My F3J project - takes flight - Page 3 - RC Groups
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Dec 19, 2009, 06:31 AM
Deniable plausibility
Shedofdread's Avatar
Absolutely beautiful, elegant piece of work. However, you must have waaaaay too much free time!

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Dec 19, 2009, 07:45 AM
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Tuomo's Avatar
Originally Posted by Shedofdread
Absolutely beautiful, elegant piece of work. However, you must have waaaaay too much free time!

roydor is building, not hanging here in RcGroups
Dec 19, 2009, 07:48 AM
In F3J size does matter!
roydor's Avatar
Had i had as much free time as I would have liked, the plane would have been completed already...
Dec 19, 2009, 07:31 PM
Registered User

thanks for the detailed info on the Elevator. Re the LE joggle - I imagine you need to trim the herex on the top surface so the the bottom overlap glues directly to the outside skin?

Hope to see it flying soon.

Dec 20, 2009, 06:49 AM
Registered User
gmo78's Avatar
This is a very interesting project. Thanks for the ideas your post gave to me!
Dec 20, 2009, 06:03 PM
Daryl Perkins's Avatar

Again, very nice work. Thanks for doing the write up on the stabs. I can do just about anything with composites... but you throw some of that balsa stuff in there... and I'm all screwed up.

How are creating the ribs with the carbon outer?

I plan on making some stab and fin molds and playing with this process, soooooo... your help is greatly appreciated.

Thank you sir

Dec 20, 2009, 07:24 PM
Registered User
Tuomo's Avatar
From quality to quatity... How many planes you will produce for the coming season?
Dec 21, 2009, 03:57 PM
In F3J size does matter!
roydor's Avatar

The ribs are very simple actually, you take a 0.2 carbon UD fabric with epoxy and vacuum it on a smooth surface, I used a 10 mm thick glass 40 x 40 cm, I also used pill-ply on one side to give it some roughness for better joining to the balsa at later stages. After removing it from the glass and peeling the pill-ply fabric, I cut 1.5 mm wide strips using a sharp knife.

I cut the ribs to size and glued them in place making sure no warping in the elevator.
I placed the carbon strips with the pill-ply roughened surface facing down on the table and using a clear tape I tap them from above and they practically jump up to the tape (static electricity helps) and get stuck to the middle of the tape. I put some thick CA on the strip, now with the roughened surface showing, and use the tape to position the strips in place over the 1.5 mm balsa ribs. I place the strips so that only 3 mm of the strip is on the D-box. The remaining strip is on the rib and sticking out beyond the TE.
After drying for about 15 min I cut the left over strip and remove the tape, I repeat the process on the other side of the rib next, ending up with a Sandwich rib of carbon balsa and carbon.
Naturally I do all the top cap strips at the same time, let them dry, turn the elevator over and do the other side. there are ways of making lots of ribs with cap strips already on them at a mass production amount but that is a different story.

I actually used this process on free flight models I had built several years ago, over ten years later they are still in flying order.


I'm actually building two models simultaneously, one full carbon 90 (~2150 gram) one full Kevlar 60 (~2000 gram).
I’m also building an electro fuse, more on this later when it’s finished (I molded the two halves of the rudder this weekend, I will close it and post pictures soon).
I’m thinking of building another model with a lighter spar as a super light model (~1800 gram) but I am not getting into that until I finish the two I’m working on.

Dec 22, 2009, 11:10 AM
In F3J size does matter!
roydor's Avatar
A couple of photos to show the cap strips
Dec 22, 2009, 03:01 PM
In F3J size does matter!
roydor's Avatar
I thought I would go through the boom making process as I had gotten some private massages on this matter.

The tail boom was rolled over a piece from a fishing pole which I covered with a thin layer of petroleum jelly and covered with Mylar for separation. I made several booms of different layups. The standard boom was made of carbon 120 g/m^2 in 0-90 deg orientation, UD 80 g/m^2, carbon 120 at 45 deg and 50 g/m^2 glass for finish. The layers were cut wide enough for one revolution around the boom plus 5-10 mm of overlap. The layers were staggered or shifted relative to one another so the overlap does not happen all in the same place but spread around the circumference of the boom, giving a fairly constant thickness to the boom. The layer of glass helps me also to place the layers on the boom since its wider then all the carbon layers on top of each other by 10 mm at least in every direction. I basically use the glass layer to pull the carbon on to the boom as I roll the boom over the carbon.

