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Author Topic: LAB (Life After Balsa)  (Read 9279 times)

Offline Mike Alimov

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LAB (Life After Balsa)
« on: November 02, 2021, 06:23:07 PM »
I don't know about everyone else, but our flying season is pretty much over (by "we" I mean myself and my boys, Luca and Gabe).  As promised, I am starting a thread that details the construction method used for our latest airplane, as it generated a lot of interest and questions.
In short, it will be a thread about building a fully take-apart, all-composite airplane in home conditions and on a reasonably controlled budget (read: no carbon fiber and no CNC machining). 
I will welcome questions and constructive comments.  However, if you believe that the stunt world will crumble when people stop building stick-and-tissue airplanes, and that electric power and composites are evil, then read no more. This thread is not for you.

Part I.  Motivation

A bit of recent history is in order to explain how we got to this point, and what motivated us to look at composites.
My boys learned to fly stunt very quickly.  In July of 2018, Luca (then 12) picked up a handle and started flying level.  In late September 2018, he won Beginner Stunt in Cleveland flying the full Beginner pattern, and on October 11, 2018 he completed his full complete AMA pattern.
  His brother Gabe (then 10), after teaching himself a full RC Sportsman Pattern in 2018, decided to switch to control line stunt in June of 2019.  He learned normal and inverted flight on day 1; all round maneuvers by late July (during 2019 Nats); full pattern by August 2019, and entered the 2019 F2B Team Trials in Tulsa on Labor Day weekend.  That's about 2 months from first flight to Team Trials! Needless to say, I was doing everything possible to keep the boys equipped with airplanes that fit their skill level, but the demand was outpacing supply.

The 2020 season was fairly successful but also tough.  We had 3 crashes back-to-back (not necessarily pilot errors, more like accidents), and after i backed my car over Gabe`s Vector, our arsenal of competitive equipment was depleted.  Luca wanted to re-create his profile airplane that flew so well for him (Initiation, which is essentially an SV-11 with profile fuselage).  We gathered all the supplies, and it looked like there was enough stuff to make an exact copy, except for one part: cheek cowls.  They serve not only aesthetic purpose by covering up all the ugly wiring inside the motor compartment, but also help reduce drag and therefore battery usage.  The original (Initiation) had cheeks carved out of blocks of 1" thick balsa.  I realized that my supply of 1" thick wood was exhausted, and trying to find light wood in this size is nearly impossible these days.
I had a feeling that this was not the last time we would need these cheek cowls, as Gabe was talking about building a backup airplane too...
In desperation, I decided to try molding the cheeks out of fiberglass.  The bucks and molds were fairly easy to make (see pictures), but my first attempt produced a cheek that was flimsy and weighed about 3 times more than I was hoping for.  It used 2 or 3 layers of 6 oz glass cloth (can't remember exactly).  Another attempt was made using a single layer of 6oz carbon cloth. Another failure, lots of pinholes and too flimsy. 


« Last Edit: November 02, 2021, 07:58:10 PM by Mike Alimov »

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #1 on: November 02, 2021, 06:24:50 PM »
That's when I decided to try the true composite formula: two layers of light glass cloth with a thin layer of foam in between.  (I will be detailing everything later).  This worked out great, and soon enough we had cheeks that were light and rigid -- and required absolutely no balsa!  This was the missing link.  The reincarnated airplane was named Education (for all the lessons we learned during 2019 and 2020), and eventually came to earn Luca his first Senior trophy at the 2021 Nats.  Gabe, flying a different design called Skittles, also used the composite cheeks from the same molds, and took the Junior trophy alongside his brother.

Offline 944_Jim

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Re: LAB (Life After Balsa)
« Reply #2 on: November 02, 2021, 06:57:40 PM »
I'm glad that your boys are going full at it! My own boys built their own, flew (and crashed), and flew a few more times. Then they basically backed away...quickly.

May suggest the next plane is called "Teacher," as you instruct us in new building methods? Thanks for sharing.  I'm subscribed to this thread!

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #3 on: November 02, 2021, 07:12:01 PM »
As the season was nearing its end, it became obvious that the boys will need more and better equipment going forward.  Each one of them wanted to have the primary competition airplane, a backup plane (just in case... ya know), and a "beater" - i.e. a practice plane that could be taken to the field and back, inevitably develop hangar rash, dings and dents, and yet fly the same as the primary airplanes.  I'll do the math for you: that's at least 5 identical airplanes, all to be built in one winter!  My mind starts racing.  Kits? ARCs?  Which design should we build? How long will it take us? How much will it cost? finally, will we be able to build and finish everything so quickly? 
Going the kit, ARC, or foam wing route would cost us at least $1000.  (as of this writing, Brodak is out of SV-11 ARCs, and foam wings from Robin's View Productions are no longer available). 
But here we were: 21 years into the 21st century, trying to find exotic wood imported from Equador, smelly lacquer called dope (?!), and something that looked like doctor's exam tissue paper.  And then hundreds of hours of rubbing the surface with a small piece of sandpaper.  I knew that it was not going to happen. More importantly, it was becoming clear that putting a decent finish on those planes would make this plan an impossible dream.
Let's do quick math here.  A typical full-size stunter has about 2 sq.yds of surface area.  During a typical dope finishing process, bare wood to clear rub-out, a plane gets completely sanded by hand at least 10 times (in reality, much more, but let's be conservative).  So that's 20 sq.yds of area to be sanded.  Times 5 airplanes = 100 sq. yards = 900 sq. feet, that's about the area of my driveway!  By hand. With wet 600 paper.  Now add the number of times it needs to be coated by dope, whether that's brush or spray (not to mention toxic load of this stuff, no matter how good we think it smells).  I think we're looking at about 1500 working hours.  Considering that my boys maintain very good grades and participate in sports and band during the school year, they have maybe 4-8 hours per week left for airplanes.  End of discussion; mission impossible.  Where do we go from now?

That's when I floated the idea of an all-composite plane.  Luca said that it's not going to happen, and started to build another profile.  Gabe was intrigued and excited, but had no idea how everything was going to work. Neither did I.  I always thought that all-composite airplanes were only possible if made in an undisclosed location near some Ukrainian aircraft factory, using CNC-milled molds and a sprinkle of fairy dust.  Mortals like myself should not apply, I was told.

Desperation makes people do desperate things. 
Over the winter of 2020-2021, we made bucks (plugs), molds, and learned how to make composite parts.  On the Labor Day weekend of 2021, those parts were assembled into the prototype airplane, and first test flights were made.  At 660 square inches of wing, it weighed 62 oz ready to fly, did not use any carbon fiber (except pushrod), and a total of 2 sheets of balsa (will be replaced later).
Gabe entered the plane in Advanced at Cleveland and Before the Snow Flies, and won both times.  Every Advanced and Expert level flier that was given an opportunity to fly the airplane immediately went to put a complete pattern on it, with very positive feedback.  Some guy named Bob Hunt flew the plane and declared "I could fly this every day!" (maybe he hasn't flown any decent planes before?).

In the next installment, I will start detailing the building process.  Believe it or not, but the first step in making a 21st century electric all-composite take apart stunter was... a trip by my good friend Joe to an Amish wood supply store somewhere in Geagua county, Ohio.
Stay tuned.

« Last Edit: November 02, 2021, 07:42:03 PM by Mike Alimov »

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #4 on: November 02, 2021, 07:21:03 PM »


May suggest the next plane is called "Teacher," as you instruct us in new building methods?

Haha, too late.  The composite plane prototype got named "Ripslinger".  Watch the Disney animated movie Planes - it has an evil character (looks like a Reno racer) named Ripslinger; the boys liked the looks of it, and that's what was put down on paper for fuselage outline...

Offline Ken Culbertson

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Re: LAB (Life After Balsa)
« Reply #5 on: November 02, 2021, 07:45:35 PM »
Haha, too late.  The composite plane prototype got named "Ripslinger".  Watch the Disney animated movie Planes - it has an evil character (looks like a Reno racer) named Ripslinger; the boys liked the looks of it, and that's what was put down on paper for fuselage outline...
I am just posting to get you on my threads list.  You are a very lucky man to have sons that share your passion for the hobby and from what I can gather, you may be actually sharing theirs!

Post a lot.  I go through the build threads a lot, especially when it is new ideas!

