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Jun 05, 2020, 05:56 PM
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Original Nutcracker Build


So, I am going to do a build on an original Jim Kirkland Nutcracker kit. Through the great advice and help of doxilia I decided to use 1/32" balsa on top of carbon fiber veil to create a composite skin. So, I went and bought the CF, and cleaned out the wood section of the hobby store and started to join the balsa sheets. Then I went into the box to get the foam cores and ran into an interesting situation. There is a core and not cores. I really need some advice here.

It seems the wing cores were cut from a single block of foam. I was thinking I had 2 cores and then 2 shucks. Not the case. Plus the leading edge is already molded. So to those of you with experience with these type of cores, just how the heck do I sheet them? Yikes..... Any and all help would be appreciated.

The airplane will have a OS Max Special Edition with a muffler. Mechanical retracts from OK Model complete the package.
Last edited by kdunlap; Jun 05, 2020 at 10:45 PM.
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Jun 05, 2020, 08:01 PM
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There are several ways you can go here, big question I guess is do you have a vacuum system? You can cut the shucks so you end up with 2 bottoms which is all you need if you vacuum. I would cut the front 1/4" off the cores you have and use a balsa leading edge but a lot of guys love the leading edge cut in like you have. You can form the balsa sheeting by wetting it with hot water and ammonia and hold in place with Ace bandages to get the leading edge curve without cracking. Back when that kit was new, we would roll one piece of posterboard on the cores using contact cement like Sorghum or 3M-77. Balsa is a much better sheeting if you form the leading edge first. As a last resort, you could cut new cores, pretty sure I have the templates. Let me know if this makes no sense and you need more detail on anything.
Jun 05, 2020, 10:35 PM
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Making the Sheeting


So I have decided to make 1/32" sheets that are 26"x36". Using 18x18 tiles to weigh the sheets down for drying. The plan is to wrap them around the cores in one continuous piece. Too late tonight to figure out if I will vacuum bag these or put them in the shucks. That's tomorrow's project. In the past, I used to CA the sheets together because I was impatient. This resulted in a seam that was a bear to sand flush. On these sheets I am using Aerobond white glue and am extremely satisfied with the seams. very strong with minimal glue needing to be sanded, especially on the side of the seam that was held together with tape. I omitted showing the part where I trimmed each sheet with a straight edge and razor blade.
Last edited by kdunlap; Jun 05, 2020 at 10:45 PM.
Jun 06, 2020, 10:53 AM
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When I vacuum bag wings, I don't glue the sheeting before it goes in the bag. True the edges of the sheeting and tape together. I use tape to hold everything in its place - ( some pin ). When you take it out of the bag, remove the tape. I also glue leading and trailing edges at the same time. The wing panel needs very little sanding. I have used both polyurethane glue and finish resin with wonderful results. That method has yielded great sheeted foam wing panels that are much less work than any other method I have used. It also saves a lot of weight as the vacuum allows you to use much less glue. I have even built panels that came out of the bag weighing exactly the same - to the gram.
Jun 06, 2020, 01:56 PM
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Really appreciate all the advice so far.

Last night I tested bending a wet sheet of 1/32" balsa around the leading edge. Looks like the balsa is OK with wrapping around without shattering. So, current plan is to swiftly apply a wrap around layer of CF veil to the core and then overlay the balsa onto it. The veil will be pulled taught. David has a great advice and I hope he won't mind if I share his email later on how to do this, along with the formulas he uses for thinning the epoxy. Still need to think through doing this in shucks vs. splitting the big block vs bagging. Right now I need to cut out the landing gear box and aileron servo tray. So, I have time to think about this. I'll be using my own design laser cut landing gear boxes and will do a step by step on these.
Jun 08, 2020, 12:51 AM
DavidsPlanes
doxilia's Avatar
Ken,

Great to see youíve started a thread.

Foam cores with the LE cut in already are a little trickier to sheet but have the advantage that their LE are matched and uniform. Of course, if the cores are sub-standard and the LEís donít match, oneís better off sanding balsa. But letís assume your cores are good and therefore Iíd recommend keeping them as is.

Now, on the wing sheeting process especially considering the composite layup, I think youíll find it easier to manage all the glue requirements and layering if you skin the cores in sections:

First you want to do the LEís by using a 2-3Ē width of balsa so you wrap around the LE to a point where you can then mate the top and bottom skins from this LE ďcapĒ back to the TE. Before doing any glue, make sure your LE skin is molded to the core by soaking the skin in hot water and leave it drying while wrapped in to the core. You can use rubber bands to hold it in place while wet. Draw reference lines on the top and bottom of the core where the LE cap will end and trim your molded skin to match the lines.

