Fairings & NACA Ducts

The first time I started making fairings, I used a block of blue foam which I tried to shape to fit.

Lots of work and questionable results..



So I decided to try X-30 pour-in-place polyurethane foam. You mix equal volumes of two liquids and it expands to a foam with 30 times the volume and a nice small cell size that is easily shape.

I used "poster board" (a very heavy weight paper) to create dams which I held in place with duct tape. I mixed the foam and poured it in and an hour later I could start the final shaping process. The poster board easily strips away, and what little sticks to the foam will sand off as you shape the foam..

A few things I learned along the way:

Unused portions of X30 should be stored in a refrigerator. It will keep several years. If stored at shop temperature (70-90 degrees F) it will become unusable in six months.

You have less than a minute to mix X30. Once poured do not try to move or spread it around. If you do the gas that is expanding the foam will escape and the cell size will not be uniform. You'll have to cut out the material and redo the pour.

Better to do multiple pours rather than one really big pour.

No matter how hard you try to seal up around the construction board dam, the X30 will leak out. It's not a big deal.

Shaping the foam is lots of fun! I really enjoyed this part of the project.

Tap Plastics has good prices on X30.



Once the foam is in place and shaped, I treat it just like any other foam; fill the surface with MICRO (epoxy and silica microballons), and put a protective layer of BID on it. I always use peelply over the lay up, and when everything is hard I strip away the peelply and use West System filler (epoxy and 410 microlight filler) which if far easier to sand smooth than MICRO.



Here is how I did the top of the wing.


Shortly after I completed the canard root fairing, Bob Farnam and his wife Jan, paid a visit. In the kindest way possible, Bob pointed out that my beautiful trail edge shape was an aerodynamic disaster. I had tucked the fairing in under the fuselage creating a constantly decreasing radius funnel like structure. Apparently this causes the air leaving the rear of the canard to accelerate, wasting energy.




So, once I had the fuselage inverted so I could install the speed brake, buildthe fairing under the canard and put the primer on the bottom, I extended the canard fairing. If you look closely at the photograph, you can see the outline of the original fairing just right of center. The extended fairing is a constantly increasing radius where the fairing joins the fuselage. Hopefully I understood Bob's critique, properly.


The fairing at the root of the vertical stabilizer was fun to make.

All sorts of tools are used to shape the foam. I often use hack saw or band saw blades to do the initial cut because they are flexible and cut the foam cleanly.

During the sanding phase, I use sections of the foam used to insulate water pipe. It's available in many diameters and I bought adhesive back sand paper that I would wrap around the pipe insulation. If you look carefully at some of the earlier photos, you'll see section of it laying around.


It took me a long time to decide what to do with the top (end) of the vertical stabilizer. I originally was going to leave it flat, but somewhere I read that this was not very aerodynamic.

When Philippe Blanc came to help on the project I asked him to create some kind of fairing.

In Philippe's mis-spent youth he had worked for Hobie the fiberglass sailboat company, so he had all the skills necessary.