Ken
AMA 15382
If it is not broke you are not trying hard enough.
USAF 1968-1974 TAC

Offline Brent Williams

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Re: LAB (Life After Balsa)
« Reply #6 on: November 02, 2021, 07:46:42 PM »
Excellent! 
Looking forward to the rest of the build process!
Laser-cut, "Ted Fancher Precision-Pro" Hard Point Handle Kits are available again.  PM for info.
https://stunthanger.com/smf/brent-williams'-fancher-handles-and-cl-parts/ted-fancher's-precision-pro-handle-kit-by-brent-williams-information/

Offline Will Hinton

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Re: LAB (Life After Balsa)
« Reply #7 on: November 02, 2021, 07:57:37 PM »
Mike, I'm lovin' it just as much as I loved judging your guys at the FCM.  (There, now I'm on the thread.)
John 5:24   www.fcmodelers.com

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #8 on: November 02, 2021, 08:44:35 PM »
Part II.  First experiments in mold making

I discussed my plans to make composite molds with my friend Joe, and mentioned that I want to carve bucks (aka plugs, or master models) from wood.   I used pine (essentially, a piece of 2x6 from Lowes) for the cheek molds, and learned a lesson: do NOT use pine!  Or anything else that has alternating hard and soft layers of grain.  It is almost impossible to get rid of that texture, no matter how much you sand.
Joe recommended basswood.  Where do I get basswood?  In a few days, Joe called me via Facetime from an Amish wood supply store.  He was standing next to a couple of pretty serious pieces of lumber, and asking which one should he pick.  After some discussion, we decided to get both, despite some defects being present here and there, and sort/cut wood once we get it to the shop, in better lighting.
After cutting the boards into usable lengths (and leaving out the worst knotted area), we put the pieces through a planer, alternating sides, and ended up with fairly flat and square boards.  There were also a few smaller pieces left after cutting the big stuff.  We put them through a planer too - why waste good carving wood!
I always try new techniques on something small.  It is cheaper and less intimidating.   So I decided to start with.. wheelpants.  What, you don't start your plane with wheelpants?  Well, maybe you should.  If they come out nice, maybe the rest of the plane will too.  Or not. BUt at least it will have wheelpants, and they make every plane look better.
So, let's start.  Step 1 shows a side and top view drawn on paper, and after being approved by The Boss (your 12 yr old), those views get transferred onto the block of wood.  Make sure the block is nice and square, or things will get ugly (as in: asymmetrical).

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #9 on: November 02, 2021, 08:49:24 PM »
Step 2: Carve the block per the top view, and finish to a 220 grit level.  No need for surface perfection at this time, but it needs to be accurate to within a pencil line thickness.  Having a benchtop sander helps immensely, as having the part on the table guarantees that your sides are sanded 90 degrees (plumb, or square) to the top and bottom surface.
« Last Edit: November 02, 2021, 09:41:03 PM by Mike Alimov »

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #10 on: November 02, 2021, 08:50:45 PM »
Step 3: repeat for the side view. Again, plumb and square is more important than smooth. Within a pencil line thickness.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #11 on: November 02, 2021, 08:52:24 PM »
Step 4: Using a carving knife, carve the piece until desired shape is obtained, and finish with file and sandpaper.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #12 on: November 02, 2021, 09:09:01 PM »
Since this was done quite a while ago, I can't find pictures of the mold making process for the wheelpants.  I will be doing a detailed mold making process for the fuselage later on.  The rest is a very abbreviated tutorial on how to make molded parts.

Step 5: finish the wood buck with 600 paper (dry!), apply 3-4 coats of Minwax Polyurethane with sanding in between.  Apply 4-6 coats of Partall mold paste, buff between each layer and dry 1-2 hours.  Install parting board, seal the gaps with modeling clay (no, not Playdoh!) and apply an even layer of Partall PVA (poly vinyl alcohol), let dry a few hours.  Prepare 6-8 layers of heavy (6-8 ox fiberglass), weigh the fabric, and mix an equal amount (or a little more) of laminating grade epoxy, like West Systems 105.  Apply each layer of fabric by brushing epoxy first, and then applying fabric until fully saturated.  Again, I will be posting a more detailed process later, with pictures.

Once the molds are made, use the same process for applying release materials: wax, dry, buff, PVA, dry.
Prepare silghtly oversize pieces of fabric.  I know, I promised no carbon, but Gabe wanted just a little bit of flash, so he talked me into using 6 oz carbon plain weave fabric.  I hate using carbon, it's much harder to work with than glass, but I admit that it looks awesome.  I promise, we will not be using carbon anywhere else except wheelpants.
Again, same process: weigh fabric, mix epoxy, brush on the molds (carefully! PVA layer is very fragile), lay the fabric.  Then everything gets covered by a piece of plastic sheet so it won't stick to the bag, followed by some breather cloth (paper towel, etc), taped in place to prevent shifting, and loaded in a vacuum bag.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #13 on: November 02, 2021, 09:11:00 PM »
Step 6.  Turn on the vacuum.  As the air gets evacuated from the bag, make sure there is enough slack in the bag to allow all the remote corners to get properly pressed in.  Leave overnight and hope for the best.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #14 on: November 02, 2021, 09:21:28 PM »
Step 7 (not fully shown, sorry).  About 12 -20 hours later (depending on the temperature in your shop), release the vacuum and remove the molds.  Using a single bevel knife or a wide chisel, trim the edges of the molded halves until flush with the parting surfaces.  Do NOT remove parts from the mold!  What is shown in the picture was a mistake - I removed the pieces, because I wanted to see what they looked like on the outside.  Do not worry - if all previous steps were done correctly, your parts will come out fine.  The hardest part of this entire process is that you won't know how the parts look until halves are joined together, everything is cured, and parts are extracted from the molds.  Two outcomes are possible:  everything turns out fine, you wipe the cold sweat after extracting your first shiny, beautiful part, and pat yourself on the back (I've tried that, but find it awkward, hard to reach my own back), and get a cup of coffee, while continuing to admire that part for some time. 
Another outcome is that something doesn't come out right.  As long as the mold is not damaged in the process, it's OK!  You can make many more attempts, and you will eventually get it right. 
OK, so we trimmed the two halves.  Mix small amount of epoxy (like 10-15 g) with cotton flocks, dispense a bead onto the edge of one of the parts using a Monoject dental syringe (bead about 1/16" thick), bolt the two molds together (I used 10-24 screws, they were on sale) and again wait until cured, 12-20 hours. 

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #15 on: November 02, 2021, 09:30:42 PM »
Step 8 (final).
This is the scariest and most rewarding (or frustrating) step.  Pry the two mold halves apart and extract the piece.  Do not damage the mold or the piece in the process!  I've never done that... er... well, anyway, it's possible to get overly anxious to see the result and damage things in the process.  Don't use sharp tools at this stage.
Extract the piece - some recommend a quick shot of compressed air to help. Can't really hurt anything with that.  I haven't tried that yet, but I intend to next time.
The part will have a very thin layer of flashing.  This is the extra epoxy that oozed out after the two parts got pressed together.  A small amount of flashing is a good thing: it means your epoxy has fully connected the outer layer of the layup.  Too much flashing means you put too much epoxy.  No flashing is worse: you won't know how much epoxy is holding the two halves together, if any.  According to Murphy's Law (in which I am a firm believer), you will find that out when the wing pops apart at the seam on the 4th corner of the hourglass in your best flight of the Nats qualifying rounds.
Flashing can be removed carefully using a simple file.  Voila!  Molded wheelpants.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #16 on: November 03, 2021, 05:29:20 AM »
If you've made it so far (reading, not molding of course), and are interested to learn more - from other, more trustworthy sources than your humble scribe, here is a great source of basic pictorials on this topic:

https://cstsales.com/tutorials.html

It's a one-stop shop: it shows you the basics step-by-step, and you can buy the supplies right there at their store. 
Having said that, there are of course many, many other composite supplies stores out there, but I tend to support smaller businesses run by modelers like ourselves, unless it is something specific that they don't carry. 

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #17 on: November 03, 2021, 06:17:29 AM »
With the whole process debugged using wheelpants, it was time now to gather courage and begin the real work. 
We will start with the fuselage mold, arguably the most difficult part of the airplane.  My reasoning was this: if we can make molded fuselage, everything else is easier.  If not, we are only lost a couple of months' worth of work, and a Plan B is in order.

There are two methods of making two part negative molds: using a single piece or two-piece buck/plug.  The wheelpant plug I showed earlier was made as a single piece - it's small and simple enough.  The fuselage, on the other hand, is a much larger and complex piece, so I decided to try a two-piece approach.

With Joe's help, the two larger pieces of basswood were planed to the target thickness (each one half the final width of the fuselage, with parting plane vertical).  (Not shown here): mark and drill blind holes in each half of the block in matching locations, so that the halves could be joined and separated any time using pieces of 3/4" wood dowel.  Once done, pull the pieces together so that they form a single block.  Using a band saw, cut to a top view, and then a side view outline.  The final piece should look like this.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #18 on: November 03, 2021, 06:22:02 AM »
It is now time to enjoy some basswood carving.  It is made even more enjoyable by using sharp tools, such as a small block plane, a wide chisel, or this fabulous spokeshave from Lee Valley tools that I sharpened for Joe to shave in the morning when he runs out of those BiC razors.
Carefully mark the fuselage areas for the location of all the center lines, nose ring, and the wing and tail locations.  I prefer to leave those areas flat, in order to avoid the headache of joining two complex curved surfaces once the wings are attached.
You will be removing lots of wood at this stage.  Be careful not to take very large cuts - it's easier to take more cuts, much more difficult to fill if you went too far. 
In order to avoid damage to the piece itself during this stage of shaping, I attached a simple block of 2x4 lumber underneath using Kreg Jig pocket hole method, and used bench vise to clamp onto the 2x4.
Once the surface is 1/8" - 1/16" within desired, stop carving and start smoothing using a file and then progressively finer sandpaper.
« Last Edit: November 03, 2021, 07:02:15 PM by Mike Alimov »

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #19 on: November 03, 2021, 06:38:27 AM »
Your goal is to arrive at two identical mirror copies of the fuselage halves.  To make them as close as possible, I bought a profile gage at Lowes to compare curvature from one half to another at marked stations (every 2" or so along the fuselage).

