** Kline-Fogleman (KFm) Airfoils - Advanced Theory/Science **

Welcome to the ** Kline-Fogleman (KFm) Airfoils - Advanced Theory/Science ** discussion thread. The purpose of this thread is to try and advance our knowledge of Kline-Fogleman airfoils. The thread is not meant just for engineers and scientists. It is for everyone who is interested in how Kline-Fogleman airfoil variants work, what they can and can't do, and their best applications in our hobby.

It would be absolutely fantastic if someone with access to a wind tunnel, or a computer with fluid dynamics software, would offer to help us in our search, but I expect we will end up doing what we have always done: build R/C models and conduct our tests with them.

To get empirical data we need to: use data gathering hardware such as the Eagle Tree flight data recorders, apply tufts (little bits of yarn) to our test aircraft and video the tufts in flight while we video the plane from the ground, or some other analysis method that may give us data. We need to build multiple models that vary in only one area (such as size, shape, or type of KF variant) at a time. We need to fly multiple times with each variation, collecting as much information as possible.

Anyone interested in helping conduct tests, please chime in. If you have already conducted tests, then by all means please pass on what you have done, and what you learned. Anyone who has hardware/software that might help us gather data, your help will be greatly appreciated. If you are an Aeronautical Engineering student looking for a good project, let me recommend this one. If you are just curious about what we are doing, then please read on. It is my fervent hope that this thread will spark people to build and fly some very interesting aircraft in hopes of advancing what we know.

If you just want to see what really interesting KFm models people are building, then please go to the ** Kline-Fogleman (KFm) Airfoils - Building/Flying Discussion ** thread that can be found here: https://www.rcgroups.com/forums/show....php?t=1117276

Roger

I want to start off the discussion with two images. The first is a drawing of the first KF airfoil variants as they are typically constructed from foam and used in R/C models. The second shows the eight KF variants currently in use or proposed.

Please note that the second drawing shows curved airfoils not variations from flat plates. I believe that we need to investigate both flat plate KF airfoils and KF variants applied to existing/documented airfoil shapes. An example of what I am talking about would be to build a wing with a Clark Y airfoil, on which much data is available, and collect data with that wing, then test KFm2 and KFm3 variants of the same wing and conduct the same series of tests. The tests might investigate the load carrying capabilities, stall characteristics, maximum angle of attack, speed envelope, CG envelope, etc. We are only limited by our imagination.

Roger

Roger,

Glad to see that you got this thread started so we can all discuss the KFm airfoils in a little more deatil without disrupting the builder's thread.

I'm always interested in a little science, math, data, test results to cure my interest in learning.

I do not have sophisticated test equipment, but I do design and build a lot of model airframes.

My first addition to your thread is observations in the differance between 2 wings I just built.

First was a 265 SqIn 33.5" wingspan undercamber, 10 degrees dihedral for rudder/elevator/throttle, three channel. Made from roughly 5.5mm Blue Fan Fold Foam. This plane was a copy of the Blue Baby.

Second wing, uses a KFm-3, 270 SqIn, 34" wingspan. 2 degrees dihedral for aileron/rudder/elevator/throttle, four channel. Again made from roughly 5.5mm Blue FFF. This plane is also a copy of the Blue Baby, but a few small alterations.

Neither wing has spars, but both have some basswood joiners in the middle joint, about 5 3/4" long. The UC wing uses 2 lengths of strapping tape on the bottom to help strength, the KFm-3 wing has no tape, or other reinforcements.

I'll post some pictures and more details as testing goes on, but initial observations on the wings so far is.

Both wings exhibit good lift, and lift stays good at lower airspeeds.

The undercamber wing seems to retain more lift at really slow airspeeds.

The KFm-3 wing has way more pitching motion, and appears to have greatly reduced drag, yet retains good lift.

Not surprising the KFm-3 wing has real good lift inverted, way more than the undercambered wing.