Given the fact that the LE has no thick harder balsa LE which provides some ding resistance, the use of CF will come in handy here as it will result in a very strong core and LE. A wing strike, if not too violent, should go no deeper than the 1/32Ē of the balsa skin. And even the balsa skin will be hardened from the epoxy used to apply it. Youíll want to first bond the CF to the core and allow it to cure prior to anything else. It is possible to do the carbon and LE skin at the same time but it's trickier so Iíd suggest you do the LE CF first and once cured, lightly sand (don't get into the veil) and mold the balsa skin and epoxy in place once trimmed to suit. You can use the shucks to hold the skin tightly against the LE (one core at a time). Make sure you leave an epoxy free clean edge where the rear skins will mate with the LE cap. Passing your finger dipped in alcohol along the edge usually is sufficient to clear it of any excess epoxy that may have escaped the cap section. Once the two LEís are veiled and skinned, you can do the easier aft top and bottom of the cores at the same time although I'd still suggest doing one core at a time given your single shuck.

It appears that the TE of the core is flat rather than feathered so I assume your kit has you apply a balsa TE for a hinge line. Itís up to you whether you do this before or after skinning but I prefer doing it after skinning as one can use the sheeted core as an airfoil reference for block sanding the TE. Make sure you apply masking tape along the sheeting at the TE junction so you donít accidentally sand hard into the skins while shaping the TE. It looks like the ailerons will have to be made from aileron or sheet stock unless you decide to frame them up using 1/16Ē skins, ribs and a balsa LE (1/4 or 3/8). The framed up versions are typically lighter and stay truer as the framework provides straight parts that donít warp but using sheet wood is fine too if you prefer that. Either way, I'd recommend that you cap the root and tips of the ailerons with 1/32" ply. This will also help them to not warp. Finally, you can add a 1/4" width of veil atop on of the aileron skins along the TE to keep those ding resistant and also makes them very true.

Anyway, thatís a start on how Iíd go about these cores. Just be watchful of the veil, when and how you apply it because it becomes very hard once the epoxy has cured for an hour or longer. Of course make sure you use slow cure epoxy (i.e., finishing resin).

Cheers, David
Last edited by doxilia; Jun 08, 2020 at 11:32 AM.
Jun 08, 2020, 10:34 AM
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David,
Thanks very much for the help. I'm going to try an experiment with some foam that I have laying around and some specialty hardener. What I want to see is if I can do all of this in a 1 step layup. So, my initial idea is to use West Systems 209 Extra Slow hardener. That will give me 40-50 minutes of pot life at 70 degrees and perhaps an hour or two to align the cores in the shucks.

So here's the notional plan. Line the shucks with several coats of PVA to prevent the cores from sticking to them during curing. Prebend a continuous sheet of 1/32" balsa around the core to get the leading edge shaped correctly. Let it dry. Apply CF veil to the cores in one continuous piece using a 50/50 epoxy-alcohol mix as you suggested. Then apply the prebent balsa sheeting using 100% epoxy. Shove into the shuck and do core 2 using another batch of epoxy.

Will try this out first before I inflict this to the cores..
Last edited by kdunlap; Jun 08, 2020 at 10:51 AM.
Jun 08, 2020, 11:53 AM
DavidsPlanes
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Quote:
Originally Posted by kdunlap
David,
Thanks very much for the help. I'm going to try an experiment with some foam that I have laying around and some specialty hardener. What I want to see is if I can do all of this in a 1 step layup. So, my initial idea is to use West Systems 209 Extra Slow hardener. That will give me 40-50 minutes of pot life at 70 degrees and perhaps an hour or two to align the cores in the shucks.
Ken,

I'm sure it goes without saying but if your epoxy has a pot life of 50 min, that's at most the working time you have to align everything correctly and allow the cores to cure in the shuck. You don't want to displace the carbon or skins once the epoxy has gone highly viscous and has begun tacking. That said, 50 min should be plenty of time if you have everything tested and prepped before applying epoxy.

Quote:
So here's the notional plan. Line the shucks with several coats of PVA to prevent the cores from sticking to them during curing. Prebend a continuous sheet of 1/32" balsa around the core to get the leading edge shaped correctly. Let it dry. Apply CF veil to the cores in one continuous piece using a 50/50 epoxy-alcohol mix as you suggested. Then apply the prebent balsa sheeting using 100% epoxy. Shove into the shuck and do core 2 using another batch of epoxy.

Will try this out first before I inflict this to the cores..
Although trickier to do IMO, that should work too. Just make sure you skin is fully molded to the core and the LE is not brittle in any way. The last thing you want is to have applied epoxy and have a skin crack.

