Experiments with KFm airfoil on 'Elf' DLG glider - RC Groups
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Nov 20, 2011, 12:17 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar

Experiments with KFm airfoil on 'Elf' DLG glider

After taking 'stick' for ages for never having built a KFm airfoil plane I decided to make up a KFm wing for my Elf DLG. I used the Elf as a testbed because of a few reasons:
  • I already had the Elf!
  • Being rudder and elevator the wing would be straightforward to build.
  • The Elf standard wing used a high performance Mark Drela airfoil which i thought would make an interesting comparison to the KFm.
  • Elf wing is easily and quickly removable which would make field changing of wings simple.
  • At 1m span it's big enough for the performance differences to be easily observed but not too big as to make construction difficult.

The airfoil I used was a 'KFm-3' (two top steps). Construction was from 3mm Depron. Total wing thickness was 9mm which is close to the original Drela airfoil. Wing planform shape was made identical to the standard wing. Two 1mm carbon rod spars were added for stiffness. Inserts were added to accept the wing hold down screws and a carbon throwing peg was added to the left tip. Wing centre was glassed for strength.
Weight if the original carbon composite Elf wing was 47g, the foam KFm wing was 50g, this at least shows that it's possible to build a foam KF wing almost as light as even a high tech carbon wing, albeit that it's not quite as stiff.

Photos of original Elf with it's carbon composite wing and the KFm-Elf attached.

Initial flight testing and videos shown in post #29:

More flight tests and video in post #88 and #90:
Last edited by JetPlaneFlyer; Jan 10, 2012 at 04:53 PM.
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Nov 20, 2011, 12:35 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
I took the Elf with it's two wings out for a brief test flight this afternoon. I didn't have enough time for back to back testing against a stopwatch but i could gain some first impressions.

There was no difference in trim between the two wings. I thought the KFm might need some extra up elevator trim but that wasn't the case. General handling seemed to be very similar, stability appeared unchanged and response to rudder was also unchanged. i was actually quite surprised that the elf flew in such a similar way despite the totally different wing.

I had just enough time to do few full power discus launches with both wings. I managed to scratch around in weak lift for a couple of minutes with the standard wing. The KFm wing seemed clearly to have a noticeable higher sink rate, flying speed was similar but it didn't seem to have as good a glide slope. With the KFm wing i wasn't able to make use of any lift that may have been there, flights were more 'throw it up and glide down'. My father came flying with me and he also noticed unprompted by me that the KFm wing had a noticeable worse glide.

It's probably too early to draw any conclusions, perhaps the stopwatch will show that the difference in glide performance is less than seemed apparent from first impressions.. Watch this space to find out, and hopefully I'll get some videos.

Nov 20, 2011, 02:24 PM
Just call me crash for short
Quick61's Avatar
Nice job on the foamie wings and thanks for the post. If nothing else, your report has given me encouragement for my line of thinking that building prototypes with KF wings to proof a design is right. Cheap, fast, easy, and effective. As you have stated, the glide is not as good but the handling is close enough to being the same. Looking forward to your next flight reports. Should be interesting, and if anyone will pick over the small details, it'd be you.


Nov 20, 2011, 02:31 PM
springer's Avatar
Interesting early data. About what we'd expect comparing KFm to high tech carbon, I'd guess. The wing looks, great, a nice build. Some guys fuss about the extra lines on the KFm steps, but I think they look good, especially when swept into the plane's lines, as you did. I found in comparing a "typical airfoil" vs KFM2 on my OSG that the former seemed faster, but once in lift, I couldn't tell the difference in glide slope or speed between thermals (between flights, of course, which made it tougher). I suspect that while I've fooled with the KFM wing a good amount and think I know how to trim it, I haven't had the same experience with the conventional yet, and may not have really optimized it (I do recall they didn't accept the same trim). You may be having the opposite situation, eh? Of course the OSG has a motor on front to get 'er up there so the initial climb/glide isn't as critical.
Nov 20, 2011, 02:58 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
What I think I have proved so far is that it's easy enough to build a KFm foam wing that's strong enough for DLG launching and virtually as light as high tech wings. The 1mm CF rod spars were the secret, they worked really well.