Fred

Fred,

You might want to consider adding a KFM5 strip to the bottom of the under cambered wing to evaluate the changes made to the same wing. Or you could make a separate KFM5 wing and swap them out. The idea being the only change would be the addition of a KF strip to the bottom of the wing. You can then observe the change in performance with only one change in variables.

Roger

I've been thinking that we are going to need some aircraft designed for our testing purposes. The have to be easy to build, and capable of carrying any data recording equipment we might put aboard. There will have to be different types of aircraft as well. A high wing for trainer/cargo aircraft, and a mid-wing for aerobatic/high speed/high alpha performance.

The high wing is not hard to design with replaceable wings, but a mid-wing aircraft will need wings that are easily built and attached, without being too complicated. Existing designs that can be bent to our purpose are easier than designing from scratch. Does anyone have recommendations?

Roger

This was from an email I received form one of the Kline Fogleman Airfoil Designers, Mr. Richard Kline. I felt compelled to add this here right up front, so others can not only benefit from it, but maybe it will spark some ideas to apply some science and testing to prove out why the KFm airfoils work as they do.

PLEASE, take the time to read this message from Dick Kline, inventor of the KFm airfiols. Lots of data all in one place. Should motivate some real interest....


Fred...

I've compiled a list of all the various reports that have been posted in the RC Groups on the KF airfoil.

Thought you might like to see what others have experienced...

– Dick

Here is a list of the positive characteristics that the KFm airfoils have shown to date in the RC community. They are reports from all around the world from people who post their findings in the RC Groups on the handling of the KFm airfoils.

#1 – The KF airfoil handles a wide range of speed from very slow to very fast.

#2 – Normal airfoils are designed to either generate a lot of lift in order to carry more weight, which means they must be thicker to produce more lift, or they need to be thinner in order to fly faster thus sacrificing lifting heavier loads. The KF airfoil can do both of these jobs extremely well.

#3 – The KF airfoil has a much greater range for its center of gravity (CG). A conventional airfoil would normally have a CG about 33% back. The KF airfoil can be moved back as much as 40%, thus allowing it to carry a heavier payload. The further back the center of gravity is the more desirable it is because of the center of balance. And, the KF airfoil can also handle a shifting CG and still give you good control authority.

#4 – The KF airfoil has great control authority. It's control surfaces remain effective through a large range of angle of attack (AOA). Even beyond a 60º AOA. This is also evident by its ability to have a very fast roll rate with full aileron and rudder control at very high angles of attack.

#5 – Normally, all flying wings require some reflex in flight. Reflex, which will produce some drag, means that the ailerons are angled upwards slightly in order to keep the nose of the aircraft pointed up. It is also well known that flying wings are extremely unstable and stall easily in flight. The KF airfoil requires zero degrees reflex on a flying wing which means no drag from the reflex action. It is a very stable platform. It is capable of flying without stabilizers or rudder, which means that two components which cause drag are eliminated. And it requires no dihedral for stability.

#6 – The KF airfoil appears to have better air penetration. High winds seem to have little effect on how it performs. It is a wind tamer. People have reported not being able to fly their aircraft during strong wind periods while the KF airfoil continues to fly smoothly in extremely rough and turbulent air. It appears as if it rides on rails.

#7 – In addition, the KF airfoil doesn't appear to be affected by weight increases. It has been reported that the handling of the aircraft doesn't seem to notice additional weight. In one case, 4.5 oz. of lead was added and it was able to handle the added weight with ease. In another case, a KF glider which was facing 22 mph wind, flew very well when nearly 100% lead weight was added to it.

#8 – At high angles of attack, the KF airfoil remains unstallable. The center of gravity and center of pressure appear to move backward thus preventing a loss of lift. It has been noted that during a steep climb, once the throttle is cut back the plane will just settle down and fly level with the ground. Apply throttle and it will begin to climb again. There is one maneuver called the "parachute maneuver" where on a flying wing at 60º AOA, by cutting power it will gently float down under full aileron control and land at your feet.

