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Sep 03, 2010, 08:15 AM
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I've decided to use my old Slow Stick as a flying testbed. It was used as an AP ship, and has plenty of power for carrying loads.

I plan to build a 50% KFM2 wing that can be converted to a 50%-75% KFM3 and a 50%-40% KFM4. If I can get through all the testing maybe I'll make a KFM1 as well. Each version will be tufted and videoed from the air as well as the ground. I have an Eagle Tree FDR that I will use to record what data I can. Angle of attack will have to be approximated with videos. Really high alpha will most likely stall out the pitot, so airspeed data will be unreliable in that regime.

I'm thinking that a rectangular plan form wing with ailerons and zero dihedral would be the best to start. I have not yet decided on wing dimensions. Does anyone have any alternative ideas or other recommendations, before I begin cutting foam? I have a number of tests in mind, but would appreciate any testing suggestions.

If someone could do the same kind of testing with a flying wing, and a delta wing aircraft, we could compare the data returned and try to make some educated recommendations from what we have discovered. Of course there are a lot more KF variations out there that need testing, but I thought the top four would be a good start.

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Sep 06, 2010, 12:27 AM
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Hi Roger!

It's been a while since I have been on RCGroup. I didn't think I was ever going to get logged in.

Anyway, its good to see a thread on theory and science on the KF airfoils.

I read over your ideas for testing the different KF airfoils and you should perform them with your Slow Stick. You are already familiar with the plane and you should be able to get a better feel and understanding of the different KF airfoils. I would also go with the plank wing without dihedral. This will allow the testing to be conducted on the simplest of wing planforms.

I have tried a few of the different KF airfoils on a SkyFly. The plane survived the testing, considering the weight of the plane. The KFm1 was not performed due to the weight, but I have a pretty good idea how it would due. I even threw in a stepped variation that was not the typical KF spanwise step.

This brings me to a question. After reading over the beginning of this thread, are you only going to focus your testing on Tony's 4 airfoils or Dick's 8 airfoils that they have provided, as is. What I'm getting at is the steps are for the most part, 40%, 50%, and 75% of the chord. Are you up for different step placements? Are you after stepped airfoil theory and science or simply want to test the existing airfoils?

Last edited by sharpewing; Sep 06, 2010 at 12:39 AM.
Sep 06, 2010, 07:53 AM
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What I am after is information. The more the better. The problem is, that unless we get more testers attempting to do the same kinds tests with different configurations and/or platforms, I could spend forever testing variations. That's why I decided to start with the first 4 types of KFM.

The KFM5a and b are better suited for testing on a slow platform that typically uses an under cambered wing. The Blue Baby series of airplanes is a good candidate for this testing, although there are many others. Someone with experience flying a number of airplane types who has experience with an under cambered Blue Baby would make an excellent candidate for testing the KFM5a and b on the same plane. Pilot reports will have to be a big part of our data even though such reports are subjective.

The KFM6 is said to be great for flying wings. I'd love to have some wing pilot offer to test the KFM4 and KFM6, and compare them to both flat plate and symmetrical or semi-symmetrical airfoiled versions of the same airplane. Once again we are looking for how the KFM performs against the "competition". In all cases having some objective data, such as maximum and minimum attainable airspeeds, max angle of attack, etc would be of great benefit.

The KFM7 and KFM8 haven't really been tested. It would be great to get some data on them. The one benefit I can see of those wings is that they provide greater opportunity for adding the structure necessary to build a strong wing. This is important in heavy lift and high aspect ratio wings. What else can they do? Would be nice to get some real data on them. Anyone want to volunteer to do some testing?

Lastly Bill, we have what you were asking about and more: the size of steps. Is a 62% step better than 50% or 75%? What happens when the step size varies from root to tip? What about step thickness? What happens to a KFM wing when the step thickness is increased? How does thickness impact performance at various speeds? Does wing aspect ratio have any impact on the utility of a KFM airfoil? What is the optimum wing tip for a particular KFM wing? Notice how nice and curved the KF airfoils are on the "KFm Family of Airfoils" jpg? Many people build their KFMs dead bang straight. Not a curve to be found (unless maybe the round the leading edge a bit). So do the beautifully curved airfoils fly any better (or worse) than the straight ones? We can only guess until someone conducts a real test to find out.

