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Sep 26, 2009, 08:38 AM
Jack
USA, ME, Ellsworth
Joined May 2008
18,350 Posts
Kline-Fogelman Airfoils - Figures and References

If the KF airfoil is new to you and you're interested in a little of the history of it, this wiki page has a good summary of all the details:

https://en.wikipedia.org/wiki/Kline_Fogleman_airfoil

This post will be updated from time to time and has figures of the variations on the Kline-Fogelman airfoil (Kline-Fogelman modified or KFm) as they have been and are being used by many RC builders. The figures for the various KFm airfoils include a comment about the recommended wing thicknesses expressed as a percentage of the root chord.

Using the KFm3 as an example, the recommended thickness is 9-12%. That would be the recommended range for wing thickness at the thickest point and the thickness is a percentage of the root chord.

The KFm3 wing has two steps, the first or 50% step would be at the thickest part of the wing, and the 9-12% distance would be the distance from the wing bottom to the top of the 50% step.

On a KFm3 wing with with an 8"/203mm chord the thickness would be 9-12% of 8"/203mm. And that point would be at the first step. And 9-12% of the 8"/203mm measurement means that the wing thickness at the first step could be in the range of 0.72-0.96" or 18.3-24.4mm.

To continue the example, if the KFm3 wing were built with 6mm thick FFF foam, there would be three layers of foam at the first (50%) step and two layers at the second (75%) step. And the wing thickness would be 18mm at the first step and 12mm at the second step.

Those percentages and the resulting step heights are approximate and can vary, your materials really decide exactly what you will wind up with for wing thickness and step heights.

The attachments include Rich Thompson's excellent comparison study of the KFm1 to KFm4 airfoils to Symmetrical, Clark-Y, and Flat Plate airfoils.

# Images

Last edited by jackerbes; Aug 23, 2012 at 09:41 AM. Reason: Up KFm Family image
 Sep 26, 2009, 08:40 AM Jack USA, ME, Ellsworth Joined May 2008 18,350 Posts A Message From Mr. Richard Kline To the RC community: I have so many people to thank for their contributions to the KFm airfoil concept, and the first person on the top of this list is Tony Bernardo (Tony65X55). Tony is the man who brought the Kline-Fogleman airfoil concept out of the shadows and into the light by applying this idea to a Zagnutz flying wing. This occurred back in August of 2006. And, it has taken off from there and traveled around the world. I feel very fortunate to have made a contribution to the RC community with this idea. I also discovered the greatest group of people in the world, who are from all over the world. Their spirit, inspiration, help, guidance and willing to share information is something the rest of the people in the world could learn from. This has been one of the greatest experiences of my life. It will be nice to know if someday we'll have an impact on the larger universe of aerodynamics by the work that is being done here in the RC. Who knows? – Dick Kline (Dickeroo) Last edited by jackerbes; Sep 26, 2009 at 01:37 PM.
Sep 26, 2009, 08:45 AM
Jack
USA, ME, Ellsworth
Joined May 2008
18,350 Posts
Tony65x55's Theory - The KFm and why it flies the way it does...

On August 20, 2006, in the ** Kline-Fogleman Airfoiled Flying Wing ** thread, tony65x55 posted the following explanation and figure as to how he thinks the Kline-Fogelman airfoil works. It is as good an explanation as we have ever gotten.

Tony65x55 wrote:

I've been thinking long and hard about the KFm and why it flies the way it does and I may have some answers. More likely, new questions.

I think the step is important as it creates the lift but not in terms of the size of the step but more as the angle between the step and the LE. That is to say, the height of the step determines the angle of the airfoil's underside.

I think the underside angled portion of the airfoil does the lifting. On the KFm Wing the lower wing root angle is +4 degrees. Air striking the LE parts and goes up and down. The air flowing over the upper surface is deflected 0 degrees but the airstream hitting the lower surface is deflected downward under higher pressure at +4 degrees, creating the upper/lower pressure differential and pushing the wing up (viola..lift!).

