Molded Wing Repair ( Rock Impact Damage)

[Konrad]

I suffered a laps of judgment at the last F3F race at the slot. Well, my Sanda Mach 2 suffered!

I got caught out with no lift. I was forced to make a way off field landing. From 300 meter above and about 1 Km out the landing looked good. It wasn’t until I got to the landing site that I see I had found the only place on the valley covered in rocks the size of my head. Really this was the only place, the rest of the area was tall prairie grass.

It was asked of me how would one repair such damage? I will try to document my repair of these wings. Unlike the spar damage we saw with this 60” racer, most of the damage is limited to the Leading Edge (LE) of the wings.

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

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...

forum.alofthobbies.com

The first thing one needs to do is relieve the stresses on the wing. Unfortunately I did a lot of this on the slope, before taking any photos. The top part of the first 2 cm of the LE at the impact point had curled under. This was distorting a lot of the wing LE. So I uncurled this area as soon as possible at the slope.

The next step is to relieve as much stress in the foam skin as possible. This means often picking at the crushed area trying to get as many of the fractures to realign. Once you have the wing looking like wing ( no stones, straw or loose pieces of foam and epoxy distorting the wing) I then heat the area with an iron set at about 85°C to 90°C. I like to use a wet paper towel to help with the heat transfer and to make sure that I don’t over heat the area. This heating relieves a lot of stress in the epoxy and fibers of the skin.

Next I try to stabilize the foam skin with CA. Do make sure that the parts and fibers are aligned as you don’t want to glue the parts out of alignment. Note; this CA is not to try to glue the wing together. CA is a weak glue. For strength we will be using epoxy and fibers to regain any of the lost strength.

The photos are trying to show the damage to the LE. This damage should be rather straight forward and well within the capabilities for most modelers to repair, other than the paint job. Now what does concern me is that the flap horn area is showing a lot of deformation. This means that the flap took some loads as the wings shifted on the fuselage. I landed clean with no flaps.

 

[jvaliensi]

I poked a hole in my Xplorer wing tip from hitting a weed. The skins are so thin. This will be good to follow.

 

[Konrad]

We can only hope.

If you have any questions please ask. I may be able to take a detailed photo. Right now I'm just guessing what areas folks might have a concern about when repairing a composite wing.

When repairing a wing we are concerned with two things, structure and fidelity to the airfoil. With the wing tip I've glued the top and bottom together with an epoxy and cabosil mixture. You can see where the missing pieces have been filled in with this mixture. I've rough sanded the wing tip to get a rough idea of what the wing will look like and to show the low spots. Now this might be counter intuitive but now I'll sand the low spots to remove all the paint. We are not at the block sanding stage of the repair. Remove the paint, as paint makes a horrible surface for glueing. I'll then put a dry mixture of epoxy and cabosil to fill the low spots. I'll then block sand this area trying to maintain the original airfoil.

Other than the sanding blocks these are the tools I use. Please note that it is very important to wear gloves when working with Epoxy. Epoxy is an accumulative poisson. You might not have a reaction for years but given enough exposure you will develop an allergic reaction.

 

[Konrad]

What is often overlooked in these composite repairs is that much of the strength comes from the inner layers of glass under the foam or balsa skin. This really is the only difficult part of the repair, as it is much like building a ship inside a bottle.

After getting the foam stabilized I wanted to get some fiber across the damaged area from inside the wing. Luckily there was some delaminated LE to allow me access to the inside of the wing. I choose to work the top inside skin first. To protect the bottom inside of the skin and any area I didn’t want to get any epoxy on I mask these area with pieces of tape. You can see some of this tape in the above post, this is the piece at the orange and white paint line. I saturate my fiber patch with epoxy on a piece of wax paper. I then try to slide this wax paper and wet fiber into the wing. I work it trying to get it to lay down across the damaged area. With any luck the patch will stick to the inside of the wing allowing me to roll off the wax paper. I add extra epoxy as needed to get as much of the patch to lay flat. (This might not be the best use of epoxy weight but as I won’t get another chance at reworking the inside of the wing I’ll take the weight hit.) I then clean up any epoxy that when cured might interfere with the wing closing up, I pay particular attention to the delaminated LE part line. I unmask the opposite inside wing skin and wait for the top inside wing patch to cure. I’m sorry to say I wasn’t able to get any good photos of this part of the repair.

I’ll do much the same with the inside bottom skin. But I won’t mask the inside upper skin, no need. And with this mixture of epoxy I will repair the LE delamination.

With the skin damage repaired from the inside I’m now able to sand the outside skin. Recall that this was pieced together with CA glue. I want to undercut these damaged fibers to allow the replacement carbon a place to lie down. We can’t just put the carbon on top of the skin as it will then change the shape of the airfoil. Now this wing will cause me some problems with block sanding. I like to span the damaged area with a ridged sanding block. I then sand the patch until I just touch the undamaged area adjacent to the patched area. This top skin tear is right up against the polyhedral break which means I can't use the outboard part of the wing as a sanding guide. This will in effect be a cantilevered sanding block repair. I can only use the inboard part of the wing to guide the sanding of the top wing skin patch.

At the end of the repair I will have a very good load path of fibers on the inside. I’ll also have fibers across the tears on the outside. This repair will be stronger than originally as there will be more fibers in the repaired area.

All the best,
Konrad

 

[Konrad]

With the power of digital enhancement I was able to get something that shows a little bit of the inside carbon patch.

I'm also trying to show that I did not contaminate the bottom skin (top in photo) fracture face with epoxy. The leading edge and skin tears still fit together well.

