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Q-talk 91 - FUN WITH FOAM: X-40

Jim Masal Carrollton, TX

Ironically for a guy who is death on Q mods, I find myself assisting the rebuild of a very seriously modified Q2. As I have always said, to do so requires deep pockets and inordinate amounts of time. And I'm not wrong in this case.

This aircraft was successfully built and flown as a Q2. The owner has flown many hours in this aircraft, but has found its performance to be much less than thrilling. This mod is a typical drive for more speed and better aerodynamic performance.

Two items on the agenda in pursuit of speed are 1) a re-shaping of the rather large wheel pants into a more pleasing (and less draggy) shape, and 2) Installation of a 6 cylinder Jabiru engine. I want to spend some time on this latter endeavor because of a great deal of activity out in Q2 land over the use of alternative engines.

An engine change often requires 2 challenging components. The design and fabrication of a suitable and sturdy engine mount is the first. Next is fabricating a cowling. Both of these items can be very time consuming to get exactly right. We believe we have significantly reduced cowling construction time by the use of X-40 foam available from Aircraft Spruce.

X-40 is a two part foam that when mixed in approximately equal parts will expand to 30 times its original volume in about 30 minutes. It is an easily sandable urethane foam. Sanding and contouring can begin in less than an hour. X-40 may seem expensive enough that some would be tempted to use the common lumber store expanding foam aerosol cans, but it is not as suitable. Aerosol foams take much longer to cure and may outgas or "grow" for days at a barely noticeable rate. The texture is not as uniform in density as X-40, and aerosols are "aggressive" in that they tend to resist or push against their containers. X-40 can be contained within a hastily fabricated cardboard dam with only a very small tendency to push out the sides, and a major push to grow toward the opening of the dam. This feature allows you to play fast and loose with some pretty flimsy cardboard dam materials.

What we did to cowl the Jabiru was to first attach a plywood disc to the front of the engine representing the spinner. The area between the perimeter of the firewall and the perimeter of the spinner would be filled with foam then carved to shape. This cowling would have round "nostrils" (air intakes) so we tacked a horizontal board to the spinner so we could hot glue or tape short sections of a packing tube in a pre-determined equidistant position right and left of the spinner. The tube diameter was an educated guess.

Next, the entire engine was wrapped with large plastic garbage bags or other plastic film. We were generous and careful; we didn't want to be scraping foam off of these parts later. Packaging warehouses have a stretchy saran-type plastic on a roll used to seal cardboard pallet boxes. We used this but slick package tape works well, too. We protected the firewall with plastic as well.

To assure clearance between the cowl and protruding engine parts, we had some 3A" blue Styrofoam planks that we taped around the engine and over any part or component that stuck out. This would give up a sanding "marker". Thus while sanding the brown urethane we would stop whenever blue foam appeared and reconsider our contouring.

Once everything was protected, we found a large piece of cardboard, crimped it lengthwise in a brake about every 4 inches, so that it would take a curved shape. Then we cut it to a size that would run from halfway up one side of the firewall, around underneath and halfway up the other side. This was taped securely to the firewall. The other end has to be secured to the spinner area so we had to get creative in how to make the cardboard neck down. We were generous in building this "dam" as it is easier to cut off excess foam than to add more later... though adding more is not all that difficult. The foam will stick to the cardboard so it is helpful to line the dam with waxed paper or a garbage bag.

At this point the contouring effort could be helped by creating ribs from firewall to spinner on top and bottom and one on the side which would serve as "limits" for the general shape of the cowl that is envisioned X-40 comes in 2 parts which are mixed in equal volumes...by eyeball is sufficient. It is not a fussy material and is adequately mixed by hand for about 30 seconds. Once it "kicks," it is a fast mover. It is very fluid at the outset and will pour through any weak spot in an improperly sealed dam. Best bet is to make the first pour a small one so that any bad sealing can be held by hand until the foam expands, firms up and seals the spot for the next pour. Expanding foam can be moved around with utensils to a small extent, but it really doesn't like to be disturbed. We pour successive 8 oz. batches so we can direct the expansion into needed areas.

In about 30-40 minutes, X-40 can be ready to cut into rough shape with a hacksaw blade or other sawing tools. First we had to remove our dam material and do a couple smaller damn-pours to fill void areas that appeared.

Once the X-40 is well cured (about an hour) it was time to bring out our hidden Michelangelo talents and remove any foam that didn't look like cowling. For us, this was the fun part. If we cut too deep or needed to change a contour, we just did another dam-pour.

In contouring, we found it is best to do one side at a time. Once the right side looked good, we could cut a scrap cardboard template of a spot, flip it over and recreate it on the left side with sanding blocks, knives, files etc. While fun to do, a certain fussiness is required, too. Complication: The fuselage tool at the factory did not perfectly duplicate the curves on the right and left sides. Therefore a template from our right side did not exactly produce what our artistic eyes wanted to see on the left side. This is where individual artistry must be exercised.

As the foam shaping took place, we began to sand down to our blue foam clearance markers. These were stop points which caused us to consider, artistically, what a pleasing shape of the cowling curve could be fore and aft of that point, and, mechanically, how much clearance we needed.

In the foam stage, we got the cowling shape very close to the desirable end result. Next, it was time to protect the foam and do any detail finishing of the shape.

To be continued in the next issue of Q-Talk...

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