the “stones” are glued onto. The cheaper sandpaper uses poor quality backing paper, which can be inconsistent in thickness and not very durable. Next, is the glue which is used to glue the “stones” to the paper. Again, the cheaper the sandpaper is, the poorer the quality of glue that is used. Finally, there are the “stones”. There are two important features of the stones. One important feature is what material they are made from. Stones that are made from hard material make the sanding process easier and more precise. The second important feature is the size of the “stones”. The size of the stone is characterized by the grit of the sandpaper. Sandpaper comes in a variety of grits, usually ranging for 60 to 1500. 60 grit is very rough and 1500 grit is very smooth. A table below shows the sandpaper grit number and the corresponding “stone” size.
Image
Also, if one puts the piece that is being sanding under the microscope, they will also see similar mountains and valleys.
At the low grit numbers, I find that the stones tear the wood and one needs to be careful not to tear the balsa wood because this can create large valleys.
Making a Sanding Block
Making a sanding block is easy once you know how. First, get a piece of balsa wood and cut it into a block. Then, shape the top and sides of the block to make it feel good in your hand. Next, make the flat sanding portion of the block by using your flat sanding surface. Typically, you want a very flat, smooth surface on the sanding block, which you attach the sandpaper to. To get your sanding block flat and smooth, use sandpaper, an aluminum plate and some double-sided tape. Attach a large 8.5”X11” piece of sandpaper to the aluminum plate using the double sided tape and then just take your sanding block and sand the surface flat. See Photo 5. Be careful not to rock the sanding block as you move it across the sandpaper.
Photo 6: Sanding fixture/jig to sanding wing taper from root to tip
Photo 7: Sanding from high point of airfoil to trailing edge. Tape at high point allows one to sand an accurate high point shape, the tape at trail edge sets trailing edge height.
Happy sanding!
For more articles about model aircraft, please visit AMA Glider’s website.