After positioning the carbon and making sure I have no folds or air trapped beneath the layers I wrap the boom with peel-ply to press everything together and to pull out all the excess resin. After the peel-ply I wrap the boom with an elastic film which applies pressure and helps the resin to be forced out of the carbon and into the peel-ply. The penalty in this method is that the boom gets a rough surface finish which needs to be sanded down. The benefit is in the fact that you get a much better fiber to resin ratio making the boom stronger for its weight.

After the boom dries I sand it down and use the layer of glass as a material to fill in any depressions by basically sanding all the glass around the depression, I sand the glass until I start to feel carbon and were there isnít enough carbon, the glass remains. I later paint the booms with clear varnish, the paint adds about 2 grams to such a long boom but the result is spectacular.

The boom dimensions are 740 mm in length, 30 to 28 mm in diameter (that is what the fishing poll allowed, I would have preferred to go down to 22 mm) the average weight of a boom is 48 grams.
My lightest boom was 40 grams, in that boom I changed the 120 g/m^2 carbon to 90 g/m^2 and used a 25 g/m^2 glass for finish.
I even made a 30 gram boom which was surprisingly stiff but eventually buckled under load while testing it, the wall thickness was simply too low and it was simply not durable enough. I guess there is a limit and I found itÖ
There might be a way to reduce weight without getting such a delicate boom, it has to do with making a thick boom wall from ligher material, I was thinking of a sandwich material, either balsa or herex but I donít think the extra work justifies a 10 gram reduction in the boom weight. A smaller diameter in the tail end would also reduce weight, I calculate about 4 to 5 grams.
Jan 09, 2010, 09:24 AM
In F3J size does matter!
roydor's Avatar
It flies!

Finished it yesterday, Used 761 for rudder, elevator and flaps and 378 for ailerons. A 5 cell 1500 2/3 A battery and a AR7600 RX.
The fuselage came in at 580 grams including elevator.
Ready to fly weight is 2020 grams or 71.5 oz.

The weather today was very calm and the field was very wet so thermals were nearly nonexistent, just light lift here and there, at the end I had some good air and was able to make a 30 minutes flight which happened to be by far the longest of the day as the two other fliers at the field had a tough time getting over 8 minutes.

The Gremlin flies great!

As the only model I can compare it to is the Perfect (no F3J supras, aspires, explorer in Israel...) I can say itís just as good. It's got great legs, nice handling, itís very agile and responsive and reasonably forgiving. Launch isn't set yet but it shows potential with a stiff wing and some nice tension in the last launch as I got my elevator and hook almost where I want them. The flaps are as effective as I suspected and my setup was nearly perfect from the start, hardly changed it the entire day (about 6 or 7 flights), the most significant change was reducing the rudder throws as it is VERY effective.

The gremlin signals lift well and can slow down nicely, itís almost too easy to turn on the tip and at times you find yourself turning a bit tighter than is best for the air youíre flying in, going easy on the rudder helps to avoid this.

All in all A very nice plane, time will tell but I have a feeling itíll be just as competitive as my Perfect.
The only thing I need to fix is some right roll that I needed to trim, I believe my wings are not entirely at the same incidence, a small adjustment at the root and that should be gone completely.

Next on my list is an electro fuse which should be ready in a week or so and a full carbon gremlin which should take a couple of months to complete.
Jan 09, 2010, 09:41 AM
Registered User
R.M. Gellart's Avatar
Great read Rydor, you are a craftsman!

Jan 09, 2010, 10:17 AM
Daryl Perkins's Avatar
Beautiful Roydor!!! Can't wait to see it fly.
Jan 09, 2010, 10:30 AM
Detail Freak
target's Avatar
Congrats on the success!

Jan 09, 2010, 11:17 AM
Registered User
woow very beauiful , great work it seems thats this baby has a nice planform and overall performance seems good and also built exactly 2,02kg flying weight whats very good also . Interesting wingtips elevator taper . It reminds me the New Ceres lift .Interested in coming testflight reports also )

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