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Building The U. Maine Trainer

- Reinforcing the fuselage -

(last update: 15 Sep 2012)

A foam fuselage is not particularly strong, so it needs some reinforcement.

From previous work, a piece of fiberglass cloth had been cut out for fuselage reinforcement. Now is as good a time as any to get that cloth wetted to the fuselage.

BE SURE TO USE GLOVES WHEN WORKING WITH FIBERGLASS AND EPOXY.

The cloth we have been using is 2.0 oz cloth. Which is fine. However, it is likely overkill in this application. It might be better to find some 0.6 oz to 1.0 oz cloth instead. The construction would be a bit lighter overall, and more glass could be used over a wider surface, which would greatly enhance the stiffening of the overall structure.

But to get the fiberglass cloth, regardless of weight, to stick to the fuselage, some epoxy will be needed.

It is best not to use cheap 5 minute epoxy for this. First of all the working time will be way too short. The epoxy will begin to harden long before the cloth has been applied. Secondly this type of epoxy is too thick. It does not wet the cloth or the foam surface well. The heavy viscosity also tends to make thick heavy layers of epoxy, thus making the resulting structure unnecessarily heavy.

No... this is a time for a higher quality product designed for the job. It is thinner, has a longer working time, and will bond better. Once again we will be using WestSystem products...

Two sacrificial cups are marked at equal heights. One cup is for hardener, one is for resin. This will not be a 1:1 mix as used with typical 5 minute epoxy. WestSystem products should be used/mixed according to their instructions. Typically a 5:1 ratio of resin to hardener is used. In this case that means 5 parts of "105" resin, and 1 part "205" hardener. For longer working times the "206" hardener can be used in the same ratio.

Measuring/mixing epoxy.

The idea is to use one cup five times (105 Resin), and the other cup once (205 Hardener). Pouring and mixing into a separate sacrificial container. The plastic cups shown above are one possibility. Often paper cups are used. If the paper cup is too deep, just cut some of it away.

Mixing epoxy.

The tub in the photo above has been used for mixing epoxy many times...

Wet the bottom, and part way up the fuselage first. It just needs to be damp with epoxy not dripping.

Wetting the fuselage.

Now try and center the reinforcing glass on the bottom of the fuselage...

Start laying the glass.

Then it is a matter of working the glass flat, and tamping it down with as little additional epoxy as you can manage...

Tamp down the bottom side.

... and working down the sides...

Tamp down each side next.

Folding down starboard side.

... make sure there are no air bubbles or lifts...

Touch up the application.

Wrap the glass around the tail as best you can. But don't obsess over it. Some of the cured glass/epoxy and or foam will be trimmed away later.

Here is a shot of what it looks like up front. Note that the "fingers" will be cut away and sanded. Good cloth covers the edges of the wooden firewall.

Fiberglass and firewall interface.

Unfortunately the 2.0 oz glass does not like to lie flat against the firewall edges. This is due in part to the stiffness of the glass cloth, and also to the compound angles of the firewall edges. There are ways to get around this. However, for the purposes of this aircraft, the areas not attached will be either filled with epoxy after this layup cures or will be sanded back until they are gone. After that a small patch of glass can be applied to make up for the loss, if so desired.

Smooth both sides with wax paper.

In the above photo a second sheet of wax paper is applied, and both sides are gently smoothed to eliminate bubbles and lifts. At this point the epoxy is beginning to "kick" (harden)...

Hanging to cure.

In the above photo the fuselage is hanging from a pipe, while it cures. It would have been better to tape the wax paper closed on the bottom (the fuselage top - it is hanging inverted), against the force of gravity. However, at this point the epoxy had already begun to harden, and there didn't seem to be much point. The tape wrapping around the nose is there to encourage those edges of the firewall to have glass lie flat against them, while the epoxy cures.

Not much more we can do now... best to wait a few hours for the epoxy to set well.