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Damage repair?

Mr Charles

Well Known Member
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
 
...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:

cowling_rivet_repair1.GIF


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:

cowling_rivet_repair2.GIF


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.

cowling_rivet_repair3.GIF


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:

cowling_rivet_repair4.GIF


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.

cowling_rivet_repair5.GIF


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.

cowling_rivet_repair6.GIF


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

cowling_rivet_repair7.GIF


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

cowling_rivet_repair8.GIF


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.

cowling_rivet_repair9.GIF


Thanks, Bob K.
 
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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:

cowling_flox_rivet.GIF


Thanks, Bob K.
 
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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
 
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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.

b6rn6d.jpg


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.
 
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
 
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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

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.
DSC03346-1.jpg

DSC03337.jpg

Lots of time with pressure washer, and I now think few gelcoat cracks...mostly paint crack issues. Here is before and after pressure washer...
DSC03326.jpg

DSC03356-1.jpg

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?
DSC03368.jpg

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
 
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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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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.
 
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.
 
New??

Just an observation, but the volume of work that will be necessary to get this cowl back in shape will probably exceed the work necessary to fit a new cowl.
 
Another possibility may be to fill the old rivet holes in the cowl with flox and redo with solid rivets but drill/countersink new holes for the rivets half way in the space between the existing rivet holes. This way the rivets will be in solid fibreglass rather than the flox repaired old holes. To make it bullet proof (possibly overkill) you could add a layer or two of fibreglass on the cowl edge extending along the rivet line to give a bit more fibreglass thickness for the squeezed rivets.

Fin
9A
 
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.

I found nothing else that would touch the saturated oil etc on the inside of the cowl! I have a hot water pressure washer...the hot water, almost steam most of time, makes a huge difference in a pressure washer! I'll never go back to a cold water one! Anyway, I digress! I will definetly set out for drying...good point. I assume washing paint off of gel coat will be ok? Assume no water penetration thru gel coat? What about an acetone rinse? Will that help?
Charles
 
Just an observation, but the volume of work that will be necessary to get this cowl back in shape will probably exceed the work necessary to fit a new cowl.

Well, that might possibly be so, but I wouldn't recommend going that way for a number of reasons:

  • I see that there are a couple of blisters and bumps on this cowling, presumably to enclose engine features unique to this aircraft. If this builder needs to recreate those features on a new cowling, it is going to be a lot harder than on a more standardized RV.

  • I think that the problems we see with this cowling are all pretty localized and manageable. A few minor repairs will make the cowling serviceable, and that might be enough to make the airplane flyable.

  • I've watched a couple of builders fit cowlings to RVs, and sometimes things go right together, and sometimes there is some back-tracking.
Thanks, Bob K.
 
I ran into a similar problem

The current RV-4 plans call for a .063 plate with 4 screws on the top and riveted to the bottom. To me it looks like the hinge allows some movement and that just hammers those hinges behind the spinner.

Chris Olsen
N2ZK RV-4
 
The current RV-4 plans call for a .063 plate with 4 screws on the top and riveted to the bottom. To me it looks like the hinge allows some movement and that just hammers those hinges behind the spinner.

Chris Olsen
N2ZK RV-4

Just a note...even though this cowl has issues, I was pretty happy that the original build held up well for 19 years!!! The hinges, other than one behind spinner and three rivets at end of side hinge, are still solid! I attribute the failure to the amount of aerobatic flying and higher stress from that, that I have done over the last year.
I will add the plate, or joggle and screws, to the inner. The outer hinge ends I'm gonna redo anyway to update to a small access cover, so only leaves a very small portion of hinge to secure...like the idea of adding b/t existing rivets...hate to "de-bond" something that has held for 19 years! Sooo...repair a few holes, install some additional squeezed rivets b/t existing, re-do inner area, remove one un-needed bump-out, pretty it all up and repaint with my new color...wow! Can't wait to get to OSH again! As I read elsewhere,"I park my plane there to make all the others look better!":D
Charles
 
Bob,
If not West systems, what would you recommend ?

Chris, for general purpose composites and laminating, I like to use Jeffco 1307 with 3176 hardener. I use a variety of other resins and hardeners for specialty applications, but 1307 is the workhorse in my shop.
 
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Oil door

Charles,

The oil door on my -4 looked just about like yours. Decided the best bet was to start from scratch and make a new door. Results here. Didn't do a good job in the photo quantity department.

