Fiberglass

Fiberglass Time!

2/8/2016

So I am continuing to work on the elevator and HS (horizontal stabilizer) fiberglass tips.  Fitting, attaching and blending in.  I have decided to cover the joint and blend in with a strip of fiberglass.  Also decided to "close in" the open framework between the elevators and HS.  Many builders have reported a 2-3 kt. speed increase after closing them in, which is understandable.  Left open they are fairly un-aerodynamic.

The front tips of the elevators came first.  The tip comes off the frame at any angle, I wanted a smoother curved transition.  Also, there was a unacceptable gap where the aluminum bulged.



So I mixed up some epoxy, made a little flox to fill the joint, and laid down some fiberglass.  This was then covered with a mix of epoxy and micro to give some thickness to shape and contour.  I can sand more later, and fill or add additional material if needed.  For now this gives me a good general shape.



So at this point I will cover my fiberglass, epoxy and filler procedure.  First, I transferred my epoxy and hardener (catalyst) into squeeze bottles.  Way easier to dispense small quantities accurately using a cheap digital kitchen scale.
I have both fast and slow hardeners, which give me a little flexibility.  Although the fast still requires 6-8 hours for a complete cure.

For structural filler I use a mixture of epoxy and flox.  Flox is simply finely chopped up cotton fibers, with the resulting cured mixture being very strong structurally.

For top-coating I use a mixture of epoxy and micro, which are microscopic glass beads.  This makes a very lightweight structure that is easy to sand and shape.  Not meant for anything structural.



Here is a mix of flox ready to go.  It can be mixed to whatever consistency is convenient.  I also have a tub of Cab-o-sil, which is fumed silica.  Very lightweight stuff that thickens mixtures also.  It can be used with any mixture to thicken it up and prevent a runny application.






So here is how I am doing the majority of fiberglass layups.  Most of this sequence is on the same part, although I might have gotten a couple of photos confused.  First I prepare my fiberglass components, and cut a couple pieces to rough size.  Lay down a piece of plastic wrap.  And don't do this in the kitchen.  The number of layers of fiberglass depends on the application and strength requirements.



Place a piece of fiberglass on the plastic, mix up some epoxy and wet it out.  Here I placed another layer, wetted it out also, and covered with plastic wrap.



Then with both sides protected by plastic wrap, you can use a mini-roller to squeeze out any air bubbles and excess resin.  Extra resin doesn't add any strength, just makes for a heavy part.


After rolling out, you can mark and cut the plastic sandwich to fit your need.  It can be handled fairly easily without smearing epoxy all over the place, which is a big help.

For this small layup I will cover the end of one of the HS fiberglass tips.  It has been shaped as needed, then a piece of blue urethane foam (Home Depot variety) has been shaped to fit the opening.  I started using styrofoam backing material, but it is a pain to shape.

The blue urethane foam is excellent to cut, shape, slice and sand.  It is fairly rigid and friendly to use.  The piece below was floxed in, with a fillet of flox and layer of fiberglass on the back.  I then dug out the front edge a tad and roughened up the fiberglass.  This will be filled in with flox and edge of fiberglass, giving a strong bond to the edge of the layup.
















First I use the part itself to draw my outline on the top plastic wrap layer of the layup.  After tracing around it I cut out the piece with a sharp pair of barber's scissors.  Other cutting tools work, but this works best in my hands.



I shoved some flox into the edges of the tip, and lay the fiberglass on, working the edges into the flox.



The last step is to lay on a piece of "peel-ply", which is basically just a piece of nylon.  After curing, it is peeled off, which results in a micro-fractured surface, ready for further epoxy work like a layer of micro.  Without it I would have to sand, which is messy and a pain.  It has some other advantages, like it tends to blot up excess resin which helps the layup.  And if working on a finished part tends to "fair" the edge of the layup to the substrate making a smoother transition.



All of the above is not horribly difficult, it just takes time.  Sanding is messy, so I am doing my best to avoid as much as possible, and be cleanly when it must be done.  Small parts I take outside the shop to sand which helps a lot too.

So I filled in the ends of the HS with foam, and then a layup tucked in with flox......



I sanded and scuffed the inside of the HS around the opening, to increase the epoxy adhesion.  Additionally I placed flox on each rivet tail or head which would give mechanical retention as well.  Then the foam was placed and the layup on top of that.  The edge was tucked in with a tongue depressor stick and the remaining void will be filled with flox.  Seriously should never come loose.



I calculated the thickness of the foam and fiberglass to leave me about 1/16th of an inch to fill and finish with a micro mix.  That should give me a nice surface to sand and finish off while keeping weight to a minimum.  To keep it in place and flat while it cures I taped it in place with a layer of peel-ply and then another piece of foam. That should keep everything in place.



And because I am a masochist....I did some more work.  I want to fiberglass in the tips and cover the joint so it looks seamless.  I started by grinding off a bunch of the gel-coat from the tips, because you don't get a very good structural bond to it.  It is rather soft compared to the epoxy/fiberglass layers.  In some places I just want to add micro to blend in the tips with the elevators.  At those areas I did just scuff up the gel coat well.  For micro it is fine.  And then I added some small aluminum strips so the blind rivets  had something solid to set against.

You can see the scuffing and aluminum strips in this shot.