There are so many questions that need answers. The amount of testing necessary to answer any of them isn't huge, but to answer all of them it is. That's why I'm looking for people to join in on the testing. Pick a question that they would like to see answered, design a test plan, carry out the tests, and report everything here. If multiple people test the same question, so much the better. More data is always better (even when it conflicts).

We have so many excellent modelers who build and fly foamies. It would be wonderful if some of these people would get interested in helping find answers to the myriad of KFM questions.

Sep 06, 2010, 08:14 AM
jackerbes's Avatar
"..focus your testing on Tony's 4 airfoils or Dick's 8 airfoils that they have provided..."

I have had questions before as to how to get the airfoils to be smoothly shaped and rounded and exactly as they are seen in the second figure with nine airfoils and not in the first figure (tony65x55's image) as line drawings showing the foam layers.

That second image was generated by HopalongX (IIRC) when we were getting the Building/Flying thread started. In case it is not clear to anyone, the airfoils in that newer image are slightly artistic or stylized renditions of typical build results of the various airfoils.

Tony's original image showed the foam layers and actual build details more realistically (spars are seen, etc.).

New builders often got a little wrapped around the axles on the details and in trying to copy what they saw there in the original figure. But I think it is fair to say that time has proven that Tony's drawing is not a building plan, just a helpful projection of how to build.

In the original figure I consider the airfoil presentations to be inaccurate or impractical for the following reasons:

1 - The curves on the top surfaces and the leading edges show shapes that are not usually or realistically attained with the material most are using.

2 - The spars are not realistically scaled relative to the foam thickness,

3 - The spar placement and center panel width in the KFm4 is not realistically going to produce a flyable wing,

4 - At least one of the specifics, the 9-12% thickness on the KFm4, has been reduced to 7-9% in the newer figure as the 9-12% wing simply did not fly as well,

And there are probably other things like that there too.

So the airfoils in the newer figure were intentionally drawn in a less specific manner and the intention is that the build attain the outer wing profile however the builder's material and techniques dictate while building the wing to generally meet the step locations and wing thickness mentioned below the figure.

For the more scientific testing here, it might be a good idea if the participants agree to a set of line drawings for the outer wing profiles with more precisely defined step location, heights, and wing thicknesses.

And it might even be a good idea that the line drawings be based on the arrangements of the materials most often used and the profile that would actually occur. That would be that foam thickness is just about 1/4", spar thicknesses and widths be that of commonly used balsa, basswood, or other materials, leading edges be more realistically like those actually produced (two layers stacked and rounded, the factory folded edge, etc.).

The point of all this is that the wings being tested be more similar in shape and configuration from one tester to another.

Or not? I'm not even sure yet what the goals of the testing will be. Will it be to see how the ribbons flutter? And what produced the "best" flutter?

Sep 06, 2010, 04:18 PM
Addicted to building...
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I posted a similar question and observation in the flying thread, so If you want this post removed just ask.

But I had made 2 Blue Baby undercambered planes for combat with my buddy. Standard design was followed, even down to GWS IPS motors, except that we are using modified "E" gears and the ducted fan motor (4.8 volt). Our power output on a 9x7" prop is 2.8 amps, instead of the 1.9-2.1 amp 'A' drives.

So the blue Babies (BB) fly fantastic. You do have to work it a little similar to a full scale aircraft in gaining altitude, throttle management, no brushless zoom straight up kind of thing. But this gives me, a RC pilot a great feel for the flying abilities for the design.

Landings can be very slow, or normal. Wind penetration is work, but undercamber has some advantage too in the wind (which we have lots of). Yes it can blow around a litttle, but kite like hovering is so much fun. Aerobatics are limited to rudder/elevator only, and yes some throttle bursts add some fun variations.

Combating, great fun. Loops, loops and ending in a half roll, stall turns, even a sort of hammerhead, all great fun. It almost makes it more fun with the skills and 'slowness' of it all.

THEN........ Most of the posts are doing flying wings. I am posting on a conventional top wing type.