As AoA increases, the airfoil maintains its upper/lower angle of attack differential, always deflecting the air downward 4 degrees more than the angle of attack of the upper surface.

On a conventional airfoil the air must follow the sloping rear section of the airfoil, which at 0 degrees AoA is already at a 8-10 degree slope away from the airstream. This slope away from the airstream make it much easier for the airflow to detach itself from the upper surface and when it does...stall.

The KFm airfoil would have to reach an AoA 8-10 degrees greater to place its upper rear section at the same AoA as a conventional airfoil. At these extreme angles of attack drag becomes a much greater force and the airfoil can no longer maintain it's altitude without a great deal of power and so begins to sink. However, it has not yet stalled and so it maintains it's extreme AoA and mushes down, without the nose breaking through the stall. Simply reducing the extreme AoA restores a lower drag and the airfoil simply resumes normal flying.

This would account for the KFm's extreme stall resistance. Unfortunately, the by-product of the step required to create the angle on the underside of the wing is drag. The air rolling around in a vortex behind the step is drag city. Fortunately, the step is relatively small in the big scheme of things and the aircraft can still perform normally. As drag increases as a square of velocity it will, however, have a limited top end to the speed envelope. But within it's regular performance envelope of slow speed flight, it does perform very well.

That's it, I finished my silly theory. I'm probably full of it but after two weeks of pondering this, that's what I came up with. Any (polite) comments are welcome.

Tony

# Images

Last edited by jackerbes; Aug 23, 2012 at 09:44 AM.
 Sep 26, 2009, 08:48 AM Ground Penetration Tester United States, VA, Waynesboro Joined Dec 2005 6,829 Posts Subscribed .... you may want to post a clicky for the parent thread. Latest blog entry: Hyperflight Sapphire / Horejsi Q12
 Sep 26, 2009, 09:09 AM Jack USA, ME, Ellsworth Joined May 2008 18,350 Posts Welcome aboard! Great idea! Thanks. Jack
 Sep 26, 2009, 09:41 AM Geaux Saints Grafton, Il Joined Nov 2007 1,913 Posts Excellent. Mike
 Sep 26, 2009, 09:53 AM “There’s no place like Foam” United States, LA, New Orleans Joined Jul 2002 24,713 Posts JB, although that pdf was great for the KF, it didn't mention the Simplest KF which is the single layer added to the wing with No spar(as you have depicted above ) .. 50%(glide) on top or 40%(pitch stability) on the bottom , both of which offer numerous advantages over the more traditional Foamie airfoils (FP and UC )... A device of which we've personally had huge success for the past 10 years ...
 Sep 26, 2009, 10:21 AM just Some Useless Geek Chicagoland Joined Oct 2008 2,557 Posts Comparisons? So, are we just going to talk about KF foils, or can we discuss the relative merits of the other common foils compared to KF? I know this topic has been done to death over the years, but perhaps we can finally draw all this knowledge together into one discussion thread. The characteristics I am interested in comparing are: - Relative lift/stability/speed improvement over, say, flat panel wings - Construction ease/time to build - Criticality of dimensions/tolerances - Mass (weight) of materials used to add foil - Relative delta in wing stiffness/strength with addition of foil I am sure there are lots of data out there to support the various conclusions people have spouted on this board, but I'd like to see it. In God We Trust. All Others Bring Data. [Note to jackerbes: Jack, if you need to incorporate this list of characteristics into some larger post than please do. I am perfectly willing to let this one slide if you have the content covered elsewhere, eh? -- Marty]
 Sep 26, 2009, 10:24 AM Jack USA, ME, Ellsworth Joined May 2008 18,350 Posts gpw, We are going to add newer and better figures and references to that post as they are generated and surface. You mention an A-12 in your post over on the Zagnutz thread, a link to it here for more details would be a great idea. Jack