Ch 10 - Canard - Part 5

Bottom spar-cap (14.0 hrs)

The process of filling the spar-cap trough for the Roncz canard is identical to the one used on the Center Section spar with one exception, the number of plies and their length is left up to the builder.

“Say what!?”

I kid you not, the directions tell you to use the 3” UNI tape, and to lay enough staggered pieces (with 60º cut ends) to fill in the trough until flush… no more, no less… it is up to you how much fiberglass gets in your spar-cap.

Since it has been done this way for over 30 years, and no canard failures have happened (that I know of), it must be a safe enough method. However, telling whether the trough is full or not while in the midst of fiberglassing, did not seem like a great idea to me. I wanted to know how many plies, and how long, way before that stage, and have them precut and ready to go.

This required mapping the trough depths, and determining how many layers would fit where. Using the bottom contour template and a gauge block set, I measured the trough every few inches, and came up with a surprisingly uneven and slight asymmetrical trough floor.

Mapping the trough geometry 

After measuring the thickness of the UNI tape, I divided the trough measurements by the UNI thickness and determined an approximate layup schedule, then I forced it to be symmetrical. 

A more accurate method would have been to glass 7 test plies, then measure the final thickness and divide by 7. 

Figuring out how many plies are needed

With the final UNI count on hand, I rounded the actual layup schedule this to be 2 full length UNI plies, and 5 smaller staggered plies, each of which would be 16” shorter than the previous (8” each per side). 

As close as this was to my measurements, it still turned out to be slightly proud of the foam, but just barely. I will take care of this minor issue during finishing.

Sanding the shear-web before glassing

UNI plies staggering schedule

Cutting the full length plies

All shorter plies were terminated at 60º 

I used flox to fill the corners because I worried about micro ending up between my structural layers

Laying the thick UNI tape

Hard to see, but I was cutting the red string that holds the cross threads together.

I pulled the cut red strings halves out of both ends

With the red string gone, I removed the cross threads.

Cross threads removed

Layar #1 (full length 108")

Layer #2 (full length 108")

Layer #3 (92")

Layer #4 (76")

Layer #5 (60")

Layer #6 (44")

Layer #7 (28")

Bottom spar cap layup completed

While always an option, I elected not to continue with the glassing of the bottom skin of the canard at this stage. This was not mandatory even on the CS spar, so I decided to save my back to “fight another day”, and peel ply the spar-cap instead.

Clean up was a definite pain and took many hours, mostly because a lot of tape edges had gotten caught under the glass.