Once satisfied, final sand to 320-400 grit, and go through the same process (Minwax poly, apply and dry, sand, repeat).  Your goal is to get rid of the wood texture.  This is where I really appreciated Joe's advice to go with basswood - it doesn't have annular ring texture near I could tell, just the fine grain of the wood itself.  I did not have any need to use any fillers or primers; several layers of Minwax provide a good enough surface - but should you feel a need to use those other materials, this is the time to do it.  In the end, the surface quality you achieve at this stage is what will be imprinted into EVERY layup you pull out of the molds.  If you are unhappy with the surface, fix it now, don't count on fixing it later.  I'm talking about any surface texture rougher than 400 grit and any surface imperfections that can be seen with a naked eye in oblique light.  Don't worry about shine, it will come later.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #20 on: November 03, 2021, 06:44:19 AM »
For the next step, you need to locate material for the parting board, and cut it out to shape, that is the fuselage contour plus about 1-1/4" around.  There are multiple materials you can use for parting board.  Main requirements: flat, reasonably rigid, reasonably smooth.
Since we were running short on time, I broke down and bought a sheet of phenolic faced plywood at the local Woodcraft

https://www.woodcraft.com/products/phenolic-faced-plywood-18mm-3-4-x-24-x-48?via=5d112bea776f6f4e030006e2%2C5d112cc9776f6f6542000015

Here Gabe is waxing the parting board in preparation for the next steps.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #21 on: November 03, 2021, 06:57:00 AM »
Once the parting board has been waxed several times, go ahead and install the first finished buck onto the board, using some wood screws driven from the back side.  Be careful to plan the screw lengths so that they don't protrude through the finished surface or split the grain.  Since the thickness gets much smaller towards the rudder, you will need to go to much smaller screws, like #2.

Once the buck has been pulled firmly towards, the parting board, force thin beads of modeling clay to completely close the remaining gaps (which should be no larger than 1/32", more like 1/64" and less).  Once satisfied, wax the entire surface with Partall wax several times, buffing out with towel in between each layer. 
Make and install smaller parting boards to cap the nose and (in our case) the future separation line for fuselage take apart.  If you are not making a take apart fuselage, leave out the parting board in the middle of the fuselage.
Do not forget to install at least a couple of peanut-sized indexing pins (I shape mine out of modeling clay) and a few wedge-shaped pieces (also clay) to serve as future prybar pockets for separating molds.
The final step is the application of the PVA release layer.  I found that the most cost effective tool to apply PVA are pieces of Magic Eraser sponge cut into a wedge shape.  Do not reuse those - once PVA dries, throw them away.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #22 on: November 03, 2021, 07:06:45 AM »
For the next stage, you will need quite a bit of materials.  I'm talking at least a gallon of epoxy, and a several yards of heavy (6-8 oz) fiberglass, plus some other things which I'll mention later.
We begin by mixing batches of epoxy (quality stuff, folks, like West Systems 105, not the 1:1 stuff from hobby shops) in roughly 2 oz batches, adding some black pigment and Cabosil thickener along the way until the mix stops dripping from the mixing stick, and spreading the resulting paste quickly onto the surface using 1" wide disposable chip brushes (source: Harbor Freight).  Make sure to start with the tight corners where the buck joins the parting board(s), as any air bubbles trapped there will show up as voids on the inside of the mold. 
Once the surface has been evenly coated with roughly a 1/32" thick layer of this gel paste, sprinkle liberal amounts of cotton flocks onto it.  Some amount of cotton fibers will sink into the gel and provide a great bond to the following layers of the mold.
12-20 hours later, once the epoxy is fully set, vacuum off the excess flocks, and you will get surface that resembles felt.
« Last Edit: November 03, 2021, 10:04:07 AM by Mike Alimov »

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #23 on: November 03, 2021, 07:21:42 AM »
I did not take pictures of the following steps - it's kind of hard to do when your hands are covered in epoxy and fiberglass (you are wearing nitrile gloves, right?).  Start mixing epoxy in 2 oz batches and applying layers of heavy fiberglass, rotating grain direction 45 degrees on each layer (which means that they should have been cut "on the bias" during prep stage).  After 4-6 layers, apply core material called Lantor Soric XF (a flexible honeycomb foam core fabric), and then 4-6 layers of fiberglass again.  The core material serves to build up the layup thickness to the target 3/16"-1/4", instead of the heavy and expensive glass and epoxy layers.
Once cured, use an oscillating cutter to trim the edges neatly.  Do NOT remove the mold from the buck at this time!  Prepare the front section of the buck with release materials, indexing pins, etc, and repeat the process.  Once done, you should have a mold that looks like this.
« Last Edit: November 03, 2021, 10:07:04 AM by Mike Alimov »

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #24 on: November 03, 2021, 07:26:42 AM »
Now join the two bucks using those dowels we provided for alignment, apply glue, and join the halves into a single buck.  When dry, patch and sand the seam until perfect. 
Re-install the one-piece buck into the existing mold, apply the usual release materials (wax, PVA), and repeat the process making the other side of the mold.
Before you separate the newly made mold halves, drill holes every 2" or so around the perimeter of the parting surface for the bolts that you will use to clamp the molds together when making parts.  Again, I used 10-24 screws and nuts, but other similar sizes should work.
If you remembered to install those little alignment tooling pins, you now have mold shells that will align perfectly to each other every time you close them!  It's really cool.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #25 on: November 03, 2021, 07:33:52 AM »
Once done, wash away the remnants of the PVA layer with water and inspect for defects (hopefully, none).  I'm talking about cavities and other serious defects. 
Assuming you did a good job, now is the time to put some real shine on the mold surface.  Starting with 600 wet and going progressively finer to 3000 wet, sand the inside surface of the mold, being careful not to round off the sharp edges of the parting line.
Buff, wax, and admire.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #26 on: November 03, 2021, 07:40:36 AM »
It is now time to plan the actual layup.  Here's what we currently use, in the order of the layup in the mold:
Two layers of 1.4 oz plain weave (aka "E glass") - one layer along the axis, another on the bias (45 degrees), then a 1/16" thick sheet of blue foam (from Lowes) - more on how to prep the foam later - followed by a single layer of 1.4 oz E-glass.
The first two layers are laid in the mold (all the usual release prep: wax, PVA) and allowed to cure until tacky (same feel as a Scotch packing tape).  A pre-formed foam shell is installed, followed by the inner layer of glass, and the entire packet is covered up by plastic release film, paper towel as breather cloth, and vacuum bagged until cured.
Once cured, edges are trimmed with a sharp tool to the parting surface. 

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #27 on: November 03, 2021, 07:43:50 AM »
Once again, avoid the temptation to pull out the shells out of the molds - until assembled, the edges are extremely vulnerable.  Instead, dispense the bead of epoxy+Cabosil+cotton flocks, and close the molds until full cure.

If everything is done right, you will be rewarded with a shiny, strong, and light part.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #28 on: November 03, 2021, 07:46:21 AM »
Do the same for the front part of the fuselage, installing take apart elements (we print ours on a 3D printer) along the way, using molds as alignment jigs, along with other internal parts like bulkheads, motor mounts, etc.

The fuselage as shown weighed 8.2 oz.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #29 on: November 03, 2021, 07:52:35 AM »
A quick note on cutting composite fabrics.
I highly recommend doing so on a cutting mat, and using a rotary tool (we like Olfa brand) found at any fabric supply stores like Jo-Ann fabrics.  Do not use scissors.
Fiberglass cuts cleanly and easily.  Use cardboard templates prepared according to the mold size and shape.
Carbon fabric will fray very easily.  Before cutting carbon fabric, run a narrow piece of masking tape along the cardboard template positioned over the fabric, and then cut in the middle of the masking tape. The tape will hold the fibers from fraying.
Eventually, the masked edges will spill onto the parting board outside of the finished part, and will be cut away during the trimming process.

Offline Crist Rigotti

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Re: LAB (Life After Balsa)
« Reply #30 on: November 03, 2021, 08:43:46 AM »
This system is incredible!  I saw the parts first hand before they became an airplane.  I was amazed on the quality of the parts.  Thanks Mike for the detailed instructions.
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Offline john e. holliday

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Re: LAB (Life After Balsa)
« Reply #31 on: November 03, 2021, 09:24:09 AM »
Mike, the old DOC is going to follow this to the end. Reminds me of when I was trying to compete in F2C and we were supposed to make our own air planes.   We had fibre glas fuselages with no balsa in them.  Another secret, what ever your boys do don't raise your voice to them.  I can see they are doing great work. D>K
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Re: LAB (Life After Balsa)
« Reply #32 on: November 03, 2021, 10:18:21 AM »
Mike, the old DOC is going to follow this to the end. Reminds me of when I was trying to compete in F2C and we were supposed to make our own air planes.   We had fibre glas fuselages with no balsa in them.  Another secret, what ever your boys do don't raise your voice to them.  I can see they are doing great work. D>K

DOC, I don't raise my voice much anymore.  I simply threaten to take away their cell phones (and sometimes do).  To a modern teen, this is akin to a day in solitary at Alcatraz, trust me.

I've always wanted to race F2C, but it became a competition amongst most talented machinists, metallurgical engineers and engine designers.  Stunt remains the last control line discipline where a widely available powertrain can be competitive all the way to the top.  It is probably why F2B is the most popular amongst all F2 classes.  I might try my hand at Clown Racing at Brodak's next year.  :##

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Re: LAB (Life After Balsa)
« Reply #33 on: November 03, 2021, 11:41:51 AM »
Excellent, excellent work.

