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Oct 07, 2018, 12:48 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
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Discussion

Molded Carbon Fiber Stick Plane (now with Video)


This project is to do a molded carbon fiber plane, similar in size to a Vapor Lite. But, the goal is to be lighter. This plane is to practice and continue getting better at molded carbon fiber rod construction. And, it is to try molding more complex shapes, hence the scalloped trailing edges.

My first attempt at a molded CF plane can be found in this thread in the F3P forum. These techniques were developed by people who compete in F3P. I got a lot of advice from people on that forum, and learned a lot by reading the threads over there. In particular I got a lot of advice and help from Ruslan Ermolin and Ampbomber (George). In fact, my Carbon Bender is my version of a Ruslan design, just scaled up a bit larger. And, I've made my own tweaks to the methods given the equipment and what I have in my workshop. Here is the thread on my first CF plane.
https://www.rcgroups.com/forums/show...olded-CF-Plane
I am posting this plane in this forum because it is a simpler rudder/elevator controlled plane, and smaller. But, if you want to jump ahead on some of these techniques, you can see them in the above thread on my Carbon Bender.

Other differences are all my molded CF rod on the Carbon Bender used a technique of forming them in grooves lazered in foam similar to Depron, and the CF rods were "bare" and not in tubing. On this plane I am using the "heat shrink" method where CF tow with not fully cured epoxy is encased in heat shrink tubing, which is shrunk, and then placed into openings lazered in posterboard.

Here is a picture of the Carbon Bender, and a quick drawing of what the Carbon Stik will look like. Essentialy it will have a wing that looks like a cartoon version of a Fokker Dr-1 top wing. It will not have ailerons, those will be drawn on the covering. The rudder will be full floating like a Dr-1. Construction and assembly should be much simpler than the Carbon Bender. Equipment will be two MicroflierRadio servos, the older ones that have 1/3 the torque of the newer ones; a Deltang RX, and some sort of geared drive, probably a geared 4mm drive, but it might be 6mm. Time will tell on that.
Last edited by Gordon Johnson; Apr 02, 2019 at 09:54 PM.
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Oct 07, 2018, 01:06 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
Thread OP
The first step is to mold CF rod into a shape other than a straight rod. Straight CF rods are available from many sources. Curved CF rods you have to do yourself. A common question is why can't you just heat up a commercial CF rod and bend it into the shape you want, and then let it cool. That doesn't work. Try it to see. So, the basic technique is to take CF tow, which is lots of strands of CF filament, wet them out with two-part epoxy, and then mold them using some kind of form, wait for the epoxy to cure, and voila you have a CF rod in the desired shape. Well, it's actually a bit more work than that.

The messy details:
CF tow is available from a variety of sources. The F3P planes are pretty much made from 0.5mm CF rod these days, and that's what size I used and molded for my Carbon Bender (except the side bracing on the fuselage which is 0.3mm). 3K CF tow has 3,000 strands of CF filament, and this is what results in 0.5mm CF rod, whether straight or curved. I get mine from The Compsite Store (CST Sales) out in California.

Like many other people I'm using West System epoxy. This is mixed 5 to 1 (5 parts epoxy and 1 part catalyst). I use syringes and do it by weight on my scale, and then mix throughly.

For the method of heat shrink tubing I'm using tubing I get from DigiKey, and am still trying different varieties. I've used black heat shrink, which works well, but is hard to differentiate between black tubing and black CF when slicing the tubing off later. I'm using clear heat shrink on this plane, but it seems to take more heat. The process I use is to CA the upper end of the CF tow so it's glued together at the tip. I fold a piece of Gaffers Tape over it (the tow slips out of it when I used duct tape). After punching a hole in the tape, this is what the wetted out tow will be hung from a nail in the rafters of my workshop.

At the bottom end I CA the tow to a 0.5mm diameter length of piano wire. This is what the heat shrink tubing will be slid over, and then when holding the bottom of the wire, on up over the wetted out CF tow.

These are stretched out and taped down over wax paper. I pour the epoxy mixture into a small zip-log bag (an idea I borrowed), cut off one tip, and then drizzle the epoxy along the lengths of CF tow. Using my finger, I smooth the epoxy into the CF tow to get it good and wetted out.

The short length of CF tow is two strands of 3k tow combined to get 6k tow, and this thicker length is for the landing gear, which will be 1.0mm molded CF rod.