Not complaining, but finished a Blue Baby 33", turned int a 35 1/2" wingspan, and did a KFm3 wing (per plans). I used torque rods since it is a smaller plane, and the adverse yaw is so bad that there is absolutely no roll going on. Give it full aileron and the downward side drags so much, that the plane acts like it will turn the opposite direction, 10 degrees of wing tilt (and the tilt on the wrong side!), and skids along going straight. It is so bad that I have never seen worse!

So I add some rudder coordination and can get it to turn normal, like a full scale cub, or WWI biplane. Too much rudder and it will spiral into a dive. So It can be flown, but took away all the relaxing fun a BB is known for.

I'll bet the answer is in tons of differential aileron. I was able to get a little bend into the torque rods on the assembled wing, but not much. Guessing I need as much as 50% or more.

It has been so much extra work to fly this plane, that it is hard to tell the differance in lift, landing speeds, or anything else I would want to tell you about my test. I'm not giving up, but initial results are not making me feel real easy about this yet.

Anyone else notice this, and the fact that the steps are causing blanking of my airlow over the top rear of the wing.

Sep 07, 2010, 04:31 AM
just Some Useless Geek
Originally Posted by jackerbes might even be a good idea that the line drawings be based on the arrangements of the materials most often used and the profile that would actually occur. That would be that foam thickness is just about 1/4", spar thicknesses and widths be that of commonly used balsa, basswood, or other materials, leading edges be more realistically like those actually produced (two layers stacked and rounded, the factory folded edge, etc.).
Yeah, that's the smart approach, Jack. Presuming that the participants in this thread don't have access to multi-million dollar instrumentation, yada yada, the easiest way to create reproducible experiments is with the common denominator in materials. As you pointed out, FFF, balsa, and basswood are available almost everywhere people who join this thread can be found. If we can all build the same thing to the same dimensions then we can check each others' work and verify the results of experiments.

This is basic scientific principle: a repeated experiment produces the same results. Somebody here tries something and gets weirdness as a result then the rest of us can try the same thing the same way. Pictures, video, and simple measurements are the validation tools for backing up our observations. Who needs millions of bucks tied up in wind tunnels and instrumentation when you gots fellow flyboys on the Internet? Heh, heh, heh.
Sep 07, 2010, 08:28 AM
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Thread OP
Freddy, your post really does belong in the building/Flying discussion, but your point about the adverse yaw is well taken. I've seen it too. It is just another thing we should research.

Sep 07, 2010, 08:57 AM
Addicted to building...
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Originally Posted by maguro
Freddy, your post really does belong in the building/Flying discussion, but your point about the adverse yaw is well taken. I've seen it too. It is just another thing we should research.

Want me to remove it?
Sep 07, 2010, 08:57 AM
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Thread OP
Jack, I agree with you about the airfoil shapes. The airfoils depicted in the "KFm Family of Airfoils" are stylized representations and not depictions of what we would ever build. They do however give one the idea of how a streamlined version of a KFM airfoil might look. Consider it modern art.

The way I see it, there are two schools of KFM construction: 1) Basically flat surfaces possibly with a rounded leading edge, 2) curved upper surfaces that attempt to mimic accepted airfoil shapes. This may include tapered trailing edges such as Viking60 uses. Since both are very commonly used, both should be tested.

The more I think about, the more I agree with you, that we should publish our test plans, including the planned design and construction of the wing. This will allow others to provide input, and possibly use the same design/construction for additional testing. The value of any test will come down to how well it is documented. Part of the documentation should include detailed diagrams/photos of the construction of the wing being tested. The documentation should provide enough information, that another person can duplicate the experiment, or carry it further.

Sep 07, 2010, 11:20 AM
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Jack wrote: " And it might even be a good idea that the line drawings be based on the arrangements of the materials most often used and the profile that would actually occur. That would be that foam thickness is just about 1/4", spar thicknesses and widths be that of commonly used balsa, basswood, or other materials, leading edges be more realistically like those actually produced (two layers stacked and rounded, the factory folded edge, etc.).

The point of all this is that the wings being tested be more similar in shape and configuration from one tester to another. "

This is where I would like to offer a different viewpoint as 'food for thought'. I don't want to step on any toes here, or dampen anyone's enthusiasm... I just want to add in some observations from my perspective. And from that perspective, letting the dimensions of commonly available materials dictate the shaping of airfoils seems counter-intuitive and counter-productive. How much MORE performance can we achieve by allowing ourselves to think ouside of that box?

