** Kline-Fogleman (KFm) Airfoils - Advanced Theory/Science ** - Page 119 - RC Groups
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This thread is privately moderated by maguro, who may elect to delete unwanted replies.
Aug 08, 2017, 08:18 AM
OpenTX University Staff
maguro's Avatar
Quote:
Originally Posted by JuulRNLAF
Isn't this subject for the building thread? Just sayin'.

About the vortex generators, there's another thread about that:

https://www.rcgroups.com/forums/show...tex-generators
You're right, of course, but the thread has been dead for a while, and the topic of tip plates was a reasonable one to discuss.
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Aug 08, 2017, 08:55 AM
gpw
gpw
“There’s no place like Foam”
gpw's Avatar
Isn’t Science based on careful observation and postulation ? Couldn’t the KF airfoil be “observed” from real life flying experiences , the part of this that affects us as modelers ? .. Perhaps having a “control” of an identical plane without a KF. Fly both , over and over , note the differences… not unlike running experiments in perpetuity collecting data of observations… After many years of flying FFF Foamies , with and without the simplest of KF airfoils , we do notice a difference in performance , it’s Subtle , but observable , and that’s not even considering the “structural” benefits…

Is it the Best airfoil we’ve ever run across in over 50+ years of modeling , er No !!! But it sure the heck is HANDY for these FFF Foamie planes … and that’s a fact …
Aug 08, 2017, 11:19 AM
OpenTX University Staff
maguro's Avatar
Quote:
Originally Posted by gpw
Isn’t Science based on careful observation and postulation ? Couldn’t the KF airfoil be “observed” from real life flying experiences , the part of this that affects us as modelers ? .. Perhaps having a “control” of an identical plane without a KF. Fly both , over and over , note the differences… not unlike running experiments in perpetuity collecting data of observations… After many years of flying FFF Foamies , with and without the simplest of KF airfoils , we do notice a difference in performance , it’s Subtle , but observable , and that’s not even considering the “structural” benefits…

Is it the Best airfoil we’ve ever run across in over 50+ years of modeling , er No !!! But it sure the heck is HANDY for these FFF Foamie planes … and that’s a fact …
We know the KF works. Certainly it beats a flat plate airfoil. As my old aero professor happily told me, "a KF airfoil works on models because of their greater thrust to weight ratios than full scale aircraft". He is correct, up to a point. Unless we fly sailplanes, most of us don't mind/notice a bit of additional drag. The KF's ability to lift, gentle stall characteristics, and ease of construction outweigh the minor drag penalty. None of this explains how the KF airfoil works, nor does it explain how to best utilize KF variations. What types of known airfoil profiles best lend themselves to being modeled as KFs? Under what conditions do the disadvantages of a KF outweigh the advantages? There are dozens more questions, and building and flying models in varying conditions just will not provide accurate data. Believe me I've tried. One needs controlled, repeatable conditions for testing and gathering data. Observation has to be totally objective. Even the Wright brothers built a wind tunnel to gather data. Then they built and tested in the real world. If something worked better than something else they wanted to know why, so they could make it even better.

I want to understand how and why a KF works. It is my nature. I know that gravity works. I understand the equations of gravitation. That doesn't stop me from wanting to know why it works. If we understood why it works, mightn't it be possible to generate our own, or possibly negate it?

