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Jul 26, 2019, 04:25 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
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Discussion

Molded CF Stik Citabria (now with Video)


Time for a new molded CF plane, hopefully done in time for NEAT. Details in following posts.
Last edited by Gordon Johnson; Sep 07, 2019 at 01:49 PM.
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Jul 26, 2019, 04:31 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
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This will be my third plane using molded CF rod. It will be similar to this sorta Ugly Stik plane I finished up in the spring. Here is the thread.

https://www.rcgroups.com/forums/show...-with-Video%29

My first molded CF plane is in this thread in the F3P forum, and it is a 4-channel aerobatic sorta 3D/F3P plane.

https://www.rcgroups.com/forums/show...olded-CF-Plane

Lots of details on molding CF rod can be found in the two threads above, and also in other people's threads in the F3P forum.

Here is what will be different on this plane than the one in the first thread above. Wing span is the same 13 inches. But, that plane had 6% camber. On this one I have doubled the camber to 12%, which should result in it flying slower. I have done a couple of skeletonize foam planes with fairly high camber, and they do fly slower.

On my previous two molded CF rod planes I painted the pieces after covering them with 2um thickness clear mylar. On this plane I wanted to paint the mylar before covering, mostly because with the huge camber on the wings painting them with laser cut painting masks would be very problematic. I also wanted to try painting one color on one side of the mylar, and the other color on the other side of the mylar.
Last edited by Gordon Johnson; Jul 26, 2019 at 07:50 PM.
Jul 26, 2019, 04:36 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
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I started with the wing. The shape of the wing is kind of like a Citabria wing, with the angled corners at the trailing edge wing tips. I wanted to mold the perimeter of the wing in one continuous length of CF (on my previous stick plane I molded two wing half perimeters, and then joined them). And, I wanted the 12% camber airfoil to continue all the way to the wing tips, which were flat on the last plane. This meant I had to design up a fixture to mold the rod, which would have the dihedral, and also curved wing tips with 12% camber.

I designed some pieces from 1/18 Ply, and then glued 1/64 ply on the top to give the wing shape, with 12% camber and dihedral.
Jul 26, 2019, 04:39 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
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On my last plane I laser cut CF rod molding forms by laser cutting a slot 0.5mm wide in art paper, which is about 0.5mm thick. In this case I had to laminate this piece of art paper with the slot cut in it on top of another piece of art paper, and then roll this over a dowel to curve it. After taping it in place I layed the wetted out CF tow in the slots and let it cure for a couple of days.

The final step of the molding was to bake it in the oven for about four hours for the final curing.
Jul 26, 2019, 04:45 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
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On my last stick plane I used one molded CF rib at the dihedral joint and one rib half way to the tip on each wing half. On this plane I want to make sure I get the full 12% camber everywhere, so this plane has two ribs in each half of the wing. I used a laser cut Depron jig with shallow slots to hold the wing perimeter in position, and holes cut where the ribs are glued to the LE and TE.

The wing structure with the five ribs glued in place weighed 0.45 grams.
Jul 26, 2019, 04:50 PM
in pursuit of low wing loading
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With the wing skeleton done it was time to paint clear mylar. I laser cut painting masks from art paper stock. The mylar is stretched out smooth and taped down. Then, the painting masks are very lightly dusted with 3M77. I'm still learning how to airbrush mylar. On this plane I thinned the paint, dusted a coat on, and then let it dry for 10 or 15 minutes. I repeated this for about 4 or 5 coats. This airbrushing does not have any of the mottled paint that my previous planes had. My friend Steve (Crossup) clued me in on the need for multiple dusting coats, and letting it dry between them.