Note that your epoxy alcohol mix depends highly on what epoxy and hardener you use. It comes down to the viscosity of the resin but we've discussed that. My approximate ratios are for Z-Poxy finishing resin.

I can think of a few drawbacks to the approach you're suggesting and thought they were worth mentioning:

1) Applying the veil to the entire core prior to skinning means applying epoxy to the core. I generally don't favor this as the core is not as uniformly flat as balsa skins are. You end up "pooling" a fair bit of unneeded glue. Applying the veil just to the LE in a 3" width only adds this extra glue to the LE are which is not a bad idea anyway since you want this area to be hard and the CF thoroughly impregnated.

2) The veil can be pre-applied to the rear skins allowing them to cure "molding" in place on the core placing wax paper between the veil and the core. Once cured, you can apply epoxy to the hardened veil and bond to the cores. Incidentally, using wax paper (wax side toward the CF) will result in a very smooth carbon sheet once the epoxy has cured. The veil wants to be uniformly damp but not wet.

3) Applying epoxy to a molded wrap around skin is rather tricky as you are basically brushing glue into a parabolic "tunnel" if you follow. Having a pseudo flat skin to apply and control the resin quantity is much easier IMO. With flat skins I don't use a brush but instead just pour the epoxy on to the skin and use a silicone scraper (or CC) to spread it evenly and then collect back any excess. But in the interest of full disclosure, I have never done the skins in one wrap around pass so I can't say from direct experience. It just strikes me as trickier and harder to spatula back excess resin into the container once applied. I typically remove a substantial amount of glue from the skins prior to bonding.

The one thing I've learned from skinning cores that makes a big difference absent vacuum bagging is the pre treatment to the cores. The job always turns out better if the cores are HAND sanded applying very light pressure using 320 & 400-600 grit paper. I like to use a full size sheet for this and spread my hand wide to make contact with the entire sheet. Using a sanding block inevitably leads to core and foil deformity so I don't do that. I often do the last pass with a few strokes using the shoe shining technique using 600 grit holding the paper across the core from LE to TE and moving along the span. I find this technique respects the airfoil best to finish them up. Use weights to hold the core in place.

Your cores look like they're cut from fairly open cell foam (which should make them light) so watch applying glue to the cores as substantial epoxy could sink into the open cell structure.

I hope this helps.

David
Last edited by doxilia; Jun 08, 2020 at 12:02 PM.
Jun 08, 2020, 12:45 PM
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I been through this with my Komet wings. Don't use those cores. Cut new or have eueka cut some from your templates. I tried cutting my shucks apart and it was terrible. Couldn't get strait wings. I found no good way to do this
Jun 08, 2020, 02:47 PM
DavidsPlanes
doxilia's Avatar
With the dual core, single shuck situation, I'd think it might be doable to work on the cores by doing one at a time leaving the other virgin core in place.

I'd imagine it would be easier to first work on the bottom core leaving the top one inserted and once complete, insert the bottom readied core into the shuck and swap out to work on the top one.

A bit of a little additional time required but perhaps worth a try.

David
Jun 08, 2020, 10:52 PM
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I have had to on occasion cut new shucks for sheeting. As the original was damaged and it was the perfect solution for my situation. It may work well for you as well.
Jun 09, 2020, 11:05 PM
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Little late to the party....back in the day we sheeted a set of cores cut just like the ones you have for the Nutcracker. We ended up marking a center line from the root to the tip length wise on the leading edge for each core. We then used a hot wire to split the shucks into 3 pieces. You get one top shuck, one bottom/top shuck and one bottom shuck. As mentioned above.... do one at a time is best. A lot easier then trying to slide the cores in from the side with keeping everything in place. T-pins will be your friend here.
Jun 10, 2020, 03:45 AM
PhoenixFlyer
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Not trying to hijack this thread. Your cores are the old kits from Skyglas, and maybe others. You can wrap the LE easy, dampen the sheets, use common sense here.
Pictures here, maybe too many, as I learned from an expert years ago. This works for me, bag and vac is great also. Observe or delete.
Vince
Last edited by PhoenixFlyer; Jun 10, 2020 at 06:50 AM.
Jun 10, 2020, 08:41 AM
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Thread OP
Phoenix, this is gold! Thanks. Need to study these in detail. The CF veil has been delayed in transit, so I have more time to think!
Jun 10, 2020, 09:30 AM
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As an alternative to CF veil, Skyloft (formerly Dave Brown, now Ohio Superstar) works very well. I have a good, detailed how-to article from an old NMPRA newsletter on sheeting cores using a Skyloft/balsa skin composite I can post if you like. I'm pretty sure epoxy is the only adhesive that will work for this method.


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