Scaling up the FT3D involved some engineering challenges for me. Basically keeping everything stiff enough. I scaled up to about 130% (41.5" ws) and have built several of them now and am mostly happy with the way that they fly - sooo much better than the original size. The one that I'm flying now has a carbon fiber arrow shaft for a spar and I think that really helps the stiffness of the wing. Getting the tail feathers to be stiff enough and straight (not warped) is a challenge. The fuse behind the firewall has been weak on mine and crushed on several crashes making it difficult to repair. The one that I'm flying now is double walled foam behind the firewall (gorilla glue lamination) and has been breaking more cleanly in crashes making it easier to repair.
My current one has a thicker and wider wing cord and thickness doesn't taper toward the tip. Aileron width is tapered. The fuse has 1/16" ply where the arrow shaft fits through. It came out heavier than my last ones and therefore I used a bigger motor (emax 2815-6), making it heavier yet.
My current one comes out to 744g w/o battery vs. my previous ones were coming in around 615g w/o battery. I usually use 2200s which are 184g.
I get some good 3D out of it - KE, HAKE are nice, inverted, harrier, it will hover, but it would hold a hover better if it were lighter. Vert is ok and I can do a version of a pop top and snaps. Roll rate has never been great on these. Flat spins, not so much - spins were about the only thing that were more fun with the original size. Because of the weight, you have to keep ahead of this one with throttle. Finally settled on an APC 12x6E which seems better for high alpha stuff than the 11x5.5. I'd like to run some kind of SF, but the emax 2815-6 didn't like that at all!
Next one I'm going to try to go back to lighter, but keep the cf arrow shaft and wing mods. Only 37g of the extra weight came from the bigger motor, so I know that I can save weight somehow (and keep the bigger motor
). I think I used too much hot glue when modding the wing and I think I can make the ply spar reinforcement lighter. I get pretty good flight times, so going with 1800's would save a lot too.
Right now, I'm so busy building balsa planes that I'm not sure when I'll get back to making another one, but usually totaling the one that I have is the motivation to build another...
For this 200%, you'd really have to give some thought to the wing spar and tail feather stiffness....