Each wetted out CF tow, with its piano wire attached at the bottom is hung from the rafters, and with folded pieces of paper towel have excess epoxy removed. Then, following others (Ampbomber on RcGroups) technique I "palm" the wetted out tow between my palms, and then paddle it with sanding blocks with wet and dry sand paper installed backwards so the paper side helps smooth the rods, and remove excess epoxy. When, the wetted out CF tow begins to get less tacky, I slide the heat shrink up each length of tow, shrink it with a heat gun, cut it loose, and form it into the notches layered in posterboard.
Last edited by Gordon Johnson; Oct 07, 2018 at 01:24 PM.
Oct 07, 2018, 01:15 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
Thread OP
Each length of CF tow, in the shrunk tubing, is cut off at each end and formed into slots in the posterboard. A bit of trial and error determined the outside diameter of CF tow in the heat shrink after shrinking. And, a bit more trial and error determined how far apart two lines needed to be for my laser to cut a notch that after "kerfing" would be the same width as the OD of the shrunk tubing with CF tow in it. The other method people use is to CNC mill a slot in MDF wood with a milling bit the right size to make the slot equal to the OD of the shrunk tubing. But, since I have a laser cutter I came up with this method.

Here are all the lengths of of CF tow in their heat shrink formed into the slots in the poster board. After curing over night, these still in the poster board are placed between two sheets of glass, and baked at about 150 degrees Fahrenheit in the oven for A couple of hours.

The next step will be to laser assembly forms out of 2mm foam, and start gluing the pieces together. The wing will have some complexity since it uses curved ribs, rather than just a straight length of CF rod.
Last edited by Gordon Johnson; Oct 08, 2018 at 04:35 PM.
Oct 07, 2018, 03:53 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
Thread OP
After baking in the oven, to speed the curing time, it is time to "slice" off the heat shrink. This is done very carefully. Using a new hobby knife, the top side of the tubing is sliced off, with the knife almost parallel to the tubing. With 0.5mm CF rod it is easy to put too much pressure, or too steep an angle with the knife and slice through the CF rod. After the top has been carefully sliced off for the entire part, the remaining roughly 3/4 of the heat shrink is peeled off.

Shown here is the landing gear formed from 1.0mm CF rod, and the left over sacrificial heat shrink tubing. This is basically how you get CF rod in whatever shape you want. There are limitations, however. It's not possible to do super tight corners. A general rule of thumb is anything less than 10mm diameter curves will be problematic.
Last edited by Gordon Johnson; Oct 07, 2018 at 06:56 PM.
Oct 07, 2018, 05:07 PM
Aedificem ergo sum
cybermike's Avatar
Beautiful work, as always! Which West System epoxy are you using - the 105/206? And What diameter of heat shrink? Last I checked, 3/64Ē was the smallest DigiKey had, but that was nearly 15 years ago. And finally - just regular olí posterboard like we finger-painted on in kindergarten - sort of a thick cardstock?
Oct 07, 2018, 06:45 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
Thread OP
Mike, so you will dive right in ...

Posterboard is 1.5mm thick, from Staples. Yup, ordinary posterboard.

Heat shrink is from Digikey. You want one that the shrunk inside diameter is 0.5mm. And, you want the wall thickness to be as thin as possible so it is flexible. Digikey has a table that allows you to set parameters and filter it down. There are items in there that are custom made, and 1,000 units. Others are not custom, but high minimums. So, I've gone with what they will ship in small quantities. I have three of four black heat shrink that work, and only one clear that works, for 0.5mm CF rod. If you want to mold 1mm CF rod then same thing, 1.0mm or 0.9mm shrunk inside diameter.

Here are Digikey part numbers (check them in the Digikey table) for 0.5mm rod.
3/64" Clear QKY364C-ND
1.4mm Black X4-1.0-0-SP-01-ND (which must be the size before shrinking)

West is 105 Expoxy, and either 205 (fast) or 206 (slow) hardener. Don't buy West from CST Sales. They ship it an expensive, stupid way. You can get it in some hardware stores, and you can also get it on Amazon via Prime.

3k CF tow I got from CST Sales. It can be found on Ebay sometimes, often in huge quantities, however. I got my syringes from CST, also wood mixing sticks. Skip their pump system for mixing in the correct ratios. I got that, but it was a waste of money. We mix up small quantities. For what you see above it was 10g of epoxy and 2g of hardener. I use the syringes to squirt it into a dixie paper cup that I mix it in. Dixie cup on the scale, zero out, precisely add the amount of epoxy. Then, zero out again, and add the hardener. Do it by weight and you get consistent results (most of the time). Get some baby food jars, pour some epoxy and harder into those, and suck it up from them via the syringes. Keep using that jar of epoxy and pour more in as needed. You can make a huge amount of CF rods from the quart of epoxy you will have to buy. Cut off only a small bit of the tip of the syringes so when squirting into the paper cup it comes out slow. Excess goes back in the respective baby food jar, and the syringes are stored in their own labeled zip-log bags till the next batch.

