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Wednesday, August 28, 2013

Ch. 8 - Rollover structure - part 4

Attaching the roll-bar (9.6 hrs)

Fabricating the roll-bar was pretty intense but, while the end result turned out better than I expected, this job won't be completed until metal is attached to, and has become part of the fuselage. 

Following the plan of action outlined at the end of part 3, I taped off all the places where I did not want the flox to go, and sanded where I wanted it to make a good bond with the underlying structure.

Taping off no-flox areas

Sanding longeron to prep for flox application

After applying more “release tape” to the inside of the lower L brackets, I buttered them with flox, and attached them to the longerons. 

Lower L channels taped. Flox about to get mixed.

Flox applied to Lower L channels over the tape

Lower L channels floxed into position

After cleaning up the flox overflow, I carefully lowered the roll-bar, and clamped everything in place overnight.

Roll-bar mounted and clamped overnight 

The next day, I removed the roll-bar and the lower brackets, peeled off the tape, and used a Dremel tool to trim chunks of hardened flox that I hadn’t been able to get to while wet.

Grinding excess flox off

Before drilling the bolt holes through the longerons, I cut 5 layers of UNI (times 2) and positioned them over the longerons. Their purpose was to simulate the thickness of the next 5 layers that will cover the depression later, while allowing me to drill the holes at the proper height now.

Spacer UNI layers over the longerons

Roll-bar reattached for drilling

Mounting holes being drilled

My plan has always been to make this roll-bar removable, so you might be wondering how I planned on retaining the bolts on the outer side of the longerons once they are buried in the foam. I decided to use nut plates (K1000-4) riveted to a 0.050” (1.3 mm) 4130 steel strip, the purpose of which is to tie the nut plates together, preventing them from moving in relation to each other.

Nut plates are meant to be attached with aluminum rivets, formed with a rivet gun, or a rivet squeezer, neither of which I own yet since I’m not building an aluminum plane. So, I tried the Neanderthal approach, using a hammer to form the head of the rivet, with terrible results.

With hundreds of dollars and a multi-day delivery separating me from the proper solution to my immediate problem, and having a brand new TIG welder standing by, I decided to try something different, and weld the nut plates to the steel strips.

To make this work, I ended up having to abrade the steel strips and the nut plates to bare metal, and then cleaning everything with acetone. This treatment finally yielded a fairly stable arc, and a molten steel puddle that could be better controlled using less than 30 amps. Even then, the nut plates had the tendency to melt like butter, so it was pretty tricky welding.

Trying a different approach

Nut-plate welded. The bolt is a spare, used to keep the nut plate lined up with the hole during welding.

Right steel strip drilled and "nut plated"

One of the issues that still needed to be addressed at this point, was how to ensure that the whole strip would not rotate as the first bolt is tightened. You have to remember that the steel strip will be buried under some comparatively soft foam, which could give way under the bolt tightening torque loads. Two mounting solutions, and one operational consideration presented themselves. 

Operationally, it makes sense never to tighten just one bolt at a time, but to start at least two of them concurrently. As far as mounting solutions, I could drill a couple of blind holes from the outside and install screws into the wood holding the steel strips, or drill a multitude of lightening holes on the steel strips, and rely on flox to squeeze through, locking them in place.

Given that I didn’t want to drill any more holes into my longerons, I decided to drill the holes into the steel strips instead.

Coming up with a lightening hole pattern

Adding "lightness"

Swiss cheese steel plate

Left mounting bracket in place

Steel mounting bracket close up

Right mounting plate

Right mounting plate close up

A quick walk around the roll-bar


I had the nagging feeling that I was not too clear explaining why I drilled so many holes in the outer tabs, and the issue that could arise. So, I made a short video to illustrate the problem that I am trying to avoid.

Roll-bar tabs issue explation

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