My KFm3 Delta Less-Than-One-Sheet-DTF - RC Groups
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Jul 04, 2011, 01:42 PM
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My KFm3 Delta Less-Than-One-Sheet-DTF

Despite this being a KFm3 with 30" WS and 20" length, it required considerably less than one full sheet of Dollar Tree Foam (DTF).

I built it without any plans or a build log, so if it's successful we'll have to reverse engineer it. If it isn't successful, feel free to take whatever design features you like and use them for your own.

Hopefully I will maiden it this evening, barring storms and this heat and humidity.

It currently uses a 3S 25C 1000mAh battery, 12A ESC and a 2712-12 1700KV motor with an EMP 7x4 prop. I fear it may be underpowered, but hopefully the KFm3 will provide plenty of lift.

Some photos before first flight:
This last photo is a closeup of the left wingtip and its 5gr lateral counterweight to the 1000mAh battery that's mounted just to the right of the fuselage.
The current CG is just behind the step of the top KFm plate.

Comments welcome!
Last edited by soundguy63; Jul 04, 2011 at 10:19 PM.
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Jul 04, 2011, 07:51 PM
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Success!! It flew extremely well and was beautiful in the air! It wasn't too far from neutral and was very stable. It is underpowered for any vertical aerobatics but has an excellent roll rate. Launching was no problem.
It will be a nice plane to just fly around the field and relax with. It might be a tad nose heavy. I will try moving the battery back 1/4-inch at a time for the next session.
Jul 06, 2011, 10:37 AM
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In studying this construction I know I should have taken the time to sink the servos into the wing. That would not only have been more aerodynamically clean, it would have given a better geometry to the pushrod movement with the control arm and horn, in order to get the same throw both up and down.
I used premade pushrods that I had bought from a sale of spare parts, just to have a bunch of premade rods/clevis that were a good length for this type of build.
However, that locked me into a set pushrod length that I then compromised on the location of the servo and control horn. I can see now how it could have been done better all around and to obtain more elevator up deflection.

A question on servo adjusters: Do they allow you to reset where center is or just to position the servo at its center without having to use the TX/RX?

I know I could pull the arm off the servo and realign, but it felt like one of the arm screws stripped when I installed it, so I put a tiny drop of CA on each screw and if I can adjust the servo center without pulling the arm off that would be good.

I see a number of ways I can fix all this, just trying to determine which will be easiest to accomplish.
Jul 07, 2011, 04:22 PM
Sometimes it work some don't
RCSquale's Avatar
Sorry I don't really get your question on sercos adjusters but what you can do now is to cut out disposable spoon to be glued on to of your servos body in order to hide the square blue color of your servo, will allow the airflow to go around better and it should look cool as well. I did that on my B-Rang build made by Quick61.
On servos horn I using thos types of adjusments that are screwed on the horn and will hold firmly the pushrod with a screw. I found it easier then trying to make sure to have exact length of pushrods with bends on both side etc...

I like this Delta plane of yours, is there a plan for it? I may want to try one myself.
How does it handle in the wind? Can it fly in medium to high wind?
Jul 07, 2011, 06:34 PM
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I wonder if you removed the paper, and made it a kfm2ish with a 620mAh battery that you wouldn't have plenty of vertical. Looks a lot like a superfly or firefly. Simple delta planes are lots of fun. I always have one around. Nice job.
Jul 07, 2011, 10:33 PM
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Thanks! I flew it again this evening. It is so smooooooth in the air and handles a normal wind without any trouble at all.
I know most people do remove the paper, but I haven't done that on any of my builds yet.
This plane, with the paper on, the laminated KFm construction and the leading edge beads of hot glue is super stiff without any spars of any kind.
The only area that has any flex is the rear deck between the prop slot and the control surface cutouts. And that's only because for hand launching the tail is the major grip and throwing area.
I know it's a little heavy, but it does handle a breeze without any jumping around. And it lands smoothly at a low speed, just need to keep a little power on until flaring.
I'm already making plans to build a 40" to 45" WS version using several sheets of DTF laminated with overlapping joints. It will also have a considerably more powerful motor and a better arrangement of the servos and control horns as well as using raw pushrods and quick connectors.
I'll probably change the sweep of the middle KFm plate as well, with less angle on the trailing step.

The servo adjuster/tester I'm speaking of is the GWS MT-1. I'm not sure of it's full capabilities because the online manual isn't totally clear to me.

