KFm wings a basic explanation

by davereap | April 20, 2017 | (5 Ratings) Posted in How To

 

This article will not go into the science behind the idea , but try to give a simplistic explanation of the KF wing. And how it works.

I will say at this point that KF wings work superbly for all powered models, they offer benefits in stability , they reduce stall and give smoother flights. Ive been sticking a KF wing on almost 100% of my foam models for several years now. If you build your own models and lack the ability to hot wire cut a wing then a layered KF wing is a must build. Give them a go.

KF by the way stands for Kline and Foggleman who were the designers that have been credited with the basic idea,

A KFm wing uses laminated foam sheets to make a layered , stepped wing. On rcgroups.com there are long threads for both general building and scientific discussions on the topic.

There are a number of KF variations , These use a combination of layers and positioning to produce a wing that can be made to emulate what are generaly considered normal wing airfoil profiles.

Drawn up by dick Kline these give some basic guide numbers  for the various combinations that have been tried. The guide numbers however are open to changes, going thinner will always work, going a bit thicker will often work, go too thick and it wont work. I personally won't go above 15% and I prefer to keep the percentage down as close to the recomended numbers as possible. Or alternatively I go thinner. Position guide numbers can also be varied, I have a habit of leaving the aileron width out of the full chord measurement, so in effect my steps are moved forward on my wings, the step then is kept clear of the aileron hinge line , I dont want a vortex to affect my control surfaces.   As a good result they all fly

Note the percentage thickness refers to each sections total thickness vs the total chord of the wing, and not the height of the step. The height of the step is only determined by the thickness of your foam sheet. Steps can be raised to give more height and the step effect increases proportionaly with both height and air speed.

The most used sections being KFm2, KFm3 and KFm4. Used because they are the quickest and easiest to make, and they fly well.

If you look at their shape and then add a line joining the top of the steps going down, or up, to the trailing edge you will see the airfoil profile they are emulating. The basic step idea is that there will be a vortex produced behind each step that fills in the missing gap and lets the air flow over over the wing as though  the gap was solid and profiled.

So a KFm4 is emulating a symetrical section, as is the KFm6

The KFm2 is emulating a lifting section , as is the KFm3, KFm9 and KFm11

Going back a while flitetest made a good video on airfoils, and the KFm3 wing is tested from about 4 minutes into the  video.

Despite their previous good results flite test has gone forward with a different type of wing construction that is well suited for paper covered foam board. However I do feel that a KF will work as an alternative build. Its a simpler build that will always fly well although it will be a fraction slower and more stable than the more usual alternative profiles

Pros and Cons

Pros. For powered models a KF gives extra stability, reduced stall, good smooth flights. Its a fast build. The basic KF sections all fly well, so I recommend it for all powered sport models

Cons.  Maximum speed is reduced , for unpowered gliders the extra drag spoils the performance, for scale the steps interfere with the looks. For indoor light weight models the extra layer adds unwanted weight.

 The KF wings are superb on all sport powered models, they are not so good on some of the other model types.

Ive used a KF on all my most recent builds, and the majority of my models have a KF wing.

Some KFm forum threads. There are more on rcgroups.

building. https://www.rcgroups.com/forums/showthread.php?1117276-**-Kline-Fogleman-(KFm)-Airfoils-Building-Flying-Discussion-**

theory. https://www.rcgroups.com/forums/showthread.php?1296458-**-Kline-Fogleman-(KFm)-Airfoils-Advanced-Theory-Science-**

Links to over 200 KF wing builds.   https://www.rcgroups.com/forums/showthread.php?1122108-Please-link-your-KF-designs-or-KF-mods-here

 

Here is just one of the many examples that make use of the KF wing.

The KESL is a 32"  KFm4-Epp  Swift Like wing by rdjay. It is an older build, using a 6mm main layer with two 3mm outer layers, the steps are raised with a balsa spar.  todays simpler build method would use three 6mm layers , directly laminated, with no extra raising of the steps. at under 36" this will be perfectly fine. the step lines are marked at 50% of the chord.

Parallel chords, tapered chords , all work fine, its just a matter of adjusting the step position as per the guides, and the COG will also need setting correctly

 

The above KFm4 wing and the various tapered wing designs available can all be made using most of the KFm layouts, KFm1, KFm2, KFm3, KFm4, etc. On;y the undercambered wings are not so suited.

Remember that top steps will produce more lift, top and bottom are classed as more symetrical builds. So all you need to do is to build up a KF profile that looks like the standard airfoil shape that might have been used on your model, build it and it will work. One example would be a typical trainer design, these usually have flat bottom wings with a curved top profile. They use plenty of lift for self righting.  To replace that profile you should choose a KFm2 or a KFm3, top steps for lift.  Progressing on, your aileron wing trainer would be more semi-symetrical, with more curve on the top but with some curve on the bottom. That shape can be simulated in two different ways, either by using a KFm7 (see above) or by using two different thicknesses for your step layers, 6mm to the top and 3mm to the bottom. Either will be good.

Try to stick to the simpler wing builds, at least when you first start, there is no point in going too mad with your layers, the simple KFm2 , KFm3 and KFm4 are most used. The simplest, the KFm2 is used on a high percentage of the models.

A typical plan is shown below. For a small size a KFm2 is good, going large a KFm4 will give a thicker wing which will better suited for the size, all will work fine. Spar reinforcement , I use either a CF rod or pallet strapping . For some while Ive been using strapping glued to the top and bottom of the foam wing, its simply glued along the step lines and mirrored below to make a sandwich. this makes for a strong stiff wing

built large here it has a KFm4 wing

 

below on a fokker,a tripple KFm2 wing works well and my seajet also uses the KFm2

 

 

Here is another faster KFm2 a bugatti by Thomas B.  https://www.rcgroups.com/forums/showthread.php?2656354-Bugatti-100

Just to show that making a fast model is not a fluke.. here is a small drak look alike.   again using a KFm2 section, the easiest KFm wing build. and a link to its first post on rcgroups.  https://www.rcgroups.com/forums/showpost.php?p=37560494&postcount=9090

 

 

 conclusion.

The KF set of airfoils work great for any powered sport models, I have been using KF wings on most of my models for years and I never have problems . When all things are equal, they will fly at a slightly slower speed compared to the standard airfoils, but in general they are more stable and give smoother flights. Its an easy quick wing build method that I can fully recommend. And for speedier flights simply add more power or use a higher kv setup on a better streamlined model. More power is more fun. See above.

 

 

 

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COMMENTS

Sir Fly on May 20, 2017
So isn't the kfm- 1 KF airfoil the airfoil Flite Test planes utilize? When the wing is folded over to create the airfoil, the top is smooth and rounded off to the trailing edge while the bottom has a step formed by the edge of the foam.
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davereap on May 21, 2017
Almost but not really. the flite test wings are basically a normal /but angled airfoil profile with an accidental step , due to the build, where the lower surface meets the upper surface, back very near to the ailerons hinge line .
A step near a hinge line doesn't give any benefit, with the top step KF builds it appears to cause more problems as any vortex produced ends up on the top of the moving control surface. On a glide the problem shows as a loss of lift as the ailerons are moved..
It may be that a vortex under the control surface has no effect at all.. certainly all the flite test wings fly fine with no odd behaviour
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Sir Fly on May 21, 2017
Okay gotcha. Very interesting!
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