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Offline Matt Colan

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Re: LAB (Life After Balsa)
« Reply #34 on: November 03, 2021, 12:28:05 PM »
This is awesome stuff!! I’ve been starting to look into trying to figure out composite parts but haven’t made the plunge yet. Thanks for posting this, it’s reawakened my interest in composite parts making, I’m going to keep following this!
Matt Colan

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Re: LAB (Life After Balsa)
« Reply #35 on: November 03, 2021, 06:00:45 PM »
Part III.  BOTH FEET IN THE 21st CENTURY

You might be reading this and thinking to yourself, "Wood from Amish store?  Carving?  How is that 21st century technology?" 
And you might be right.  So read on.

It was right around January when the fuselage molds were finished, and the first successful fuselage was produced.  It had good glossy finish, and was stiff and light enough to give us hope that this technology is viable enough to continue working on.

The next large part of the airplane, of course, is a wing.  Some would say it is the most important part, because that's what is carrying the plane through the air.  After all, that was the foundation of the sheeted foam wing cottage industry: consistently straight, dimensionally stable wings.  Everything else can be built.

So I breached this subject with Gabe.  My thinking was that we could hotwire the wing bucks out of blue foam, cover them in fiberglass, sand and finish, and then pull the mold off that.  There was one limitation, though...
Gabe's ears perked up. 
- What is that?
- Well, I can only hotwire a straight tapered wing...
There was some silence for a moment, and then I heard:
- No.  It has to be elliptical, just like Skittles.
"- Gabe", objected I, "how am I supposed to hotwire an elliptical wing? It's impossible. The wire is straight."
Again, silence, and then "No".
  Have you ever had a 4-year old stop in his tracks at the sight of an ice cream shop on a stroll through town, and refuse to leave until he got a cone?  "Johnny, you can have a popsicle when we get home..."  -- "No!"
That was Gabe at this moment.  He dug his heels in and would not budge, which is not very typical of him.  Usually my technical arguments are convincing enough, but this was not the case that day. 
In retrospect, I'm glad he stood firm.  The composite elliptical wing turned out to behave very well (something that we actually knew from the original, built up Skittles), but I'm skipping too far forward.

We started brainstorming how to make a buck for an elliptical wing.  The only method that I knew of was learned by me from an RC boat modeler about 30 years ago; he was making bucks for racing boat hulls.  Not too long ago I came across Bob Hunt's TruForm molding method description, and it is very similar.  The idea is to install several plywood bulkhead-like templates at predetermined stations along some sort of a building reference board, fill the spaces in between with a material that can be carved (blue foam, basswood, etc), and carve until the surface reaches the bulkheads.  This way the cross sections are both known and guaranteed (as opposed to me eyeballing the fuselage cross sections when carving from a single block). 
I outlined the process to Gabe, and warned that something this involved could cause our build schedule to slip severely, but it did not seem to move him.  You know, this generation of kids just doesn't understand what it's like to operate within constraints (technical, budgetary, etc) - they reach for the sky and worry about the costs later. Maybe it's a good thing, I don't know.

And then he said something that almost made me laugh: 
"I'll 3D print the wing buck".
It is true, he just got a shiny new 3D printer for Christmas, a $500 Creality CR-10 Pro V2.  But wait, kid... It will take you up to a year to learn how to use it!  And then you need to learn CAD, which is probably another year.  I took CAD in graduate school.
"Nah", was Gabe's response, "it won't take that long. I'll have the buck printed before you can make your foam wing. Bet!"

The word "bet" was a challenge thrown my way, and in such a cocky tone, that I just had to teach this kid a lesson.
I said the following:
"If you can produce a useable elliptical wing buck before I make mine, you will get a fast gaming laptop for your birthday".
Needless to say, I was absolutely confident that I'll win this bet.  After all, how long does it take to hotwire a wing!  Piece of cake, huh?

Instead of cake, I ended up eating crow.  Before I had my templates made out of formica (again, thanks Joe!), Gabe presented a fully rendered 3D model of the Skittles wing, done in Fusion 360 (and Autodesk product).  He downloaded the Fusion 360 hobby version (free), watched a 10-minute tutorial on Youtube, and declared, "I got it".  At that time, I chuckled to myself, thinking "good luck, kid".  By the end of the day, he was lofting straight wing sections.  Next day, he had a crude prototype of an elliptical wing, which got refined better and better every day.  Things were looking both promising and more sour for me.
Here's a 30-second animation of his design process in CAD:





Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #36 on: November 03, 2021, 06:03:07 PM »
The next challenge was to take a 3D model from CAD and turn it into something that resembled a wing buck.  Once again, it didn't take long. 
You know, we old school folks take things one small step at a time. At least that's how I was taught.  Not this generation, though.

Gabe's very first 3D print was a section of an RC wing that he downloaded from www.eclipson-airplanes.com
It only took a mere 23 hours to print, but it came out pretty well, except for some initial warping. 
Encouraged by our success, Luca stepped in and, using his newly honed skills from his high school woodworking class, built us an enclosure to keep the temperature more constant.  This helped quite a bit, and the printing quality improved.  We now knew that it was possible to produce full-sized wing sections on our printer.


« Last Edit: November 03, 2021, 06:25:10 PM by Mike Alimov »

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #37 on: November 03, 2021, 06:08:01 PM »
In order to fit within the printing volume limits, Gabe split the wing model in half spanwise, and inserted two cylindrical tubes in the middle of the wing, such that the halves could be joined over two short sections of a cheap fiberglass tube we had laying around. 
A mere 104 printing hours later (!), which took about a week, we were holding two 3d printed wing sections, which matched with each other perfectly, and were more accurate than what a human hand could produce out of wood.

The halves were joined using epoxy, seam patched using Aeropoxy Light (I just had lots of it), and it was looking very, very real. 

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #38 on: November 03, 2021, 06:14:47 PM »
To get rid of the texture left by the 3D printing process, a thin layer of polyester spackle (Icing, Bondo, etc) was evenly applied over the wing surface and then block sanded until a very thin layer of spackle remained on the surface.  Then primer, sand, sand again with finer paper, and then wax.  The buck was finished, and then installed into a fixture made out of thick plywood (and a laminated parting board).  Hot glue was used to hold the wing in position such that the parting board was aligned with the wing centerline, and the remaining gaps sealed with modeling clay.
Notice the unusually high end caps in the mold fixture: such size is not necessarily required to make molds, but it will come in handy when it is time to align two wings for assembly.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #39 on: November 03, 2021, 06:37:07 PM »
Wing molds are produced using the exact same method as was described earlier. The picture shows the molds when they just got separated from the buck for the first time. They had to be washed of PVA, wet sanded and buffed to achieve high gloss surface.

Finally, the first composite wing was produced, and came out right around 6 oz.  You may notice a patch of 6oz carbon cloth around the spar at the root.  In all subsequent wings, this was replaced with several layers of 2oz glass, and proved to be adequately strong, easier to work with, and substantially less expensive.

Well, you know the rest of the story.  The prototype trimmed out on the first day, flew well, and gave us hope that the next batch of airplanes we will be building this winter will fly just as well.


Offline Mark wood

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Re: LAB (Life After Balsa)
« Reply #40 on: November 03, 2021, 07:32:51 PM »
Very nicely executed.
Life is good AMA 1488
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Offline john e. holliday

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Re: LAB (Life After Balsa)
« Reply #41 on: November 03, 2021, 09:53:16 PM »
Well, one thing, once the prototypes are done it should be easy to reproduce the new planes. D>K
John E. "DOC" Holliday
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Offline big ron

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Re: LAB (Life After Balsa)
« Reply #42 on: November 04, 2021, 09:58:26 AM »
This is awesome to see. But WAYYYYYYYYYY over my head. Glad to see those young men working on this.
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Offline Serge_Krauss

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Re: LAB (Life After Balsa)
« Reply #43 on: November 04, 2021, 03:42:02 PM »
Mike, this is a wonderful thread that I have read with great interest and satisfaction. It's just captivating and lucidly laid out for us. You have done wonderful work with more than just your modeling; your boys are much more than a usual source of pride, and I'm amazed at their abilities and dedication. Thanks so much for sharing this with us. Would you also care to share with us your findings relative to the wings' stiffness and any internal structure they might - or might not - have?

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #44 on: November 04, 2021, 04:46:50 PM »
Thanks to everyone for your support and positive comments.
Needless to say, this is complex technology, and is not for everyone.  It requires a large(ish) initial investment into tools and materials, although it gets better after that. 
I've shown that the bucks can be created using two methods: a more traditional (carving) and a modern one (3D printing).  Both have positives and negatives to them, but at least we have a choice!  The beauty of the composite construction is that the materials required (fiberglass, epoxy, and foam) have been abundantly available for decades, with no end in sight - unlike 4-6 lb balsa, dope, and silkspan.

Serge - I will gladly show the internal structure of the wing (not much there, just a spar) and share other details when I have a chance to take more pictures and put them through the image compression software.

For now, I'll open the floor for more comments and questions.

Offline Howard Rush

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Re: LAB (Life After Balsa)
« Reply #45 on: November 04, 2021, 10:13:07 PM »
Thanks for telling us the trick of leaving parts to be joined in their molds. I wondered how people joined parts like that, but I never knew until now.
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Offline Steve Berry

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Re: LAB (Life After Balsa)
« Reply #46 on: November 06, 2021, 08:49:43 PM »
Well, I'll ask the question many are thinking - can we see pictures of completed airframe? Ever thought of selling ARFs of it, or at least the basic components to build one of these for ourselves?