The Short Of It:

From all of my scratch-building of glider & sport aircraft wings since the early 1980s, and through all of my own KF foamie wing builds over the last ~4 years, I've developed this perspective:

The shaping of the forward 25% (to 50%) of the wing's airfoil (or lack of shaping) can have a very profound affect on a wing's ability (when the power is turned off) to cleanly generate lift, it's ability to penetrate wind, and it's ability to glide efficiently and cleanly in the power-off glide mode.

Oh yes, it's easy to overcome drag by simply adding excess power, and this is the most often recommended design approach these days on this discussion forum for the typical RC foamie.... And there are aircraft designed for different flight performance tasks - super slow flight, or heavy lifting, or 3D aerobatics, as examples, where minimizing drag and achieving optimum glide & wind penetration performance is not necessarily the priority.

But I would like to suggest that you can learn a whole lot more about a wing's actual performance when the power is turned off.

I do all of my test flying at 10,000 feet ASL, with ~16% lower air density. And I occasionally fly at locations well above 12,000 feet. Some aircraft which are said to fly fine at sea level are real 'pigs in the air' when you try to fly them up here...

The Goldberg Cub, with it's classic Clark Y airfoil, is an example of this which comes to mind... when flown at a float fly I CD'ed at Kite Lake up at 12,000, it was shown to be a terribly stall-prone wing- not at all fun to try to fly, & one which experienced repeated dunkings. (Yet it flew 'fine' at 5000 feet ASL, according to it's owner.)

Recent test initial summary:

My most recent KFm3 type wing build was started with a very fat rounded leading edge... a nice easy place to start when building... It flies OK with power on, but when the power was turned off and it's trying to glide, it was too draggy & had a very narrow range of angle of attack (adjusting balance and elevator trim setting) where it would glide somewhat decently.

Next, I took the heat iron to it and began to heat-form the wing's leading edge, shifting the airfoil's L.E. entry point lower and lowering the upper surface's profile for the forward 15% of the airfoil. I also worked in the lower L.E. surface with the heat iron, giving it more of what is referred to as 'Phillips Entry'. (Refer to the MH32 airfoil which I previously used for the Me163-e Komet build for an idea of what I was starting to work toward with this reshaping.)

After these airfoil shaping modifications, it glides far more efficiently, and it is far less 'touchy' as far as the elevator trim / angle of attack for optimum glide. I'll do further testing with the new sleek fuselage with the folding prop once I get my work schedule cleared again- (busy times!)

So what does this have to do with KF stepped structures, you ask? Just this: the KF steps can only influence the airflow aft of those steps.... but that airflow dynamic starts from just in front of the wing's leading edge, and there is so much potentially happening forward of where the stepped structures are implemented. It's difficult to evaluate possible added drag affects from step placement & height when you start with too much drag from the forward wing structure's shape!

Bottom line:

If you try to define a 'universal' testing approach where you restrict yourself to only use the profile of the rounded Bluecor / FFF panel fold as your leading edge, you may be starting with a 'drag penalty' and an inefficient leading edge shape/ contour, making it far more difficult to evaluate the drag-increasing or drag-reducing affects produced by the various configurations of KF type stepped discontinuities which you are attempting to evaluate. So for some testing purposes, I am inclined to start with a better-performing airfoil shape for the area forward of the stepped discontinuities.

I'll post reports once I get back to doing further flight evaluations with the new low-drag fuselage.

Sep 07, 2010, 11:58 AM
Account Closed
OK, well I guess we are definitely on the opposite end of the spektrum, when it comes to testing out stepped (KF) airfoils.

Dick Kline's patent uses the word "step-like" in the description to discribe his airfoil. This thread is titled "Kline Fogleman...". When I put the two together, I came up with a place to have a general discussion on stepped airfoils and to hopefully take it to another level.

These basic sections have been built over and over. At what point do we move foward? Building 10oz. planes over and over, to me does nothing for the science of KF airfoils. We know they can fly.

