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Jan 29, 2009, 02:26 PM
Registered User
On the thickness of the KF wing is 7 to 9% the over all thickness or the step height above the top of the bottom skin?

On a 7 inch chord wing 1/2 inch is 7% anf .63 inch is 9%

With blue core the wing is 1/2 inch thick without a step when it is folded over onto itself. It would need a 1/8 tall spar to make 9%
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Jan 29, 2009, 04:41 PM
jackerbes's Avatar

Thanks for the answers and the additional details.


The son of tony65x55, the designer, flies his Zagnuts a lot and Tony claims it is the first plane that the kid has not destroyed almost immediately. That says a lot for the gentle handling and slow speed capability of the plane. Tony's report on the flying characteristics in this post:


is what really get me interested in building it. I only got about four flights out of that first one before I folded the wing (and that was my fault, not the plane's). But it did have a lovely, gentle power off glide.

Once you have that trimmed up and set it to low rates, it is a plane that even a newbie could fly I think. But you can flip it over to high rates and it is maneuverable enough for even experienced fliers.

Jan 29, 2009, 06:09 PM
jackerbes's Avatar
grant31781 wrote:

"..On the thickness of the KF wing is 7 to 9% the over all thickness or the step height above the top of the bottom skin?..."

I wondered about that earlier too and figured out that it was the wing thickness. The top of the KF step is always the thickest point in the wing just by the nature of the design

"..On a 7 inch chord wing 1/2 inch is 7% anf .63 inch is 9%..."

Right, and .63" is about 5/8" (.625").

"..With blue core the wing is 1/2 inch thick without a step when it is folded over onto itself. It would need a 1/8 tall spar to make 9%..."

You are right of course and it raises some interesting questions. But the quick reply is that is a OK to use the 1/4" square there and exceed the thickness spec in the figure.

Now I'll go on and on about that a little. :>) But full discussion of that is welcome of course.

First, I've bought two batches of blue core (Dow Performance Board III fan fold bundles to be moe exact). On the first the material averaged out to about .185" (almost 3/16" or.1875") and on the current bundle is is closer to .220" (about 7/32" or .21875"). The blue core is wavy and varies in thickness by .050" or so from place to place on the same sheet. So .200" or .210" is a better nominal size for the foam than is .250" or 1/4".

On the first Zagnutz I built I followed the plans on page 3 and used a 1/4" square spar. So that, with the .220" foam I have now, gave me a wing thickness of .690". I didn't "crunch the numbers" on that at the time and I now realize that that is about 16% if you use 11" as the chord length.

Why use the 11" number for the chord? I did that because I noticed that the dimensions of the 40% KFm1 strip on the plans (4.5" at the root and 2.25" at the chord) are right for a wing with an 11" chord.

So there we had a wing that was right on the money for the chord percentage but way off as far as the wing thickness. And that has been built numerous times and always flies well.

I've read this whole thread and remember some of the bits and pieces of info that have gone by me here. I seem to remember that while it is acknowledged that a 1/2" step works well on a KFm1 wing, it is too high and does not work well on a KFm2 or KFm3. On the latter, a lower step of 1/4" or so (to a max of 3/8" maybe?) seems better.

I'm building another one of these now changing from the KFm1 to a KFm3. I had to work out the details for the step height, KFm strip lengths and locations, etc., for that. I the wing partially finished and have some notes and photos on that build and intend to post those here soon.

Here is a photo I took of the wing tip of that wing with the spar mounted and two pieces of foam scrap laid on it to show how the KFm strips will be there. I used pieces of 1/8" x 1" and 3/8" x 1/2" basswood to build that spar. Those were chosen because they gave me a 5/8" (.625") high spar and, with two layers of .220" foam, the wing thickness will be 1.065" and that is 10.3% of the 11" chord.

The piece of 1/4" square balsa seen in the photo will be replaced with strips of foam cut to about that same height. So on that wing, the steps on the back edges of the two KFm strips will be about the thickness of the foam or 1/4" and that is a step height that is known to work with the KFm3 wings.

