Splitting the canopy into two pieces is something of a milestone. Although it may not be immediately obvious, it is vital to secure the Plexiglas canopy so that as the big cut proceeds, the canopy remain perfectly immobile. If not adequately secured, each large piece becomes very flexible. If not supported, uneven stresses can be generated on the Plexiglas and as the big cut proceeds those stresses will load up and concentrate at the edge of the cut. At some point those stresses become unbearable and that canopy can crack in an instant. That very thing has happened to more than a few builders.
There are any number of ways to secure the canopy prior to making the big cut.
How you secure the canopy is not as important as securing it adequately. Some people have stuffed a number of pillows, foam or strategically fixed blocks of wood inside and out to support the canopy during the cut. Others have used duct tape. Some do it right on the airplane. As I considered the problem back in 2001, I developed a technique using kite string. It seemed to work perfectly on the two canopies I have split to date. When the two pieces finally separated on the work bench, each large piece continued to remain relatively immobile, separated only by the kerf width of the fiber cutting disk.
It occurred to me that according to a recent VAF poll, 1/3 of RV builders who participated reported a canopy crack developing at some point. By any measure, that is an awful failure rate. Many new builders are completely unfamiliar with Plexiglas and could become better acquainted with its special properties by simply becoming more familiar with it. That is easy. It starts by removing a good sized chunk of the canopy waste flange and practicing technique on it. As you can see, the four corners of this -8 canopy had large amounts of waste material that needed to be removed. Once removed, drill and countersink the waste 'glass with free and reckless abandon. Try different things, test it to destruction. Drill holes up to full size and experiment using different countersinking techniques. Personally, I use a zero flute countersink bit that produces a high quality countersink with no chance of cracking the glass. The builders manual suggests and most builders end up using a standard piloted countersinking bit. As a review of the archives may reveal, this practice seems to be a common cause of canopy cracks. Is this suggestion by Van's driven in part by an economic pragmatism given that builders already have piloted countersink bits in possession? Tell you what. While practicing countersinking technique on a simulated full size hole drilled through Plexiglas waste material, intentionally place a side load on that standard piloted countersinking bit and
see what happens. Chances are excellent the Plexiglas will crack. Keep in mind that on the real thing, a crack generated while countersinking can occur following the slightest moment of inattention. Classic local stress. Don't take it lightly. These and other exercises may help the new builder to develop a realistic *feel* that may serve later when the builder must drill and countersink holes in that very expensive canopy, a time when the builder has only
one chance to get it right. No secret here. Care and attention to detail here means increasing your chance of joining the 2/3 of RV builders who have not experienced the frustration and displeasure of a cracked canopy.
Tip: Canopy Prep..BIG SPLIT & Beyond
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