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Sep 16, 2019, 08:51 PM
Thread OP

Carbon Fiber Kadet Senior Frame ...


Using the SIG Kadet Senior Kit plans I created a 3D model wire diagram to create tiny 3D printable brackets that would allow someone to use carbon fiber tubes to build a Carbon Fiber Kadet Senior airframe.

I haven't explored how to design a wing, but I figure the wing ribs might be carbon fiber rods since bending isn't too extreme for a NACA 4212 airfoil.

I just don't know if anybody would want to build one other than myself.

I figure the 3D printed brackets would have to be numbered so the builder would look at the plans, then just slip everything together in an hour or so and once the airframe is straight and true it would be simple gluing all the brackets in the joints of all the carbon fiber tubes.

The strength would be maybe 100x or more stronger than the balsa plane, but maybe cost $300 or so in carbon fiber tubes.

The brackets would need to be printed on a SLA type 3D printer since they're small and need to be very strong (still the weakest point in the airframe for easy repairs) ...

See the attached .jpg of my first airframe sketch done before I got the plans to get the measurements corrected. The second lighter image is the SIG Kadet Senior Kit from the plans ... I am thinking of adding more to the frame so that it will be extremely stronger.

Any ideas? Suggestions? Best price for 5mm carbon fiber tubes? Should I make it even larger? Smaller? In the 3D CAD modeling software I can easily resize everything in seconds.

Once this is proven to be as good as I expect, then I can design other airframe models for carbon fiber tubes and little brackets. Right now I just don't know if this is just an exercise or a total waste of time or a great idea.
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Sep 17, 2019, 07:39 AM
'Douglas' to his friends.
An intriguing method; well done for coming up with the idea (whether or not it works...).

Just a thought... Do the brackets slide along the four principle longerons running full length, or is every piece of tubing a separate piece between any two brackets..? I would suggest that the former would be far easier to asemble and keep straight than the latter, with greater inherent strength.

Would I make one..? I have a 'standard' 3D printer, for printing with PLA filament. I can print small stuff, but it would be best if they were attached to a 'tree', or 'sprig', similar to plastic model kits still in their box. I'd certainly have a go, trying out the result with little bits or offcuts of CF tube before launching into a whole 'plane. Wing ribs..? CF tube is pretty hard to bend, so I'd be wary of that, but would consider 3D printed ribs, or a foam wing.

Idle speculation for now, but I like the concept. Are the brackets visible on any of the 3D sites (Thingiverse etc...)..?
Sep 17, 2019, 10:28 AM
Registered User
It's a great idea. I searched and searched and couldn't find anything on a carbon space frame, so I've come up with my own ideas over the last few weeks.

This is a proof of concept fuselage for a 25" Telemaster.

I'm threading 1K carbon tow in a "loom", which is equivalent to the very thinnest 0.010" rod, and having some success. I built two sides from tow but have joined them with CA and regular rod sections. I can tell, even though it isn't finished, that this is viable for an indoor model. And with 3K tow it would be almost beefy.

What you see so far weighs .96 grams.

I don't know much about 3D printing and plastics but would like to see you proceed with this. I'd encourage you to try much thinner rod first.
Sep 17, 2019, 01:24 PM
Thread OP

My plan is very tiny ... too small for FDM printing brackets that slip inside the cut to length hollow carbon fiber tubes. The brackets would hold the entire shape of the airframe straight simply because they direct each carbon fiber tube exactly where it should be relative to the other carbon fiber tubes attached to that bracket. Hence once the entire frame is plugged together, I think it would be perfectly or very nearly perfectly straight and aligned front to back and side to side.

What is the best glue to use I don't know. The capillary action of the glue would be great if the glue can hold things together, yet with enough tug, the joint could be pulled apart for repairs if needed.

This is why I think once the parts are all SLA printed and the carbon fiber tubes are all cut to lengths and everything is numbered a kit builder should be able to assemble and glue the airframe up in one day if not just an hour or so.

I could design the brackets for FDM 3D printing to slip over small wooden dowel rods to make an airframe built up of maybe cheap 3mm dowel rods. The airframe would be a little heavier than the balsa version, but with electric motors today, that's not a serious problem. This is my idea of the first test airframe build up to see if the brackets really do make a good airframe.

