Skin repair of molded models (60" glass and rohacell racer)

[Konrad]

It pains me to admit this in front of everyone, but I damaged my little 60" moldie racer. The good news is I don't think I damaged the main spar. You can see that the wing skins are showing damage. At the flap hinge line both the upper and lower skins are showing tension damage. A head of the spar the lower skin is showing a substantial compression wrinkle. These lead me to think that there was a lot of forward motion of the wing tip as the plane came to a sudden stop! And that this resulted in relative motion around the spar.

The tension failures look easy enough to repair. Saturate the tear with thin CA. then sand the surface to remove any paint trying not to disturb the glass fibers. The removal of the epoxy paint allows for the introduction of 0.5 oz cloth to be laminated over the tear. Sand with a hard sanding block and feather edge as one would prior to paint.

Now the compression damage is a bit more involved. I first drilled a hole in the LE to allow me to use a 0.027" music wire to lift up the skin to be co-planer with the original skin. This meant I had to put the wing under stress (tension) to unload the skin. Once the compression valley was 90% removed with the wire I drilled a CA vent hole where the wire had penetrated into the rohacell foam. I removed the wire and poured thin CA into the channel made by the 0.027" wire to harden the damaged rohacell foam trying to give the skin some support from inside. Again sanded away the epoxy paint and did a lay up of two layer of 0.5 oz cloth, to gain some fiber strength in the skin over the failed area. Feather edge the glass and epoxy, paint to match.

If any of you guys have a better way please let me know.

All the best,
Konrad

 

[Konrad]

I'm happy to report that the repair is sound. We just had our MOM 60" race yesterday and was able to test the wing at full ballast (300 grams). There was no change in trim and the wing stalled just as it had prior to being damage. The structure performed great against the loads of the classic split S start. This is an inverted dive from an altitude of about 400 feet to gain speed at the start line. No flex or weird handling was noticed.

Now I did run into another structural problem latter in the race. It is called the pylon. Again the repair performed well as it showed no distress from the shock of impact.

Now a failure mode I wasn't expecting was that the thin "weak" servo leads would do as much damage as the actual impact with the pole. The yellow wing (a double carbon layup) was involved in a mid-air where it cleaved the other guy's wing. While going through the wing it got snagged on the servo lead. (That is the other guy's servo) This servo lead cut the LE, top and bottom skin and most of the spar.

On my red wing the servo leads cut the bottom skin as the wing was separating from the fuselage prior to the Multiplex 6 pin connector letting go. I'm shocked that these thin copper wires can cut through these composite wings with little or no damage to the servo leads!

While I don't think there is much that can be done about impact damage, maybe a solid carbon LE or fiber across the LE part line. I'd like to learn what if anything you guys do to protect the wing from the servo leads as they pass into the fuselage. I'm thinking these flush contacts may have value other than convenience.

P.S.
Are the ArmSore contacts no longer available from Aloft?
https://www.hyperflight.co.uk/produ...TACT&name=compression-contact-wing-connectors

 

[Wayne]

Like a pull string to open a package.

I do agree, I like to have some carbon on the leading edges of my wings. It is kind of a double edged sword though, can cut both ways. The carbon can help avoid damage to small bumps and bruises on good landings, but it can also cause a lot wider damage area when you really nail something. So it is a bit of a tradeoff.

 

[Konrad]

Trying to repair a damage (not just dented) 60"moldie wing.

I've stabilized the structure by repairing much of the compression and delamination damage with thin CA glue. I inserted two 0.3mm thick by 50mm carbon blades to span the broken carbon cap strips. I had the cut away part of the top wing to get to some compression damage on the spar toward the tip.

I plan to lay up two pieces of carbon roving to span the damage of the spar. These will encapsulate the damage and add to the shear webbing.

What I'd like to learn is how do you guys deal this the 40mm of missing wing that was pulverized by the impact?

Do you insert a chunk of foam and carve away what doesn't look like a wing? Or do you shape some thin foam (like floor underlayment) and inlay this in the voids of the wing skin?

After I get some rigidity to the skin I'm planning to block sand the foam and glue to get as close to the airfoil as I can. I then plan to lay up two 1/2 oz layers of glass and epoxy. Block sand this and fill in any low spots with (?)? Block sand again. Lay up one layer of carbon tissue and two more layers of 1/2 oz cloth. Block sand this to final size.

Laterally balance the wing and test fly.

