KFm airfoil 48" Flying Wing - RC Groups
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Jul 09, 2011, 10:43 PM
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gatorlynch's Avatar

KFm airfoil 48" Flying Wing

Here is my Coroplast version of the KFm airfoil flying wing.

Specs - 48" WS, 23 oz. AUW, 1200kv 450 motor, 3S 1600mah, 8X6 APC

I couldn't get her to balance to a good CG during my dry fit, so I had to cut a slot & move the entire motor mount assembly forward. I had started off with the entire motor aft of the body.

She was very stable, although a bit sluggish I thought. This is my first wing, so I really don't have anything to compare to. Changing from the 9X5 the 8X6 helped noticeably in the speed department.

I will max out the throws on the Tx, plus cut some slots along the hinge lines to make sure I'm getting good free movement of the control surfaces.

As evidenced in the video, I did experience a large degree of flutter (how about the entire ship) at higher speeds. I believe I need some additonal supports/stiffeners back by the motor. I will also put in a doubler/splice plate on the underside, where the 2 foldover pieces butt along the centerline - to reduce overall side to side flex.

Thanks to Tony65X55 for the design - https://www.rcgroups.com/forums/show...0&postcount=38

KFm Coro Wing Maiden (2 min 51 sec)
Last edited by gatorlynch; Jul 10, 2011 at 07:33 PM.
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Jul 11, 2011, 04:54 PM
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earlwb's Avatar
Nice wing, it looks pretty good up in the air too.

You could try putting in some lengths of carbon fibre tubes or rods into the wing. You can get a size that slides snugly in between the flutes. the CF rods do a pretty good job of stiffening up a wing. it might be all you need.
Jul 11, 2011, 11:58 PM
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JohnsPop's Avatar
You also might try trimming off the inside corner of that aileron next to the prop at the same angle as the outside corner and if I was going to use two aileron servos, I would have put them in the middle of the aileron and used extensions instead of putting them so far inward like that...... take out some of that flex. Awesome plane, though!
Jul 12, 2011, 12:04 AM
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JohnsPop's Avatar
Oh, ya know.... I wonder if the way your hinge is cut through there at an angle like that if some funky air currents are passing through the ailerons and causing the flutter. Usually the hinge line is right in a flute and the side of the flute blocks air from passing through there. I wonder if a piece of packing tape or scotch tape put down in there and pressed in with a craft stick or something and then folded over would make a valid test to see what would happen if you stopped the air from passing through that hinge line.... Easy test and if it passes you might be able to come up with something more permanent. Although the way I fly, a piece of packing tape would probably hold up ok until the wing met its demise.
Jul 12, 2011, 07:44 PM
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gatorlynch's Avatar

Spar Added

Thanks for the comments, guys.

I decided to use a scrap piece of 3/8" for a lateral spar. On my next build, I will insert some CF rods into the flutes before I fold & glue the wing.
I also added a piece of CF on top of the motor mount stick running forward, to help stiffen longitudinally.

Next time, I will also move the servos further outboard, to center them on the elevons. I was trying to keep them as far forward (which means inboard, due to the wing sweep) as I could for balance, but I think the elevons need them more centered.

I'll put some hinge tape on & see if I notice any difference.

I flew 3 batteries today. I think I've got her dialed in close enough for now. To offset the weight of the new spar, I added 1 oz. of lead at the tip. She needed a whole bunch of left elevon up trim/centering to keep from rolling to the right - perhaps due to to torque of the motor/prop? This being my first wing, I don't know if this is normal.

Oh, I almost forgot - the flutter is all but gone. It only showed up briefly one time during some high speed manuvering, as the video shows.

KFm Wing with spar (2 min 15 sec)
Jul 14, 2011, 08:46 AM
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gatorlynch's Avatar

Got Bomb ?

Added a Quanum Bomb Drop System to my wing -

I'm going to try to move the base further back closer to the CG, so it won't impact flight performance so much.

Got Bomb ? (1 min 55 sec)
Jul 14, 2011, 06:30 PM
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gatorlynch's Avatar
More frivolity & mirth with bombs!

Bombs Away ! (2 min 9 sec)
Aug 23, 2011, 08:22 PM
Registered User

Coroplast Zagnutz Wing

Hey Gator,

I just found your Coroplast 48" wing. I been into SPAD for two years and
have been on the lookout for this type of wing.

I have downloaded Tony's plans and plan to start shortly on this build.

I have two questions:

1) Have you stuck with your original power plant?
If so, do you have a Hobbyking or other sites stock number(s) as I have
trouble crossing motors to get the correct size.

2) With your set-up, where did you end up with the CG as measured
from the nose?

Thanks In Advance
Aug 23, 2011, 11:28 PM
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gatorlynch's Avatar
I am on my second wing. I felt the first wing needed some improvements, and it was just as easy to cut a new wing out than to mess with opening up the fold, etc. on the first one.

