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peterz(at)zutrasoft.com
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 Posted: Mon Aug 29, 2022 5:40 am Post subject: Classic wing (walk area) delam. |
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Bud,
In your experience, what forces are involved in an injection repaired delam bubble failing? Is it the low pressure air flow?
Cheers and thx,
PeteZ
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budyerly@msn.com
Joined: 05 Oct 2019
Posts: 284
Location: Florida USA
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| Posted: Mon Aug 29, 2022 8:07 am Post subject: Classic wing (walk area) delam. |
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Pete,
It has been a mixed bag. If the delamination was due to excessive sanding, long term storage depressions or impact damage. It is often best to take the delamination out mechanically, then rebuild fully.
Small dents from long term storage I have filled over and if the dent was smooth and simply took a set from prolonged pressure, simple filling was all that was necessary.
Minor delaminations from impact from non mechanical such as saw horses or prolonged setting on a hard wing stand for instance, I have simply done an injection by filling the void using one entry for the epoxy and another for the exit. In many of these if you cool the surface of the bubble, it shrinks to nearly flat, so I fill with epoxy, shrink with cold ice pack, then use weight (like a thin flat shot bag) to keep pressure on it and it has lasted years and only showed two small pin holes. The repair is only as good as your analysis of how and why the delamination occurred.
Injection repair is very dependent on the underlying surface. If the bubble is injected, then a vacuum bag placed over the top may just collapse the underlying foam on a stab or Classic wing/aileron or flap.
If a flap, aileron or stab (which is glass on foam sandwich) has a delamination due to an impact, the foam is crushed so a vacuum bag repair will actually leave a depression after cure. Problem is then the depression may pop back up, as now you have a flexed glass with damaged foam, and flight stress and thermal stress from sunlight heat may just cause the sucked down patch to pop like stretched sheet metal will and leave another void and worse yet another hump that now has filler over it. I prefer to simply cut out the skin, sand off the paint and use micro or expand cell and or glass to repair the underlying damaged foam then patch over the top to return the skin strength. Since the Classic and other glass on foam hand laid structures are made in the amateur's shop, a fix in another shop done by hand will give acceptable results using proper technique and in the sun will not show as much.
On thick glass on foam sandwich like the biax on the Classic wing root or on a Varieze (Rutan glass airplane) I find that grinding off the paint to the filler, then bevel the glass at least an inch back from the damaged area to expose the underlying hollow foam structure is best. That way the underlying foam lightening hole structure can be reinforced then patched over properly, as the foam is actually a structural shear element and must be sound. Once the underlying foam is repaired/filled and then sanded and prepped scrupulously clean your are ready for the glass repair. A patch with glass orientation laid exactly like the piece removed and properly allowed to cure is best. I prefer to do a scarf type repair, but I also overlap on the top. Again, I'm the department of redundant redundancy. Therefore, I will have a humpy repair surface from the repair even with proper overlap between layers. Sand, fill, sand, prime, sand, prime again, prep the paint overlap area and final paint. Sometimes the whole upper surface of a wing gets a repaint as blending time is not worth it.
If the prepreg autoclave wing skin of an XS is delaminated and or cracked/flexed where a separation has occurred, then the damage should be cut out and an inner layer should be fabricated and laid inside the skin as a support to work on. I've laid up sections then Reduxed in a two ply inner patch to start with, then filled the foam layer, smoothed that out and then glassed over the top using the appropriate uni. There is going to be a hump with any repair. I'm afraid you have to sand back the finish on a large enough area to smoothly fill over the top to have a nice smooth looking repair. ( It ends up looking like how I do my access holes.) Remember, on prepreg structures, prep is everything. Use a proper structural laminating resin and cure IAW the instructions. It is best to do the repair with proper heat control and pressure of either mechanical or vacuum to get the air bubbles out of your epoxy from a hand layup.
I have followed the industry standards such as:
General Aviation Composite Repair by
Larry L Mansberger
The University of Texas Arlington
Department of Mechanical and Aerospace Engineering
I'm afraid that larger repairs require a good eye and deft hand to assure a proper repair is made. I don't allow auto body shop repairmen to grind down the finish. They apply too much heat and pressure with their grinders and more areas delaminate. Be patient and slowly remove the finish. Use peel ply over the final layup to assure the epoxy flows out to seal the edge of the glass as fillers and paints have VOCs that are essentially paint removers and the edges of the glass may lift.
Another issue is sanding and buffing the final paint out. One can bubble the paint and your new underlying surface using a buffer too aggressively. Go slow and watch the pressure and heat.
I hope this answers your question as it is a good review for me on how I'm going to fix my wing root.
Best Regards,
Bud Yerly
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