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Kfm4 Funjet clone OR Build a plane while your wife watches a movie.

thenated0g

Drinker of coffee, Maker of things
Mentor
#1
Introduction

I have been reading about kfm airfoils for some time now. Several things about them seem very attractive for rc plane building. Mainly it is the speed of construction and the ease of construction that appealed to me. While surfing around on my phone i came upon ProjectFlightDesign's youtube channel. And so i decided to follow his build of a funjet clone and use whatever parts i had in my office.

Here are his build videos.


I followed almost everything he said to do with a couple exceptions. One, i used layers of foam board as i do not have or have access to 1/2 inch solid wall insulation. And the second thing i did was i accidentally made the fuselage 20 inches instead of 24.5 inches.

The build


One thing i was initially not looking forward to was the lack of plans printed out. But after finishing it, i wish everybody would make build videos like this. It was so much easier and faster to do the cutting this way. Compared to printing out 50 pages, aligning them and taping, cutting them out, attaching to foam, and cutting out again. My cut lines are so much nicer looking. Or i am really really bad at the other way! You could probably have the entire plane cut out, taking your time too, in less than 10 minutes.

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I made the fuselage the same width all the way up to save time. It comes out to about the same thickness as the foam he used and all those layers of foam and hot glue make a super solid structure. The 4 kfm pieces are very easy to cut and attach. Basically its just half the size of your existing wing and glue it down.

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The leading edge of the wing is made up of 3 pieces of foam. I first cut the top and bottom with a normal 45 degree bevel cut. Than i repeated cut more sharply angled cuts until i had a point in the middle board. I used black duct tape to protect the leading edges.

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Next i began forming the shape of the nose. Or as my friend put it "attacked it with a machete." I really have no experience in carving so i just kept cutting until i had something that looks smooth. Than i wrapped it in 1 layer of black duct tape. It feels very strong with all those layers of paper, foam, glue, and tape.

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The main body is completed. It is very light and very firm. No empty cavities from a shaped wing, it is a solid foam plane.

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The plans call for the CG to be 15.5 inches back from the tip of the nose. Being that i had made my plane 20 inches instead of 24.5 inches, that wasnt going to work. So i took the original length of 24.5 and subtracted 15.5 to get 9 inches. So i used 9 inches from the TRAILING edge, specifically the motor plate, as my CG. Or in the pics, slightly forward of the servo wires. I made a little hatch to go over the battery and help keep the wind from fighting to break it free from the velcro.

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The electronics i had on hand that went into the plane:
Hobbyking 20amp esc
Altitude Hobbies Suppo 2208 1450kv motor
Stoneblue Airlines Emax 8x4e prop
ReadymadeRc 1100mah 3s 30c batteries.
Hobbyking 9g cheapo servos​

Thrust on this setup i believe is around 650 grams.
AUW ended up around 470g

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Before and after paint pictures. It had been raining for several days here in northern California so i just threw on some black/Windex mix with my airbrush. Side note here is that i pretty much religiously used min-wax before this but decided to "see what happens" and just painted right on the paper. It worked really well. No side effects that i can see.

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MAIDEN

Maiden flight was excellent. Really really nice flyer. I started out with my 9x rates around 50 and midway i turned them down around 30-35. I think next time i go i will set them to 25 with my switches set to 15. I think 10 or 15 may be the sweet spot. Seriously, this thing doesn't need much throw at all.


Conclusion

Man, i am sold on kfm builds. If you have built any of the flitetest planes or speedbuild kits, this plane and wing style will be very simple and fast for you. I am already looking at my next kfm. Probably a wing. Cant recommend this build style enough.

NEXT

Have you seen those EFX Racers that go 100mph out of the box? You can buy the motor, esc, and props for around $45 on hobbyking. Really want to see this thing go fast. In a totally unrelated story, watch for my upcoming video where my funjet explodes when i hit dirt at 100mph.