 

[Konrad]

I'm loath to do this, as so many folks try to learn the tricks of the trade and forget to learn the trade.

When working with catalyzed resins I find I have two issues. One is that the resin doesn't cure. and the second is that the resin sets too fast. The Tap Plastic 4:1 super hard resin has a 10 minute pot life. You would think this was long enough. Well, it often isn't enough time to get all the fussy stuff we need to do. To extend the working life I spread out the epoxy on a flexible sheet so that the exothermic reaction is slowed down by the cooling effect of all that surface area. If the epoxy won't flow, such as when it is mixed with Cabosill, I place the pot on a a frozen brick. The cold will extend the pot life of the epoxy.

All the best,
Konrad

 

[Konrad]

Ok, I now think the repair is structurally sound. Both inside and outside skins have some fiber ( carbon) over the tears.

Now comes the hard part trying to regain the airfoil shape. What I’m showing in this post is how the repair looks before I start to block sand. Please note that all the sanding* has been done with sanding blocks, but this is not what most would think of as block sanding.

What I’m trying to show is what looks good and bad at this stage of the repair.

I’d like to point out that Sanda Models is using a very fine layer of 0.5 oz glass cloth to provide a nice surface finish. The weave in the carbon cloth under neath this glass would result in far too much epoxy being used to fill the coarse weave of the carbon.

The first problem is that there is still some of the 0.2 oz carbon tissue on top of the original surface. this means that it is adding unwanted material to the airfoil shape.
The second problem is that I’ve sanded through all the carbon tissue in a spot or two. This is because the foam was distorted badly from the impact (we knew that). But I didn’t under cut these areas enough prior to adding the carbon tissue. I will need to make up another lay up adding a layer or two of carbon tissue.

I’m sorry I forgot to take a photo of the area with the raw lay ups of tissue. In this repair I used 5 layers of tissue to give me something to work with. While it looks like I may have sanded through these layers in spots the surface as it is shown now is close to its finished shape. That is the airfoil is true to about 0.15 mm. I point to the fact that we can still see the polyhedral kink line. After I take care of the foam break through this 0.15mm will be block sanded using primer as a filler and build up material.

* The only place I've hand sanded without a sanding block was to remove paint from the low spots prior to adding the carbon tissue.

All the best,
Konrad

 

[jvaliensi]

I have an Xplorer 2.5 Light. The skin is so thin and light. I did a hand toss with the glider. I did not throw it hard enough and although it did not stall I never got enough airspeed to get control authority. It landed in the weeds. Much to my shock there was a hole in the leading edge of one wing tip.

I made a patch from Vladimir's carbon vail. I put epoxy on a sheet of plastic and then laid the carbon on top of it. Then I brushed some epoxy on the wing, where the patch was going. I placed the plastic on the wing and taped in down good. I was thinking this would be a good patch. It seems strong enough but it did not come out as smooth as I wanted. It is less than 1.5% of the wing span so I doubt I will notice any difference in the air.
I do not like these super light gliders. I'm sure I'll destroy it sooner or later.

 

[Konrad]

I feel your pain! This Mach 2 has only been out maybe 12 times. To dent it this badly, this early hurts!

While I like to have the latest and greatest products and systems. There are times that even I know they would be wasted on me. This Mach 2 was one such model.

What I like about Wayne is that he has never tried to up sell me. He will sell me whatever I want but he tries to direct me to products he thinks are more closely suited to my abilities and temperament.

What scares me is when I get a hold of models like the Aeroic Gremlin and this Mach 2.

I’m going to admit that much of the off field landing was my fault. More my fault than bad luck! I found that in the panic of loosing altitude right after launch I had left my TE in “Thermal” mode. It was mentioned by the guys helping me look for a landing spot that I was near a stall. I knew that when in sink one need to get the ship moving (flying) and to range looking for lift. I don’t know why it didn’t dawn on me that I was still in launch mode (TE down). I should have clued in on what was happening when the guys were telling me I was near a stall.

Yes, I had some bad luck finding the only boulder field in the valley. But a better pilot would have had plenty of time and height to have ventured out looking for lift. With my TE down I really couldn’t get the Mach 2’s nose down and speed up to cover ground. I did find what felt like dust devils but really couldn’t work them.

Wow that veil really does look like it is a loose weave. Is there a reason you didn’t place the weave biased at 45° to the LE? As I always want to block sand my repairs to try to bring back the original airfoil shape, I almost never use the plastic cover method trying to get that gloss look.

Note that my patch is using uni directional carbon on the inside wing skins to try to maintain span wise stiffness. For the outside skin my carbon fibers (very light tissue) are in a random order across the tears. This is done to try to tie all the little pieces together.

(I don't know about you but I crash just about all my models sooner or later! I try to make it the latter!

All the best,
Konrad

 

[Wayne]

Like the ice block idea.. Too often I run into much shorter working times as the addition of fillers to the resin just seems to insulate and allow the epoxy to get hotter, thus shorter pot life. Interesting..

 

[jvaliensi]

Actual my temp patch was better. I had some self adhesive targets in the car when I poked the hole in the wing. I cut a square the wrapped it around the leading edge, the same as the carbon patch. It actually did a really good job of maintaining the leading edge shape. I thought I would get the same results with the carbon.
It is so thin, that I can sand it off in a few strokes.
We'll see if the sailplane survives the contest at TOSS this weekend. No sense in getting too excited about it fixing it.

 

[Motowncali]

I like the ice idea...
I use 30 minute pot life epoxy and once I am happy with the work and set it aside to cure, I either put it in the cab of my truck if it’s daytime, or I wrap it with an electric blanket if it’s evening time...