CST Sales is a good source for straight CF rod. They have 0.3mm, 0.5mm, 0.7mm, 1.0mm, etc. And, they have 0.7mm CF tube, and larger sizes as well. They are a good consistent source for CF rod.

sheets of glass in small square size can be found at Home Depot. These are good for baking in oven. 1.5 to 2.0 hours at 150 degrees Farenheit is all it takes after it cures naturally for 12 hours or so. Bake it still in the heat shrink, and still in the posterboard forms. Otherwise it will tend to relax and not hold its shape as well.
Oct 07, 2018, 09:07 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
Thread OP
Mike,
Don't lay in a supply just yet. Wait till I get further. This particular batch the heat shrink is not coming off as easy as the last time. This time I'm using clear, not black. Or, maybe I didn't let the epoxy set up as far. Regardless it's not coming off easily. That's why this plane is for me to experiment with the heat shrink method. And, I'm not done experimenting yet.
Oct 08, 2018, 01:35 AM
Registered User
Nice!!! Where the heck was I during the first thread ?/ I surely don't remember it. great reading for me today at work!
Oct 08, 2018, 11:07 AM
Registered User
Great information on the technique! I figured you were working on a DR1 when I saw the moulded shape of the trailing edge last month. I haven’t worked with West Systems since the early 90’s when I built a Kayak, as I recall it didn’t smell and had a wide range of cure times available.
Oct 08, 2018, 03:18 PM
Team WarpSquad
Brainstorm's Avatar
What a fantastic project, Gordon! I sincerely appreciate your effort to simplify the carbon molding process and make it more accessible here in the Scratchbuilt section. Good stuff!
Oct 08, 2018, 04:37 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
Thread OP
Quote:
Originally Posted by hogan
Nice!!! Where the heck was I during the first thread ?/ I surely don't remember it. great reading for me today at work!
Sorry. The first thread was in the F3P forum, so the people who gave me so much help over there could continue to help me.
Oct 09, 2018, 09:11 AM
Registered User
found this you tube video of it-
https://www.youtube.com/watch?time_c...&v=YPNcTIauATw
Oct 09, 2018, 09:37 AM
in pursuit of low wing loading
Gordon Johnson's Avatar
Thread OP
Quote:
Originally Posted by fmhans
Great information on the technique! I figured you were working on a DR1 when I saw the moulded shape of the trailing edge last month. I havenít worked with West Systems since the early 90ís when I built a Kayak, as I recall it didnít smell and had a wide range of cure times available.
Fritz,
It was good to see you at NEAT. Yup, that molded wing was for this plane. Baby steps. I'm not ready for a molded CF full fuselage Dr-1 triplane yet. But, it's well known I love triplanes.
Oct 09, 2018, 08:45 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
Thread OP
Well, as I said this plane is about learning more about CF molding. I was not happy with the parts I molded a couple of days ago using heat shrink. The clear heat shrink I have seems to need more heat to shrink it than black shrink. So, I start shrinking before the epoxy is really set up, and even then it starts hardening from the heat. And, in this batch it was very difficult to slice off the heat shrink, possibly because I shrank the tubing while the epoxy was less hardened.

To recap, on my last plane I first used the method of laser cutting poster board templates, waxing the edge to keep the CF tow from sticking, and then molding the epoxied tow around the outside edge. Next I tried the method of scoring grooves in Depron, waxing the grooves to keep the tow from sticking, and then carefully laying partially cured epoxied tow into the grooves (which generally required bits of masking tape as it would tend to slide and pull while I was getting it all in place). This second method is what I used to mold all the curved pieces for my Carbon Bender. Pictures of these two types of methods from the Carbon Bender are below. Methods are all adaptations of other people's methods.
Last edited by Gordon Johnson; Oct 10, 2018 at 12:40 PM.
Oct 09, 2018, 08:54 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
Thread OP
So, I got to thinking, what about dispensing with the heat shrink entirely. When I laser cut poster board there is charing and black burned dust on the edges of the cut. That might help prevent the wetted out tow from sticking. The poster board I've been using when cutting it for heat shrink is 1.5mm thick, which is about the thickness of the tow when encased in shrunk heat shrink.

So, I went through and changed my CAD file for the wings to have the two lines 0.5mm apart to cut a slot in the posterboard almost exactly 0.5mm wide. I had some card stock in the workshop, so I laminated two sheets together and got a stacked sheet 0.8mm thick, close enough for this test.

I laser cut the slots, mixed up a small batch of epoxy, and made a single length of wetted out tow. After Palming it, and paddling it between sanding blocks (back side of the sand paper) then began the waiting. It took about an hour and 15 minutes (with the fast hardener) until the wetted out tow was not sticky any longer, but still very flexible. I put the epoxied tow in the slots, used bits of tape to hold it in place as I went around the shape putting it into the slot. I've found that care must be taken to get it in without crushing or squeezing it too much or that part of the rod will end up flat and deformed.

After curing for almost 24 hours, I baked it for 2 hours in the oven at 150 degrees tonight. And, the pieces came out of the slots with no sticking. They literally fell out of the slots. Shapes are very good. And, the laborious task of slicing off the heat shrink has been eliminated.

Next I'll go modify my CAD files for the other parts, cut out templates, and re-make those parts using this method.


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