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #47 on: November 06, 2021, 10:16:05 PM »
 Steve, the completed airframe is shown in Reply #3 of this thread. A fully painted version is yet to be built.

I have not thought about ARF yet, but a question back to those who might be potentially interested would be: what price would you be willing to pay for a composite ARF?
I need to know whether it's a viable thing for me to spend my time on.

Offline Ken Culbertson

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Re: LAB (Life After Balsa)
« Reply #48 on: November 07, 2021, 12:24:38 AM »
Steve, the completed airframe is shown in Reply #3 of this thread. A fully painted version is yet to be built.

I have not thought about ARF yet, but a question back to those who might be potentially interested would be: what price would you be willing to pay for a composite ARF?
I need to know whether it's a viable thing for me to spend my time on.
I would think that a price between $2,500 and $3,500.  More if something can be done about the BOM rules.  I recently lost 5 PA planes in a housefire and our insurance had no issue with the $3,500 @ claim price on the newer ones.  I would certainly think that wing/flap sets could be marketed.

You are in a unique business position.  You do not have to recover your development costs and you, at this point, have no investors to please.  This leaves you with the proposition, can I make one of these from my existing molds cheap enough to be worth the time - OR - is the actual manufacturing process after making the molds simple enough that you could pay someone to learn it and do it for you (NOT CHINA).

Bottom line, if you are willing to make the development process one that refines the designs for your own use and you want to sell some of those to others, it might be viable as a source of revenue to fund your hobby.  Outside of that I don't think you could survive success.  That might change if the BOM rules were relaxed but for now.....I wouldn't want to be at a contest where your planes were legal and it's clone was not.
 
Ken
« Last Edit: November 07, 2021, 07:40:21 AM by Ken Culbertson »
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Offline pmackenzie

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Re: LAB (Life After Balsa)
« Reply #49 on: November 07, 2021, 05:59:28 AM »
It is now time to plan the actual layup.  Here's what we currently use, in the order of the layup in the mold:
Two layers of 1.4 oz plain weave (aka "E glass") - one layer along the axis, another on the bias (45 degrees), then a 1/16" thick sheet of blue foam (from Lowes) - more on how to prep the foam later - followed by a single layer of 1.4 oz E-glass.

Did you ever detail this step?  I read though the thread a few times, but didn't spot it.

Thanks,

Pat MacKenzie
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Offline Steve Berry

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Re: LAB (Life After Balsa)
« Reply #50 on: November 07, 2021, 07:04:47 AM »
Steve, the completed airframe is shown in Reply #3 of this thread. A fully painted version is yet to be built.

Yeah, apparently I skipped right over that. My bad.  It is a good looking aircraft.

I, too, would like to see how the blue foam gets prepped and such. This is such an informative thread, with clear concise documentation. Thank you for doing it.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #51 on: November 07, 2021, 08:07:16 AM »
OK, let's talk about foam.
First, a bit of theory.  The purpose of the layer of core material in a composite structure is to create spatial separation between the outer and inner fibrous layers, and to take up the compressive load when the finished structure is being stressed.  The thicker the  core layer, and the higher compressive rating (measured in psi) it has, the stronger the whole composite sandwich will be.
However, for our applications, there are limitations, and some compromises have to be struck.
First, the thickness: the thicker the core, the more it will weigh, no matter the material (foam, balsa, etc).  Also, thicker material is harder to bend to small radii which are found in places like the trailing edge, some wingtips, etc.   If the core can't bend, it will crack, and we generally don't want that.  I empirically determined that the foam thickness of about 1/16" is about right for my choice of material and my layup schedule (i.e. the number and gage of fiberglass layers).
As it turns out, not all foams are created equal.  There are different types of foams (XPS - eXtruded PolyStyrene, EPS - Expended PolyStyrene, polyurethane foams,  expanded polymethacrylimide (PMI), and many others), and then there are different densities within the same type.  Everyone knows the large pink foam sheets made by Corning and sold at Home Depot, but did you know that they come in different grades?  For most people the R-value (thermal insulating value) of the foam is most important, but for us it is density and compressive strength.  Here's a table detailing different grades of Corning Foamular pink XPS foam:
https://dcpd6wotaa0mb.cloudfront.net/mdms/dms/EIS/10015703/10015703-FOAMULAR-SI-and-I-P-Units-for-Selected-Properties-Tech.-Bulletin.pdf
I had a sheet of blue XPS foam from Lowes that I bought long time ago, and it is similar in properties to the Foamular 250, which means it is a 25 psi foam with a roughly 1.5 lb/cu.ft density.  Read that again: 1.5 lb foam - can you find balsa this light?  And if you did, how much would you need and how much would it cost? 
When I sliced half of this 4'x8'x2" board into 3/32" slices the other day, I got over 60 wing panel -sized sheets.  The entire board costs about $35 plus tax. Not bad. 
Here's another interesting fact that I've learned from the Corning foam table.  While the foam density (lb/cu.ft) increases by only 16% when going from Foamular 250 to Foamular 400, the compressive strength goes up by 60%!  Which means it is more benefitial to use stronger foam.  If you can find it, that is.  Regular home improvement stores don't carry those foams, they are considered specialty material and are sold by specialty stores, and cost quite a bit more.
Don't even get me started on specialized brand name foams like Rohacell, Herex, Airex, etc.  A 2'x4' sheet of 3mm (~1/8") Rohacell is listed at $99 in small quantities.  Some of these are difficult to find, too, and I've never seen them in thicknesses that would work for us (1 - 1.5 mm).  If you have a source, let me know.
Anyway, once you hotwire your blue or pink foam to about 3/32" thickness, it comes out with a layer of crust (melted foam on the surface) covered with long whiskers.  Both the whiskers and the crust need to be sanded off.
Here Gabe is sanding away using a cheap Harbor Freight sander. 
I highly recommend doing both the hotwire cutting and the sanding outside (pick a nice day), if you don't want to be evicted by your wife or develop some disease as a result of exposure to toxic fumes and foam dust.

I will continue when I have more pictures.


Offline Mark wood

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Re: LAB (Life After Balsa)
« Reply #52 on: November 07, 2021, 08:27:57 AM »
OK, let's talk about foam.
First, a bit of theory.  The purpose of the layer of core material in a composite structure is to create spatial separation between the outer and inner fibrous layers, and to take up the compressive load when the finished structure is being stressed.  The thicker the  core layer, and the higher compressive rating (measured in psi) it has, the stronger the whole composite sandwich will be.

Nice job explaining the purpose of the core. Balsa simply would be a waste used in a core. The reality is that by the time you bulk up the section to handle the compression load using balsa for our models, the balsa will handle the load negating the need for the carbon. Well, up until you scale up to wind turbine blade size that is.
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Offline pmackenzie

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Re: LAB (Life After Balsa)
« Reply #53 on: November 07, 2021, 08:29:10 AM »
CST has Rohacell in 1mm thickness

https://www.cstsales.com/a-rohacell-ig-51.html
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Offline pmackenzie

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Re: LAB (Life After Balsa)
« Reply #54 on: November 07, 2021, 08:31:33 AM »
Nice job explaining the purpose of the core. Balsa simply would be a waste used in a core. The reality is that by the time you bulk up the section to handle the compression load using balsa for our models, the balsa will handle the load negating the need for the carbon. Well, up until you scale up to wind turbine blade size that is.

Pretty sure the Yatsenko models use balsa cores.
Balsa is a very hard material to replace :)
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Offline Mark wood

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Re: LAB (Life After Balsa)
« Reply #55 on: November 07, 2021, 09:00:01 AM »
Pretty sure the Yatsenko models use balsa cores.
Balsa is a very hard material to replace :)

That simply adds weight as the carbon carries all of the load. All the core does is keep the carbon in column. Without a core the carbon would buckle under compressive loads. It takes a working knowledge of the Euler buckling equations to fully understand.
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Offline pmackenzie

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Re: LAB (Life After Balsa)
« Reply #56 on: November 07, 2021, 09:11:26 AM »
From what I have seen Yatsenkos are glass/balsa/glass, not a lot of carbon in them.
The main thing that balsa will provide over foam is ding resistance, since that mainly comes from the core material compressive strength due to the thin composite layer.
Rohacell is better than the extruded PS foams for this.
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Offline Avaiojet

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Re: LAB (Life After Balsa)
« Reply #57 on: November 07, 2021, 04:33:07 PM »
If I was going to build a model using no balsa, I would have a conversation with this guy. Istvan Travnik, and from 2012!! Wow! does he ever have experience.

https://stunthanger.com/smf/building-techniques/the-bluefoam-model-or-how-to-build-a-stunt-plane-without-a-bit-of-balsa/

Remember him? I'm sure many do.

Nothing like being ahead of the wave with experience, and this guy has plenty of it.

No secrets, BS, dead ends, or searching for articles either, he spells it all out in a great photo build.

And for those two individuals who will attempt this masterful craft, he can be PM'ed.
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Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #58 on: November 07, 2021, 07:34:10 PM »
...
The main thing that balsa will provide over foam is ding resistance, since that mainly comes from the core material compressive strength due to the thin composite layer.
...