#9 – The possibility exists that the KF airfoil may be more fuel efficient because one quarter of the airfoil produces little friction due to the trapped vortex pitting air against air. A normal airfoil produces friction over the entire surface. Only the first 25%-35% of a conventional airfoil generates most of the lift, whereas it is possible that the KFm3 produces lift from leading edge to trailing edge due to the vortex attachments helping to provide lift. That is perhaps why the KFm3 is a heavy lifter.

#10 – Unmanned aerial vehicles are extremely expensive aircraft. The loss rate in the military is rather high – over 20%. The KF airfoil is a minimal structure which is very strong and simple to build. It would greatly improve on the survivability of these planes because of all the reasons outlined above. In addition, because it penetrates the air so well and is extremely stable, it will produce better photography. Because of a better center of gravity, it will be able to carry more weight, either extra cameras or extra batteries for longer flights. And, because of its ability to penetrate air better, it is not affected as much by higher wind gusts. It has been shown to fly in strong winds when all other RC aircraft were grounded.

And, now for the most amazing aspect of all... the KF airfoil has no apparent bad habits, vices or negatives that have shown up. All airfoils have their positives and their negatives, because each one is designed to do a specific job. Thus, a given airfoil can do certain things well, but do poorly in other areas. To date, nothing negative has appeared on the KF airfoil.

The concept behind the Kline-Fogleman airfoil may be hard to understand because it is so different from present aerodynamics. The KF concept uses a vortex, which attaches itself to the KF airfoil behind the step and becomes part of the airfoil. This means that one-quarter of the KF airfoil is air against air. This also means less friction and greater efficiency penetrating the air. It is important that the step be between 7% and 9% at 50% of the chord on top. When the step is on the bottom, the step should be at 40% of the chord. When the step is on top, it will produce higher lift than on the bottom, but the airfoil works well right side up or upside down. With the KFm4 airfoil, which has the step on the top and bottom, 50% of this airfoil is air against air, which means no direct friction or drag.

(-My special thank you Dick, for this data of observations would be so hard to compile on my own research efforts. This is great stuff, and I will refer to this often as I'm designing new models....

Fred)

Originally Posted by maguro

I've been thinking that we are going to need some aircraft designed for our testing purposes. The have to be easy to build, and capable of carrying any data recording equipment we might put aboard. There will have to be different types of aircraft as well. A high wing for trainer/cargo aircraft, and a mid-wing for aerobatic/high speed/high alpha performance.

The high wing is not hard to design with replaceable wings, but a mid-wing aircraft will need wings that are easily built and attached, without being too complicated. Existing designs that can be bent to our purpose are easier than designing from scratch. Does anyone have recommendations?

Roger

Roger,

I am currently building and testing wings and other things on the Blue Baby design by Tony65x55, in scratch built foamies forum. Interesting as there are at least 4 published size plans too. It's a high wing, but I have found it very quick to build, and although short coupled, with a low aspect ratio, it flys really good!

A link is here, although it is also a Sticky:

Blue Baby Thread.
https://www.rcgroups.com/forums/showthread.php?t=681556

Today goind to hit 15,000 posts! but check it out. also easy to modify the design, and there are also plans for building floats.

Fred

Subscribed!

Good job Roger!

Jack

I am very familiar with Tony65x55's designs. I've built at least three Blu-Babies, and I used an update of Tony's 96" KFm3 wing on my Blu-Guppy sailplane. The Blu_Baby design is definitely a good candidate for the nigh wing platform. It may require widening to allow for data recording equipment. A few more mods would also be required in order to video air flows at some point.

Roger

I'm hoping that some brave soul (with lots of time to cut wing steps) will conduct step width experiments on various shape wings. On my straight wings I've always used 50% step width. On my swept wings I've been varying the width and have gotten good results from 35% to 50%. However, I haven't collected data on these wings and compared them side by side.