I didn't create any plans with this first version, but I will try to do better on the second. The shapes are pretty easy, I just spent a lot of time making sure my cuts were straight and clean.
Jul 08, 2011, 12:36 AM
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Paper or no paper is like chevy or ford. Its what you like. However, it is lighter to use a spar. Weight is the enemy in small planes. I have flown my simple delta numerous times in 20mph winds. It is also much easier to repair if do not use paper. Simply pour boiling water over the crumpled areas and they will come back to about 95%, then reglue. DTF paper doesn't like dew very well either. But to each is own. If you like it, that is all that matters. Besides, it only cost $1 for the foam. I usually always use foam core for my initial simple builds and then move over to epp once they are worn out if I want good bouncy stuff. IMHO a little flex is better than something rock solid. Ever wonder why so many people that are drunk don't get hurt in car wrecks, same goes for little rc planes. Bytheway, some hobby lobby craft stores carry big sheets of foam core board. You nearly must remove its paper though as it is very heavy, but the foam is better quality than DTF.
Jul 08, 2011, 12:11 PM
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Nice plane!

Is the motor a tower pro?
Jul 08, 2011, 01:13 PM
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Thanks! It's this one from Heads Up RC:
It can fit directly into a plastic stick mount, so you don't need any plate at all.
I'd recommend something larger unless you keep weight to a minimum.
I need to weigh my plane's AUW and report back.
Jul 10, 2011, 05:35 PM
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Looks like you are running a 2 cell Lipo.

Based on the Heads Up web site, this motor could easily handle an 8 x 6 prop. Wonder if that's the easy way to more ummph (a technical term) from your power system?

I know this is not your exact prop, but Heads Up says:

The GWS EP 7035 prop produces about 8 ounces of thrust at 4.5 amps with a pitch speed of 34 mph.

The GWS EP 8060 prop produces about 12.5 ounces of thrust at 10 amps with a pitch speed of 38 mph.

Looks like a 50% thrust increase ??

Good luck!
Jul 10, 2011, 05:38 PM
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For those of us new to foam construction, can you give an example of a "spar" from foam? I was thinking of just embedding a carbon fiber rod across the back end to provide stiffness, but I'm intrigued by the idea of an all-foam construction.

Jul 10, 2011, 06:04 PM
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A spar for me is carbon fiber, fiberglass rod, bamboo skewer, bamboo shade, or music wire. I usually run a spar a the end of a kf section and sometimes in front of elevons. Sorry, I didn't mean a spar from foam...
Jul 10, 2011, 07:38 PM
“There’s no place like Foam”
gpw's Avatar
You know , Hippo, remember him , years ago experimented very successfully with foam spars... I tried it on his Xbow... still have one around here... Worked fine ...
Jul 10, 2011, 10:19 PM
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I'm using a 3S batt with this motor, so I think the 7x4 prop I'm running is the most I want to try with the physically small 12A ESC. I don't want to have to worry about running WOT even when it's high ambient temps.
I'm guessing it's making about 14 or 15 ounces of thrust.
After flying it again this evening, with 10+ mph wind, I'm still very impressed. I know it would do better vertically if I fixed the elevon throw (it lacks enough up throw to do a zippy loop) but I'm tempted to just leave it alone and not mess anything up right now. Either that or do some outside loops, it has plenty of down throw.
In the meantime I'll be doing fast laps and rolls, which sound really awesome as the wing beams the prop-in-slot sound with each rotation.
Jul 27, 2011, 09:33 PM
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A few updates:

First I added two short bamboo skewer spars under the rear deck to reduce the flex in this area during hand launching. This mod was successful and I flew it a number of additional times.

Last week I was going to work on the control throws and while handling the plane noticed that the plastic stick motor mount was still rigidly hot-glued to the paper, but the paper was pulling loose from the foam and would have probably resulted in loss of the plane on the next flight!

I reglued the motor mount and also added a thin washer under the small prop nut to better secure the propellor. Both those mods were successful.

Lastly I should have left the control throws alone! I was trying to balance the up throw and down throw, as well as add a little more total throw.
I realigned the servos and control rods and I moved each elevon clevis down from the 2nd hole to the 3rd hole.
These changes produced enough additional throw that I quickly began over-controlling the now out-of-trim plane soon after launch. It crashed hard just 18-inches into the lake after making it about 90 yards from the launching spot! Everything was still there, but was all soaking wet and the airframe and one side of the wing was badly smashed.

Oh well, it's a good thing I have several other planes under construction, but I really did like the way this one flew (except for such lazy loops).
Last edited by soundguy63; Jul 27, 2011 at 09:42 PM.

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