KFm-4 airfoil for speed and stability

by mankick | October 19, 2014 | (9) Posted in Projects

I really enjoy a great scrach build and thanks to Jake Marshall and his team of builders he has provided a wonderful platform to exploring prop jets that not only look great but fly exceptionally well. MesarcFoamfighters uses a variety of techniques from score cutting foam boards for exact folds to glue joints that provide excellent structural strength. They also use a variety of wing designs from the simple flat wing found on the MF-22, to the low profile armin wing found on the MF-18.

After reviewing his work I decided to build the MF-18 Hornet. I have always loved the agressive look of the Hornet and the profile it displays in the sky is just incredible.

The plane designed by the foamfighters was very scale and had that intimidating appearance I could not pass up. The build was simple enough with the exception of adding a few reinforcements on areas that get worked like the battery door and belly pan, since it is a belly lander. I also noted the flight charateristics being very docile and it had a tendency to want to fly upright. The undercambered wings, typical to the armin wing, also found on many of the flitetest wings provided that lift and almost self-righting tendency, which is great for beginners but takes away from the want to do manuvers typical of a jet. 

So how can such a great plane be improved? A good friend, with a wealth of knowledge guided me to the KFm wing designs (Thank you Frank M.). After looking at a few youtube videos and researching the benefits of their simple design I was hooked! It was then clear to me to improve the flight flexibility of the plane while maintaining if not improving stability and increasing the speed envelope, in theory, would be turning to the KFm airfoil. 

To give a quick background of the KFm airfoils: This airfoil design is very simple to build, being nothing more than layered foam, that creates a step on the wing, and improves the strength up to 3x without a spar. Here is an example of a KFm-4 airfoil (step on bottom and top making a symmetrical airfoil).

Depending on the version you choose the step may be on the top, bottom, both, or even multiple steps on either of the surfaces. What this does is create a bubble of air behind the step to the trailing end of the wing in which air flowing over the wing from the leading edge then slips over the bubble, instead of dragging over the entire wing surface. As Dave from RCsuperpowers stated "their is some magic that you dont find in the other wings" that happens and is immediately felt at any speed. The only downside to this design is the appearance. This airfoil does not look scale!. 

Here are some great links that have helped me to understand the KFm airfoils:

how to build: http://youtu.be/MHvVk2bcf5A

Testing: http://youtu.be/6H-s7yuJcwk?list=PL6A15E9415D2EF835

Types of KFm airfoils: http://rcfoamfighters.com/blog/?tag=kf-airfoil

Why to use: http://youtu.be/ROHE-YpV4Kk?list=UUFOM5TvYMxeExoyxoUp8ipg


The Changes:

To accommodate this new airfoil changes had to be made to the build and design of the MF-18. I wanted to have the same plane (dimensions, size) with enhanced flight characteristics and a greater speed envelope. In other words I want to go faster, and I want to have the ablility to go slower, or fly with less effort/throttle. I decided on the KFm-4 air foil, or as some choose to call it "the speed wing" This wing has a symmetrical airfoil, found in may acrobatic and sport planes. This wing does not provide lift with speed like the armin wing, but being a intermmediate pilot I do not need that crutch anymore. Also, I did not want to have to compensate with down elevator when doing a highspeed pass. 

The KFm-4 airfoil was easy enough to make by measuring the existing wing cord from the center edge and the wingend edge, then halfing both measurments and connecting the dots across the wing to have a line measuring the center of the wing cord. The next step was to transter the half line, leading edge and middle/wing edge to foamboard. This made the shape of a half wing. The mirror image was drawn adjacent to the original sketch with a 1/2 inch spacing between. This new shape was then cut out and folded over the main wing cord from the leading edge back to make the airfoil. It was a very simple process and can be shown in the links above. 

Since the spar was no longer needed the bottom plate of the main wing was eliminated. Slots for the belly pan were formed into the bottom of the wing, instead of knotches, and the wing neck was removed from the main wing and reattached by creating a slot in the center of the leading edge, lining up to the center edge of each wing. 

Also, since this is not a hollow core wing spaces for the servos had to be cut into the bottom on the wing then the servos were sunk in with less than a mm of the aileron servos extending off the bottom.

The modifications were simple and kept the demensions of the plane identical. The fight characteristics, however we greatly enhanced.

The pros: The plane flew faster, was more stable, and tracked like an arrow when accelerating. The roll rate increased and the plane required less power to fly level than it did with the armin wing. This thing will float! Also, the structure of the plane felt more solid, and stronger.

The cons: The plane does not generate lift with speed and will drop a wing should a sharp bank be performed without speed. Also, the plane glides at a faster rate and does not spill speed easily, requiring a larger area for landing.

The verdict: Using the same power set up from the original build and the same floor design but different air foil I am convinved that there is some magic behind flying a KFm airfoil. I was happy to meet my goals of bettering a already wonderful plane. The plane was faster, more stable, and the speed envelope was increased.

To complete the project I added endplates to the wings. They are small but help to prevent the spill off of air running from the the center of the plane to the wing tips. A great example of this technique used is found on the versa wing. The enormous end plates act as vertical stabilizers as well as a guides forcing the air to maintain direction from the leading edge off the trailing edge. They were made using gift cards, painted and glued to the wing tips.

Check out the videos for more info about the plane and the maiden.

Happy flying!


sailorJohn on October 20, 2014
Very impressive!
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Den4wa on October 20, 2014
Nice build and maiden flight!!!
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Dick Kline on March 12, 2015
Just watched the video. Very pleased with the results. I'm guessing that the magic might be partly due to air pressure needing to equalize in the vortices which help to provide a more stable flight. This concept came from paper airplanes I created in the late 1960s. You've made my day!
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planefun on December 19, 2015
just started experimenting with KFm here too. KFm-6 on the inner 5/8 of wing and KFm-4 the outer 3/8 worth. a couple degree worth of attack angle, a few degree dihedral, ailerons about 65-35 behind the KFm-6 and KFm-4 sections. saw a comment about ailerons being really mushy then over-reacting with KFm-6, so KFm-4 wingtips where ailerons have more leverage might minimize mushy behavior someone had described, is the hope anyway. gotta love the rigidity of whats so simple to slap together and experiment with.
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KFm-4 airfoil for speed and stability