If, compared to previous testing done by others, I'm seeing a larger difference between the KFm and the 'normal' airfoil (if full testing confirms that to be the case) I suspect that it is because the 'normal' airfoil in this case is an accurately made very specialised and high performing glider airfoil. I think most of the KFm tests that have been done in the past have been against airfoils that are only very rough approximations of 'typical Clark-Y type' airfoils. I'm pretty sure that if I made a rough 'Clark-Y approximation' airfoil then I'd see a similar performance loss to that which seems (on first impressions) to be the case here.

Anyway, more testing to do before i can draw any firm conclusions; and yes, put a motor up front and any differenced in glide performances become pretty much academic.

Attached is the KFm-3 airfoil i used.
Last edited by JetPlaneFlyer; Nov 20, 2011 at 03:06 PM.
Nov 20, 2011, 04:59 PM
Onward through the fog.
Cybernaught's Avatar
Originally Posted by JetPlaneFlyer
put a motor up front and any differenced in glide performances become pretty much academic.
In gliders, Drela's foils, Selig/Donovan and Eppler are surely the way to go to maximize performance. As you note, with a motor up front things are a bit different. Your Elf is very nice and if she performs half as good as she looks it must be a terrific plane to fly.

I appreciate the trouble you are going to in order to broaden our understanding of the right applications for the KF foils. I'll be following your experiments with a lot of interest.


Nov 20, 2011, 05:02 PM
springer's Avatar
Perhaps not quite academic, the motor eases the issue of getting the plane up in the air, and can solve the problem of not finding sufficient lift in a particular area, but the challenge is still to keep the plane up in the air as long as possible without having to turn it on! (also saves old geezers arms!)

I like how you did the KFM. It's the classic version that most all are using regardless of the plane type, and will, i'd think, be a more correct (in that it represents what most who provide the anectodal reports are flying) comparison than when one takes a conventional airfoil and cuts the step into it.

I noticed you left the TE full (untapered). That's also typical, an artifact of the "quick/cheap" build philosophy. If your interest stays piqued, you might want to try tapering it after wringing out the current setup, and see what that does to the glide slope.(one of the nice things about foam, easy to change!) In my anectodal experience, the first wings I made for the OSG were squared TE, but after losing the first one and tinkering with KFM2's on the second version, I started tapering TE's to less than half DS foam thickness (about 3/32) and the plane seemed to like it. I guess this agrees with Viking's experiences as well.
Nov 21, 2011, 06:05 AM
Registered User
davereap's Avatar
Lovely work on that wing, your early flying results are much as I would expect, the 3mm build KFm3 will not beat the original high performance wing..

I have found a 4 layer KFm9 to be better than the KFm3, I went mad and bought some 2mm for glider trials, it makes for a thinner section which is faster on the glide..
However I would still not expect the KFm9 to equal the special original sections performance..

where it does compete is the simplicity and cost of the build.. even I can make a flat foam layered wing, when a high performance CF wing is well beyond my abilities

Thats why Ive always said, KF is more for sport flying, strong, quick and easy wings that work well on powered models of all sorts.. some performance loss, perhaps in some ways, probably more draggy, but with improved stall and generally perfectly acceptable flying characteristics.. on jet types and fast aerobatic models a KFm4 smooths the flight..

It looks like you've a few options still to try, like tapered trailing edges, and it will be interesting to get some numerical results on the performance..

Did you put any tape reinforcing on the wing, I find the trailing edges are easy to tear, stress tears are common, any sudden stop will start a split from the back edge .. I am thinking thats a problem that might develope under launch stresses..
Last edited by davereap; Nov 21, 2011 at 06:17 AM.
Nov 21, 2011, 06:48 AM
Registered User
Nice build & nice experience, the ELF wing will somthing that would be really hard (imposible I guess) to beat for the kfm in the glider world.

Another issue I've seen with my kfm experiments is the tickness of the wing. In a normal wing the root is thicker, but due to the depron technics used to build the kfm wing, the thickness is equal on all the length, this is penalizing for the drag imho.
Maybe having a step ending at the half of the wing would help to reduce this thickness issue.