Many people are happy in just knowing that if they drop a stone it will fall to the ground, or that just slapping two pieces of foam together in the form of a KFwill make a heck of a lot better flying airplane than just a flat plate. Those people are not interested in the subject of this thread. Many see it as a waste of time, and some seem to be threatened by what we want to accomplish. I say it is our time to waste. If someone wants to gather subjective data to explain the KF, I say go for it. I am an engineer, an aerospace engineer by training, and I like data. So I will still hold on to the hope that someday we will be able to get the kind of data we need to satisfy the question. Why?
Aug 08, 2017, 04:56 PM
Registered User
Michael V's Avatar
Quote:
Originally Posted by maguro
We know the KF works. Certainly it beats a flat plate airfoil.
Ah wait just a minute. It most likely beat flat plates on some planes, but it's not always true.
I fly slope (not powered and thus glide ratio, AOA stability (airfoil stall) and drag are important issues and matter quickly). I love KF airfoils, have used them almost exclusively. However, I have a "flat plate" airfoil plane and very low aspect ratio to boot. More precisely it's a RM1 airfoil (for reference) which is a flat plate, with a down bevel at the front top and an up bevel at the back bottom. and that things performs flawlessly, much better than the same plane with KF foil (it's a flying wing).
Kfm in this case would be too draggy (I know I have another plane with it), too thick, not as fast. So my plane with the Kf is better in stronger wind where low glide ratio is not as critical, and more drag is OK too (it's heavier), it is nicely stable, with good stalling behavior which makes it smooth.
But in lighter wind, my super light "flat plate" plane is better, no doubt (actually the Kf planes can't fly, even the light one, too much drag). Granted the 2 bevels got rid of the "square" behavior of a flat plate, and it could be argued that it's not really a flat plate anymore, rather some kind of hybrid, even an "airfoil" to a point (so who's to say it's not an airfoil after all, is it just because part of the top and bottom are parallel). Fine, but in the end it is one sheet of foam, 2 bevels and I've got a stable, fast for the weight/AR/size, very maneuverable, very light plane.
The plane is similar to an Alula, and I've compared the 2. The Alula is marginally better (it has fully analyzed and molded foam airfoil), but in cost/performance or difficulty to build/performance the differentiating factor is huge. Alula is marginally better, but I can make 80+ planes for 1 Alula, not that big of a difference in behavior, it really is marginal.
And you'd be very surprised at the maneuverability, like I was. Not expected of a flat plate, which is either tracking or being completely stalled. Part of the deal is Re, inertia (lack thereof, thus no stall) and smoother separation boundary (because of bevels instead of hard edges in low pressure fields).
See I think now that I wrote this and think more, that's the key, I have no inertia (so light) therefore no ability to introduce and maintain any AoA. In which case the beneficial factor of smooth stalling of the Kfm is not there (I'd need AoA), nor needed, yet I still get the penalty of the drag for it anyway.