Bingo!  Exactly!  Balsa core is used to increase dent resistance, which is a purely utilitarian consideration, not a structural requirement.  Stunt people are obsessed with shine and good looks, always have, always will. Sometimes to the detriment of the plane's flying ability (16 oz shiny paint jobs, anyone? I'm not making this up).

The beauty of our class (F2B/stunt) is that we are not competing for the fastest / biggest / strongest / highest / longest-flying airplane (insert any other superlatives here).  If we were, then all stops would come out, and I would be using the highest grade of materials possible - carbon, honeycomb or Rohacell, etc, and so would everyone else.  Look at all free flight classes, F2C team race, and RC pylon racing - there is not much balsa wood being used there (that is to say, effectively none).  But for stunt we don't need the plane to be the most-est in anything (strength, weight, etc) - we just need it to be light ENOUGH and strong ENOUGH to take us through a pattern, and dent resistant ENOUGH to handle a few seasons' worth of competitions and practice and still look pretty good.
 
As a result, we have the luxury to choose from several technologies available to us.  The most established one, of course, is a built up balsa frame finished in tissue and dope.  Then there is the sheeted foam, now with epoxy finish. And the composites of different kinds.
 The esteemed brothers Yatsenko have set a pretty high bar when it comes to quality and durability of their equipment.  But their method and choice of materials can hardly be considered accessible /attainable by an average home modeler, or else everyone would be copying this formula.
When I was developing our composite methods, one of may goals was to keep the entire process attainable as much as possible.  That's why carbon and Rohacell were not in consideration.  My foam sheets end up costing pennies per sheet (not counting time/labor, of course), and 1.4 oz E-glass ended up being pretty close to a $1/yard, not much more than toilet paper.
To compensate for the lack of compressive strength in XPS foam, I simply doubled up the outer layer of glass - again, not because a single layer would not be strong enough for typical aerodynamic loads (it is), but purely to increase dent resistance.
So, there you go - it is possible to achieve about the same result using a combination of thin glass (3/4 oz?) with balsa or Rohacell, or thicker glass (1.5 - 3 oz) with XPS foam.  I hope this does not devolve into another "what's better- pipe or 4-2 break engine" argument - as it is apparent that more than one composite layup formula can be viable for our application.

Lauri has mentioned something that I also wanted to address - that balsa core composite wings are more rigid, and how apparently it is better.  Is it, though?  Yes, when I finished the first composite wing (with only a single spar at the main location), the rear part of the wing felt like it would have a little play if subjected to mild vibration (and vibrations are present in all airplanes, including electric).  Then I remembered that back in the ole days when everyone flew combustion powered airplanes with soft wing covering (tissue, Monokote, etc), we didn't have to worry about installing vortex generators, trip strips and the like.  The slight vibration of the wing surface was doing a good job shaking the stagnant boundary layer off our wings and keeping the plane tracking very predictably (Mark Wood had a separate thread going for that effect).  When the rigid foam wings came, and people learned how to create those stunning 20-point mirror finishes, combined with a smooth electric powertrain, we started seeing more and more of these devices on wings and tails.
So I decided to skip ribs and secondary spars of all kinds, and let the back of the wing vibrate a little bit.
We have not had a need to install any vortex-generating devices on the Ripslinger, and it is tracking pretty well.  Coincidence?

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #59 on: November 09, 2021, 09:36:56 AM »
It is time to put some controls in that new shiny composite plane.  As has been discussed here in great detail (and pretty much agreed on - a rare phenomenon on this forum), electric-powered planes have an even higher need to have smooth controls, as there is no vibration to shake them loose.
Here's how I make controls these days - maybe not the best way (I welcome any suggestions on how to make it better), but accessible to everyone, and using widely available materials and tools.
Start out by buying an electrical connection block at Lowes.  Using side cutters, detach one segment, remove two retaining screws, and free up the metal insert.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #60 on: November 09, 2021, 09:40:18 AM »
Secure the metal part in Vice-grips and cut in half with Dremel tool.  Additionally, I mount the newly formed piece on a long M3 bolt (that's metric 3mm thread, folks) and grind a little more to remove burr and give it a more slender and noble shape.

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Re: LAB (Life After Balsa)
« Reply #61 on: November 09, 2021, 09:50:05 AM »
Pre-cut 1/8" music wire to appropriate lengths as needed for your design.  I chuck each piece into my little lathe (a drill press will also work) and polish the surface starting with 600 paper and progressing to a polishing compound.  This surface will eventually ride inside nylon bearings.  Do this step for both the flap and elevator horns. 

Obtain the following supplies:
- long (40-50 mm) M3 screws; I recommend plated, not black oxide, as they solder much easier;
- 3 mm horn bearings from Hobby King (they call them Unilateral Hinge 5x3x20);
- a block of nylon.  Scraps from machine shop.

The horn bearings will need to be drilled out to accept 1/8" music wire.  Use index drills and pick one that provides for free rotation but no slop.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #62 on: November 09, 2021, 09:59:43 AM »
Make the first 90 degree bend in the horn wire, using your favorite wire bender.  Then slide the metal joiner and the nylon hinge(s) in whatever order makes sense for your airplane.  My pictures reflect a horn being made for a profile plane, hence the horn will be offset.  In a typical full body stunter, the horn will be centered and flanked by the two nylon hinges.

Using the same cut-off wheel on Dremel tool, grind a little flat on the horn wire in such a way that the screw, when driven into the brass joiner and positioned over the flat, locks in a vertical position.  If a little off, re-grind the flat at a corrected angle.  Be careful not to grind too deep into the wire, no more than 1/4 of the wire diameter.
When satisfied, apply soldering flux designed for steel alloys and flow some solder, locking the screw in place.  A little solder should flow out of the brass joiner and along the music wire.
Let cool and clean the flux off with baking soda and water.

Then mark and make the second bend (making sure that all your bearings are installed first!)


Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #63 on: November 09, 2021, 10:23:43 AM »
The last step is to make a small nylon block with three holes: one tapped for M3, and two others drilled for the diameter of your pushrod wire (I recommend 3/32" music wire, with polished ends of course).

The elevator horn can use a piece widely known in the RC community.

The best feature of these horns is that they provide a very fine adjustment of the arm length: unlock the pushrods, turn the piece 180 degrees (half a turn) and get a 0.25mm (approx. 0.01") height adjustment.  Fly and try - if you want more, turn more; if you want to go back, simply go back 1/2 turn.  Predictable and reliable. 
Here are some benefits compared to traditional horn construction:
- does not require brazing;
- no metal-on-metal friction points that require lubrication, all pivots are metal on nylon;
- very fine and repeatable adjustability.

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #64 on: November 09, 2021, 10:38:25 AM »
Our bellcranks are made out of 1/8" phenolic board (bakelite? garolite?).  Earlier versions had a simple hole for the axle (1/8" music wire), while the Ripslinger was the first plane to use a bellcrank suspended in ball bearings. 

That's pretty much all there is to the controls.  Let me know if you have any questions / comments.

Offline Brent Williams

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Re: LAB (Life After Balsa)
« Reply #65 on: November 09, 2021, 09:57:52 PM »
Have you had any fear of failures with those dubro clevises?

Monty Sumach recently lost his beautiful ship to the elevator clevis opening up.

Laser-cut, "Ted Fancher Precision-Pro" Hard Point Handle Kits are available again.  PM for info.
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Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #66 on: November 10, 2021, 08:46:31 AM »
Have you had any fear of failures with those dubro clevises?

Monty Sumach recently lost his beautiful ship to the elevator clevis opening up.

Brent, no, I'm not familiar with Monty Sumach, his airplane, or the circumstances of this accident.  So, after hundreds of trouble-free flights using these Dubro kwik-link clevises, I was living without fear...
Please tell us more details: was it a control line or RC airplane, combustion or electric powered (I think I see a pipe brace in the fuselage belly?), was the brass locking plate present in the clevis when first discovered?  Etc...

And even if the community agrees that this is not the best clevis to use, we can switch to an all-metal Sullivan version, or a single-sided nylon block similar to the one we use on the flap horn...

Offline Brent Williams

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Re: LAB (Life After Balsa)
« Reply #67 on: November 10, 2021, 09:07:26 AM »
If you're on facebook, the Alberta control line page has the details of the crash.  Monty's plane was a large, piped, control line stunt scale ship.  From his report, it had a persistent vibration which possibly led to the pin backing out.

Perhaps the reduction in overall vibration in electric planes would make clevis type connections more trustworthy.
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Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #68 on: November 10, 2021, 03:03:21 PM »
Brent - I would never recommend using RC clevises in internal combustion planes; that's a disaster waiting to happen. 

Lauri - very interesting and valuable input.  I could simply insert a partial-span foam spar about halfway between the main structural spar and the trailing edge, and that would stiffen up the back of the wing.  Otherwise, as you noted, the thicker foam core makes up for the lack of compressive strength, and the wing is as strong as anything else out there.

Great discussion, guys.

Offline Howard Rush

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Re: LAB (Life After Balsa)
« Reply #69 on: November 12, 2021, 07:38:47 PM »
Bingo!  Exactly!  Balsa core is used to increase dent resistance, which is a purely utilitarian consideration, not a structural requirement.

The core might be taking some shear.  I'd have to check with the stress man.

To compensate for the lack of compressive strength in XPS foam, I simply doubled up the outer layer of glass - again, not because a single layer would not be strong enough for typical aerodynamic loads (it is), but purely to increase dent resistance.

