Make your own carbon-fiber landing gear

Model Airplane News, Feb 2000 by Syme, Thayer

Lightweight, sturdy and simple

FROM TIME TO TIME, a model comes along that does not lend itself to landing gear made with traditional materials. Especially with the very popular slow and park flyers, weight is critical. This prevents you from using heavy anchor blocks for torque wire, multiple pieces of braced wire, or sheet aluminum. When confronted with a situation like this, custom landing gear made out of carbon fiber and epoxy makes a lot of sense. The use of composites is uncommon for landing gear. This is a shame, as it is quite possible to make stronger and lighter landing gear with these materials. Let's look at a typical park flyer to trace the steps of making and installing carbonfiber landing gear.

The Diversity Aircraft* Dragonfly is a 48-inch-span, 450-square-inch foam-wing park flyer that has a direct drive S-400 for power. Its flying weight is 15 ounces. The original landing gear was made out of music wire sandwiched between two layers of plywood in the motor mount. The gear would often split the mount because of the inevitable bumps and bruises of landings while pilots learned how to fly. The gear's excessive flexibility also allowed the wings to drag while landing Or taxiing.

To avoid these problems, I decided to try a new set of vacuum-bagged, carbon-- fiber gear and mount it on the carbon-- fiber fuselage backbone and plywood frame. Mounting the new landing gear on the fuselage instead of on the motor mount helps stabilize things, but it also changes the plane's balance and ground handling. I will look at all of these concerns as I complete the project. Vacuum-- bagging eliminates the need for any excess resin, making the part even lighter and stronger.

THE MOLD

Make a very simple mold using a piece of scrap pine 2x4. Keeping the intended track and height of the landing gear in mind, I made my mold 3 inches tall and 8 inches wide. I cut the edges of the block by making a gentle arc from the midpoint on the top to the bottom on each side. After sanding the block, I covered it with MonoKote*, which does an excellent job of sealing the mold's surface and also acts as a release agent for the carbon fiber. Even without wax on the MonoKote, the finished product was released with little trouble.

You should prepare everything before you mix your epoxy. Clean up your workspace and cover the bench with cardboard or plastic. Measure the total length of the landing gear going around the curved surface of the mold, add a little length to be safe, and then cut all the carbon fiber that you'll need for the layup. Decide how much material it will take for the project, either by analysis or by trial and error.

After you've made some carbon-fiber parts, you will be able to estimate what you will need without doing either. I decided on three layers of cloth cut into 1-- inch strips. While cutting the cloth, also cut a few narrow 1/2- to 1/4-inch-long scraps that you'll apply "cross-grain" near the axles and at the center; this will increase the strength of these high-stress areas. Be careful: any loose particles of carbon fiber will float around the workshop and create a hazardous environment. A respirator and a fan are very good ideas.

When all the carbon has been cut, dispense one pump each of resin and hardener into a plastic yogurt cup. Mix them thoroughly for about a minute.

Work with just one layer at a time when you lay up the landing gear. First, place the bottom layer of carbon on the curved surface of the mold and apply some epoxy. With the disposable brush, carefully work the resin into the fibers using a stippling or poking motion. Eliminate any air bubbles or voids. Be careful to keep the fibers straight, and fully saturate them with resin. It is OK to use a little excess epoxy at this point because it can be worked into the upper layers as you go.

Now place the next layer of carbon on the form, and work any excess resin from the first layer up into it. If necessary, add more resin to fully saturate the new layer. Continue to add your layers in this manner, gently stippling the resin thoroughly into the fibers. Reduce the amount of resin you add as you get near the end of the layup process; when the fibers are saturated, any additional resin only adds weight and reduces strength. Vacuum-bagging helps to remove any remaining excess resin as well. When the top layer is saturated, apply the cross-grain fibers at the axle mounts and the center. If you are not planning to vacuum-bag the gear, set the mold aside and let the resin cure overnight.

VACUUM-BAGGING

It is best to vacuum-bag the landing gear to achieve the optimum resin-to-fiber ratio. In addition to the vacuum pump, you will also need Peel-Ply*, bleeder material, some sort of plastic film and window-- caulking putty. Peel-Ply is a Teflon-coated nylon fabric that allows excess resin to migrate out of the layup and into the bleeder material. In addition to absorbing excess resin, the bleeder material helps get all the air out of the sealed system. The air can easily flow in and around the fibers of this material and escape. Without the bleeder, one area of the mold can be sealed off from the rest, and this will prevent the whole mold from being in a vacuum. For small parts like this landing gear, you can just use a couple of layers of paper towels for the bleeder material. To seal a small, hard mold like this, it is easiest to use a plastic bag; in this case, a gallon-size Ziploc freezer bag worked well.