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Old Apr 01, 2011, 09:58 AM
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Hi guys.

I read some pages and the articles. All of my 3 buildings (3 pipers, by the way) I did with KFm #3 airfoil.


I started a discussion in other forum, here in Brazil, to help and disseminating the airfoil.

But, one member asked "Is there a real plane with this type of airfoil?!".

I did a basic search and I found this movie, on youtube:

DARPA Oblique Flying Wing (0 min 26 sec)


Could you help me?! Exists a real airplane with this airfoil?!

Thank you. I'm sorry for my english issues.
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Old Apr 01, 2011, 10:30 AM
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Quote:
Originally Posted by Matheus Gratz View Post
Could you help me?! Exists a real airplane with this airfoil?!
No there is no full size aircraft that uses a KF airfoil.
NASA and others did some wind tunnel testing back in the 70's that indicated at full size aircraft scales the KF airfoil (as described in the 70's patents) had poor lift/drag performance and for that reason they considered it not to have any practical application, so it was not adopted in full size aviation..

Not to say that it doesnt have some merit in RC model scale applications....

Steve
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Old Apr 01, 2011, 10:31 AM
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That is not the same airfoil. The plane in the video has a regular airfoil. What it does is rotate the wing, so one half is ahead of the center, and one half is behind. Sometimes called a scissor wing.
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Old Apr 06, 2011, 10:14 AM
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I woke up from a dream last night about an old plane called the 2x4, sold by House of Balsa. It was one of the first small field flyers with a 48" wingspan, but a much deeper (meaning more chord) low aspect ratio wing.

The concept is simple enough. say you wanted a glider with 4 square feet of wing area, or 576 sg in. (144 x 4). You can get that same wing are several different ways. The common way at the time was to build a wing 6 feet across (72") which is near 2M, and then build it with an average chord of 8" (576/72 = 8). What the 2x4 did was to take the wingspan down to only 4 feet (48") and then extend the chord to 12" to get the same area.

The thing is, the aspect ratio of the wing also determines how well the wing performs. That 6' wing has an aspect ratio of 9/1, while the 4' wing has one of 4/1. The "fatter" lower AR wing of the 2x4, gave that glider the same wing area as it's thinner winged cousins (thus keeping a nice low stall speed), but at the cost of more drag.

A lot of the flat plate 3D foamies use this trick to their advantage (heck some of them even add parts to cause MORE drag). They really float because their wing loading (wing area/weight) is so darn low. But try and loop one with it's power off, and you're in for a surprise. They create so much drag, that they cannot retain any energy. You dive one for 30 feet, pull up sharply, and it will come around until its pointing back straight up, and then get sluggish and fall off.

Any way, my point is simply that a particular KF airfoil might conceivably work better (or worse) on a low aspect ratio wing, when compared to another airfoil, but when both airfoils are placed on a high aspect ratio wing, the reverse might be true.


Just food for thought. As if we needed more.

And if someone mentioned this already, and I missed it, my apologies to them.
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Old Apr 06, 2011, 11:47 AM
just Some Useless Geek
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I hope you guys are keeping in mind that the KF has more applications than just gliders. We've been using KFs for any/everything in R/C, including sport scale, heavy lifters, hotliners, 3D, yada yada yada. Wing efficiency is not the be-all, end-all driving force in developing useful KF adaptations. The definition of "useful" changes every time a member of the audience enters or leaves the room.
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Old Apr 06, 2011, 11:56 AM
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Quote:
Originally Posted by A Useless Geek View Post
I hope you guys are keeping in mind that the KF has more applications than just gliders. We've been using KFs for any/everything in R/C, including sport scale, heavy lifters, hotliners, 3D, yada yada yada. Wing efficiency is not the be-all, end-all driving force in developing useful KF adaptations. The definition of "useful" changes every time a member of the audience enters or leaves the room.
You forgot propellers. Wish more research and development would of come out of that. I need to find a new friend that has on of those $10K+ 3D printers.
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Old Apr 06, 2011, 12:18 PM
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Quote:
Originally Posted by A Useless Geek View Post
I hope you guys are keeping in mind that the KF has more applications than just gliders. We've been using KFs for any/everything in R/C, including sport scale, heavy lifters, hotliners, 3D, yada yada yada. Wing efficiency is not the be-all, end-all driving force in developing useful KF adaptations. The definition of "useful" changes every time a member of the audience enters or leaves the room.
Of course they do. Gliders are handy for certain examples (like in my post above about aspect ratio), and because by removing the power system, they offer less variables to remove when comparing similar airfoils. But they are not the only, or even the best use, of any airfoil.
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Old Apr 06, 2011, 03:04 PM
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My KF test wing has almost exactly a 4/1 aspect ratio. The Blu-Guppy has an AR of 10.66/1. Both very different yet both flew well (The wing of the test ship first flew on a Slow Stick). I have to agree that the lower AR wing performed better, but I think that may also have to do with the thrust-to-weight ratio being so much higher. I know that when under power for take off the Blu-Guppy was a tiger.

The new test ship has an aerobatic fuselage, and will be tested a low, med, and high (for this plane) speeds, at varying angles of attack, and even inverted. The object is to find out how the KF variations perform relative to each other, and how they perform relative to a more typical airfoil. What people do with the information, once we have it, is food for the other KF forum.

