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Wednesday, December 09, 2015

Center section spar - part 10

Bottom spar-cap (17.0 hrs)

Finally something structural to talk about!

I was a little apprehensive about this step, mostly because I had never worked with spar-cap material before, which is basically very thick UNI-style fiberglass. It turned out, I shouldn’t have stressed over it, because although it took 9 hours to finish the bottom side, I found the process strangely enjoyable (perhaps a reaction to the epoxy fumes in a closed garage).

Laying the spar cap

It all started with the application of clear tape to the aft section of the spar, where a dam would be erected. This was perhaps an unfortunate choice, because it made cleanup more difficult later, as you shall see.

Upside down spar with clear tape added to its aft face

I then used duct tape to cover areas I wanted to keep clear of epoxy.

Starting to mask everywhere other than the trough

Having plenty of "useless" Urethane foam laying around since I started the project, I decided to use it to build the dam that would isolate the bottom trough, and contain the spar-cap material.

As I sunk the razor knife into the Urethane, I couldn’t stop laughing. I forgot how soft this material really was, it was as easy to cut as it is to rip paper apart. As expected, the statically charged foam dust started migrating everywhere, quickly making a mess. I’m so glad I didn’t use this material anywhere on my plane.

This material is just ridiculously soft. It took a long time to get this razor knife to stand long enough for this picture.

Anyway, because the few pieces I had cut were making dust if you just looked at them, I decided to enclose them in duct tape, and vacuum up the mess.

I chose to use a glue gun to attach them to the clear tape already on the spar’s aft face (another unfortunate choice).

Using hot glue on the duct tape covered foam

Foam dam applied to the entire aft face

The hot glue solidified quickly, and built up to a discrete thickness, thus a gap developed between the foam dam and the spar's aft face. This gap got filled with flox later requiring much sanding and cleanup.

A closeup of the dammed trough

Meanwhile, the time had come for cutting the actual spar-cap material. Thing is, when the plans were first written, the UNI you could buy was a little thicker than it is now, as a result using the original layup schedule would have resulted in a thinner structural member. 

Original UNI schedule from the plans

Luckily for us, Burt recognized this potential issue back in 1980, and published an alternate schedule in CP#25 (page 6), just in case thinner UNI was used.

Additional spar-cap plies due to thinner than plans' UNI thickness

Since the BizMan (aka Beasley) had graciously given me his leftover UNI tape, I already knew it was of the thinner kind, and as a result I cut the UNI layers by the plans, plus the additional layers from the Canard Pusher newsletter, and set them aside.

Beasley's leftover UNI tape going to a good cause

Funny thing is I just had enough of it to finish the bottom spar-cap, and not an inch more.

I also copied something I saw on my friend Wade's blog, a way to funnel extra epoxy away from the work and into a spare cup. I did this on both ends of the spar.

Left duct tape funnel

Same thing, right side.

After doing all of this prep, I called it a day even though it was still early, since I wanted to get started at sunup the next morning, and keep going until the bottom spar-cap was done. But I should have checked my epoxy as well, because I found the hardener all gelled up in the morning, and barely able to come out of the can. 

So, the day started by making coffee in one corner of the kitchen, and heating up epoxy at the same time in the other in order to make it usable again.

Oh well!

Un-gelling the E-Z Poxy resin and hardener

When I finally got all my stuff together, I began painting pure epoxy on the shear-web.

Wetting the shear-web with pure epoxy

Then, I mixed some flox, and filled the gap between the foam dam and the spar itself.

Using the pastry bag method to lay flox over the shear-web's rounded edge

Flox completely filling the rounded gap with the rear dam

I then grabbed the first full length piece of UNI tape, laid it in the trough, and made sure all the fibers remained straight.

Laying the first UNI strips of many to come

Straightening out the UNI tape

The UNI spar-cap material has cross-threads holding the UNI fibers in place. Well, actually the cross-threads are really one long fiber weaved though it, and held in place by a red “cross-cross-thread”. 

Close up of the red threads (three UNI strips depicted)

Removing the longitudinal red fiber unlocks the cross-thread that can, and should be removed by pulling it straight out of the side of the UNI tape, and discarded.

Removing the red thread

It actually came out pretty easily

Removing the one-piece super long cross-thread.

At this point I was left with only the longitudinal UNI fibers.

Those white spaghetti will actually support the whole plane one day

Concentrating on straightening out the UNI strands

It is very important that these fibers be as straight as possible in order that they might develop their full strength, so I paid close attention to this fact from this point on, and made extensive use of a secret weapon.

To be totally honest, I discovered my secret weapon accidentally just that morning, but it worked beautifully. 

Perfect size!

Who knew they actually made spar-cap combs?

Please don’t tell my wife what happened to her comb, because she’s still looking for it ;-)

The next few steps were repetitive, time consuming, but pretty simple. I just poured pure epoxy on the UNI, spread it around wetting all the fibers, squeegeed the excess off the ends, combed the fibers straight, and moved on to the next layer.

Removing the excess epoxy

UNI strips getting smaller

Shorter and shorter strips of UNI are laid staggered

Many hours into this job

I'm very happy with how straight the fibers laid

Here's a good shot of the middle section

As I was expecting, I came up a little short thickness wise, and I ended up adding two full length plies, then I peel-plied the whole thing.

Peel-ply over the top

All the red threads and UNI cross-fibers added up to a small mound, and since Wade started the "UNI monster" tradition a while back, I decided to honor it (as my friend Ary already has in his blog), and create my own monster, and on Halloween 2015 of all days… perfect!

"Trick or treats!"... I mean... "Bwahahaha!!!"

I was really excited about removing all the damming materials the next day, and reveal what my spar looked like.

All foam and duct tape removed

Yeah, it needed some major cleanup, but it looked very good overall. Unfortunately, the clear tape on the aft face of the spar got overrun by the flox fillet I had laid, and did not come out in one piece, leaving nearly invisible pieces of clear tape right where a structural layup would eventually go.

This was obviously a big NO-NO, and I had to be 100% certain I removed all of it. So, I spent two days working meticulously at cleaning up all traces of extra flox and tape. This turned out to be a very difficult detail oriented job, since I had to be careful not to damage the shear-web beneath tape and flox.

Close up of the issue

Sanding through flox and spar-cap is very tough, but care must be exercised to avoid damaging the shear-web.

Two days later... Cleanup complete

Bottom spar-cap done! (spar upside down)

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