I made some Kevlar-Foamular-Kevlar shells that were kinda dent prone.  I put some .5 oz. carbon mat on the outside surface.  It made a nice surface finish and helped the dent issue.  I used Epon 815 epoxy.  Some higher modulus epoxy like that in prepreg might also have kept it from denting for less weight. 


Lauri has mentioned something that I also wanted to address - that balsa core composite wings are more rigid, and how apparently it is better.  Is it, though?  Yes, when I finished the first composite wing (with only a single spar at the main location), the rear part of the wing felt like it would have a little play if subjected to mild vibration (and vibrations are present in all airplanes, including electric).  Then I remembered that back in the ole days when everyone flew combustion powered airplanes with soft wing covering (tissue, Monokote, etc), we didn't have to worry about installing vortex generators, trip strips and the like.  The slight vibration of the wing surface was doing a good job shaking the stagnant boundary layer off our wings and keeping the plane tracking very predictably (Mark Wood had a separate thread going for that effect).  When the rigid foam wings came, and people learned how to create those stunning 20-point mirror finishes, combined with a smooth electric powertrain, we started seeing more and more of these devices on wings and tails.
So I decided to skip ribs and secondary spars of all kinds, and let the back of the wing vibrate a little bit.
We have not had a need to install any vortex-generating devices on the Ripslinger, and it is tracking pretty well.  Coincidence?

A local guy puts more VGs on his open-bay Flaming Dog Head airplanes than on his all sheeted ones.  I'll set him straight. 
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Offline Doug Moon

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Re: LAB (Life After Balsa)
« Reply #70 on: November 15, 2021, 02:48:55 PM »
Can you show the inside of the wing and how you set that up? Can you show how you joined the two shells together? 
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Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #71 on: November 15, 2021, 04:36:57 PM »
Can you show the inside of the wing and how you set that up? Can you show how you joined the two shells together?
Hi Doug,
here are a couple of shots of the inside of the first wing prototype (the plane is in storage now, hard to get to), which is pretty close to the final version.  Not much inside the wing - primarily a spar made out of 1/8" balsa - we may be replacing that with a composite sandwich in the future.  Besides that, there are 3 balsa inserts about 1"x1" in the trailing edge for hinges, and the take apart hardware (the root rib has cut outs for that), both not seen in this photo.  The take apart hardware will be specific to your chosen system (if you go that route); otherwise just glue a couple of basswood or plywood joiners and glue the wing halves together.
As you can see, there is only one spar - no further structural elements.  I didn't see a need for those. I'm being told otherwise (not for structural, but for aerodynamic benefit).  I have a feeling that only those who consistently fly 580+ point patterns will feel the difference.

I don't have pictures of how two shells are being glued together.  You will need to wait until our winter building cycle begins, which hopefully won't be long now.  Gabe might even upload a few videos to his YouTube channel, since some things are easier to show/explain live rather than in a pictorial. 

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Re: LAB (Life After Balsa)
« Reply #72 on: November 16, 2021, 11:34:02 AM »
Looks great!

Thank you
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Offline Donald Main

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Re: LAB (Life After Balsa)
« Reply #73 on: November 21, 2021, 06:38:17 PM »
I just read thru the whole thing and enjoy everyone's hard work. Your dedication is awesome and your kindness to share shows how humble you are. I know Gabe and Luca will keep excelling for a long time into the future. All three of you are ambassadors for our sport. Thank you. 

Offline Dennis Toth

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Re: LAB (Life After Balsa)
« Reply #74 on: November 22, 2021, 06:12:43 AM »
There's a material call Depron foam that is a rigid sheet, sandable, light and in 1 - 4mm thickness. Midwest use to carry it, It is the material that the foam dinner plates are made of. I have used it for small catapult gliders, it glues well with Elmer's and takes waterbased acrylic paint well. This type of foam could be used for ribs and wing sheeting with a good I beam type spare. Problem is I can't find a supplier who has any in stock.

Best,    DennisT

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Re: LAB (Life After Balsa)
« Reply #75 on: November 22, 2021, 09:25:35 AM »
Don - thank you for the kind words.  We always enjoy your company at the contests, and look forward to flying with you in the future.  It is my intention to share whatever I know with the community, as a way to give back for all the years that I've been on the receiving end of mentorship and help.

Dennis - yes, I was aware of Depron, and have a few sheets.  However, I did not pursue this material for the same reason you mentioned: it is simply not available in the US.  I just checked, and the two top sites (RCfoam.com and RCdepron.com) do not have 1mm foam.  Other thicknesses are not acceptable due to their increased weight and stiffness.
So, until/unless someone decides to sponsor us by shipping a few dozen sheets of 1mm Rohacell or Airex, we will continue to use the blue (pink?) insulation foam.

Offline Mark wood

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Re: LAB (Life After Balsa)
« Reply #76 on: November 22, 2021, 12:19:16 PM »
Mike,
While I think that nothing serious can be made from Depron, its original use is an insulation layer under those cheap laminate parquets. So maybe a good source would be someone selling/installing floors.
As main spar I'd like to see something better, a proper I-beam. So, 3-4mm vertical grain balsa core, perhaps secured with +/-45 degree thin glass on both sides and good hardwood or carbon strips as spars. L

Well, I would debate that some. The trouble with this structure, which is actually holding up, is from buckling and not shear. While vertical grain wood would be good the horizontal grain is likewise adequate provided the intergranular shear strength isn't exceeded. Since there has not been a failure indicating a short fall, increasing the difficulty of manufacture and weight isn't directly warranted. From reading the only "problem" is some buckling causing some profile deformation.  It doesn't take much to prevent buckling provided the section modulus is great enough. Thin carbon won't do that. It will carry lots of tension but by the time you fixate it for the buckling the need for the carbon no longer exists or there is far more carbon and weight than necessary. Wood and fiberglass have similar stiffness so if I were going to add a spar cap, it'd be spruce but the if part of this is that I probably wouldn't, that balsa shear web spar seems be enough and if'n I was worried about it, I'd simply switch from 1/16 to 3/32 and head down the road. Think of it in terms of a stress skin structure. Keep the skin in alignment and all is good.

Of course there is always the coolness factor and carbon fiber certainly fills that requirement.
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Offline Mark wood

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Re: LAB (Life After Balsa)
« Reply #77 on: November 22, 2021, 01:41:57 PM »
Mark,
I’ve gone through that development from the very first Shark versions (foam or lenghtwise grain balsa spar, foam ribs) to the latest versions (as I explained in previous message).
The first ones were really annoying, a cracked rib or spar leads to a structure that does not work correctly, ALSO with this kind of monoqoque structures.
But I must add that my experience is with IC engines, with electric it may be less sensitive.
Anyway, they are details that are difficult to fix afterwards, so
it’s better to make it right in the first place. And it’s really not that much more work.
Another thing is that in a take-apart model, the take-apart components in wing are easier to anchor to a good spar(cap) than just to skin. L

Well, Lauri, I can't disagree with that.  I went away from vertical grain webs in almost all of my designs as the shear in the web isn't all that high. I also don't do monocoque structures as my stress analysis skills and patience aren't up to the task. I prefer a D Tube I beam spar design as it is easy for me to analyze. Of course I don't do that on models as they are kind of a known quantity to plagiarize. For models my preferred wing on CL models is a hybrid between the Nobler style and how Yuriy builds the classic wing although I wasn't aware of his construction before making mine. The web I use is more for a construction alignment aid than a full structural member. Being a long time free flight builder I am hyper aware of weight and challenge every decision when adding structure. Carbon fiber is great, however it is 15-20 times stiffer than fiber glass or balsa wood and much more dense than balsa which means that, in order to satisfy Euler enough of balsa or foam needs to be used or a section of carbon weighing more than the balsa needs to be used. Generally by the time you satisfy Euler, Poisson is no longer in the neighborhood when using balsa.

I'm not saying it is a bad idea as there are plenty of examples of it working and most of my FF models include CF hybrid spars which are joined to CF D tubes and CF capped ribs. As you know from your engineering background that the trouble begins when you mix materials of differing stiffness. That's my point primarily.
« Last Edit: November 23, 2021, 07:41:37 AM by Mark wood »
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Offline Howard Rush

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Re: LAB (Life After Balsa)
« Reply #78 on: November 23, 2021, 04:19:34 PM »
Carbon fiber is great, however it is 15-20 times stiffer than fiber glass or balsa wood and much more dense than balsa which means that, in order to satisfy Euler enough of balsa or foam needs to be used or a section of carbon weighing more than the balsa needs to be used.

Another story:  I was building Snorts awhile back and wondering if a 1/4" carbon spar cap with staggered plies would suffice.  A structures guy (nobody you know) was at the house, and I asked him. I knew that 1/4" square spruce tapering to 1/4" x 0 worked, so we looked up some parameter of spruce (I forget what) and decided that the carbon strip had the same amount of it.  I made the spar and something seemed wrong.  I went back and looked at the book and noticed the "x 103" at the top of the spruce column.  I ended up with carbon-honeycomb-carbon spar caps.  This was far from the goofiest Snort issue.   
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Offline Mark wood

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Re: LAB (Life After Balsa)
« Reply #79 on: November 23, 2021, 06:33:07 PM »
Another story:  I was building Snorts awhile back and wondering if a 1/4" carbon spar cap with staggered plies would suffice.  A structures guy (nobody you know) was at the house, and I asked him. I knew that 1/4" square spruce tapering to 1/4" x 0 worked, so we looked up some parameter of spruce (I forget what) and decided that the carbon strip had the same amount of it.  I made the spar and something seemed wrong.  I went back and looked at the book and noticed the "x 103" at the top of the spruce column.  I ended up with carbon-honeycomb-carbon spar caps.  This was far from the goofiest Snort issue.   