My Blu-Guppy design is an 8 foot foam KFM3 sailplane I based on the Guppy. I also built a KFm2 Blue Beagle variant, a KFM3 AP plane borrowing from Tony65x55s design, and now a KFM2 version of the Quick 1. All my planes were built with different design goals in mind and all performed well with KF airfoils. Would they have performed better with other airfoils? Maybe, but the KF wings have the advantage of being very easy and cheap to build, are very strong, and they perform better than I would ever have hoped. That is especially true of the Blu-Guppy sailplane.

The intent of this thread is not to develop the optimum KF sailplane, it is to learn what we can about KF airfoil variants, and to share what we learn.

Roger
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Old Apr 07, 2011, 12:35 PM
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Seeking info on the curved (undercambered) KFM5s

Hi Guys,

Holy cow, there are a lot of pages on several threads about KFM airfoils! I am particularly interested in learning more about the KFM5a and KFM5b. While my mind is reeling from all I've read about their flatter cousins, I have not yet come across much about these curved variants.

I'd appreciate it if anyone could lead me to information, discussion, or builds using either of these airfoils.

Thanks,
Dan
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Old Apr 07, 2011, 02:55 PM
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The KFM5 started out on the Blue Baby, a high wing trainer that started out with an under cambered wing. Eventually people started using the KFM2 on the Blue Baby, and a KFM3 on a 60" AP version of the BB. Lots of people building the BB continued to use under cambered wings. Eventually one of those people tried gluing a KF strip to the top of the wing. The performance of the plane improved and the KFM5 was born. Later someone, who didn't like the looks of the step on top, put one under the wing, and the 5b was born. This link (http://www.rcgroups.com/forums/showp...postcount=4390) is to one of the earliest KFM5s I could find, but it is not the first.

I doubt you will find any analysis done on the KFM5s, as this thread is the first (but not the only) I know of, that is actually trying to do some kind of analysis on KF airfoils on RC planes. Most people just build one, and see how it works for them in their application.
Most people who have tried the KFM5, like it, because it gives better stall performance, while stiffening the wing. High alpha/slow speed performance can be further improved, by adding tip plates. They reduce the spanwise flow that creates tip vortices. Many people don't use them, because they don't like the looks.

Roger
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Old Apr 07, 2011, 04:59 PM
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Actually, the first post of a KFM5a was here: http://www.rcgroups.com/forums/showp...postcount=2186 But at that point it didn't have a designation, as I just wanted to make a UC wing at the 42" span that didn't need struts and tried doubling the forward 40% of the wing on bottom. It worked pretty well. (plane and wing still flying). It first got called the KFC wing for KF cambered, before the KF step proliferated and number designations came to be. The original wing for the OneSheetGlider was a KFM5a, and there are several pictures on how I did it in the thread first few pages. Basically it acts like a UC wing, nice and floaty, the thicker LE may help soften the stall from a single thickness UC wing, and it's decently strong, though a bit flexible (gives variable dihedral at times....).
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Old Apr 07, 2011, 08:43 PM
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Thanks, guys!
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Old Apr 11, 2011, 06:47 PM
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I maidened the KF Test plane this weekend. It is one sweet flying machine. The only problem I had was a buzz that came from the motor when I attempted a high speed pass. I landed, tightened the prop nut and spinner, and took off again. Got the buzz the next time I tried a high speed pass, so I reduced throttle and kept flying. Tonight I'll check the balance on the prop, and spinner. One of them vibrates when the airspeed gets high enough. Full throttle in a vertical climb is no issue, so it has to be speed related.

Rolls are axial. I even did a power off roll, and it worked OK. I should have pulled the nose up a bit before entering the roll, but what the heck. It flies nice. I think I'm going to do a lot more shakedown flights, before covering the wing with tufts and knuckling down to serious work. It is just so much fun to fly.

The second landing in the video occurred in strong wind. I was getting a ton of lift on the first pass at landing, so I went around, and then hit sink like crazy at the end of the field. Made for a lousy landing. I'm glad the grass was soft.

I've decided that I can't keep calling the plane "The KF Test Plane". It needs a real name. I'm open to recommendations. Dick you are the founder of the feast so to speak. You get first crack at a name, should you want to.

Still have about half an hour to go on the Youtube upload, so I'll post the video in a little bit.

Roger
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Old Apr 11, 2011, 08:27 PM
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Here's the video. I got rid of the sound on the second flight, because a large helicopter was flying near the camera. All you could hear was the scream of the heli motor. Maybe I'll just put some music on it.

Roger

KF Test Plane Maiden Flight (6 min 45 sec)
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Old Apr 12, 2011, 01:32 PM
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Looks good Roger!

To make the comparison to a 'conventional' airfoil more meaningful will you be making a proper accuratly reproduced airfoil shape? IMHO in the past too many of these comparison tests have been compromised by the builders taking a flat slab of foam, sticking a 'hump' on the top then calling it a Clark Y.. Virginius E. Clark would be turning in his grave!

For an aerobatic model a Clark Y would not be the one to go for anyway, a symmetrical NACA would be better.

Steve
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