A similar idea would be for the wing halves ... using ancient technology to steam dowel rods or slats even to airfoil shape and connect to brackets that create the entire wing shape with dowel rod spars.
Sep 17, 2019, 01:27 PM
Thread OP

Really nice looking airframe ... for something that small your system is probably better, but I'm designing for much larger airframes that allows for 5mm or larger diameter carbon fiber tubes or rods.

That build style takes too long for my tastes ... not much different than balsa building and seems a lot of work.

Good job!
Sep 17, 2019, 09:43 PM
Registered User
I'd like to see the wood truss too. I think it should work well. IMHO truss is not used enough or fully understood in our hobby.

By the way I looked at my inventory of tubes and rods, which is pretty decent, and I think 3mm carbon fiber tube would be a conservatively large size for the scale of what you are doing. Still quite light, and testing it with my hands it is very low flex for 6 or 8 inches. Use 3mm throughout and I'd bet you could sit on the fuselage with it across 2 chairs.
Sep 18, 2019, 10:30 AM
A man with too many toys
Originally Posted by CGordon
I'd like to see the wood truss too. I think it should work well. IMHO truss is not used enough or fully understood in our hobby.
That's because modern power system are very powerful. When I design an airplane these days I just use sheet balsa for the fuselage. If you look at the old time designs they use wood truss construction. I have built a few of those. They are very low weight and strong - especially when you covered them with silk and dope. I use to fly at SAM contests.

Sep 18, 2019, 11:06 AM
B for Bruce
BMatthews's Avatar
Originally Posted by wperko
..My plan is very tiny ... too small for FDM printing brackets that slip inside the cut to length hollow carbon fiber tubes. The brackets would hold the entire shape of the airframe straight simply because they direct each carbon fiber tube exactly where it should be relative to the other carbon fiber tubes attached to that bracket. Hence once the entire frame is plugged together, I think it would be perfectly or very nearly perfectly straight and aligned front to back and side to side....

I could design the brackets for FDM 3D printing to slip over small wooden dowel rods to make an airframe built up of maybe cheap 3mm dowel rods. ...

I seem to be missing the answer to Dad's question. Would you be using full length main longerons?

I'd suggest that you want to do so. The strength from carbon fiber comes primarily from the nice long full length fibers that allow the tubes to be strong and flex evenly. If you use short connecting pieces you lose that long fiber consistency and strength. And then you're relying on the SLA and whatever glue you use for the main strength of the fuselage. Plus by using long full length main longerons with the brackets slipping over the tube you can get by with a less strong adhesive bond without concerns that the joints will be pulled apart in a bad landing.

Issues I have with connectors like this that slide over the main longerons and that the diagonals plug into is that the connectors are fatter than the tubes. So now we end up with a bunch of bumps when we go to cover the fuselage structure with our covering unless we attach even more structure in the form of outer corner stringers to support the covering. And that's adding more work. Plus another advantage of the wood structure is that the longerons are bulky enough to provide a good contact area for the iron on or glued on covering to adhere to the structure. So what would you be doing to allow for this? A simple and quick sketch suggests that you might be able to have the brackets open to the outside corner and get by with a 3/4 wrap around the main longerons so the outer 1/4 of the longerons are left bare. Is that something you have in mind?

I'd skip using dowels. Try to find good straight ones at the lumber yard and you'll quickly see what I mean. It is rare to find a straight one. And they are not often all that round or consistent in diameter. If you're going to make these connectors up to give a proper fit you might as well start with the carbon rod or tube and get the fit you want.

There's still a place for the sticks and truss style of building. Both for the lighter weight and slower stall speed it give us as well as the look of it when used with transparent or translucent covering materials. I know that it's easy to get a good degree of power these days. But the weight of the model affects more than the climbing ability. The kindly low speed manners provided by light airframe weights is often nice to have.
Sep 18, 2019, 11:18 AM
Balsadustus Producerus
For decent wood doweling, you could try:

These arrow shafts are very straight. I usually don't have to straighten out more than two shafts per dozen, and I've tossed maybe three or four shafts as unusable out of 5 dozen orders. Port Orford Cedar is what I use, as it's very light and tends to stay straight, or to stay straight after straightening. Various kinds and grades of wood are listed. Generally, longest length is 32". You could, of course, splice as needed for correct length. They sell tools for that.