I did not want to have any movement of these so the joint would stay closed.  So I used G/flex resin which is a variant of the epoxy I am using.  It has a much higher modulus of elasticity, so it can flex a bit.  I think it might be advantageous when I am trying to keep the skin from flexing and breaking the bond from the fiberglass.  It can't hurt.




So after "wet-riveting" the tips on with the G/flex resin, I placed a couple of strips of fiberglass between the tip and HS to close and strengthen the joint.  Last I saw it the peel-ply was still in place.




Next up will be a lot of micro and shaping.  Maybe I can find a surfboard guy......

January 26, 2016


Once I finished riveting the tailcone, I decided to give it a rest and work on the elevators and horizontal stabilizer.  After each elevator is mounted on the HS, the leading edge of the tips are lined up with the HS.  Hopefully the trailing edges are then lined up.  Mine were spot on!  I was relieved, having expected to call 911 after my heart attack.



Then the connecting elevator horns must be final drilled and match drilled for connection.  (without that they would not move in unison).  That was a little nerve racking, as the instructions say to "drill as straight as humanly possible".  Sheesh, what does that mean?  Anyway, it all came out fine.

I figured this was as good a time as any to attach the fiberglass tips to the HS and elevators.  The fiberglass was not perfect out of the mold, have heard a lot of grumbling by other builders on this.  Sure enough, they did take some trimming and sanding.  I made a small sanding jig to sand the reveal that fits under the aluminum skin.  It was not very refined, but this made it very crisp:

You can see where there are uneven sanding marks, indicating how poorly this would mate against the skin.  A few minutes with coarse sandpaper and they were all nice and happy.

 These are blind-riveted to the skin edge, since there is no access inside.  (also known as pop-rivets).  However, that means the rivet sets/pulls against the fiberglass edge, not very sturdy. I know when we rebuilt John's elevator for his RV-6 that a lot of the fiberglass had cracked and the rivets pulled through.  So like many others, I glued an aluminum strip to the inside mating edge so that the rivet would have a strong spot to pull against.  It took a little extra time, but well worth the effort I believe.

This shows the strip held in place while it sets.  (I used RTV silicone for this).  The adhesive wasn't very important, as it only holds the strip in place until the rivets are set.  Wasn't worth breaking out the epoxy.

So the tip was set in place and final drilled.  Then the skin dimpled for flush rivets and the then tried in once more.....

It is amazing how fast the fiberglass dulls my drill bits and countersink bits.  Might try my drill sharpener (thanks, Mike), can't make them any worse than they are now.







And finally they get set into place with pulled/blind/pop rivets. I think pop is actually a brand name, these are official pulled rivets from Vans and Aircraft Spruce.





And here is the HS with the leading edge fiberglass tip clecoed into place.  It was oversized and I had to trim the back edge enough to allow the elevator to clear.  There are a lot of open spaces and rough joints, some fiberglass and epoxy sessions are in order at this stage.  I have not done a lot of fiberglass and resin in my day, but a little.  However, I purchased a practice kit and this weekend will experiment on non-aircraft pieces to get the process down pat.

This is also where I have to decide to do the minimalist approach, or glass the entire joint and cover all of those rivets.  Decisions, decisions.









2/29/2016

The last I left off I had wet-riveted the HS tips on with epoxy.  After removing the peel-ply, this is the "fractured surface" that remains, excellent for bonding to.  The gel coat has also been sanded down to expose the fiberglass, which gives more strength as the gel coat bonds weakly.  Nevertheless, some areas were sanded down and touched up, but the peel-ply helps to avoid a lot of sanding.

Then a thick mix of epoxy and micro goes on thick.  Adding afterwards is a pain, it is easy to sand off.













After curing overnight, it is sanded off and down to approximate shape.

I also finished putting micro on the close-outs of the HS, and the elevators.  Again, very thick paste of micro, most everything will sand off.

















You can see to the right one of the elevators sanded down.  Most all of the micro is gone, and the thin layer even shows the fiberglass underneath.  Trying to keep the thickness to a minimum and weight down.















To the left I am futzing with the surface between the HS and elevator.  I taped up the elevator to avoid getting epoxy stuck to it while I shaped the tip interface.

And to my dismay the elevator tips were warped and did not line up with the trailing edge.  So I finally decided to just buzz them down quickly with my angle grinder and sanding pads.  It goes very quickly, until they are just under flush with the trailing edge.  Leaves a big ugly hole and gap.










I filled the end of the hole with a flox/epoxy mixture which is very strong, stuffing a bit into the spar area so it won't wiggle away from the edge.  Over that I put a layer of fiberglass onto the surface, and finish with peel ply.












The peel ply pulls off easily the next day.  It gives a nice flat surface, and micro-fractures the surface to give a nice bonding surface to the next layer which will be some micro.























So with the elevator tips fixed, it is time to lay on a layer of micro to flush out the HS tips and elevator tips.  There are discrepencies that I just couldn't completely sand out.  I locked the entire assembly, and sanded rough for good adhesion.





Then mixed up a big batch of micro, and covered the tips so I can make everything flush.  I went ahead and just covered up the joint between the two.  When it is starting to set up, I can cut between the two with a razor blade.









The next day I sand both parts as one single unit, and am able to keep the contours match perfectly.  I will come back and touch-up the mating surfaces when done with the underside.  Will update this when they are completely finished, getting close.