Chapter 4 - Bulkheads

What's Up?

In this Chapter, we are going to build various cross members of the aircraft. They are the Instrument Panel, Seat Back, Landing Gear Bulkheads, Firewall and a couple front panels namely F22 and F28.  Since this is the first chapter, all the parts are flat and free of curvatures. This give us plenty of opportunity to get some hands on training. Most of these parts are non-critical parts - that's good, because I am not sure I know what I am doing yet. 

 

 

Tracing Out the Drawings

The first order of things in this chapter is to trace out the drawings for the bulkheads (M4-8). My glass dining table at home was perfect for this task. I placed a mechanics light on the floor (shining upwards), got a roll of Mylar from a local art supply store and started the tracing process. I followed the instructions by flipping the half drawings - left or right as required. The end result is a complete drawing of the bulkheads on Mylar. I preferred to have a complete view of the part (on drawing) before cutting the foam. This way, I can plan on the materials better especially when the dimensions are tight. You do not get much ‘excess materials’ from the kit – think FRUGAL! Supplied fir wood for the firewall later in this chapter is a good example. Now the building begins!

 

 

Transferring Drawings onto the Foam

I transferred the drawings onto the foam using a tracing wheel they use for sewing (I learned this from the Cozy forum & various web sites). Refer to tracing method. I tacked the drawings on the foam with thumbtacks and ran the tracing wheel along the lines on the drawing. The spokes on the wheel put a string of ‘indents’ on the foam. Surprisingly, the tracing wheel tracks a straight line quite well even in free hand. Remove the drawings and connect up the dots with a fine felt tip pen. Though the tracing wheel leaves a bunch of holes on the drawings, they are readable and can be reused – wonder how I know. Regardless, this method worked very well!

 

 

 

 

Front Seatback

This is the first REAL part since the practice lay-ups. The foam I got from Aircraft Spruce was large enough to accommodate the entire seatback and I did not have to glue two pieces together. I cut the foam with a hot wire except the center hole for the maps.

It worked especially well for the 45o angle cut. I placed the foam flat along the edge of my workbench. I then clamped an aluminum straight edge on top of the foam, parallel to the table edge and recessed .75”. Ran the hotwire along the entire length of the seat back – straddling along the table edge and the straight edge; a perfect 45o cut can be made. I did make a directional error on one of the side-angled cuts but was easily repaired. Getting mad at myself was more damaging than the repair work itself. The lay-up part was straightforward. 

Lesson learned – I gave up finding the right time to knife trim. Instead I used the FEIN tool and learned how to make a tight cut along the edges. I also learned to use the plastic peel ply method as discussed in detail on John Slade’s Web site – Thanks John!

Forward Bulkheads

The forward bulkheads consist of 3 pieces – the F22, F22-doubler and F28. The high-density foam was difficult to cut with a blade – especially with any precision around the curves. I ended up using the Dremel tool with the router bit. Instead of raising the foam above the table – such that the router bit will not gouge onto the tabletop, I did the following:

I set the depth of cut by placing a thin paper between the tabletop and the end of the router bit as shown. Once set, I routed along the traced lines. When completed, only a paper-thin foam is left connecting the bulkhead cutouts and the residual foam. Break off the bulkheads pieces as needed. Just a slight brush off with 100 grit sandpaper will result in smooth edges for the bulkheads.

 

 

 

 

Instrument Panel

This is the first part that I had to glue multiple foam pieces together using the hinged method. Surprisingly, this method was quite easy and worked well. The biggest challenge was to get symmetrical & consistent rounding inside corners. These corners also have different radii. I tried wrapping sand paper around various diameter wood dowels without great success – because I do not have that consistent ‘touch’ from one corner to another – as a professional carpenter. I ended up with transitional bumps between the rounded corners to the straight edges. Its’ probably OK, functionally. However, if I have to look at the imperfection every time I get on the plane, I’ll hate myself for not doing something about it early on… As a result, I had a set of angled templates with rounded corners made, resulting in a nicely rounded inside corner every time!

Making the vertical stiffeners was challenging. I followed the ‘plastic wrapped wood strip’ and BID tape recommendations by others. It was done, but the quality will definitely blow my Grand Championship Quality Award!

Lessons learned – BID tape and rounded corner templates.

 

 

Landing Gear Bulkheads

One thing to look out for is the correct type of foam to use. The foam supplied by Aircraft Spruce was not marked. I had to go through the process of elimination to get the right foam. Adding to the confusion, their ‘tan’ color medium density PVC foam was .274” thick instead of .25” per plan. I had to get assurances from several experienced builders before cutting the foam.

Due to the slight thickness of the foam and somewhat aggressive squeezing of the BID layers for the hard points, they ended up too thin. With some careful measuring and estimating, I ended up with 27 layers of BID cloth instead of the 22 layers per plan. I did get a nice flush between the hard points and the foam. You can see the peel ply that was partially trimmed and pulled back as well as the plastic peel ply.  

Oops #1 (I expect to have a few of these Oops down the path, now is a good time to catalog them) – Whatever I did, the quality of the upper forward bulkhead was poor. Post cure inspection revealed many dry spots and hairline streaks from its edges. The part looked fine after lay up and prior to cure. I posted the problem on Canard Aviation web site and received quite a few good pointers. I believe the probable contributors were aggressive squeezing and low temperature curing (~60F). Needless to say, I rebuilt the upper forward bulkhead with much improved results! Listening to others does pay… In addition, I built a localized heat tent to keep the parts warm throughout the curing process.   

Firewall

I traced out the drawings for the firewall as the other bulkheads and cut out both the temporary and permanent ones per plan. Once made, I clamped the firewalls together and performed the edge sanding and hole drilling together. This way, hardware positions on the temporary and permanent firewalls will be the same.

 

 

 

 

 

 

You're probably wondering what in the world are those round inserts – instead of the 1” square inserts per plan. Based on the Wayne Hicks’ and others web site, it was suggested that a larger insert (~1.25" sides square) might be worthwhile. Therefore, instead of the 1”x1” square inserts, I had 1.75” diameter (diagonal length of a 1.25" square) aluminum discs made in their place. Besides, it was easier to cut a round hole in the firewall (using a hole saw) for a nice clean fit.

As I pushed the MS24694-S54 screws into the blind holes, they felt quite snug. Therefore I did not do anything more than per plan. I ground a flat on each screw head, countersunk the holes and installed with flox. Waited for the flox to cure and glassed the forward sides.

 

 

 

 

Part Weight

Here are my part weights. Some still have peel ply and plastic peel ply on them. They may be an ounce or two heavier than eventual usable weight. However, it’s a good relative measure.

   

Lb.

Oz.

Front seat back   

5   

2.6

Forward bulkhead F22   

2   

3.0

Forward bulkhead F28   

0   

8.0

Instrument Panel   

3   

15.8

Forward landing gear bulkhead (upper)   

0   

13.0

Forward landing gear bulkhead (lower)   

1   

9.2

Aft landing gear bulkhead   

2   

14.8

Firewall top   

3   

5.4

Firewall left   

0   

5.0

Firewall right   

0   

4.8

Firewall bottom   

3   

11.0

Generally, my parts are slightly under the average. That’s good, but I won’t have the lightest Cozy… based on current projection – but its along way from over yet.

CHAAARGE … Chapter 5!