** Kline-Fogleman (KFm) Airfoils - Advanced Theory/Science ** - Page 122 - RC Groups
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Jan 27, 2018, 11:06 AM
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Michael V's Avatar
Originally Posted by maguro
What I didn't see on the site was the Reynolds Number. It makes a huge difference in the results. The fact that the KFM has a huge drag penalty is nothing new. For most RCers drag is not an issue. The improvement in lift of the inverted KFM3 was news though. I'd like to see a lot more of their data.
Careful here, the improvement is "compared to a symmetrical airfoil". You take off some of the symmetry (underside bulge) and introduce some "flatness" or "undercamber", so yes there will be an improvement. The improvement is not tied to the step. IMO, more to the aero shape change. We'd need to compare to a similar airfoil shape with no steps to be precise.
Were you to compare with a flat or semi symmetrical airfoil, I believe you'd see a drag penalty, the same way the upstep Kfm3 has more drag than the symmetrical airfoil.
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Jan 27, 2018, 01:02 PM
jackerbes's Avatar
Originally Posted by Michael V
Agreed, but they do it to have some reference point. So not fully valid for our purpose, but it gives some comparison point, again not fully representative

Agreed, and semantics. I didn't mean penalty in the sense it's bad, I meant penalty in the sense that there are solutions that can give me the same lift with less drag. So unless I'm interested in other characteristics of the Kfm (easy to build, smoother stalling behavior, higher AOA tolerance) then I'm not necessarily willing to pay that price. I'm a glider flyer, so for me drag is always a penalty since I don't have "free" power to spare. My Kfm planes always need more wind to fly (slope soaring).
As a power flyer, you're right, model planes nowadays are so overpowered to start with, that who cares about a little more power consumption, it won't make much of a difference, so for those drag is not really a "penalty", although it is (less flying time on the battery)
When I built a glider with a KFm wing and tried to discuss it on glider forums I quickly lost interest in trying to talk to the narrow minded folks that seem to buy all their planes and sneer at anything that is not just like them or competitive with them.

I did not expect my gliders to be like theirs and it was not like theirs. But it flew well, could rise in thermals, and it introduced me to one of the most relaxing and enjoyable venues for building and flying I have encountered.

If I wanted to compete in any way with other flying venues my first start would almost always be to see if I could build one because I enjoy that as much or more than any other aspect of the hobby.

It would be interesting to see some reports that compare our designs, materials, and building techniques to the other that are usually mentioned.

Jan 27, 2018, 01:23 PM
flyin' fool
goldguy's Avatar

I've tried many of the KF family foils. My findings have been that the 2 and 4 are closely matched in performance with power flying, except that with the 4 you end up with a more rigid but heavier wing. This is a comparison in flying mild to wild, including 3D.

The wonderful feature of the KF foils is their simplicity to build from sheets of foam and they work amazingly well............... even with the increased drag of the airfoil.

But a KFm9? That's a lot of work with many layers of foam, glue and a pain to give it a finish of some sort.

Gliding and sloping is a different story. The KFm2 works OK in higher winds with ballast onboard. But, nothing like a proper airfoil like the PW51 for planks. I can cut out a proper airfoil specifically designed for the use I need with my simple hot wire cutter in 11.25 minutes both side at 25% of the weight of the 9 and have a flat surface to apply the finish.
Jan 27, 2018, 02:29 PM
jackerbes's Avatar
I use colored 2.2 mil package sealing tape to both add strength and apply color to foam. And the layers were, in part, to allow me to embed tubes for joining a 96" wing made in three 32" panels.

Big Blu 96 - 96" KFm9 Winged Glider - https://www.rcgroups.com/forums/showthread.php?1470211

Jan 27, 2018, 04:13 PM
Build straight - Fly twisty
Whiskers's Avatar
Originally Posted by Dickeroo
I recently came across this on the net.... It seems to indicate some positive results for a UAV...
They say, "The step on the upper surface of the airfoil generates a laminar flow vortex"
That seems to display a profound ignorance. A vortex is the opposite of laminar flow.
Jan 27, 2018, 05:22 PM
OpenTX University Staff
maguro's Avatar
My Blu-Guppy is also a 96" bird. I used a KFM3 shaped into a MH32 airfoil. I designed it about the same time that Jack did his. This is where you find the Blu_Guppy theaad. About 7000 copies of the plans have been downloaded to date. I received notes (and some videos) from happy builders around the globe. Not too shabby for a draggy KFM airfoil. I'm guessing that Jack has a similar story.
Jan 27, 2018, 06:24 PM
Build straight - Fly twisty
Whiskers's Avatar
I've always liked the looks of the Blu-Guppy, and I love the side by side drawing and photograph.
Has anyone built it without the steps, and if so how did it fly?
Last edited by Whiskers; Jan 27, 2018 at 07:28 PM.
Jan 27, 2018, 07:57 PM
jackerbes's Avatar
I remember the Blu Guppy 96 too, it is a lovely thing for sure.

I wanted to make a solid foam fuselage from 2" sheet foam rather than a built up fuseage and also wanted to make the wing easy to transport so I got off onto another tangent on building.

But looking at the wing profile in a close shows it as a classic KFm3 other than a little finer entry at the nose. So I'd be very surprised that it did not fly similar to the Big Blu 96. I got a lot of help from the SailPlane Calc spreadsheet in finalizing the build details too. BB 96's specs on it's maiden were:

AUW: 1460 grams = 51.5 oz.
Wing Loading: 9.4 oz./sq. ft. or 28.5 grams/dm2

That wing loading put it in the same class as many good RTF gliders and it flew well. I did not have access to a good thermal flying area (I have more continuous/less thermal coastal winds here) but it flew well and was a lot of fun to fly. There is some data on climb and descent rates (collected with an eLogger) in the thread.

I don't consider the KFm9 to be a great advantage, if any at all, over the KFm3. I just wanted to build those two wing panel joiner rods into the wing and the extra layer of foam helped with that.

She met her end in a RX failure that put her down in a spiraling dive that damaged the wing badly... (sob!)

Jan 28, 2018, 09:29 AM
OpenTX University Staff
maguro's Avatar
Jack, I expect both sailplanes had very similar flight performance. Might have been fun to fly them together for an afternoon of fun.

Whiskers, someone did build the Blu_guppy without the steps. He provided information and photos of his build. I don't remember him giving flight reports, but I expect it performed better than the stepped Guppy, because of the reduced drag.

The Blu-Guppy got it's start from my old aerodynamics professor at Va Tech. He was a professor emeritus by then, and I asked his help getting a wind tunnel testing done in the Va Tech small low speed tunnel on full size KFM wings at RC model airspeeds. I thought it would be a great senior project. His response was that the only reason KF airfoils worked in RC models was their huge thrust/weight ratios employed. He was so pompus about it, I vowed to prove him wrong. Thus the Blu-Guppy was born.

The Blu-Guppy was a fun plane to fly. I especially enjoyed thermalling it with a flock of vultures. It was also very easy to spot land (without diving) even though it had no spoilers or flaps. I would never fly it with a "serious" RC sailplane person. They would not understand why I would even fly such a plane. But most foamie builders would understand right away.
Jan 28, 2018, 03:32 PM
Build straight - Fly twisty
Whiskers's Avatar
Quote: He was so pompus about it, I vowed to prove him wrong. Thus the Blu-Guppy was born.
There has never been a better reason to build a model airplane

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