Welcome to the ** Kline-Fogleman (KFm) Airfoils - Building/Flying Discussion ** thread.

This thread is intended for discussions about building and flying aircraft with Kline-Fogelman wings. We invite and welcome the introduction of new and different K-F winged builds, if you want to document a specific build and/or have sustained exchanges about it, we recommend you give it a name, start a thread for it, and post a link to the thread here too.

For more advanced discussions about and relevant to the theory and science of the Kline-Fogleman airfoils please visit the ** Kline-Fogleman (KFm) Airfoils - Advanced Theory/Science ** thread:

https://www.rcgroups.com/forums/show....php?t=1296458

This is a continuation of the discussions of the KFm airfoils that used to take place in the ** Kline-Fogleman Airfoiled Flying Wing ** thread.

All discussions and attachments are still in place in the parent thread, many of the figures, studies, and other attachments will be or have been moved to post #2 here.

Post #4 has tony65x55's explanation, in layman's terms and with an accompanying figure, as to how he thinks the Kline-Fogelman airfoil works

The parent thread can be read here: https://www.rcgroups.com/forums/showthread.php?t=558321

For a thread that has a listing of all or most of the plans for all of the Blu Baby related plans and for many other related KF winged airplanes, this thread is a compiled list of those plans:

** Tony65x55 ** planes and plans. - www.rcgroups.com/forums/showthread.php?t=997484

Jack

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.

08 July 2018 - I've added another document that has more info on the KF airfoil, this one is from the Defence University of Sri Lanka.

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.

2 May 2021 - I've added a document that describes and give the specs for the Dow foam we call "FFF".

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)

THE KF STORY

THE KF STORY

Back in 1964, I was in the process of developing my paper airplane which demonstrated the stall-resistant characteristics that the KF airfoil became known for. At about that time, two scientists were working for Bell Labs in New Jersey, and they came up with one of the greatest discoveries in the history of man. When a television station went off the air at night, people saw snow on their tv screens. However, this snow they were looking at wasn't meaningless. It was something that was profoundly important. It was from the big bang, the beginning of the universe. And, it was bouncing all around the universe in the microwave background and showing up on TV sets. That light could be traced all the way back to the very beginning of the universe some 13.7 billion years ago. Millions of people had seen this snow on their TV sets, but nobody had connected the dots until Arno Penzias and Robert Wilson discovered what it actually was.

Here is the reason for the Penzias/Wilson story...

Some people have said that the steps on paper airplanes have been used for a long time before I did it. But it wasn't until I made a more pronounced step that you could actually see the stall resistance kick in. And, it was Floyd Fogleman, who watched my demonstration in a parking lot and commented on it as something he had never seen before. A paper airplane that resisted stalling. He was a photo-retoucher by profession as well as a pilot and model airplane builder. I was that of an art director working in advertising. At that time I had no knowledge of aerodynamics.

Since then, I have learned a few things about aerodynamics. Air must travel faster over the upper surface of an airfoil in order to lower air pressure and thus produce greater air pressure underneath. This is the basic concept of Bernoulli's Principle. However, Isaac Newton's concept also comes into play when the bottom of the airfoil pushes against the air, the air pushes back also creating lift. These explanations, however, pertain to traditional airfoils.

At present, there is no scientific explanation for how the KF airfoils produce lift. The traditional airfoil generates lift roughly from the leading edge back to about 25% - 35% of the chord. The remaining portion of the airfoil does very little actual lifting.

With the KF airfoil having a step at 50% on the upper surface, the air gets tripped up and tumbles into a cavity behind the step. It becomes attached to the surface of the airfoil and since is it rotating faster than the air beneath the airfoil, it reduces the air pressure on the upper surface. Therefore, most of the length of the KF airfoil chord is producing lift even at high angles of attack. This might be the reason that the CG needs to be set further back on a KF airfoil as opposed to a normal airfoil. Also, one quarter of the KFm2 airfoil is air against air and does not produce any direct friction or drag.

The KF airfoils do not conform to conventional aerodynamics. They operate on using trapped vortexes that turn the negative force of drag into a positive force by employing that energy. The claims that have been made for the KF airfoils still remain intact. They have a wide speed envelope. They resist stalling. They are less affected by strong winds. They provide great stability. They are very responsive with excellent control authority. And, they make it possible to explore many different ways of constructing an airfoil. They invite creativity. However, a claim has never been made that they make great gliders. They make good gliders, but not competitive gliders. The step requires thrust in order to fully engage the trapped vortex.

