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Tuesday, March 04, 2014

Ch. 8 - Rollover structure - part 5

Burying the nut-plates and restoring the fuselage outline (17.7 hrs)

This time around I got to take care of old business, and while there was no real rush to accomplish this, I had grown tired of the unfinished look of the rollbar attachment to the fuselage. 

As you might recall, I had carved depressions in the fuselage in order to bury nutplates below the skin, and these needed to get filled with foam, then glassed.

Right depression

Left depression

Here’s an updated cross section of the main plan...

Items to be installed in red

To make sure the nutplates won’t move in the future, I floxed the tabs holding them to the sidewalls, allowing the flox to mushroom through the lightening holes.

Adding flox to the right nut-plate bar

Nut-plate bar held in place by the roll-bar bolts

Repeating the process for the left side

Flox curing

After a few hours of curing, and before the flox hardened completely, I flattened the mushroom heads some more with my fingers, in order to increase the flox grasp on the metal. Then I left it to complete the cure overnight.

Gummy flox flattened by hand

The next morning, I gathered old scraps of foam left over from cutting the fuselage sides.

Nothing goes to waste with this construction method

After mixing up some micro, I glued the foam scraps into both depressions until I filled them up, and then some.

First scrap glued into the sidewall depression

Straight pins unwittingly donated by my wife

The pins could not be saved afterward

Fighting gravity. The whole thing was slowly sliding downward.

Belt and suspenders concept

Once again, I left things to cure overnight.

After getting back to the fuselage, I spent most of the day sanding the soft foam, mixed in with fairly hard micro, until I matched the profile of the rest of the fuselage.

It felt like I was sanding all day

First contouring done

Notice the rough shape of the foam and micro

The many holes still needed to be fixed before moving on to glassing

All of that rough sanding really took its toll on foam and micro in the form of deep gouges, and holes. In order to fill them up I mixed up a new batch of West-System micro, and reapplied it.

Patching the holes

"What can I say? There were a lot of holes!"

One of the advantages of the West-System, is its fast cure time, meaning I could start sanding the same day. So, once the micro cured, I started smoothing things down all over again, until I was satisfied with the final contour.

Final contouring done

Smooth enough for glassing

At last, I was able to take a brake from sanding. 

I marked and cut all the necessary layers of UNI. First, 2 layers cut with a 30˚ bias, and laid over the depression crossing each other, then 3 layers from 52 to 48 inches long, staggered 1”, with a 0˚ bias over most of the longeron.

UNI marked for cutting

UNI before adding epoxy

2 UNI plies cut at 30˚ crossing each other, restoring the original structural integrity.

Left depression filled, and glassed.

3 UNI plies cut at 0˚ extending 52" over the longeron

Right longeron getting reinforced

Since the strakes will eventually attach there, I decided to peel-ply everything, then I reinstalled the very tight fitting roll-bar, but only after the epoxy got tacky. I did it this way in order to prevent the epoxy from being mechanically squeezed out of the fiberglass, while still allowing the fiberglass to conform to the bottom surface of the roll-bar.

Adding peel-ply

Peel-ply and roll-bar added

Left side peel-plied

Roll-bar protected with packing tape, and installed in position after the epoxy became tacky.

Overnight cure, peel-ply removal, and sanding the rough edges came next.

Removing peel-ply is very enjoyable...

...sanding isn't.

Lots of peel-ply strands, and some unevenness, required much sanding on both sides. 

The final shape came out very nice

Sand, sand, sand.

High spots were lowered a bit

Very nice profile

Recessing the roll-bar into the longeron worked out quite well, the right side ended up 0.037” (0.94mm) below the top of the longeron, and the left side just 0.016” (0.41mm). Not too bad for carpentry work.

Slight gap between the top of the longeron, and the top of the roll-bar.

0.037" gap (0.94mm)

Even smaller gap on the left side

0.016" gap (0.41mm)

Roll bar walk-around