And your professor is correct. KFm trade thrust/drag for stall stability in most cases. That's why KFm3 can handle more load, not because of more "intrinsic" lift, but because more power is used and more AoA is possible without stalling completely, thus more weight can be carried as more lift is created through AoA (both by the wing through aerodynamic effect and by the increase in thrust angle, a double whammy). If one doesn't have thrust (like us slopers) we trade wind speed ability (raise the necessary range) for more stability, and less airplane speed for smoother behavior. All valid trades if that is what one is looking for.
Last edited by Michael V; Aug 08, 2017 at 05:03 PM.
Aug 08, 2017, 05:05 PM
OpenTX University Staff
maguro's Avatar
Quote:
Originally Posted by Michael V
Ah wait just a minute. It most likely beat flat plates on some planes, but it's not always true.
I fly slope (not powered and thus glide ratio, AOA stability (airfoil stall) and drag are important issues and matter quickly). I love KF airfoils, have used them almost exclusively. However, I have a "flat plate" airfoil plane and very low aspect ratio to boot. More precisely it's a RM1 airfoil (for reference) which is a flat plate, with a down bevel at the front top and an up bevel at the back bottom. and that things performs flawlessly, much better than the same plane with KF foil (it's a flying wing).
Kfm in this case would be too draggy (I know I have another plane with it), too thick, not as fast. So my plane with the Kf is better in stronger wind where low glide ratio is not as critical, and more drag is OK too (it's heavier), it is nicely stable, with good stalling behavior which makes it smooth.
But in lighter wind, my super light "flat plate" plane is better, no doubt (actually the Kf planes can't fly, even the light one, too much drag). Granted the 2 bevels got rid of the "square" behavior of a flat plate, and it could be argued that it's not really a flat plate anymore, rather some kind of hybrid, even an "airfoil" to a point (so who's to say it's not an airfoil after all, is it just because part of the top and bottom are parallel). Fine, but in the end it is one sheet of foam, 2 bevels and I've got a stable, fast for the weight/AR/size, very maneuverable, very light plane.
The plane is similar to an Alula, and I've compared the 2. The Alula is marginally better (it has fully analyzed and molded foam airfoil), but in cost/performance or difficulty to build/performance the differentiating factor is huge. Alula is marginally better, but I can make 80+ planes for 1 Alula, not that big of a difference in behavior, it really is marginal.
And you'd be very surprised at the maneuverability, like I was. Not expected of a flat plate, which is either tracking or being completely stalled. Part of the deal is Re, inertia (lack thereof, thus no stall) and smoother separation boundary (because of bevels instead of hard edges in low pressure fields).
See I think now that I wrote this and think more, that's the key, I have no inertia (so light) therefore no ability to introduce and maintain any AoA. In which case the beneficial factor of smooth stalling of the Kfm is not there (I'd need AoA), nor needed, yet I still get the penalty of the drag for it anyway.
Did you miss my comment that unless we fly sailplanes, we don't mind the additional drag.
Aug 08, 2017, 05:08 PM
Registered User
Michael V's Avatar
Quote:
Originally Posted by maguro
Did you miss my comment that unless we fly sailplanes, we don't mind the additional drag.
No I didn't, I know and agree. And I'm not discussing that.
Aug 08, 2017, 05:15 PM
flyin' fool
goldguy's Avatar
'Maybe' the answer can be found here.................
https://en.wikipedia.org/wiki/Kline%...gleman_airfoil
Aug 08, 2017, 06:46 PM
OpenTX University Staff
maguro's Avatar
Quote:
Originally Posted by goldguy
'Maybe' the answer can be found here.................
https://en.wikipedia.org/wiki/Kline%...gleman_airfoil
The answer to what? There is nothing there but history.
Aug 08, 2017, 11:20 PM
flyin' fool
goldguy's Avatar
Maybe we should ask this guy...................
Last edited by goldguy; Aug 08, 2017 at 11:41 PM.
Aug 08, 2017, 11:24 PM
flyin' fool
goldguy's Avatar
Or....................... read the wiki explanation.

It does propose how it works and WHY ................
Last edited by goldguy; Aug 09, 2017 at 02:40 AM.
Aug 09, 2017, 08:24 AM
OpenTX University Staff
maguro's Avatar
Quote:
Originally Posted by goldguy
Or....................... read the wiki explanation.

It does propose how it works and WHY ................
This is not news. We have been working with KF, and Dick Kline for years. A simple explanation for the masses does not provide data, nor increase understanding. That explanation is as accurate as the simplistic explanation that Bernoulli's theorem explains aircraft lift.
Aug 09, 2017, 11:42 AM
gpw
gpw
“There’s no place like Foam”
gpw's Avatar
Well then , what does Dick say ??? He created it … Mag , you got “the book” … ?
Aug 09, 2017, 11:54 AM
flyin' fool
goldguy's Avatar
My data tells me, time to chime out...................

.......... unscribed
Aug 09, 2017, 12:33 PM
Registered User
Dickeroo's Avatar
For those RCers who might be interested, there is a lot more info that you can find on the KF concept here....


https://www.rcgroups.com/forums/show...oils-at-low-RN
Aug 11, 2017, 03:26 PM
Registered User
Dickeroo's Avatar
Apparently small vortices on wing tips can make a wing more efficient, according to this mailonline article... leave it to the birds to teach us something new.

http://www.dailymail.co.uk/sciencete...VE-energy.html


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