Heart of Glass

[Toro]

Glassing a motor box is an easy way to add strength, and fuel proofing. I have glassed motor boxes for most all of my big airplanes, as I fly in a style not totally conducive to floating around, and weight has never been a real issue for the altitude I fly at. It’s very simple to do, and I thought for those guys that have never done this, a little photo how to would be an easy way to get your feet wet.

The first thing I like to do, is throw a light sanding over the whole wood structure to be glassed with some 220/400. Just enough to give it some tooth, allow for the grain in the wood to open up, and allow the resin to permeate the pores. I will then wipe off any dust with a tack cloth, or slightly damp rag to get any large dust particles off the wood itself. You want the motor box to be completely dust free as you will see why in a bit.

I have found it easiest to cut your glass as you go. This allows the ability to make cuts to fit, as you need them. Remember as you cut you’ll need to leave overlap just like monokoting. I typically try to really overlap the seams where the wood joints are. This gives an added measure of strength at the joint. This combined with dowels into the wood will give you an almost bulletproof setup. As I cut the fiber glass (typically on motorboxes I use a heavy glass with large weave) I use a SUPER SHARP exacto blade, and will replace the blade after about 3 cuts. A sharp blade will prevent the cloth from wicking (making lots of wickers IE THREADS). This will leave you with a nice clean edge to work with. After I cut the cloth to size I then proceed to spray a liberal amount of super 77 on the glass itself to prevent it from riding up when I put the resin on. Allow the 77 to dry and put the cloth on the area to be glassed. The good thing about only applying the 77 to the cloth is it allows you to pick up the cloth if needed and move it around. I proceed to cover the whole motor box in cloth, and make sure all my joints lay down without any of them wanting to raise up. If one of the joints tends to rise up, spray a little 77 on it, and push it back down.

After your motorbox has been covered in the areas to glass, its time to get the resin out and its time to “WET” out your glass. The big thing to remember while wetting the glass is more glue doesn’t = more strength. In fact the best amount of resin is just enough to completely wet the fiber out, and not allow puddling. I like to use West System epoxy system with a slow cure catalyst. I have found this product to give excellent pot life, and workability, but also give a solid cure. FACTOID: did you know that epoxy/polyester resins never really finish curing? It continues to oxidize forever, and it is this that can cause epoxy joints to become brittle over time. Especially if exposed to UV. My airplanes typically suffer from gravity storms before the epoxy degrades to such a point.

Anyways, mix your resin to manufacturer specs. Typically 4:1 or less. Pour the resin on to the glass and squeegee out the resin to cover the glass, and take out the “WHITE” look of the glass. White glass is dry glass, and offers no strength. So make sure your glass takes on that transparent look when wetting out. Set out some paper on the ground and squeegee your heart out. Get that glass nice and wet, but remember “NO PUDDLES”. After you get the glass totally wet, and you are satisfied with your results, its time to hit the yard work, or bed, or whatever you have to do for the day. It typically takes 8 hours or greater for the resin to cure, and provide a handleability. After that you may do whatever, prime, paint, throw another layer of fine glass on it to make it really pretty, but this ads weight…. I typically leave it at the heavy layer, and it has been outstanding in the strength department. I haven’t had a box separate on me yet. If you have any questions there are a ton of great builders on the site here… don’t hesitate to ask for advice, or what would you do ?

[Dangerous Dan]

more pics please

[Toro]

Ill have to see of MAXON has some more that he has stored...

[Dangerous Dan]

I was just wanting to see the finished product with motor installed. Looks good. I played with FG for the 1st time last week, was able to make my CF tailwheel bracket flyable again until the replacement arrives.

[Toro]

That plane was Max's I wonder if he took some photos of the finished product.

[Plane Addicted]

Great write up. I enjoy reading this kind of how to stuff. Share the knowledge!

John

[Matchless]

Toro, I see you laid the glass on the box on the bias.... Probably worth mentioning for everyone why you did this and how : )

[Rockoflyer]

Looks Nice Toro.

[Toro]

Bias lay ups will give you more strength in torsion, and in the direction of anticipated flex. IE forces to the rear. Essentially BIAS means multiple directions. This is what the technique is on the corners of the box

[mavacpjm71]

Nice How to Toro - thanks for doing it....looking good too

[edgewise]

Toro
I've done some glass work like that before, and used a heat gun to thin the epoxy to get more out of the weave. Do you think that was a good practice, or do you think it weakened it alot?

[yakwrecker]

I love west system epoxy. I have a corridar silica filler that allows me to mix up a white thick paste to fill gaps or work in a corner that regular epoxy would roll off and make a mess.