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  #1  
Old 04-25-2012, 09:05 AM
Mr Charles's Avatar
Mr Charles Mr Charles is offline
 
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Default Damage repair?

Purchased RV-4 a few years ago, built in 1993. Fiberglass cowling and intersection fairings need some work, to say the least! I have studied every thread I can find, and have got a pretty good crash course in fiberglass work, but most seems aimed at new fabrication...so I have a few questions...
1) Have some rivets that hold hinge section pulled thru FG, what is best way to epoxy in hole to redrill and install new rivet? Cut out and repair area much larger than old hole? Countersink and fill/redrill? What mix? i.e. flox?
2) I believe the cowl is polyester, has gel coat, and painted w two colors...I have no idea type of paint, but would guess maybe acrylic enamel? (how do I tell?)...have a few cracks in gel coat...If I sand down and do repairs to the gel coat, and use procedures outlined by Dan and others, when I get to the step of layering on a coat (2-3 coats?) of epoxy with a spreader to fill pin holes and provide the outer "shell"...where do I stop? at some point I will have sanded/repaired area abutting already painted area...how do I blend together? I plan on repainting the white areas, and keeping same design, but changing color on the second "accent" color.
3)And how about the use of the DPLF epoxy primer? Again where do I blend to old finish? Or do I clean/sand/prep entire cowl and spray DPLF over entire cowl?
4)To paint, do I need to stay with original type of paint, or get better two stage urethane etc color matched to rest of plane? If I stay with acrylic enamel, can I spray it on the K36?
Sorry for all the questions...I've done FG and painting some, but not on an airplane and am really pretty green! Appreciate whatever help you can provide...
Charles
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  #2  
Old 04-25-2012, 12:41 PM
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Bob Kuykendall Bob Kuykendall is offline
 
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Quote:
Originally Posted by Mr Charles View Post
...1) Have some rivets that hold hinge section pulled thru FG, what is best way to epoxy in hole to redrill and install new rivet? Cut out and repair area much larger than old hole? Countersink and fill/redrill? What mix? i.e. flox?...
Here's how I'd approach that repair from a practical composites perspective:

Here's what the airplane started with: a 3/32" rivet countersunk into the fiberglass, securing the aluminum hinge on the inside:



Here's what you've probably got now: A rivet with munged head and an oversize hole. Actually, you've probably got a bunch of these. If so, consider repairing every other one, and then going back to get the rest, to preserve the alignment between the hinge and the fiberglass:



Step 1 is to drill out the rivet. Be careful not to mess up the hole in the hinge, you need that to realign the pieces later.



Step 2 is to sand a conical depression around the rivet hole. For a structural repair in composite materials, you normally want a scarf angle somewhere between 20:1 (3 degrees) and 150:1 (0.4 degrees) depending on the materials and application. But for something like this, you can probably get away with about 15 degrees. If the cowling edge falls within that cone, I'd probably steepen the scarf angle on that edge to leave as much of the original material on that side as practical:



I'd probably make a 150-degree cone for my die grinder and use that to make the conical depressions.

Step 3 is to fill the conical depression with as many layers of fine fiberglass as required to restore the original thickness. I'd use 2oz or 3oz glass for this. For resin, I'd use whatever the cowling is made of; generic polyester resin (Bondo brand is fine) for a polyester cowling, and epoxy resin for an epoxy cowling. If I couldn't tell what it is, I'd probably go ahead and use epoxy. But not West system, I hate working with that stuff.



Step 4 is to sand the top of the repair smooth. Of course, wait for the resin to cure nice and hard first; probably a full day at around 70 degrees.



Step 5, back drill the rivet hole using the hinge as a guide.



Step 6, countersink using a microstop countersink. Don't go too deep, you don't want to do this again!



Step 9, install a new rivet. Use a squeezer, and set it on the underdriven side of the acceptable range. Also, if possible scuff sand the mating surfaces and use some resin to bond the hinge to the inside of the cowling.



Thanks, Bob K.
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Last edited by Bob Kuykendall : 04-26-2012 at 02:47 PM.
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  #3  
Old 04-25-2012, 12:56 PM
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Also, here's something that Steve Smith and I did when we assembled his RV-8 cowling: Between each pair of rivet holes in the hinge, we drilled a 1/4" hole and countersinked it. Then on assembly we bonded the hinge to the cowling with epoxy as well as riveting it. We also filled each of the 1/4" holes with a dab of epoxy/flox mix, and then applied a fiberglass tape over the hinge so that it bonded to the dabs of flox. That gave us a line of "flox rivets" that backed up the aluminum rivets:



Thanks, Bob K.
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Last edited by Bob Kuykendall : 04-25-2012 at 01:00 PM.
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  #4  
Old 04-25-2012, 04:08 PM
Finley Atherton Finley Atherton is offline
 
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Bob has given you excellent information an this will give a structural repair.

You could also consider drilling and countersinking for a 1/8" rivet if the glass is thick enough. Cosmetically you would not see the larger 1/8" rivet heads as they would be covered by filler during the finishing process.

Another possibility would be to take it to Step 2 as per Bob's instructions then fill with flox rather than layers of glass. Not as strong as Bob's repair but I would be surprised if it was not good enough IMHO.

Fin
9A

Last edited by Finley Atherton : 04-25-2012 at 04:35 PM. Reason: Added more detail
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  #5  
Old 04-25-2012, 04:16 PM
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Bob has described a truly structural glass repair, good for almost any part of a composite airframe.

For something as non-critical as a cowl hinge rivet I would probably experiment with cheating a little.

Cured flox is tough, with reasonable tensile and compressive strength. Do a mucho-oversize countersink on the damaged hole and wipe it full of flox. Back drill it after cure, then properly countersink for the rivet head.