Another problem with depron & KFM technic is around the shape of the LE, which is often really basic because sanded at best, but really important in a standard airfoil. Even the position of of the edge (half of the thick in your example?) will change a lot the efficiency of the kfm airfoil.
Nov 21, 2011, 08:35 AM
Build straight - Fly twisty
Whiskers's Avatar
A DLG is a great test vehicle for these trials.
So far the results are pretty well exactly what is expected, and I'm very interested to see what further sessions show.
Impressions of stalling characteristics will be very interesting.
Nov 21, 2011, 10:11 AM
High Altitude Flyer
viking60's Avatar
Yes, your initial observations are about what can be expected with that shape. Competitive narrow chord HLGs & DLGs typically are built with fairly specialized airfoil contours.

The steps implemented at ~40% of chord & aft have very minimal affect on the airflow dynamics forward of that point.

If the entry point of the LE is at ~50% of the total thickness of the forward half of a wing, and if the upper and lower surfaces are parallel for the front 50% of a (layered foam) wing build, then the resulting camber line of the forward portion of the resulting wing is a flat line.

Below is one drawing of a fairly simple to build wing structure based upon the MH32 airfoil profile [from the DANCER series development experimentation over the last 4 years] which does have a fairly efficient glide & good penetration in ~30 MPH slope winds at low wing loadings. For a 6" chord DLG wing, other airfoil profiles would likely be more suitable.

Here's wishing you the best of success in your ongoing experimentation!

Nov 21, 2011, 10:59 AM
Winging it >
leadfeather's Avatar
Excellent work JPF,

I hope you continue to develop this idea. A cheap, easy to build alternate for a DLG wing would be really good. I'd be interested to see how close (in terms of performance) the KF wing can eventually come to the exotic molded wing.
Nov 21, 2011, 01:38 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Thanks for the positive comments guys

I'm sure you are right in that if I replicated the Drela airfoil exactly, apart from a couple of small steps located well aft, then the performance impact would be much less, quite possibly negligible. That however wasn't the object of this build. I was attempting to compare a typical KFm airfoil, similar to the type used by most KFm foamy builders, against a good conventional airfoil.

Most tests to date have been either testing a KFm airfoil against some aberration mistakenly labeled 'a Clark-Y'. Or occasionally testing two conventional airfoils one of which has a a small step or two cut in. While the results may be interesting neither of these test regimes is actually comparing what typical 'KFm foamy flyers' and typical 'conventional airfoil flyers' actually use on their models.

Some might argue that what I've done is an unfair comparison and I guess I'd have to agree. Comparing any quick, cheap, home-brew airfoil against one that's been developed by a world leading aerodynamicist with countless hours on computers and in wind tunnels was sensibly only ever likely to have one outcome. The question that the tests might help to show is; just how large or small the gap in performance actually is? And is it in fact possible to make a 1M KFm wing DLG that will perform reasonably well?
To satisfy my own curiosity I'd also like to test the other claims made for this type of airfoil. There are a whole lot of claims: https://www.rcgroups.com/forums/show...36&postcount=6 so it will be interesting to see which ones hold water for me so in the future I can at least speak with the benefit of first hand experience.

As it gets dark around here at about 3:30 and I don't get out of work until after 5 then further testing will have to wait until the coming weekend, weather permitting.
Nov 21, 2011, 04:48 PM
Build straight - Fly twisty
Whiskers's Avatar
JPF wrote, "Some might argue that what I've done is an unfair comparison..."
If they argued that they'd be totally missing the point.
Thanks to you we are able to compare a 'state of the art' difficult to build wing with a very cheap and simple and 'easily doable' structure.
Even at this early stage at least the KF 'bang for bucks' claims are well supported.
Nov 21, 2011, 07:52 PM
springer's Avatar
Yeah, If we keep this in perspective, we're comparing a wing that could arguably be built by most anyone from dollar store foam and the CF tubes for five bucks? (don't know competitive price for the CF) to what I'd guess is a wing that is Pricey ($200 for the complete plane from Kennedy composites) and state of the art in design, materials, etc, etc. and couldn't be built without some sophisticated skills and probably molded/die cut parts, fixtures, etc, etc (like masks or ventilation to work with the carbon??)

JPF's already essentially met the weight target and validated it doesn't tear itself apart on launch. And there's at least a couple more things to try to optimize the wing without any serious cost or work to see if it can work the lift. Fun stuff!

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