Problem is, there isn't much in the way of diameter choice.

My other hobby is traditional archery, and I make my own arrows
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Sep 18, 2019, 03:42 PM
B for Bruce
BMatthews's Avatar
And not long enough for something the size of a Kadet Sr either. At least not for the main longerons along the belly. MAYBE long enough for the ones from the wing leading edge to the tail?

And besides, if we're going to revert to wood why not just do the square strip built up structure?

I have to wonder about how much time would be saved as well. By the time someone at a "factory" cuts all the carbon tubes, prints the joiner brackets, labels all the parts so the highly similar parts don't get mixed up, packages the whole lot up and sells it for a price that gives them a living wage the cost isn't going to be insignificant. And then the guy that receives the kit has to assemble it all. And I'll bet that there's still enough play and flex in the setup before the glue goes in that it'll still need to be jigged up to square and align the whole structure before glue is applied. After all a couple of thousands of an inch added up across a number of parts adds up to enough play overall that we can still assemble a twisted or banana shaped structure if we do so "in the air". At least that would be the case if we have enough play in the brackets to allow wicking in some thin CA to make the joints permanent. Once glued it would be amazingly stiff and strong. But before? I'm thinking perhaps not so much.

Oh sure, the grip of the printed brackets could be made tight enough to hold the ends and main tubes reasonably firmly. But then we don't have the room needed to allow the thin CA to wick into place. Or if we try to assemble the joints "wet" with epoxy there're a real risk of the tight fit wiping away the glue during the pieces being inserted. So I suspect that realistically we're looking at a light friction fit at best which means there will be some play that adds up overall.
Sep 18, 2019, 05:27 PM
A man with too many toys
An airplane can be too light. It's no fun to fly a really light weight airplane on a windy day.

Sep 19, 2019, 02:14 AM
B for Bruce
BMatthews's Avatar
Flying gliders of various sorts in some occasionally very windy conditions over the years has given me a bit of a different outlook I guess.

I'm also not talking about super light. Just on the lighter side of the range of weight for a given size of design. I've found that with the right design elements and suitable trim setup that I'm OK with just about anything provided the model can fly back and land. And in the right conditions I actually enjoy the challenge.

But I'd agree that there is a point where things shift from "challenging but fun" over to pretty much pure survival and "why did I take off in the first place?" with a super light design intended for calm conditions only. It's really not fun and a pretty high risk of tip over damage when flying such specialty models in nasty stuff.
Sep 19, 2019, 12:39 PM
Thread OP

The reason I started thinking about using 5mm tubes is because the center hole is where the bracket pins slip into and if the center hole is too small, the bracket pin would lose strength. But, I also only saw a 5mm tube on a website selling carbon fiber tubes.

Do you have a link to where the 3mm tube came from so I can get exact dimensions of the tube? I need to know exactly the outside diameter and the inside diameter to design brackets.
Sep 19, 2019, 01:10 PM
Thread OP
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The idea is to maintain maximum flex/strength for all carbon fiber members and ease of construction, so straight carbon fiber members connect between 3D printed brackets.

I think you can see how this design system makes the frame very nearly exactly straight just plugging the carbon fiber tubes onto the 3D printed brackets ... something that shouldn't take very long.

I was even thinking that with the right covering, the brackets might not have to be glued to the carbon fiber tubes ... this would be something to encourage a new type of covering so that the airframe doesn't twist out of shape during flight.
Last edited by wperko; Sep 19, 2019 at 01:16 PM. Reason: add another image
Sep 19, 2019, 01:12 PM
Thread OP

I agree that making the airplane too light makes it difficult to control in windy situations. I'm not trying to make the airplane light, I'm trying to make it so strong that the airplane can survive almost any crash.

You can put a motor on a leaf, but it's still going to flip 'n roll on its own.

OTOH, even something the size of a SIG Kadet Senior or larger could be made so light that it could be flown at much lower air speeds that it could be flown indoors.
Last edited by wperko; Sep 20, 2019 at 07:16 AM. Reason: add a fun quip

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