** Kline-Fogleman (KFm) Airfoils - Advanced Theory/Science ** - Page 73 - RC Groups
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Feb 06, 2013, 02:37 PM
Registered User
Originally Posted by dave1993
personally in my tests there was no increase in lift. imo the biggest benefit is stall resistance. i never get tired of seeing my kf foamy float along unbelievably slow at crazy aoa. at least compared to uc w/o the strips.
Yeah, probably with thin airfoils already optimized for high efficiency, there would be no increase in L/D ratios. However with thick airfoils, there is a chance to apply the KF because the flow separates from the airfoil on a thick airfoil like a NACA0024.
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Feb 06, 2013, 04:22 PM
Interesting wind tunnel vid!
...I got nothing from it.
Suppose I need an education to know what to be looking for???

Well, here's a nice vid I just stumbled upon a few minutes ago...
These people are Flying a wing!
I'm impressed! a Lot!

BMF On Tour - GO BIG Mfk Falkens bidrag - 19. maj 2012 (13 min 16 sec)

Feb 06, 2013, 06:39 PM
jackerbes's Avatar
There is a thread on the plane here: https://www.rcgroups.com/forums/show....php?t=1655952

31feet - 9,6m
52LB - 24kg
Mats are 15 and 20mm styropor covered with tape.
Powered by Hyperion 5045 on 12S 6500mah - prop APC 22x10E
Plenty of power

Feb 11, 2013, 11:16 PM
Registered User
Hey everyone, I'll be honest I'm new to this thread and have skimmed through parts of it but not in any depth. Anyways, I did a quick experiment to compare a KF airfoil to a regular airfoil and an un-profiled wing, testing lift and drag at 0 degrees and 20 degrees. It is a very basic test, but I thought it might be interesting to some of you all the same. Here is a video discussing the results:

KF Airfoil - Wind Tunnel Testing (3 min 36 sec)

I plan to come up with a better workflow for taking measurements, then test a wider variety of airfoils at increasing angle of attacks. I also want to do some airflow visualization, but am having trouble making a steady stream of smoke.
Feb 11, 2013, 11:51 PM
Nice test.
I'd like to see more variables introduced.
Thanks for posting the vid link!
Feb 12, 2013, 12:37 AM
fix-it-up chappie
tolladay's Avatar
Originally Posted by experimentalrc
Hey everyone....
Nice video. I like the "simple yet effective" testing.
Feb 12, 2013, 11:17 AM
springer's Avatar
Fascinating! Simple yet effective for sure!!!!

Have you tried any flow straightening mechanisms yet? like the soda straws or paper when you do the smoke? Love the brushless tunnel fan idea!
Feb 12, 2013, 11:39 AM
RC beginner
Originally Posted by experimentalrc
I did a quick experiment to compare a KF airfoil to a regular airfoil and an un-profiled wing, testing lift and drag at 0 degrees and 20 degrees.
im unable to view videos atm. could you provide a brief summary of your conclusions regarding comparison of lift and drag?
Feb 12, 2013, 11:53 AM
Dave, his testing confirmed all the other declarations regarding the KFm step.

a true airfoil had less drag and better lift
a KFm had a very little bit better stall at high incident AoA

The main thing here is that the KFm is ez to build for the hobbyist and will get respectable results.

I'm sure someone will come along and give a more technical critique!
Feb 12, 2013, 12:12 PM
RC beginner
thanks rusty. that was very helpful and agrees with results from my own tests couple years back. i also like the improved stiffness with the kf strip. allows higher aspect ratio wings which sorta compensates for the slight reduction in lift..
Feb 17, 2013, 08:01 PM
Registered User
Dickeroo's Avatar
For those of you who haven't seen this but might be interested...

Here is a weight-lifting test between a KFm3 and a conventional airfoil.

weight lifting part 2 (11 min 40 sec)

The final tally.

KFm3 79 oz. / Conventional 71oz.

weight lifting part 3 (6 min 10 sec)
Last edited by Dickeroo; Feb 17, 2013 at 08:16 PM.
Feb 22, 2013, 03:59 PM
Must not buy more planes!
mclarkson's Avatar
Is there a recommended relationship between wing chord and step height? Might I make my steps too shallow or too tall?
Feb 22, 2013, 04:38 PM
jackerbes's Avatar
Originally Posted by mclarkson
Is there a recommended relationship between wing chord and step height? Might I make my steps too shallow or too tall?
Not really, just the percentages in the figure in post #2 and how that happens to work out for various materials. If you have a 10" KFm4 wing the recommended thickness at the thickest point would be 6-9% or 0.6-0.9".

If there was three layers of material there and we used 0.25" FFF it would make the steps about 0.25" high and the wing would be just slightly over the 7%.

If you did the same build with 1/2" foam and the chord was scaled up and the other numbers all scaled about the same (20" chord, 0.5" steps, 1.5" thick wing) I would expect it to work as well.

Feb 22, 2013, 06:29 PM
Must not buy more planes!
mclarkson's Avatar
I was never entirely sure, reading those diagrams, just what the 7-12% referred to. The chord? The span? Something else?

I have a wing with a 12" chord, for example. So a KFm4 would need to be around 1.25" to 1.50"? So, were I making it from 1/4" FFF, I'd want to use multiple layers? (Otherwise, I'd only be 0.75" thick, correct?)
Feb 22, 2013, 06:47 PM
Build straight - Fly twisty
Whiskers's Avatar
Go for a KFm6 with steps at around 40 and 60% (or whatever takes your fancy)
At 1 1/4 inch it will be right on the money.

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