You have an old polyester cowl. I'd do the surface finishing work with a good polyester filler product...cheap and fast.

Hard to say what to do about the old paint, or painting over it. Can't go real wrong if you remove all of it and start fresh.
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  #6  
Old 04-25-2012, 04:18 PM
Finley Atherton Finley Atherton is offline
 
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I have thought about this some more from my previous post.

The rivets should not have pulled through the glass in the first place. Maybe the holes were countersunk too much or the fibreglass was a bit thin to start with.

If the glass looks thin you might want to consider carefully drilling out all the rivets in the affected area, filling the countersunk holes with flox then laying at least a couple of layers of glass over the line of holes then drilling/countersinking using the holes in the hinge as a guide.

Fin
9A

Last edited by Finley Atherton : 04-25-2012 at 04:37 PM.
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  #7  
Old 04-26-2012, 12:58 AM
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Mr Charles Mr Charles is offline
 
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Quote:
Originally Posted by Finley Atherton View Post
I have thought about this some more from my previous post.

The rivets should not have pulled through the glass in the first place. Maybe the holes were countersunk too much or the fibreglass was a bit thin to start with.

If the glass looks thin you might want to consider carefully drilling out all the rivets in the affected area, filling the countersunk holes with flox then laying at least a couple of layers of glass over the line of holes then drilling/countersinking using the holes in the hinge as a guide.

Fin
9A
Wow! Love this forum! You are all so helpful! OK, I spent some time cleaning and inspecting cowl...there are just a few areas of rivet problems...a couple at end of side hinge, and several on hinge inside of inlet.


Lots of time with pressure washer, and I now think few gelcoat cracks...mostly paint crack issues. Here is before and after pressure washer...


I found my hot water pressure washer makes a good paint remover!!!
It also appears that original builder used good adhesive/sealant between hinge and cowl...pro seal maybe? Black or dark gray in color..anyway, rest of hinge/hinges appear very solid..
The original build had hinge rivets exposed...I think I will take time to glass over them. Suggestions/Tips? I already ordered $400 worth of epoxy materials/tools/cloth etc so is it ok to do all this with epoxy, using techniques from threads...i.e Dan and others?
Next, any suggestions on straightening warped oil door?

I plan to strip off rest of paint, make rivet repairs, remove unneeded exhaust "bump out" , redo piano hinge access/securing method, pretty up inside of cowling and add paint for protection layer, scuff gel coat, primer and paint! One more question...where I patch/repair with epoxy and feather onto/over/with gel coat, any special prep? Spray DPLF over entire cowl including over original gel coat, then K36 over everything also?
Thanks
Charles

Last edited by Ironflight : 04-26-2012 at 01:03 AM. Reason: added some carriage returns for better formatting!
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  #8  
Old 04-26-2012, 02:46 PM
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Bob Kuykendall Bob Kuykendall is offline
 
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Thanks for those photos, they really clarify the situation!

It looks like you've got two different situations going on here:
  • The 3/32" rivets inboard of the inlets and right behind the spinner have loosened and pulled through.
  • The 1/8" countersunk pop rivets outboard of and immediately behind the inlets are shedding their heads and pulling through.

First off, I have to question the use of piano hinges at the joints inboard of the inlet. I know that the vast majority of RVs use screws at that location; that is what the RV-6 and RV-8 plans show. Basically, the lower cowling has a joggle that extends above the split line. The builder installs nutplates on the joggle, and joins the upper and lower cowl with screws that go through the two cowling sections and into the nutplates. What I'd suggest for this area is to replace that inboard hinge arrangement with something like almost all other RVs have.

For the long horizontal hinge along the sides of the cowl, I see that the rivets there now are 1/8" pop rivets, probably made of aluminum, and some have shed their heads. I think that those are an unfortunate hardware choice for a cowling; they don't handle the mixture of different materials and also the vibration very well. If the hinges are in good shape, maybe you can repair the holes as I suggested earlier and then replace the 1/8" pops with 1/8" AN426AD solid rivets set with a squeezer.

The rivet holes in this area don't look that much enlarged, it could be that doing a flox fill as Dan Horton (edit add: And Finley!) suggests, and then redrilling them for driven rivets of the original hole size would work fine.

To address Dan Horton's point, he is absolutely correct that a flox fill might be perfectly adequate to fill the rivet holes, and would certainly be easier than what I suggested. However, I was hesitant to suggest something like that because I am worried that hoop stress caused by the rivet swelling as it is driven might crack the flox fill. Also, if you precut a bunch of 3/4", 5/8" and 1/2" squares of fiberglass it would go pretty fast to saturate them and press them into the divot.

Thanks, Bob K.
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Last edited by Bob Kuykendall : 04-26-2012 at 05:38 PM.
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  #9  
Old 04-26-2012, 04:11 PM
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Replace all the pop rivets with squeezed rivets for sure. Ditch the short hinges at the spinner too. Add the joggle or make an aluminum splice plate with two rows of three nutplates. Put it behind the seam and attach with three #8 screws in the upper cowl and three in the lower.

Bob makes a good point about hoop stress with a squeezed rivet. I suspect a flox-filled countersink will handle it, but maybe not. Note I said "experiment with cheating"

Given the paint will fly off with a pressure washer I'd sand it all down to bare glass. The cracked gelcoat or filler in the third photo must be sanded away completely and the area filled again.

Yes, epoxy over old polyester is fine, if it's not oil soaked.

Bond a stiffener rib or channel to the inside of the oil door.
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  #10  
Old 04-26-2012, 05:28 PM
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Based on a previous experience, a high pressure washer has a tendency to force water into the glass, you may want to consider placing the cowl out in the sun for a week or so to insure it is dry to prevent trapping moisture under your new finish.
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