Another question I have is do you guys add any glass to the inside of the wing skins before closing up the wing?

 

[Konrad]

To repair the tear from the servo leads I've filled the void with expanding polyurethane glue. I will block sand this smooth to the wing skin to make a suitable substrate on which to layup a double layer of 1/2 oz cloth patch.

Looking at the broken spar I noticed that the OEM used a think layer of micro balloons to bond the spar to the skins. The removal of this mixture should allow me to actually make a wet layup of carbon fibers and epoxy over the broken carbon fiber spar caps.

I had feared that the broken spar was going to be a weak point in the repair. Now I'm concerned that the repaired area of the break will be so much stronger than the original spar, that stress riser at the end of the repaired area will become an issue. I'll see what I can do to dissipate (blend) the repair fibers into the adjacent structure (spar).

 

[Konrad]

Wow, That worked out better than it had a right to!

Has anybody used expanding PU to fill the voids (make the substrate) in large areas of missing wing skin?

 

[Wayne]

Looking good. I'm no expert on the repairs, I usually kill them rather than damage them.

 

[Konrad]

Looking good. I'm no expert on the repairs, I usually kill them rather than damage them.

LOL, I can't even do that right.

I hope I'm not hurting sales. Heck, what am I talking about, I already bought her replacement!
https://forum.alofthobbies.com/index.php?threads/mibo-dart-60.465/

 

[Wayne]

Nope, not hurting sales at all. I am looking forward to seeing your progress.

 

[Konrad]

Along with showing how things are done. I find it advantageous to show when things don't go well and to show how to correct these issues.

Since I don't have a way to ultra sonically test for damage. I'm forced to do some stress testing. While I had seen some damage to the shear web out at the tip I didn't think the carbon caps had been damaged. But while testing my interim repair, the spar in the area of this web damage failed. While I'm a bit pissed that I damaged the aileron. But I'm happy I found this damage before I sealed up the wing or worse when test flying the ship.

 

[Konrad]

I did some research (read woke up in a cold sweat) about the failure modes of stiff structures. And remembered that stiff structures that fail as a result of impact will have at least three breaks. This is because there is little flex. So to allow the impact object to pass, the structure will break at three point. One at the initial impact point and one on ether side of this point.

While I can see the impact point where the pole hit the spar and broke it. I also saw some light damage to the shear web out towards the wing tip. This damage really was an indication of a crack in the spar caps. Stress testing subsequently showed this damage.

Now I'm looking for the third break. I fear it is towards the root of the wing. As shown earlier there was some compression damage in that direction behind the spar. I now think this is an indication that part of the spar flexed aft broke allowing the shin to buckle a bit. A complication to this repair is that the wing is a Schuemann planform (a straight TE with a 3 segment LE) that necessitates a bend in the spar by design. It appears the compression lines start near this bend. I think that the manufacturing bent in the spar is masking some damage.

So it looks like I'll have to cut into the skin, AGAIN and chase this damage.

 

[Konrad]

It's always a good day to wake up to cured epoxy.

I ground the skins back looking for that third crack. While I didn't find a clear indication of broken carbon. I did find indications of stress, just like there was on the shear web where the spar failed (under stress testing).

I've made a carbon bandage that goes around all four sides of the spar in the damaged area. I suspect that this bandage has more carbon by weight than the whole wing had when new. I ended all four elements of this bandage at different locations along the span of the spar. This was done in an effort to minimize stress risers.

While I had my carbon tow out I decided to add durability to some of my other models.
As a result of the modern practice of using the wet seam there is little in the way of stiffening fiber across the fuselage seam. In the old days we used fiber tape to join the two fuselage half's together.

It has been my experience that the fuselage fails at this seam, particularly towards the leading edge of the wing with through wing spars. This is because wings try to move forward pinching the fuselage when the fuselage comes to an abrupt stop.

I find that making a continuos carbon hoops at ether end of the canopy opening add years of durability to the model. This is for all wet seam models regardless of manufacture.
Note that I could not add the rear carbon hoop to the Sanda Mach 2 as there is a factory installed radio tray in the way. But this tray actually add more support for this area of the fuselage than the fiber hoop would add.

All the best,
Konrad

 

[Konrad]

Starting the fun part. Not really but it is something that is new to me. So if I'm way off base please let me know how to correct this.