Jan 30, 2009, 12:09 AM
A real WingNut
j4ck4's Avatar
I would also be glad to see those pics of your build Jack. I was wondering and thinking that to get the best effect from the kfm that Tony65x55 used on the original kfm zag, shouldn't it be turned up? I know mine acts kind of squirrelly at low speeds and thats what I want out of it.
I know it actually flys pretty slow, but as I have stated with just starting to fly it seems awfull fast at times. Especially if its headed towards the ground.
Jan 30, 2009, 01:02 AM
Jack ,you're right into what i'm trying to figure on. if you get the kfm dimensions correctly,it will fly slow nicely,i think. please post your build pics with all measurements.now,the step will be on top.thanks
Jan 30, 2009, 06:32 AM
Went to mow a meadow
frank48's Avatar
Grant Jack & heli

Heres a calculator designed to help with the KF step. I cant take any credit for this, its something I found a while ago in these forums. Thanks to whoever made it.

Jan 30, 2009, 08:56 AM
jackerbes's Avatar

Thanks for that calculator! I ain't doing it the hard way any more. I opened that file (it is a M$ Excel spreadsheet for those that did not know that) with the free OpenOffice.org software and it is really neat.

I had to turn the protection off to change the values but that is a normal thing to have to do. Once I did that I was able to play with the numbers at will. And all the other info and the decimal to fraction equivalents make that a pleasure to use.

Jan 30, 2009, 09:04 AM
Went to mow a meadow
frank48's Avatar
Originally Posted by jackerbes

I had to turn the protection off to change the values but that is a normal thing to have to do. Once I did that I was able to play with the numbers at will. And all the other info and the decimal to fraction equivalents make that a pleasure to use.

Theres no need to turn the protection off, thats just for the example "how to".

There are tabs at the bottom of the sheet

Jan 30, 2009, 09:07 AM
jackerbes's Avatar
j4ck4 wrote:

"..to get the best effect from the kfm that Tony65x55 used on the original kfm zag, shouldn't it be turned up?..."

Tony's plans use the KF wing (also called the KFm1 to distinguish it from the KFm2, 3, and 4) design. As tested and developed the step was on the bottom and it worked. The KFm2 is essentially the KFm1 wing turned over and the step was moved back from 40% to 50%.

There are no KFm rules, what works, works. Part of the value of what we are doing here is that by us reporting the various changes we are refining the parameters, at least for this airplane, and learning more about the use of the KF airfoil. With all the variables that there are in any given airplane, and in any given build of that plane, it is almost impossible to narrow this down to a single set of parameters.

The fact that Tony's KFm1 wing has a 16% wing thickness instead of the 7-9% in that illustration and flies well is unexplained but we are happy with it. Maybe one of us needs to thin that down and see if it gets better or worse? Or maybe it will just change one of the flight characteristics and still be a good wing?

Jan 30, 2009, 09:38 AM
jackerbes's Avatar

Zagnutz KFm3 Build - Section 1

Zagnutz KFm3 Build

Here is the first installment on this build. The photos are in sequence with the narrative, I'll refer to them by number occasionally for reference.

I used Bill Seagrave's Scale&Tile java applet to scale, tile, and print the plans from page 1 here. They were taped together and cut out to produce full sized templates for the parts.

The plans are for a KFm1 wing with the KF strip on the bottom of the wing, this build will be for a KFm3 wing with two KF strips on the top of the wing. So the KF strip in the plans will not be right for a KFm3 wing but the plans and templates are still useful.

The starting point is a 24" x 48" sheet of Dow Performance Board III fan fold foam or FFF. Basic tools are a sheet rock "T" square and an unseen 18" metal rule for straight edges, a fine tip Sharpie marker, and the small utility knife with snap off tips.

As seen in the first two photos, the foam has the printed or foam side up but I work from the other side, the plastic coated side, for layout and cutting as it takes marks better and gives me cleaner cuts.

Hold the template down and just mark the ends and corners of the cuts. Use the "T" square and metal rule as straight edges for the cutting. For example, in photo e you see the marks that frame the motor and propeller area on the back edge of the wing. On the more subtle or less acute angles I make a "tick" mark at the point where two lines cross to better define the placement points for the straightedges. You can see a mark like that in photo 4 (at the aileron end near the wing tip).

The ailerons will be cut after the wing is finished so those are not marked now.

The cutting is done with the knife blade well extended. The blade is square to the cutting surface and the handle tipped well back from vertical. The low angle really helps the cutting action and keep foam from balling up on the blade tip and producing rough cuts. Keep the blade against the straight edge as you make a cut and make a shallow cut through the plastic, then make one or two additional light cuts to full depth. Photo 5 shows a typical cutting position, I find that pulling the knife slowly and always watching the point of cut works best and I stop and move my hand to hold the straightedge down firmly near the cut every 10-15" inches or so.

