Composite kit Aircraft

Aircraft built from composite materials have the advantage that parts usually weigh less and every conceivable form can be made with these materials, unlike aluminum. Sleek looking aircraft like Cirrus Design, Diamond Aircraft and DynAero are a good illustration and a large part of these aircraft are made from composite structures.

Other aircraft manufacturers apply composites, but only for certain small parts. For example: wheel pants, cowlings, wing tips and sometimes even larger sections like fuselages.

Composites are being used since the 50s of the last century, mainly in marine applications like boats, and even Boeing used 2 % fiberglass in their original 707 aircraft. Their latest aircraft, the 787 Dreamliner is large constructed of carbon fiber materials.

Composites are not for everyone, personally I prefer aluminum and try to keep as far away from carbon or other composites as possible. Its very dirty work but someone (you) has to do it!

For some parts of the aircraft, mostly curvy and rounded, you almost can not avoid these 'new' materials. So if you want to get your hands and the aircraft dirty, do read on.

Fiberglass Materials

To create a structure from fiberglass or carbon fiber you will first need to construct a negative mold where the fiber cloths will be layed up, after which resin will be applied for cure. A woven cloth has enough play diagonally, enabling you to drape it around most forms. Should these forms become too complex you will need to use smaller pieces of cloth.
You will also need a batch of brushes of several sizes, paint rollers and a lot of them as they will harden too and become useless.

Cutting fiberglass can be done on a right angle with the weave for normal lay-up, but be sure to cut it on an angle when you need to wrap around sharp corners. It helps to use a pair of scissors with teeth, they grab on to the cloth and cut very nicely.

Keep your fiberglass as fee as possible from contamination as the resin will not bond properly and the structure will not have the required strength. Delamination due to ingress of humidity may also occur later on if the cloth is not properly wetted with resin during construction.

For a composite structure like an engine cowling to be strong enough you will need several layers of cloth, experience (and advise from other aircraft builders) will tell how much layers you are going to need, expect at least three to five for any strength.

Resin

There are several resins types that can be used with composites. Generally, use the one that your aircraft kit manufacturer advises. Some examples are: vinyl-ester, polyester and epoxy resins. Epoxy being more expensive, will not react on most materials used in aircraft construction. But it might have a negative effect on your body, epoxies can be toxic.

Polyester resin dissolves some foam like materials as easily as warm water melts sugar. It hardens quickly when mixed when MEKP catalyst is added in the right amount by weight.

Vinyl-ester is a relatively newcomer and is mainly used because of the aggressive allergic problems some people experienced with the other epoxy resins.

The right choice of resin also depends on the application. If you are building fuel tanks then polyester resin might not be the best choice as it will slowly be dissolved by the alcohol in the fuel. Either use vinyl-ester resins, aluminum tanks or stay away from fuels with alcohol.

Cure temperature

Resin cures best when the temperature is higher than 75°F (24°C), that might be too warm for us to work in comfortably. The lower the ambient temperature gets the more catalyst needs to be added for the cure process to start. Look for the precise limitations on your batch of resin/catalyst. When the temperature gets too low, the resin will not react and curing might not happen at all.

Its really helpful to prepare a test sample to see how the resin will setup and cure in your environment. This gives you an idea on how much catalyst you will need, this depends not only on temperature but relative humidity is a factor too.

Workplace

Keep your workplace well lighted, good ventilated and always use protective clothing, gloves and use a face mask. These small fibers can damage skin and possible your lungs too. Some type of catalyst are very dangerous for skin and eyes.