The point of science is to collect repeatable data through accurate measurement so that you can come to some conclusion. I gotta admit I've been a slacker about the collecting part, the repeatable part, and the accurate measurement part. Other than that I've been pretty meticulous.

Thank you guys very much. I have followed the KF threads from the start.
I do fly BB w/ UC & KF 2. I will enjoy following this thread.
Keith

A Useless Geek wrote:

"..The point of science is to collect repeatable data through accurate measurement so that you can come to some conclusion. I gotta admit I've been a slacker about the collecting part, the repeatable part, and the accurate measurement part. Other than that I've been pretty meticulous..."

You sir, are in denial. If you have not done any or all of those things you have *not* been pretty meticulous! You have been, like the rest of us, too busy having fun to be bothered!

The attached study, published by Rich Thompson in February 2008, is arguably the best comparison study of KFm airfoils that has been accomplished to date.

And let me also pause here to give Rich Thompson my heartfelt thanks for the work he accomplished there. It is an important work!

The image is the results of that study and as you can see, the KFm2 was the "victor" by a narrow margin.

I'd love to see something like that done again today but with wind tunnel data (lift, drag, etc.) to give us some less subjective appraisals.

We are always going to find that individual builds and and the perceptions of individual pilots make it impossible to single out one single KFm variation as "the best KFm airfoil". So let's just see if we can crunch the numbers and make some sense and use of them.

Do we know or have acccess to anyone that has access to an wind tunnel?

Jack

Jack, you have a serious humor deficiency. I'd suggest talking to some of our Canuck neighbors about fixing that. They're just north of the border, you know. [ahem]

I've done a lot of my own A-B comparisons, but haven't collected serious data. I lack the proper instrumentation.

Originally Posted by Freddie B

This was from an email I received form one of the Kline Fogleman Airfoil Designers, Mr. Richard Kline. I felt compelled to add this here right up front, so others can not only benefit from it, but maybe it will spark some ideas to apply some science and testing to prove out why the KFm airfoils work as they do.

PLEASE, take the time to read this message from Dick Kline, inventor of the KFm airfiols. Lots of data all in one place. Should motivate some real interest....

Fred[/I]

Fred,

I'm thinking it would be very helpful if, when quoting others in a different thread, you also post the date, and the source/ discussion thread & posting # of when the quote was written. (Some items are relative more specifically to the context of a certain discussion at a certain point in time.)

Our understandings in regards to the aerodynamic affects of adding discontinuities and vortex-trapping structures to aircraft wings is in evolution and expanding constantly.... a person's statements and choices of terms and wording come from a certain viewpoint at a certain point along that progression of conceptual evolution.

Given a chance, any of us might actually choose to re-word what we had written on some previous occasion to better express some concept, to better clarify his explanations or theories.

Maybe we should keep this in mind... Once you or anyone else quotes an old posting from another discussion thread into this discussion thread on RC groups, the original author can no longer edit that re-posted essay to amend or clarify concepts that very possibly have also continued to evolve, or that would benefit from further qualification and verification. If postings from 3 or 4 years ago were marked as such, it might help minimize some confusion.


Keep up the good work!!

VIKING

Viking!

You got it! To clear that up, Dick Kline sent me that email 8/11/2010, which is 13 days ago. (Post #6, Message....) I Guess that is recent enough for our use here today, and if Dick want's to edit or change it I will help see that this is done. I know you and I have nothing but great respect for Mr. Kline and Fogleman and this whole KFm airfoil process..

When Mr. Kline compiled it, only he knows, but I'm sure it has been over several years. As Mr. Kline further states, it is 'Others Observations' reported. That is why I was suggesting that some of those 'observations' might make some research 'targets' to apply some science, and come up with 'proof' if there is such a thing. I know it would be nice when someone doubts a theroy to be able to have some data to back it up. I think that is what this thread is about to try and do.

Anyway you too keep up the good work and postings for such a great revolution in the RC hobby, KFm airfoils!

Fred