My F3J project - takes flight
I guess its about time I shared a project I've been working on for a long time.
For the last three years I've been designing and building an F3J plane I call the GREMLIN.
The concept for this model originated back in 2003 when a good friend of mine and I were together in the Israeli team for the EC in Romania. We wanted a larger model with better penetration then most at the time, a cross tail and a two piece wing to keep the inertia low. We wanted a model with a pointy nose, looking down slightly for better landing. But most importantly we wanted a model of our own.
We designed the entire model, start to finish, with varying airfoils across the span, elliptical wing of 345 cm in span and modeled the entire fuse when we had to abandon the project due to high costs at the time and lack of interest of other modelers in Israel who we wanted to enlist for the project to share some of the work.
The project regained new life in 2006 just before the WC. I was approached by members of the Omer club in Israel. They're molds were getting obsolete and they wanted a new model, out came the design and I started reviewing it in order to update it as a state of the art design, out went the old airfoils and in came Drela airfoils, the planform was tweaked and we were happy with what we had. During the WC I spoke about the project in length with Philip Kolb who suggested to help with the wing design. After reviewing our work he suggested some tweaking of the planform and the airfoils. He modified the Drela airfoils and came up with PK381 trough PK385 for the project.
The model ended up looking very similar to the High End although its design predates the High End introduction by more then a year (the High End came out around summer 2007, more then a year after we started milling our masters), so our plane is an original not a knockoff.
With Phillips inputs we redesigned the wing for the third time and Uri De-Swaan made a CAD file for the wing so we could machine it out of tooling blocks. Unfortunately the manufacturer made some rather clumsy mistakes which demanded a lot of work on our side.
First wing mold to be made was by the Omer club members who then built their own fuse and called the model Shesek, named after a Israeli fruit. The performance of the model are excellent and the model is definitely competitive with other models available today.
I teamed up with three other guys who with time dropped out due to the amount of work it involves and work obligations. I decided to go for a pod and boom option similar to my modified Pike Perfect fuse. Last week I finished the first wing out of my mold, I've already built tail booms, rudders and elevators for the model and the only thing left is the pod, it should take me a few more months to finish the model.
wing span 345 cm
wing area 70 dm
Wing airfoils PK 381...385
Elevator area 6.7 dm
Elevator airfoil HT14
weight 2000 - 2200, can carry up to 600 grams of ballast
With Time I'll try to do a write up of the building process it self.
Enjoy some pics
Very cool. Thank you very much for all the pics. I really envy you doing this
Finishing/plugs Maintaining the TE sharpness
Hello Rydor this is a very timely thread :)
Thankyou for the detailed photos.
I would like to ask some questions on finishing polishing the plugs plugs/positives.
What have you used to fill the LE channel - Wax?
I am part of a small team about to udertake the building of a f3b model using MDF CNC routered positives. Presently we will have a channel outlining the wing planform similar to you positives.
We have concerns on how to polish and finish the TE against the channel without loosing definition by rounding off the sharp finish to the TE.
Can you give us a description on how this was done with your positives.
Due to some misunderstanding, the guy who manufactured the masters for us made the LE channel to wide and to deep to use wax so I filled it with a two component polyester filler, worked well, just need to shave the excess material with a knife before its completely hardens.
The TE is going to loose some definition no matter what you do, the trick is to mill it a mm longer so you can sand it off after building the wing. If you haven't (we didn't) put something in the grove to eliminate the drop as much as possible so when your polishing the corner won't get rounded as much.
Thanks for the pics and all the best to your project, roydor!
Your project makes me appreciate the circa 1ke moldies we can buy. So much hard work goes into the delelopment and manufacturing of our plastic wonders ;)
It's a good-looking ship. Love the 2-piece wng and the tailplane.
How did you get the cloth to sit in the servo cover moulds? I did a couple and had to resort to using an insert to keep the fabric from lifting and creating voids.
Thanks for sharing your knowledge.
Nice work roydor;
Jeff, if possible, avoid making any channel that later needs to be filled. We had this mistake made on some positives and it was a nightmare to fill it correctly. The finished result was only acceptable. If the guy with the cnc understands that the LE needs to be perfect without the channel, he will be kind enough to change the mill to a flat one for the LE intersection of wing/parting board, so there's will be no channel done. It is possible to it and in fact, it's much better.
Great effort Roydor -also thanks for all those photos,
I assume you are polishing the plug, so using Roydor's suggestion would it be possible to extend the TE during milling down to a milled out land say 0.5mm or so below your separation plane level. Do the TE finishing/polishing and then mill the slot along the TE position which will give you your sharp TE.
Agreed this will require care in the indexing to line up the plug on the CNC table for the second operation, but with index points on the master, this should be possible, or even an indexed table with stops etc.
Good luck with your group.
Thanks for your replies Roy and John.
We've spoken with our machinist and he says (based or his level of experience???) that it's an "all or none" trough around the wing, just as Roy's machinist did with his masters. I think I have a solution though... refer to image.
1/ Fill with anything.
2/ Trowel in a fillet using wing surface as reference and finish surface.
3/ Fill remaining void using the parting line as a reference.
To locate the TE post finishing, the opposite side of the trough can used as a reference. Does this sound feasible?
Chet, what you suggest is a good solution, however, I dare not ask too much of our machinist, he may throw his hands in the air and say it's all too hard. We are guided by his capabilities. ;)
Ooops, is this what's referred to as "hi-jacking a thread"? Sorry Roy, but given your experience, your feedback would be helpful, too.
Would love to see more details of your build when you get to it. That stab looks light.
David - check PM.
The stab should come out around 40 grams and is VERY strong, been flying a Pike Perfect with this stab for a year now with excellent results.
A short update.
Due to my job getting in the way I couldn't get as much progress as I wanted to but I have finished the master for the pod, molded it and made the first pod.
It came out quite well and with a nice weight, very stiff. All Kevlar with some UD S-glass reinforcements. I should finish the fuse around 650 grams and rather strong.
I'll try finding the time to try and build a slightly lighter one for the second wing I'm building.
More updates soon.
In the meanwhile some pics
Very nice plane, congratulations! :)
Very nice Roydor!!! Will we see it in France?
I think my favorite part is the stab.... too cool!!!!
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