Disclaimer

This blog is for entertainment purposes only, and is not meant to teach you how to build anything. The author is not responsible for any accident, injury, or loss that occurs as a result of reading this blog. Read this blog at your own risk.

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



UPDATE

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



Tuesday, August 20, 2013

Main landing gear - part 2


“Wrapping things up” (6.8 hrs)

Last time, I wrapped the landing gear bow from the trailing edge forward, this time I will wrap it in the opposite direction. These layers of UNI cut at 30˚ add torsional stiffness to the gear bow.


Cutting more UNI

4 + 4 plies ready to go


Once again, I elected to pre-preg the UNI to make it easier to apply to the gear bow.


Pre-pregging

Pre-pregged UNI being applied to right leg (up is fwd)

Plastic wrapping being removed

UNI smoothed out by hand.


Later, I peel-plied the trailing edge, where an additional layup will be done to house the brake lines.


Trailing edge peel-plied


With the right leg completed, I repeated the steps for the left leg.


More pre-pregged UNI for left leg

Pre-pregged UNI applied to leading edge of left leg (up is fwd)

Gear bow curing after having peel-ply applied



In addition to pee-plying the trailing edge, I elected to peel-ply near the middle of the gear bow, where the gear tabs will go. These tabs will physically connect the gear bow to the fuselage, and will be made up of 72 layers of fiberglass. As you can imagine they will be the thickest layup on the plane. 


Trimming the trailing edge after curing overnight

Gear bow done, ready for fiberglass tabs.




Monday, August 19, 2013

Ch. 8 - Rollover structure - part 3

Roll-bar fabrication in action (16.0 hrs)

I have been both looking forward to and dreading this part, because this is where the time and money I invested in acquiring my new welding skills is supposed to pay off (or not), but before I test myself against the chromoly steel, I still had a little more cutting to do, namely the L channels that will clamp onto the bottom of the longerons, and the roll-bar cross-members.



Cutting metal this way is VERY dangerous.  DON'T DO IT!  Read my previous admonitions against doing it, here.


With the top and bottom L channels cut and trimmed, I clamped them to the longerons in order to record some precise measurements, before cutting the round section stock for the cross-members.


Front left

Rear left

Top and bottom L channels in position


Then, I cut the cross-members and checked the fit.


Front crossmember joint

Test fitting the basic structure before tacking


Satisfied with the way they looked and fit, I tacked them in place, locking in the geometry.


Tacked rear crossmember


I found that I could only weld one tack at a time before the metal got too hot for the fiberglass, so I alternated the tacks from one side of the fuselage to the other, waiting for the metal to cool before adding more. This kept me from damaging the structure, while allowing for a very tight fitting metal structure.


Basic frame tacked together


Next, I measured ad cut the vertical structure...


Checking for size

Paper templates of cutting profile

Test fitting after cutting


... and tacked that in place as well...


A few tacks to hold the top structure

Tacking the right side of the vertical piece lifted the left joint 1/4". Here I am pulling it back down for tacking

Vertical structure tack welded

Profile view of the same thing


I did the same thing with the two diagonal braces...


Readying the rear braces

Test fitting the rear braces

Different view of the same thing


... except now the setup was sturdy enough that I could take it to my table for tacking.


Roll bar assembly brought to the welding table to tack weld the rear braces

Getting the last tacks in before the real welding begins


With the structure secured with multiple tacks, it was finally TIG time!


Going for it!


TIG welding was challenging, and fun, and seeing the roll-bar come together before your very own eyes was priceless.






Using aluminum foil as an Argon dam for welding in tight corner

Dam is useful due to the very long tungsten stick-out needed to reach into the tight corner


There is however one weld I dislike. For some reason it got built up more than I thought it had when I was under the hood. It’s not that it isn’t safe, it’s actually stronger than the rest of them, I just hate the way it looks. I thought about grinding if off and welding the joint again, but I ran out of Argon just as I ran out of things to weld, and... I guess I’m just starting to get used to it.

Anyway, there is no real reason to rush to modify it, I can always do it later, as long as I do it before powder coating.


There had to be one ugly weld somewhere, *#%*!!!


To prevent the freshly sanded and welded metal from rusting, I sprayed gray primer over it, which will be sandblasted away before final painting, maybe a few years down the road.


Priming to prevent rust






Roll-bar back on the plane





That should do it!


Next on my list, will be mounting the roll-bar solidly to the fuselage. This is going to be an involved process with many small steps to be taken in the appropriate sequence. To give you a better idea of what's involved in finishing the roll-bar mounting, here's the checklist I created for this process...



Things to do to mount the roll-bar and close up the fuselage depression

There are still a few more welds to be done to the roll-bar in the future, like the headrest mounting tabs, the shoulder harnesses attach points, and the canopy stay mount, but those are small potatoes compared to what I was able to accomplish today.

I am very happy about the way my roll-bar turned out, the welds are my very best ones to date (all except one), and I’ve had a great time finishing this side project.