Re: Epoxy

One Sky Dog

Hi Eddy,

I am suggesting your developed method of repair is not appropriate for our aircraft.

A proper repair is to: Remove all paint and fairing to expose the damage. Remove damaged plies and core. Replace core with same type and density leave a few thousands of an inch low. Scarf glass plies 20:1 ratio. Replace repair layers one for one same direction with just enough resin, not floating. For 9 oz. cloth this will leave the visible weave of the top repair layer. Lightly scuff with fine scotch Brite to remove the shine and fill the weave and repair area with epoxy and micro balloon mix to fair in with the surrounding surface. Prime and paint.

Sailplanes are powered by “gravitational acceleration is approximately 9.8 m/s squared” weight is good unless you want to be a leaf. Drag is bad for sailplanes competition sailplanes carry a lot of water to increase their wing loading.

You are right high strength composites are made by precisely controlling the fiber volume and minimizing voids. Your laminate turns out with greater specific strength but gives up stiffness because it is thinner.

I am only worried because your repair methods are not the ones that the designers of our airplanes have approved. I can patch a hole in my Cessna with a beer can and it would probably be strong enough if I bonded it on instead of riveting, but it would not be the right way to do it according to the designer.

That is my point not designer approved, and I do know there are a thousand ways to make and repair composite stuff.


Charlie Johnson

On Dec 5, 2018, at 12:10 PM, ΕΔΔΥ . overlordmustafa@... [Q-LIST] <Q-LIST@...> wrote:

This technique I'm suggesting is not for molding, it's for placing over an are which has had a repair or even as a repair.

If you're worried microballons offer no strength, glass does.  The power ratio of the planes I've used this one is zero, it's for sailplanes and this method has a MUCH higher weight to strength ratio than using most weaves of glass.  I worry about weight WAY more than any homebuilder.  The trick to making fiberglass light and strong (lighter, it's fairly heavy) is to minimize the epoxy.  If you had an entire wing with a wet layup and added 1 layer of 0.5oz/yd^2 you'd end up with a much smoother wing without adding any epoxy.  It's free strength with virtually no added weight.  Although, this is a tangent. 

I'm just suggesting if someone is worried about sanding through a spot of fiberglass, this is a way of repairing without.  Realistically, if one uses fine sand paper, they will not compromise the structural integrity of the fin.  Sand carefully and there will not be any need for any repairs (and I'd certainly take off the primer).

On Wed, Dec 5, 2018 at 12:49 PM Charlie oneskydog@... [Q-LIST] <Q-LIST@...> wrote:

Hi Eddy,

This is a great technique for reducing print through of structural fabric weaves in molded epoxy parts. It works by trapping extra epoxy between the fine weave and the structural plies the stiffness of the glass compared to resin minimizes the surface shrinkage reducing print through.

Our goal on a repair is to scarf in and replace structural layers one for one , same weight same direction, slightly below the surface. Any fairing required should be done with a similar resin chemistry filled with glass micro balloons to reduce the epoxy weight and facilitate easier sanding prior to paint.

Grams count our power to weight ratio is way less than your RC models and our repairs can be bigger. I appreciate your thoughts and methods for model repair. Most of our guidance on repairs goes back to Burt Rutan and his pioneering work on mold less composite structures.


Charlie Johnson

On Dec 5, 2018, at 9:42 AM, ΕΔΔΥ . overlordmustafa@... [Q-LIST] <Q-LIST@...> wrote:

I've done many repairs on fibergass, I used to build RC models.
One great method to get a smooth final coat of glass is to get the absolute lightest fiberglass you can get your hands on.

After laying up whichever weight of glass (which most people use is rather a heavy weight, compared to what i'm used to using) you can place a layer of  0.5oz/sq ft  fiberglass as a top coat.  This adds virtually no weight, you don't have to add any epoxy. This 'trick' works significantly.  It's like getting strength at no extra weight.

If you're worried about sanding through the fiberglass you could add a layer of this light cloth.  Apply it differently than thicker cloth.  Brush on the thinnest layer of epoxy onto the fin then place the glass over the wet surface.  Slow cure is better over fast cure for this (I prefer slow cure over anything else).

So, if you're worried about digging though fibergass, this is an easy fix.  When you're done it's amazingly smooth.  You'll still need to sand it to get it perfect, but it's close to perfect.

You might want to even bump up the weights(of glass) a bit.  The lightest glass is so thin you can't move it around very easy when placing it (so be delicate when setting the glass down).  An advantage to using the extremely thin stuff is it goes around corners and bends like nobody's business.


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