There have been some bad lessons learned in the 4/4 aerobatic world trying to use carbon with spruce in spars. There are a few that have succeeded and a few that haven't. The problem is that carbon is so much stiffer than spruce that it will carry all of the load and the spruce won't carry any. When a load is applied to the wing it bends and  the stiff carbon doesn't stretch much which creates a large shear force along the bond interface and eventually tears loose and the wing fails. It's kind of one of those get all the way in kinds of things. By the time you get the carbon to where it can manage the buckling spruce becomes far to heavy of a core as it doesn't carry any of the load. Foam and honeycomb are much better core materials than spruce for carbon.
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Offline Howard Rush

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Re: LAB (Life After Balsa)
« Reply #80 on: November 23, 2021, 06:58:35 PM »
I guess I wasn’t clear about the spruce. Some guys just can’t tell a joke.
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Offline Mark wood

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Re: LAB (Life After Balsa)
« Reply #81 on: November 23, 2021, 08:06:50 PM »
I guess I wasn’t clear about the spruce. Some guys just can’t tell a joke.

Yeah, it doesn't come through on the typewriter... ::) Cool spar cap though
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #82 on: November 24, 2021, 11:04:40 AM »
I think we should be guided by theoretical engineering, but the limitations of practical model building sometimes alter the choice of materials, as I illustrated with foam. Maybe Euler and Poisson would demand that I use Rohacell at $50 per sheet, but my wife thinks that the 25 cent blue foam is better for the family budget.
With regards to the spar: what you saw in the pictures was a prototype wing. The real wing used three staggered layers of 2oz/yd glass about 2" wide at root and tapering to 1" and tip: first layer full span, second layer 2/3 span, third layer 1/3 span. So, the root of the wing has an equivalent of 6 oz glass spar caps, and also 1/8" vertical grain balsa about 1/3 span, laminated to the lengthwise 1/8" spar shown on the photos.
It wasn't too heavy and the wings didn't fold, while being easy to build.

What presented a greater challenge than the wings was stabilizer. We used the same layup schedule as everywhere else, but weight did not scale favorably to the smaller parts: the stabilizer alone came in at 3oz, whereas it should be more like 1.5oz. Any ideas would be welcome.

Offline Mark wood

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Re: LAB (Life After Balsa)
« Reply #83 on: November 24, 2021, 11:41:48 AM »
I think we should be guided by theoretical engineering, but the limitations of practical model building sometimes alter the choice of materials, as I illustrated with foam. Maybe Euler and Poisson would demand that I use Rohacell at $50 per sheet, but my wife thinks that the 25 cent blue foam is better for the family budget.
With regards to the spar: what you saw in the pictures was a prototype wing. The real wing used three staggered layers of 2oz/yd glass about 2" wide at root and tapering to 1" and tip: first layer full span, second layer 2/3 span, third layer 1/3 span. So, the root of the wing has an equivalent of 6 oz glass spar caps, and also 1/8" vertical grain balsa about 1/3 span, laminated to the lengthwise 1/8" spar shown on the photos.
It wasn't too heavy and the wings didn't fold, while being easy to build.

What presented a greater challenge than the wings was stabilizer. We used the same layup schedule as everywhere else, but weight did not scale favorably to the smaller parts: the stabilizer alone came in at 3oz, whereas it should be more like 1.5oz. Any ideas would be welcome.

No Euler is the buckling trouble and the separation created between the layers by the foam is what is necessary. In essence the Euler buckling equation take in to account the section modulus under compression. The foam core helps keep the load bearing material in column. Buckling occurs when the load bearing member deflects a little bit and then departs catastrophically. That is what the foam or balsa or what ever core material being used does. In lightly loaded structures it doesn't take much to do that and the very lightest foam cores should be sufficient. Of course cheap is  generally pretty light. I think your wing lay up schedule is nearly perfect for a composite structure. 

Balsa is super hard to beat in small light structures. Since the loads are low in the stab, foam spar webs and ribs could work. There are several full size aircraft that use a method where the wing is made in two half molds, the same as your molds, with ribs and spars in mirror image. he spars and ribs are basically split in half making a top and bottom pieces along the for aft center line. The spars have caps and webs on both sides made with composite material. The wing two halves are then simply glued together same as you are currently doing. Since the spilt line is basically the neutral axis of the spar the butt glue joint is sufficient. For the stab I would probably forego the thin foam and use a single biased layer over the entire planform, some tow as a spar cap and maybe another layer.

The construction schedule would be to first form the skin in the mold. The spar pieces could be made at the same time in a separate mold maybe.  They could even just be balsa. The foam ribs and spar halves would be glued in to the skin while still in the mold. The spars could be reinforced with glass at this time if needed. After curing the halves sanded flat and glued together.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Crist Rigotti

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Re: LAB (Life After Balsa)
« Reply #84 on: November 24, 2021, 12:12:38 PM »
<snip>
What presented a greater challenge than the wings was stabilizer. We used the same layup schedule as everywhere else, but weight did not scale favorably to the smaller parts: the stabilizer alone came in at 3oz, whereas it should be more like 1.5oz. Any ideas would be welcome.

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Offline Ken Culbertson

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Re: LAB (Life After Balsa)
« Reply #85 on: November 24, 2021, 12:32:36 PM »

What presented a greater challenge than the wings was stabilizer. We used the same layup schedule as everywhere else, but weight did not scale favorably to the smaller parts: the stabilizer alone came in at 3oz, whereas it should be more like 1.5oz. Any ideas would be welcome.
Maybe that is not a *really* bad thing depending on other factors.  When I switched to electric I basically built like it was for IC but a different nose.  Everything came out nose heave and I found I was putting lead or clay under the stab of just about everything.  So I started building a more robust stab which was naturally heavier and I haven't had to add weight to regain the balance since.  Just a thought.

Ken 

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Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #86 on: November 24, 2021, 02:35:29 PM »
Maybe that is not a *really* bad thing depending on other factors.  When I switched to electric I basically built like it was for IC but a different nose.  Everything came out nose heave

Ken

Well, we have 3 oz(!) of extra weight in the nose. Ended up swapping an originally planned 2826 motor for a 3525, and replacing a 5S2800 with a 6S2800. Otherwise the plane would have been at 59oz flying weight.

Offline Ken Culbertson

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Re: LAB (Life After Balsa)
« Reply #87 on: November 24, 2021, 02:43:38 PM »
Well, we have 3 oz(!) of extra weight in the nose. Ended up swapping an originally planned 2826 motor for a 3525, and replacing a 5S2800 with a 6S2800. Otherwise the plane would have been at 59oz flying weight.
Good for you!  Most just slap in lead.  I like to slap in more capability.  With electric you can always add a bigger battery!
With IC you can always slap in more lead.  wasn't implying that the extra tail weight was Good, only that it may not be a problem if you build nose heavy.

Ken
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Offline Mike Alimov

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Re: LAB (Life After Balsa)
« Reply #88 on: November 24, 2021, 04:01:21 PM »
I guess it's a difference in design philosophies, but I don't like the weight penalty of the more powerful systems if I can't take advantage of the extra capability. I've flown the prototype with both power systems described above, taping 3oz of lead when necessary, and didn't feel any difference. Same prop, same speed, same line tension, airplane behavior, etc. I think it's a case where enough power is good, but more is not better.

Offline Mark wood

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Re: LAB (Life After Balsa)
« Reply #89 on: November 24, 2021, 04:44:54 PM »
I guess it's a difference in design philosophies, but I don't like the weight penalty of the more powerful systems if I can't take advantage of the extra capability. I've flown the prototype with both power systems described above, taping 3oz of lead when necessary, and didn't feel any difference. Same prop, same speed, same line tension, airplane behavior, etc. I think it's a case where enough power is good, but more is not better.

You and me both on that one. However the larger motor is actually a little more power efficient for not too much more weight. Lead on a plane just scratches me in the wrong way.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Howard Rush

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Re: LAB (Life After Balsa)
« Reply #90 on: November 24, 2021, 05:12:10 PM »
However the larger motor is actually a little more power efficient for not too much more weight.

Then you can get away with a lighter battery.  Of course, then you'd need to add more lead.  Mind you, this is coming from a person who made his airplane's nose so light that he repeatedly broke it off while building it, then has added so much ballast that it's hard to find room for more.  The lead doublers are strengthening the nose, though.   
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Offline Mark wood

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Re: LAB (Life After Balsa)
« Reply #91 on: November 24, 2021, 05:22:10 PM »
Then you can get away with a lighter battery.  Of course, then you'd need to add more lead.  Mind you, this is coming from a person who made his airplane's nose so light that he repeatedly broke it off while building it, then has added so much ballast that it's hard to find room for more.  The lead doublers are strengthening the nose, though.   

Yeah, I thought of that when I was writing the words. It's a consolation thing. And Mike is right, better to have more battery and capacity than need than lead. Lead just scratches all the wrong places. Especially when it's in the front. Then it's not only scratching in the wrong places, it's drawing blood. 
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman


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