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Tuesday, May 09, 2017

Ch 10 - Canard - Part 11

Top spar cap (16.5 hrs)

Filling the trough on the canard with as much UNI tape as possible but no more, is super important. Experimenting with UNI tape is the main reasons I took so long to get this job done.

Top canard contour gauge highlighting the spar-cap trough

Using too little UNI tape would make the canard weaker, while too much would create a troublesome step on an aerodynamically critical component. Also, since the trough depth changes with the distance from centerline, some sort of a layup schedule had to be predetermined.

So, this past month I took time to explore different ways of determining this schedule, from mapping the depths of the trough every 3”…

Using machinist gauge blocks to determine trough depths

… to testing the average thickness of a 10 layer UNI tape layup in a simulated trough…

Simulated spar-cap layup on scrap foam

10 layer spar-cap coupon

Measuring thickness every 0.5" (1.3 cm)

See how the contour gauge is lifted off the foam on the far right (leading edge) of this photo? Not good!

Based on my tests, it appears that the epoxy does not add any thickness to a properly squeegied layup.

Therefore, the best method to determine how many layers of UNI tape can fit in the trough at a particular distance from centerline, is to use actual UNI tape stacked as in a real layup, and add or remove layers based on whether the contour gauge fits on top of the canard, or not.

Determining how many layers of UNI tape can fit at this given location

Let's paraphrase the goal of this step once more... fill up the trough with however much UNI tape one can fit in it, and end up flush on top with the rest of the foam.

To make a long story sort, I ended up with 9 layers with the following schedule (yours might be slightly different):

  • layers 1, 2 & 3 - full span 108”
  • layer 4 - 84” to 96”
  • layer 5 - 66” to 84”
  • layer 6 - 42” to 66”
  • layer 7 - 18” to 42”
  • layers 8 & 9 - 12”

If you are wondering as to why the top spar-cap uses more layers than the bottom, it is because the bottom spar-cap has the best work environment for fiberglass, i.e. all the loads are applied in tension, while the top spar cap has to be slightly overbuilt to cope with the compression loads of the upward flexing canard.

After cutting the UNI tape, I prepped the trough by lightly sanding it.

Scuffing up the surface before fiberglassing

Then I mixed some epoxy, wetted the trough…

Pure epoxy painted to the top of the shear-web (trough-bottom)

… and begun laying up the UNI tape.

Layer #1 (cross-strands were removed after this photo was taken)

Fibers must be straight for proper strength to develop

EZ-Poxy has the nostalgic brown look of yesteryears

Layer #2 being applied

Closeup of the UNI tape. The cross-strands are removed before adding epoxy.

Layer #3

Squeeging extra epoxy out at the far end

Photo taken after combing the fibers straight

Layer #4

Layer #5

Layer #6

Layer #7

Layer #8

Layer #9

Completed spar-cap

With the layup completed, I peel-plied the whole thing and went to bed. 

Peel-ply added

The next morning the “cleanup from hell” began. 

Cleaning up this mess took me a day and a half!

Cleanup took twice as long as the actual fiberglassing. 

Note to self: Next time around remove the duct-tape before the epoxy cures.


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