General KFm Theory

Tony

Tony

Staff member
Here are a few PDF's of the KFm theory and why the creator thinks they work better than a standard airfoil. These are not my design but I thought you would enjoy knowing why I chose the wing I did on the RCH Trainer and why you may want to try it out on your next plane. Have a look and let me know what you think.
 

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Not sure why i didn't see this post before, a subject that I've had much lively debate about on other forums! The claims made for KF airfpoild are to be blunt nonsense. I actually shared a few emails with the inventor Dick Kline a few years ago, he's a really nice guy but knows 'zip all' about aerodynamics. There has been quite a bit of testing in wind tunnels of the KF airfoils and without exception the results are terrible!

Having said that, for a powered trainer it's not a bad choice, mainly because it's easy to build and strong. The efficiency is very poor but that's not important for a powered trainer.

I actually did some testing years ago with my Elf discus launch glider to highlight the differences in lift/drag performance. I build a KFm wing that was exact same shape as the stock Elf glider wing and weighed the same. Flight time was a little over half (actually 60%) of the standard wing. The best i got with the KFm wing was 46 seconds which actually isn't too bad and at the time I think was a record for a KFm discus glider... but it was still miles short of what the standard wing can do.

I dredged these photos up:

Standard Elf:
a4429893-197-Standard_wing.jpg


KFm Elf:
a4429894-8-Kfm_wing_1.jpg
 
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Yea, this post is about 5 years old and the whole dynamic of KFM is based on pockets of air. When it steps off, it was believed that pocket of air had less resistance with air flowing over it than if it was flowing over a solid air foil. Same thing with a truck bed with the tailgate open or closed. Open there was no pocket and closed there was and it was proven that closed give better gas mileage.

With that said though, the ONLY reason I still make the KFM is because they are easy to make. Much easier than a standard airfoil. But I am looking to make an actual airfoil wing for the trainer and Edge 540 and get away from the KFM. I absolutely can't stand the way the KFM looks. Blocky as hell and just not the right fit. But again, they do work for now.
 
Yeah, I've hear the truck tailgate comparison before. Trouble is it's not at all valid for airfoils because a conventional airfoil is a streamline shape, a truck is anything but streamline. If you want an idea what a 'tailgate' sticking up does for a streamline airfoil get a glider with spoilers and see what they do for the glide slope when you deploy them (they are called spoilers for a reason!)

I have some videos of the two versions of my Elf glider which make the difference in performance in the real world painfully obvious if anyone wants to see them, the video quality is crap as it's an old keychain cam but it gets the point over.

But as I said, for a trainer or sports power model this really isn't a consideration because efficiency is very low down the priority list.
 
The truck tail gate was for the KFM and what they said made it superior to an airfoil. Allowing the air to ride on another cushion of air. Like I said though, the KFM is played out, this post is 5 years old and it's time for a new design.
 
Hello all: I personally like the Armin wing. It is strong and smooth and no harder to build than any other. Just my 2cents worth.
 
I have thought about updating the wing on this plane, but figured stick with what is working. Maybe there is a V4 coming lmao.
 
Check out how flute test does their wings. They have a full airfoil design that vet simple to assemble.
 
although KFM foils work great for a quick build without the use of hotwire and such, it will never outperform smooth airflow from that of actual foils. if it did you would see birds wings evolved in step ladder form like kfm :)
 
Hence why it was called a theory. The idea was the air buffer behind the ribs causing reduced drag. About 2015 people started going back to a traditional air foil shape. As you can see in the first post in here, this was back in 2012 when this was made. A lot has changed in 7, almost 8 years.
 
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