As much as possible, I put the straight edge on top of the part and the scrap side away from the cut so that if the blade wanders it will be into the scrap.

With all the cuts made, turn the foam side up and mark the center line. Mark dots at 5-1/2" and 8-1/4" back from the nose. These are the locations of the back edges of the two KFm strips.

On the tips mark dots at 2-1/4" and 4-1/8" back from the leading edge, this again are the back edges of the KFm strips. These marks are faint and not seen in photo 6.

The spars/KF strip supports are seen already in place in photo 7. Those can be built in any of a number of ways and from various materials. I used:

2 - 1/8" x 1" x 24" basswood
2 - 1/4" x 3/8" x 24" basswood

The 1/4 x 3/8 strip was centered on the 1" wide strip with the 3/8" side vertical. It was glued with five minute epoxy on a flat bench top and weighted to be flat and straight. It looks like an upside down "T". Two of these were made. The ends were angled so that the spars could butt together on the center line.

The center line dot at 5-1/2" and the dots on the wing tip at 2-1/4" were used to make marks for aligning the spars. That is the red line faintly seen on the wing tip on the left in photo 7.

Align the back edge of the 1/4" x 3/8" strip support with a line drawn from the dot at 5-1/2" to the dot at 2-1/4". That puts the back edge of the support even with the back of the KFm strip. A closeup of the spar and the aligning mark is seen in photo 8, note that even though the vertical strip shifted from the center of the 1" strip as it was glued, its back edge is aligned with the red line.

Photo 9 uses some pieces of scrap to show how the 50% (to the right) and 75% (to the left ) KFm strips will be glued to the spar and support . The 1/4" square balsa strip that is supporting the front of the 75% strip will be added later. A strip of foam could be used instead. Photo 10 shows the butt joint at the centerline.

Note the low areas on each side of the butt joint. That is caused by the high and low points of the waves that are in the foam. When I glued the spar down I laid it flat and the spar glued to the high points leaving a gap at the lowest waves. I later used a small strip of plastic to push small amounts of PU into the visible gaps and under the spar where the spar was not in contact with the foam.

I'll continue this later, the next step will be to cut the KFm strips.

Last edited by jackerbes; Jan 30, 2009 at 09:46 PM.
Jan 30, 2009, 09:56 AM
Good going guys,this is great. the kfm3 with strips on top, gotta have a run. pl by all means keep posting results and possible maiden.i do say, it'll be real nice and slow flying,if the weight is kept light,then it will be a good low performer.
Jan 30, 2009, 12:25 PM
A real WingNut
j4ck4's Avatar
Gosh I have two planes in my office now to maiden and I think I'm gonna have to start this one Jack. Build along with you, just gotta find the $ for some basswood now.
Jan 30, 2009, 02:11 PM
jackerbes's Avatar

Basswood is what was in the LHS, he is more model trains than airplanes. But if you have access to a table saw and can find nice dry clear pine, redwood, spruce, or many other woods it will work as well.

You can see the comparitive weights here:


Fir and basswood are about the same weight, cedar is lighter but not as strong, etc., etc. I think I figured that basswood weighs about three times as much as balsa (which is not the same wood as balsam, the latter is much heavier).

If I still had access to a good table saw I'd be cutting my own spars out of something (maybe aircraft grade spruce) but still buying some balsa for lightness.

Jan 30, 2009, 02:27 PM
A real WingNut
j4ck4's Avatar
I have two good lh's within 4 miles, one has a pretty good stock of wood the other none. He's into cars and trucks more than anything. Then about 8 miles got one devoted to planes and copters so I can get most of the wood I need at one or other. No friends with a table saw, and actually don't know anyone here in T Town that flies, though there is a club here.
Jan 30, 2009, 06:50 PM
A real WingNut
j4ck4's Avatar

Just got back from the field I use for flying, weather is 59deg wind about 10mph or so. Had another great flight with the kfm wing and couldn't help myself, just had to toss the 60" Zagnutz. You talk about getting my Nutz off, that thing is the fastest plane I have flown so far and the funny thing is it has my old gws350b on it. Cool though it will hang in the wind when you cut the power back and nose it up, loop like nobodies business too. May be the 9.6v battery.

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