Ch. 3 - Education - Foam cutting

Hot-wiring

Thus far, we have been working with flat, and relatively thin pieces of foam. We have had to do some minor shaping with a knife, and sandpaper, but have not had to deal with the thick foam blocks that will become wings, winglets, canard, and control surfaces. 

As you might imagine, these aerodynamically significant parts require a level of accuracy that is beyond what is possible working with such primitive tools. To our benefit, the designer has once again devised a simple yet ingenious method of achieving the precise shaping of these parts, with inexpensive home-made tools.

It turns out that the foam selected for this purpose (PVC) can be easily cut using a hot metal wire (nichrome). What is even more significant from a health standpoint, is that it does so safely. This is an important point to be made, because Urethane foam will also cut with a hot-wire, but will release hazardous gasses in the process.

Hot-wiring PVC = Good

Hot-wiring Urethane = BAD!!!

The hot-wire tool consists of a 2”x4” piece of wood 3 to 6 feet long, through which two holes are drilled, and two short pipes inserted. The wire stretches from the ends of the pipes, and can be tightened by twisting one pipe. Clamps attached to the base of the pipes provide electricity to the wire, which heats up and is then used to cut the foam. The power supply can also be homebuilt using a transformer and a dimmer.

Hot-wire cutter sketch

My setup for cutting small blocks of foam

So, we have a hot-wire. Now what?

More importantly, the hot-wire will enable us to make precise cuts of what?

This is where Burt Rutan provides his expertise in the form of paper templates. These templates need to be reproduced accurately onto some stronger material (usually wood laminate). 

Canard template (not to scale)

They later get nailed at both ends of a block of foam, and the hot-wire is made to slide over the profile of the template, thus yielding our airplane part.

Canard templates (one on each side of the foam) nailed to foam block

One benefit of this method is that advantageous features, such as wing taper, twist, and smooth profile transition, can easily be reproduced to improve the aerodynamic efficiency of the part, in a way that is almost impossible to duplicate with any other form of airplane-building, at least on a budget and in a non-factory setting.

To prove this point I will share a video of my wife Gina, and my friend Mike, cutting a block of foam with a hot-wire. The width of the piece is not important, as this was just an exercise, yet the profile is an accurate reproduction of the Long EZ canard because they used the actual canard templates.

Worthy of mention, is the fact that Gina had never seen this process done before, and was blissfully watching TV before being summoned in the shop with a ruse. She had no previous knowledge, and received no instructions besides what you see on the video, and all she wanted was to get back to her show. Against all these odds, I have to admit that she did a great job, and showed us all that hot-wiring is not as difficult as you might imagine.

Gina and Mike cutting foam with a hot-wire cutter

Top foam removed

Gina seems ready to go fly

Ch. 4 - Instrument panel - Part 2

Instrument panel (4.3 hrs)

The construction manual encourages the use of leftover BID pieces on the forward side of the instrument panel (with proper 1” overlap, of course). It also states that BID orientation is not important.

I decided to use up some of my scrap BID, and continue laying it at a 45˚ bias (main fiber orientation).

Testing scraps of bid for fit over foam panel

After designing my fiberglass pattern (above) with plenty of overlap, I carefully rolled the pieces, and laid them aside in the order they would be used.

Fiberglass ready to go

With the foam panel properly covered in slurry, I placed the BID in accordance with my schedule, and added epoxy.

This is the fun part of the construction, but also incredibly messy if one is not careful. As you can see, plastic sheeting on the table is a must, as the warm epoxy is very runny.

With the first piece in position, and properly saturated, on went the second piece...

... and the third, and the fourth.

As I’ve done previously with all second layers, I elected to cut a whole new piece of BID to cover the entire panel.

At about this time my good neighbor Lou showed up, and was promptly put to work. Epoxy mixing duty at first, but later he got some hands-on time laying the third ply. This ply covers only the top half of the panel, and since Lou did such a good job there, he was also drafted to peel-ply where needed.

Mixing some epoxy for starving peel-ply

The instrument panel backside turned out quite nicely.

The best part is that I might have found a capable assistant who only lives two doors down from me.

I’ll be knocking on your door soon Lou, thanks for your help.

Ch. 4 - Instrument panel - Part 1

Instrument panel (4.5 hrs)

In addition to going with a 2" wider cabin,  I will raise the instrument panel by 1" in preparation for an extended nose, this will allow for a smoother curvature when looked at a 45˚ angle. All of this is pretty straight forward at this point, just push the sides out and up by 1". Conversely, adjustments to the layout of the panel components necessitate some strategic thinking.

I never had an issue with the size of the Long EZ seat, but I did with the elbow room, so I planned to enlarge the side consoles/armrests while keeping the flight controls in the same position relative to the aircraft centerline.

One additional item that just nagged at me, is the unequal size of the legs cutouts, which is directly related to the consoles width. 

Typical Long EZ leg cutouts

Due to the change in console width, a repositioning of the leg cutouts would also be necessary, and presented an opportunity to rethink this area.

My experience with the Long EZ mockup revealed the right leg cutout to be adequate, and the additional size of the left one not necessary. My decision therefore was to shrink the left cutout to match the size of the right one, and use the added space in the center post for a standby instrument, or two.

I have to admit to having been a bit apprehensive about this step, mostly because I had not yet come up with a good idea on how to precisely transfer the pattern to the foam. Up until now, my bulkheads consisted in reproducing measured dimensions to foam, not transferring patterns.

I decided to design my modified panel first, then figure out where to go from there. The quickest way to do this was to use tracing paper over the original design.  

Tracing paper over bottom half of instrument panel template

Shifting the paper repeatedly, I traced each design element into the desired positions, and pretty soon I had a new wider panel, complete with identical leg cutouts.

Modified panel

The system I came up with to transfer the design from paper to foam, consisted of laying the tracing paper (with the drawing on it) over the foam, punching holes into both of them in strategic locations with a paperclip, removing the paper, and connecting the holes with a pen.

Punching holes through paper and foam

Holes in the foam

Connecting holes with pen

Adding rounded corners

It worked spectacularly, and as long as you punched the hole precisely, the resulting foam pattern would be just as precise.

I knew all those high school technical drawing classes would come in handy one day

After a quick trim, here’s the revised instrument panel:

Before proceeding to the glassing stage, I had to insert a piece of plywood into the foam for the fuel selector to attach to. This gets buried into the fiberglass, and will provide a solid anchor (compared to foam) for the screws to bite into.

The next step will involve epoxy, but before then I’ll take a moment to dream...