Homepage/Builder's log for Cozy MKIV S/N: 1671

Ch 3 - Practice
Ch 4 - Fuse Bulkheads
Ch 5 - Fuse Sides
Ch 6 - Fuse Assy
Ch 7 - Fuse Exterior
Ch 8 - Headrests, Heat duct
Ch 9 - Main Landing Gear
Ch 10 - Canard
Ch 11 - Elevator
Ch 12 - Canard Install
Ch 13 - Nose Gear
Ch 14 - Centersection Spar
Ch 15 - Firewall
Ch 16 - Control System
Ch 17 - Trim
Ch 18 - Canopy
Ch 19 - Wings, Ailerons
Ch 20 - Winglets, Rudders
Ch 21 - Strakes, Fuel, Bags
Ch 22 - Electrical
Ch 23 - Engine
Ch 24 - Covers, Fairings
Ch 25 - Finishing
Ch 26 - Upholstery

Chapter 4 - Fuselage Bulkheads

Chapter time:                                                        Total Time:

There are seven bulkheads in the fuselage of the Cozy MKIV. 

The front seatback is also a structural bulkhead.  Since it is theoretically one of the simplest bulkheads to build, it is built first in the plans.  It is constructed of 3/4" H45 foam.  This was the first actual part for the aircraft I built, as all of the previous parts were for training and practice.  It took a little longer than I expected, since this part introduced a new technique and a new material, flox.  Flox is a mixture of epoxy and flocked cotton.  Much like micro, flox creates a paste, but unlike micro, flox can be used for structural joints, whereas micro is for prepping foam for fiberglass, and filling only.

Instrument panel foam cores cut and laid out on drawing to verify

Instrument panel bulkhead with wireways (rear side) nearly finished

The seatback bulkhead introduces the technique of creating a "flox corner" where two layups of fiberglass that are prepared at different times are joined at a corner over foam.  Rather than just epoxying the two layups together, a trough is cut out in the foam behind the existing layup.  The trough is filled with flox just prior to applying the second fiberglass layup.  This "flox corner" provides a more solid joint between the separately created layups.  While the concept is great, actually creating a flox corner is not straightforward. 



Top - seatback with one layer of glass on the bottom.  Bottom - flox corner after layer of glass on top (light bluish line near bottom edge is flox corner) prior to trim.

After researching the forums, I found some advice and decided to use a razor knife to cut a 45 degree angle into the foam towards the existing layup, then bring the knife along the back of the layup to cut out a triangle trough of foam.  The instructions say that after you remove the foam from behind the glass, you still have to rough up the glass.  If your micro bond worked like it should, I can't imagine why you would need to do this, because the foam does not come away from the layup well at all.  I would have guessed that one could just remove the bulk of the foam, and any little bits of micro stuck behind would actually help the flox adhere better. Ah, but these are not the rules, and micro is a no-no in any structural joints.  So I found that a Dremel 117 cone bit works really well to get the rest of the micro and foam off of the fiberglass, while also abrading the glass as prescribed.

F-28 Bulkhead weighted down by boards and blocks during cure, and after cure, untrimmed and still in plastic (right)

I decided to use MGS epoxy for this project.  Its odor is not too noxious, it is a clear liquid with low viscocity, and a three-part system with one resin and two hardeners, fast and slow that you can ratio to vary cure times from 15 minutes to 36 hours.  Directions call for "stippling" - a rapid straight up and down motion of a paintbrush to work the epoxy into the fiberglass cloth.  I was nearly finished before I discovered, and found to be the case with several others, that "stippling" does NOT work with MGS.  The low viscocity does not react well to that, and causes lots of air entrapment in the layup.  MGS is best applied very warm with a brushing motion, then allowed to "soak in", encouraged by light application by a heat gun to keep it thin till it saturates the cloth.  Follow this with some light squeegee action to ensure consistent volume, and results come good and fast.  This chapter took about eight months of on and off work due to a lot of frustration, and a couple of restarts, largely due to not knowing the characteristics of MGS prior to starting.  I am expecting the next chapter to move a lot faster.


Pictured above are the landing gear bulkheads curing under plastic after weights were removed.  The three of these bulkheads combine to form a very rigid and structurally strong three-dimensional structure to which the main landing gear is eventually attached.  These become one of the strongest parts of the aircraft.


The green square shaped areas are solid fiberglass/epoxy composite, with no foam.  22 layers of fiberglass to make these.  They are slightly thinner than the foam.  Doing it over I might have done 24 layers, but 22 layers was the specification, so two more layers are only adding weight to a part that the cosmetics of will never be seen once it is installed and covered with many more layers of fiberglass.


Some parts of the aircraft are constructed out of high quality spruce and fir woods that are encased in fiberglass and epoxy.  The furthest aft bulkhead, the firewall, is one such part.  The firewall took a different direction with the addition of wood, and also with aluminum inserts for engine mounts.

Aluminum insert prior to Alodine conversion coating process

Aluminum in most aircraft is usually treated through a conversion coating process or anodizing.  Alodine is a commonly used conversion coating.  The alodine is a powerful acid/chromium mixture that actually converts the surface of the aluminum into a very hard, gold colored alloy that is very corrosion resistant.  It also acts as a primer for paint or epoxy to bond to.  Prior to the Alodine, phosphoric acid is used to clean and etch the aluminum.  This stuff is the real deal, like cartoon-style acid that starts fizzing and burning stuff when it gets on it.  You have to handle both of these chemicals carefully, as they are both toxic and corrosive. 

Aluminum inserts after Alodine

Aluminum inserts after Alodine

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