1.The second wing (KFm -V2) has the same electronics. The prop is an APC 8X6EP & the motor is a speed 450 from headsuprc - http://www.headsuprc.com/servlet/the...0-Speed/Detail

I am drawing 21 amps & 240 Watts on the wattmeter & the motor is only rated at 20A max, so I try not to fly at full throttle.

2. I don't yet know what the final CG will be. I usually record somewhere what I'm using for CG on my planes once I get them dialed in, but for some reason I didn't on my first wing. The old airframe with the markings on it ended up in the recycling bin & got picked up Friday.

I maidened V2 Saturday, and am still in the process of dialing her in. She's balanced about 7.5" from the tip now.

I was suprised to find out she was nose heavy, even with cutting from the same plan & locating battery & electronics in the same place. I had to move the battery (rotate it to run fore to aft). I think it's because I eliminated the wood spar in the rear & added some CF in the flutes.

I'll post some details & pics in the next day or so. I'm going out in the AM - as the winds from Irene will probably keep us grounded starting tomorrow PM till this weekend.

I have a question for you - what are your thoughts or plans for a motor mount?

I initially used the stick mount because it's simple, easy and I had one laying around. I think it also helps act as a stiffener.

I had also thought about making an "L" shaped motor mount out of gutter pipe or plywood & fanstening thru the Coro to a plate on the other side. This would allow you to slide the motor as needed for balance during your trial fit.
Last edited by gatorlynch; Aug 23, 2011 at 11:34 PM.
Aug 24, 2011, 07:48 AM
Andy H
I Scaled this down to a 36" wing, does that mean I need to go 3:4 on the overall weight as well?

I tried it last night and all it did was nose dive rather than glide,

Also do I need to have the Vert's on the wing tips. The plan didn't have dimensions for these?
Aug 24, 2011, 01:08 PM
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gatorlynch's Avatar
If you're nosing in, your balance point is off. You need to move your CG point and rebalance. Trial & error, until you can fly level hands off without too much trim on the elevons.

My wing has flown without winglets - I had 1 came off at the field one day, so I removed the other one (they were just put on with gorilla glue) so I could keep flying and too see what would happen. I didn't notice a huge difference, but I would think they're designed to be there for a reason. Plus, without a fuselage, it's about the only thing you have as a visual reference for orientation.

Print out the portion of the plan showing the winglets at full scale, then use as a template.

Your weight will not be a linear porportion versus a larger wing span. Your battery, motor, servo esc, etc. are fixed in weight & do not vary with a smaller wing span.

Hope this helped ...
Aug 24, 2011, 05:29 PM
Andy H
Originally Posted by gatorlynch

Your weight will not be a linear porportion versus a larger wing span. Your battery, motor, servo esc, etc. are fixed in weight & do not vary with a smaller wing span.

Hope this helped ...
Weight is fixed as battery + Motor +Gear won't change... which it is then, what is the heaviest that a wing of this design could fly at. For EXAMPLE If a 48" wing could cope with 200g then will a 36" wing cope with 150g (3:4). Surely the wight is a function of the area of the wing and the lift generated by the wing.
Aug 24, 2011, 06:52 PM
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gatorlynch's Avatar
Andy - I'm not able to answer your question, as it's not a simple answer. If you want to delve into the design aspect, you can lose yourself for days at a time here in these forums, and get the opion & experience of knowledgable people. Here is a link to a thread discussing the KFm airfoil - https://www.rcgroups.com/forums/showthread.php?t=558321

This being a SPAD (to me, the operative letter here is S=Simple), I just built it, knowing it would weigh a bit more than a foamie build. I didn't want to get all caught up in wing loadings, etc. I can tell you my V1 flew well, both power on and off @ 23 oz. I also feel motor/prop selection is important - you need to have enough thrust for the weight.

PM me if you want to - I'd be happy to try to help you out.

Aug 24, 2011, 09:30 PM
Registered User

Updates & Changes

Don't forget to post the changes and updates made from v1 to v2.
I have an equitable 450 I had purchased for a HOT version of rcrich's
cheap-n-easy so I think I have everything I need.
hxt 500 servos be good enough for this bird?
Also, how do get such a straight fold when folding the coroplast,
I assume you are using 4 mil, at the angle for the leading edge?
Aug 25, 2011, 07:24 PM
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gatorlynch's Avatar
Originally Posted by stumblebum
how do get such a straight fold when folding the coroplast,
I assume you are using 4 mil, at the angle for the leading edge?
My wing is cut from a single sheet of 4mm Coroplast.

I cut a small piece of scrap, to show you how I cut my wing folds.

This technique also works for cutting out the control surfaces, or cutting fuselage folds on other models. If it's a control surface or a 90 degree fold, I usually only cut one pass wide.

Here is a link to my hinging tool - http://home.comcast.net/~TipsTricksL...nging_tool.htm

The curved tips of the blades allow you to cut without worring about cutting through the bottom skin layer. A straightedge (I use a framing square) is used when you're cutting at an angle or perpindicular to the flutes. When you're cutting with the grain of the flutes, the width of the paint stick allows proper blade spacing to be able to cut & remove the entire top skin right from sidewall to sidewall in one pass.

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