I'm using some extruded polystyrene found in wood flooring underlayment. This is about 2mm thick. I'm thermally molding this flooring material over a small (thinner airfoil) section of wing. I'm using this section of the wing as a male mold. I protected the wing with some aluminum foil as a heat barrier against my heat gun.

Now I need to make it clear that the foam is just a for shape. The actual strength comes from the fact that this foam in sandwiched between two layers of fiberglass. The foam acts like a shear web between these glass layers.

So to make the inner layer of glass I used 2 layers of .75oz glass cloth wrapped around the wing that was protected with wax paper*. I then taped the thermally shaped skin on top. let it cure and now i have a nice skin to use a filler for the section of the skin missing where the pole destroyed the wing.

The replacement shell is a bit over size so a little guiding will need to be done as the skin is installed.

Now my real concern is that since most CA glues are not safe on polystyrene foam. That from this point on all future work will have to be done with foam safe materials.

Once I get the skin inlayed into the wing I'll need to work some glass fiber over the seam from the inside. As there is a section of skin missing from the bottom this shouldn't be too much of an issue.

Once the top skin is installed do much the same for the bottom skin but use a very dry mixture of epoxy and micro balloons for the leading edge.

Now the hard part will be using a very rigid sanding bar use the undisturbed part of the wing as a sanding guide to shape the top of the foam inserts to match the airfoil.

Once the foam has the correct shape, glass all the damage areas to make the top (outside) glass layer for the skin.

If anyone has a link to a proven repair method please let me know.

* Modern wax paper is not like the paper of old. Try to use a silicon paper as the epoxy barrier.

All the best,
Konrad

 

[Motowncali]

Just my two cents on what you have going on there, I would open the wing a little more and use blue foam to create a core for that section. Then the skin that you just created from the wingtip, I would use that over the blue foam. Feather it all together and crash it again.... lol ahhh I mean fly the snot out of it!

 

[Konrad]

Mo, Thank you.
I assume you are saying to use a large chunk of foam to fill the void and then sand away anything that doesn't look like an airfoil. Not sure I follow what you are saying about the glass work.

One of the guiding principles of my repair philosophy is to try to mimic the original structure and maybe even improve upon it. My understanding of these composite wings is that the skin provides a lot of the torsional resistance. This is often done by the shape of the airfoil as with the classic sheeted foam wing. But with hollow molded wings it really is the inner and outer wing skins working like the load members of a spar with shear webbing in between that resists the torsional loads.

My concern is that “Blue Foam†is nowhere near as stiff as the original Rohacell foam. I'm hoping that with actual added layers of glass across all the seams that I'll end up with a comparable performing structure, albeit a bit heavier than the original undamaged structure. I'm anticipating having to add 1/2oz of lead to the other wing. I'm not too concerned with this 1 oz weight gain as I do ballast the ship.

On the improving side of things I hope to carry some glass fibers across the leading edge seam.

Prior to glueing my inlay work I removed the mylar vapor barrier. I used this vapor barrier mylar as a heat defuser during the heat forming stage of the repair.

 

[Motowncali]

That is one heck of a repair!

 

[Konrad]

Thank's Mo. I too am pleasantly surprised that it looks like I'll actually be able to repair this wing. I honestly had written it off and was just using this core as a means to practice repair methods. I now wish I had been a bit more careful about hanger rash.

I have to ask what are you guys using for paint. I was a bit surprised that there are no 2 part paints sold in hobby shops today. I'm recalling K&B SuperPoxy. While I don't need the fuel proof properties, I've always loved how it sanded and flowed.

 

[jvaliensi]

Klass Kote is the replacement for SuperPoxy. I've used it. It is good. I've painted bicycles and airplanes with it.
http://www.klasskote.com/re-model-crafting-epoxy-paint

 

[Konrad]

Klass Kote is the replacement for SuperPoxy. I've used it. It is good. I've painted bicycles and airplanes with it.
http://www.klasskote.com/re-model-crafting-epoxy-paint

Thank You!
If Dave Platt uses this stuff who am I to second guess him. I have a concern that the primer takes 48 hours to fully cure. Have you used other faster cure primers (for block sanding). I have a background in automotive re-finishing. But the last time I professionally painted cars was 35 years ago. Things had moved on since then.

 

[Motowncali]

I have been very happy with the Rustolium rattle can stuff. Krylon works too.
I guess it depends on if you are looking for something to spray through s gun or just a rattle can. I’ve never really been a painter, so I just rattle can stuff. It it needs to look better than that, I take it to a friend.