The hull of a seaplane uses a step to lift the fuselage up high in the water so the pilot can break the surface tension and lift off the water. Without enough thrust the seaplane would not be able to lift off.

Reaction to the KF airfoils is usually disbelief at first. People have said, "Man, that looks ugly." "That thing can't fly like that." "It just doesn't look right." "It won't get off the ground." But, many others were willing to find out for themselves whether or not it works – and today the KF airfoils are flying all over the world in the RC. The KF wings are easy to build and inexpensive to make. They take much less time to construct. They create the opportunity for wide variety of experimentation. And, they are very forgiving. One of the aspects of innovation is that it shrinks time. The KF airfoil flies in that direction.

– Dick Kline (Dickeroo), August 23rd, 2012

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

Subscribed .... you may want to post a clicky for the parent thread.

Welcome aboard!

Great idea! Thanks.

Jack

Excellent.

Mike

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 ...

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]

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

Marty

Most of the data on the KFm is what we as model builders have collected by flying the Kfm wing.

None of us have the money nor a laboratory or access to the equipment to do high tech research on the KFm.
Standard wing designs with high tech testing are covered on hundreds of web sites.

We know that the KFm works because we use it.
Many nay sayers will never be convinced.
Some nay sayers that do try the KFm are surprised and become converts.

We can't statistically, scientificly or any other way prove the KFm works or why.
We have theory and practical experience.

Marty I'm sure this didn't answer all your questions, probably none.
There just isn't much scientifically gathered info.
People with the money to do so would rather ignore the KFm rather than testing its merits. Including the government.
---------------------------------
Theory is all the Wright Brothers had before there first flight.
After they had their first flight they had the practical proof.

They still didn't have high tech scientific measurements to prove that it could fly......

....but it did.
-----------------------------------
Several of us have been having discussions about the KFm at link in Post#1 of this thread.
That thread became the Zagnutz thread and then blossemed to other KFm discussions.

Jack, Dickeroo and I decided to start a new thread for discussion and leave the Zagnutz thread to the Zagnutz builders.
Dick and I volunteered Jack to kick things off. LOL
So here we are.


This thread is a work in progress.
Info will be added as it is found.
You find some post a link to it.

This is only day one.


Mike

A Useless Geek asked:

"..Comparison?..."

Marty,

I think this thread is great place for comparisons of the KFm to other airfoils. Or for getting started on doing them and finding the data at least.

I am not setting limits or declaring any rules but I think the logical way to do it is to discuss it and sort of work out candidates and broad objectives for comparison here.

Then, since someone is going to have to finalize the details and do the work, have a thread where all that is accomplished.

Can you imagine there being a number of comparisons under way and all the details being worked out or discussed here in one thread? And trying to follow the details of any one comparison? I think I see that as a bit of a bedlam.

But, in the reserved posts above I can put descriptions of comparisons, a link to the thread for it, and we will have the best of both worlds, finite discussions that are easy to follow and reference here to where that can be found.

The characteristics you mention are the kinds of things we want to know but the methodology for getting those data is obscure to me. And where much of that is found now is too.

So I would say yes, let's do comparisons. There was some mention of getting a Clark-Y comparison started in this post:

https://www.rcgroups.com/forums/show...postcount=3655

But I'm not sure that the even the initial questions there were answered or that it is going to move forward. Maybe we can see?

Pinging Lee!

Is there a plan for a Clark-Y to KFm comparison?

Jack

OK , here ya' go !!! https://www.rcgroups.com/forums/showthread.php?t=898625 Probably the simplest plane I've ever built , and scale ish too

You guys do need to consider the new guys who've never heard of this before , otherwise a lot of people go to a lot of trouble to post data, and then it's lost in the archives.. perhaps a good modern comparison and analysis and a sticky so the little fish can find it easily , and we wouldn't have to go through the annual comparison .. .... All I care is it WORKS , and it's EASY !!! Kinda like my first wife ...hahahahaha

Hey Y'all, looks like the new thread is off to a great start!

Add another successful KF wing to the list ... The A-12 ... KF strip on the bottom of the wing 40% at the root tapering to the pointed tips ... Works GREAT!!! Flies Super... Quote from GPS.

This brings up a question I have been pondering for awhile.
"Is the KF step as, or more effective when parallel to the LE or when perpendicular to the chord?"

I have had good results making steps at less sweep than the LE. I am guessing this puts the lift of the vortex (talking top step here) in better proximity to the CG. As the step gets closer to the tip it moves farther aft of the CG, on a swept wing that is.
Dickerro's paper airplanes had the step running at a right angle to the chord.
Any thoughts on this? Any builds along these lines? (bad pun )
jp