Wood/Carbon Hybrid booms are, in my opinion at least, the strongest booms you can get for the weight. When I decided to start selling the TITAN tricopter kit, it was always my intention to offer Hybrid booms as an option. However, when I started trying to make them in large quantities it became obvious that it was going to be too time consuming and I wouldn’t be able to make them for a reasonable price. The good news is that it is pretty easy, and really cheap to modify your booms yourself; you just need a bit of time.
First off, why bother? Carbon fiber is tough, right? Well, yes; in the right circumstances carbon fiber is one of the strongest materials in the world. But in other situations it’s downright weak. It comes down to tension vs. compression; carbon fiber has extremely high tensile strength but it has zero compressive strength. “HANG ON!” you say, “Carbon fiber is ridged; it HAS to have compressive strength!” well, what you are thinking of is carbon reinforced resin, the actual fibers of carbon are just, well, fibers. What makes your carbon booms ridged is the resin that holds the fibers together; the resin is the weak link that we are trying to reinforce. The hollow, square carbon booms most commonly used in multirotors are most susceptible to crushing or twisting damage, we are going to fill the hollow center of the boom with wood so it can’t be crushed, and it will give it much greater resistance to twisting as well. This only gives a marginal increase in overall weight, but it increases the strength of the boom many times over. So let’s get started!
- 10mm square pultruded carbon fiber rod with an 8.5mm round hole (this is the standard size, I am using the pre-cut booms available from Fortis Airframes)
- Gorilla Glue, use the standard polyurethane Gorilla Glue, NOT Gorilla wood glue. You can find it at most hardware and home improvement stores. You will only need a small bottle; it should be around $5.
- 5/16” poplar wood dowel, you will need a piece about 4” longer than each boom you want to reinforce. You should also be able to find this at a hardware or home improvement store; a 4ft long piece should be about a dollar.
- A fine-tooth wood saw
- A drill and a drill bit (I’m using a 7/64” drill bit)
- Sand paper (220 grit is good)
- Paper towels or shop rags
- Masking tape
- Latex gloves are nice, but not required
If you are not using pre-cut booms, cut them to the desired length first. I am using 13” pre-cut booms from Fortis Airframes.
Cut the 5/16” dowel about 3”-4” longer than your booms, since my booms are 13” I am cutting the dowels to 16” or 17” long.
Take a wad of paper towel and push it through the boom to remove any dust or dirt. You can moisten the towel with water, but I am using rubbing alcohol to make sure it’s extra clean. If your wood dowel is clean you can use that to push the paper towel through; repeat this several times so they’re spotless.
Wrap masking tape around both ends of the carbon boom; leave a little hanging off the end to catch excess glue. Make sure there are no wrinkles and that the tape is sealed well all around, the dried glue is a pain to remove. It may also be a good idea to wrap the rest of the boom in newspaper so you don’t accidentally get any glue on the outside.
Lightly sand the wood dowel with sand paper to clean it up if necessary; then wipe it down with a damp rag. It is important for the glue that the dowel is not dry. Gorilla Glue is a moisture-cured urethane, which means it is activated by the humidity in the air; the problem is that the center of the boom is so far away from the open air that it may not get enough moisture to cure, if you put a little moisture in the wood then this is not a problem.
Steps 6, 7, & 8 need to happen quickly: Use a paper towel to apply a liberal amount of glue to the entire length of the dowel…
…Then put a little bit of glue in the end of the carbon boom…
…Now slide the dowel smoothly into the boom while rotating it to get a good even coat of glue between the two. Keep it upright, and keep an eye on the end because the excess glue could spill out and make a mess if you’re not ready for it.
Once you have an inch or two of the dowel sticking out each end of the boom, you can set it aside to dry. Be sure to set your boom on a piece of scrap cardboard so the glue doesn’t stick to your workbench. You’ll see the glue start to work rapidly; it will foam up and expand. This is why Gorilla Glue is so great; it will expand to fill any voids so you can be confident you have 100% glue contact between the wood and the carbon.
After 1-2 hours, the glue will be dry enough to finish the process. First, remove the masking tape.
Now, use a fine-tooth saw to cut the dowel flush with the end of the boom.
Use the sand paper to sand the end of the dowel even with the end of the boom. After cleaning the end of the boom with a moist rag, I usually use a black marker to hide the color of the wood.
Use a drill to make a hole in the dowel, the Fortis Airframes booms are pre-drilled so all you need to do is line up the existing holes. Since these will be used on a TITAN tricopter, I will be drilling a hole for a #4 screw; I use a 7/64” drill bit which is a very snug fit, you can use a 1/8” bit if you prefer a bit more clearance.
That’s it! Now you’ve got booms with a strength-to-weight ratio that can’t be beat!
Check out the TITAN Tricopter frame kit here!