I'm going to breifly give an example of foward movement when it comes to advancing the science of KF airfoils. You may or may not have seen my Horten wing. I'm not going to go into any great details right now. I'm simply using it to relay a point of foward movement. Some of you may also disaprove of the modifications that was made to the KFm4 airfoil. The wing was designed around the basic KFm4 airfoil section and was an experiment with a modified step design to control airlow over the wing. It was also a test to remove the myth that flying wings need wing washout, dihedral, reflex... to maintain adaquate control. Well, it's flying without all that. Even after nine months and it was all done with a stepped airfoil.

F.Y.I. The design modification I used on the Horten wing will not benefit light weight wing designs. It likes to go fast. Wind or no wind.

Let's get the basic data and move foward with our designs.

Also, if you need data from fast, heavy, wind penetrating KF designs. I can help in that category.
Sep 07, 2010, 12:32 PM
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The whole idea here is to advance our knowledge of KF airfoils, not limit design options. Viking takes the KF step and applies airfoil refinements to it. WJSFLYWING you modified the KFM4 to your advantage.

No one wants you to stop what you are doing, far from it. The important thing is to document the builds, so others can duplicate your work (Viking I know you do this in spades). Conduct specific tests, collect data, make some modification, repeat. Once you are done with the particular series of tests, report your findings with as much quantifiable data as possible.

I hope this thread will help us advance our knowledge of KF airfoils and how we can best use them. It would be nice if we could advance the knowledge of how/why KF airfoils work, but without a wind tunnel I don't know how much we can do here.

Sep 07, 2010, 12:33 PM
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Freddie B's Avatar
wjsflywing, and viking60,

Wow all good points. Looks like this is prime time for both of you, and others to test different aspects of what makes the KFm airfoils optimum. No need for everyone to concentrate on the same tests all at once.

The forward of the step issue makes great sense, and testing with refined leading edge design, compared to 'standard, whatever shapes, would be very interesting data as you have explained on your recent KFm3 build. I would trust most data prepared by someone that took the time to show these differances, no matter how 'subjective' others may call it. I could see some graphics designed to refine the current 'art' showing KFm wing configurations after it is all refined

You design modifications that improve fast, heavy, wind penetrating KF designs, is a great addition. I live in the most unfriendly wind area I have ever been. Mid-West USA, great plains, no natural wind breaks, hundreds of miles of open range creates some nasty weeks on end of blowing fury. This would interest me and all my flying pals extra special.

Even scienctists argue all the time over even faily simple things. They often do not believe anything unless you can prove it, but that proof has to be proof they agree with. Get the point. Any testing and results is furthering the KFm airfoil knowledge base and welcome with open arms. We may all have to deal with some arguing scienctists, forever!

I'll always remember Burt Rutan likes to refer to NASA as Nasta! Too boggled down in pure science to make strides like Burt has over simple things he's observed, like a shuttle cock, leading to Spaceship One, and winning the Ansari X-Prize ($10,000,000.00 US isn't bad either).

Sep 07, 2010, 02:08 PM
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nmasters's Avatar
Originally Posted by A Useless Geek
These Veem guys apparently wanted to use the KF without crediting its creators, and without paying any royalties. So, they flopped the KF on its head and tried to create a "variable pitch" system via synthesized stimulated cavitation. Great.

I smell it, too.
No. It's a trailing edge wedge. There's no similarity at all: Steps, like KF but including devices like vortex flaps and split flaps, produce separation bubbles. A TE wedge can be used to correct a separation problem and add some aft camber.

Last edited by nmasters; Sep 13, 2010 at 11:16 AM. Reason: punctuation
Sep 07, 2010, 03:21 PM
just Some Useless Geek
I squat corrected. Norm, are you saying that the trailing edge wedge might be a useful feature to test out on R/C planes? The Google search you pointed to mentions this device in a number of patent references, but there don't seem to be any current aircraft manufacturers using it. Even Boeing and M-D apparently aren't using the thing, and they were supposed to be its main proponents.

If I read that first patent proposal correctly, the wedge was used as a tuning mechanism to correct manufacturing or aging defects. I'm not sure that has any real application to what we're doing; my foamies contain nothing but manufacturing anomalies. Heh.

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