Tony
 
update

Well, I have started on my project...tried to sand original paint off...36 grit on a DA wouldn't touch it! Not sure what kind of paint it was, but wow it's tuff! Imron maybe? So...I pulled out pressurized sandblaster...it took it down to the gel coat with just a little effort...EXCEPT...areas that had been filled with bondo or something similiar...and my sandblaster basically wouldn't touch it without going over a dime size area at a time! I've never worked so hard at removing paint in my life!:mad: Then, I wasn't sure if micro would adhere ok to the polyester/bondo/filler...so I ground it all off! Of course, this left some ugly areas where scoop attaches, and at every bump/blister! So, just tripled the amount of work to do!:( Also, tried to sand down and clean up the inside of the cowl, as original builder wasn't worried about appearances inside...but it bugged me! So maybe a skim coat of micro???
Mixed and applied first epoxy resin ever! After sanding down interior of cowl, had some thin spots...actually a few areas completely thru cowl, especially on the scoop "sidewalls", so taped outside and layed a few layers of S glass on inside...tomorrow I will have a solid base to layup, or contour with micro on the outside...tips or suggestions???
Question...if I place some squeezed rivets b/t the existing pop rivets, should I go back and replace all pop rivets with squeezed rivets, or if the pop rivets are still solid, can I leave em? Also, any tricks to filling/covering the rivet heads? Assume a layer of micro, epoxy wipe to fill pinholes, primer, etc...do I need to prep the heads of the rivets in any special way?
Thanks again in advance!
Charles
 
Question...if I place some squeezed rivets b/t the existing pop rivets, should I go back and replace all pop rivets with squeezed rivets, or if the pop rivets are still solid, can I leave em? Also, any tricks to filling/covering the rivet heads? Assume a layer of micro, epoxy wipe to fill pinholes, primer, etc...do I need to prep the heads of the rivets in any special way?
Thanks again in advance!
Charles

Hard to answer. Pop rivets can be difficult to hide under the filler/paint because of the hole. Try a test fill/primer over one pop rivet and see if the results are acceptable. Pop rivets can be difficult to drill out/remove and there is the risk of enlarging the hole in the fibreglass. If you are putting new solid rivets between the existing pop rivets they you should be able to remove the adjacent pop rivets for good and simply fill the pop rivet holes.

You don't need to prep the heads of solid rivets in any special way before filler/primer/paint. If some rivet heads end up sitting a tiny bit higher than the level of the surrounding fibreglass you can file the heads down slightly to flush. If they sit a fair bit higher you would need to fill around them.

Fin
9A
 
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Hard to answer. Pop rivets can be difficult to hide under the filler/paint because of the hole. Try a test fill/primer over one pop rivet and see if the results are acceptable. Pop rivets can be difficult to drill out/remove and there is the risk of enlarging the hole in the fibreglass. If you are putting new solid rivets between the existing pop rivets they you should be able to remove the adjacent pop rivets for good and simply fill the pop rivet holes.

You don't need to prep the heads of solid rivets in any special way before filler/primer/paint. If some rivet heads end up sitting a tiny bit higher than the level of the surrounding fibreglass you can file the heads down slightly to flush. If they sit a fair bit higher you would need to fill around them.

Fin
9A

OK...Got the basic repairs almost completed...I'm close to the point of putting micro on the areas on the outside of the cowl for fill...guess I still have a question on covering the rivets..I have added extra squeezed rivets where needed, but decided to leave most of the pop rivets as the bond between hinge and cowl, as well as the remaining rivets, are in great shape. I sandblasted the pop rivet heads, sanded down all high rivets, including sqeezed rivets...so....how should I cover rivets? Thin layer of micro, thin layer of epoxy or flox? With or without layer of fiberglass cloth? If I get to thick, cowl cheek shape will have odd shape, but too thin and I worry about popping off rivet head...Or will the epoxy primer and K36 filler/primer fill and cover adequately?
Thanks,
Charles
 
Wipe wet micro into any divots or holes in or around rivets. Remove all excess while wet. Before the micro cures apply a single strip of cloth over the rivet line. After cure sand the glass to dull the surface and clean up all edges, then wipe with micro to fair the glass strip into the surface. After cure sand to contour.
 
Ok...making progress! Gonna get some pics uploaded soon.
But now that I've got the hang of fiberglass work...was wondering about something...
I currently fly with no wheelpants, and have the old two piece aluminum gear leg fairings...want to replace with the new one piece fiberglass...BUT...gonna save new fairings and new wheelpants for this winter! Could I just overlay my aluminum fairings with 3-4 layers of fiberglass? Glass em right in place...micro and paint. Non removable..I'll cut it all off when I fab up the wheelpants/fairings this winter.
Just thought it might work, as I have leftover fiberglass and supplies, and gonna be shooting some paint anyway!!!
Advice/Opinions greatly appreciated!
Charles
 
My 22 year old -3 is a recent purchase. Plane has flown this way for years but not up to my standards. There are many posts which helped immensely with my repair process. Just wondering if there are any other comments or suggestions that could help me along the way. I am very much the rookie with fiberglass (I can cover a cloth plane though). Flick album has details on cowl.

https://www.flickr.com/photos/24686554@N02/albums/72157657971347703
 
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