It took two days to paint the mylar, letting the first side completely dry overnight before flipping the mylar, and painting the back side with its own set of stencils. But, two days later I had a piece of mylar ready for covering.
Jul 26, 2019, 04:56 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
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To cover the wing I used a technique where I tape the mylar down, and glue strips of 10mm Depron on each side. This gives me handles on each side, and the mylar stays flat and relatively tight. I sprayed 3M77 on half the wing structure taped it on the wing form (with wax paper on top of the wing form) and then applied the covering for one side using the "handles". Next I did the same for the other side. But, I had paper covering the covered side of the wing except the last 6mm where the covering would overlap at the dihedral bend when I sprayed it with 3M77. This resulted in a stripe of wing covering at the dihedral bend 6mm wide with 3M77 on the stripe area for overlapping the second piece of covering over.
Last edited by Gordon Johnson; Jul 29, 2019 at 07:06 AM.
Jul 26, 2019, 04:58 PM
in pursuit of low wing loading
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Here are some more shots of the completed and covered wing. With the painted mylar covering the wing weighs 0.94 grams (up from its 0.45 gram weight before covering).

Now it's on to designing up the tail surfaces and molding the CF pieces for those.
Jul 26, 2019, 06:44 PM
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Gordon:

Very nice work. What are you planning to power it with?

Ralph
Jul 26, 2019, 07:33 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
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Ralph,
The other of your 4mm watch gear drives with the hot wind 4mm quad motor. Of course!

The gear drive I got from Australia didn't provide any more thrust, could not turn a larger diameter prop. So, your drive will go on this plane too.

Gordon
Jul 26, 2019, 07:53 PM
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Gordon:

I can't remember did I put the machined prop adapter on the second drive? If not I can do that for this build if you like. Just email me.

Ralph
Jul 27, 2019, 05:06 PM
Registered User
This is neat, Gordon. I've been catching up on your previous work but it is good to see something brand new which is coming together right now!

I wish there was a way to lay out the entire wing as a single section of tow (to avoid gluing parts). It would be like one of those pencil mazes where you have to draw a line and can't lift up.
Jul 29, 2019, 07:20 AM
in pursuit of low wing loading
Gordon Johnson's Avatar
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Quote:
Originally Posted by CGordon
This is neat, Gordon. I've been catching up on your previous work but it is good to see something brand new which is coming together right now!

I wish there was a way to lay out the entire wing as a single section of tow (to avoid gluing parts). It would be like one of those pencil mazes where you have to draw a line and can't lift up.
Glad you like it. Get some CF tow and epoxy and dive in. This technique is right up your alley given your interest in structures.

I think trying to mold the entire wing including the ribs in one continuous piece would be very difficult. Instead of using 3k tow I could use 1k tow, and keep "looping" it around and back and forth so in the end it is 3k most everywhere. One problem with this is the technique involves letting the wetted out (and "dried" with excess expoxy removed) tow partially cure, generally about an hour, so it isn't sticky. So, that would mean it would not really adhere to other strands of 1k tow. The alternative is to layer 1k tow in while still very wet. But, this would mean it would have excess epoxy, and then be heavier. So, in the end, I think the method of gluing results in a very light skeleton. I'll post a picture tonight from home of this wing with ribs glued in, but very twisted like a DNA double helix. It is incredibly flexible even with just CA on the joints. Until I started building these types of structures I really didn't appreciate how flexible they are. They are very light, but require external bracing, which the F3P guys have really perfected. In fact, my friend (and noted free flight rubber modeler) Ray Harlan built a version of my Carbon Bender, and used very fine tungsten wire to save a small bit of weight on the bracing. Anyway, this wing with all five ribs glued in weighed 0.45g before covering.

BTW, many years ago I taught at Chico State, about 90 miles up the road from you.
Last edited by Gordon Johnson; Jul 29, 2019 at 07:26 AM.
Jul 30, 2019, 09:45 PM
in pursuit of low wing loading
Gordon Johnson's Avatar
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Tail pieces have been designed, and molded CF rod pieces are curing under a sheet of glass.
Aug 01, 2019, 08:09 PM
in pursuit of low wing loading
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Here is the picture I promised showing how flexible the wing structure is after gluing the ribs in place. And, this is why it needs to be braced externally.


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