** Kline-Fogleman (KFm) Airfoils - Advanced Theory/Science ** - Page 114 - RC Groups
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This thread is privately moderated by maguro, who may elect to delete unwanted replies.
Mar 31, 2017, 08:07 AM
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The airplane would be in the 8-10 lb range with a span of 7-8 feet or so. It just needs sufficient spar strength to absorb the loads both in flex and torsion. Large foam wings run the risk of flutter without the proper stiffening. It is certainly doable. The E-flute Carbon-Z Cub is a fine example of large strong foam structures. Heck, if you wanted to, you could hot wire the KF notch into Carbon-Z Cub wings.
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Apr 04, 2017, 10:33 AM
Flying one day at a time....
pardshaw's Avatar
Quote:
Originally Posted by maguro
The airplane would be in the 8-10 lb range with a span of 7-8 feet or so. It just needs sufficient spar strength to absorb the loads both in flex and torsion. Large foam wings run the risk of flutter without the proper stiffening. It is certainly doable. The E-flute Carbon-Z Cub is a fine example of large strong foam structures. Heck, if you wanted to, you could hot wire the KF notch into Carbon-Z Cub wings.


Well, until a few months ago, I too might have held a similar view regarding size (wingspan) and weight for that amount of power.


However, following a series of experiments with the KFm4 symmetrical section on 3-D/aerobatic designs, I'm now not so sure. Currently, it looks like there maybe some benefits (specifically for KFm4 3-D/aerobats) in using far higher wingloading and power loadings than is currently typical of KFm4 foamie designs.

That is the reason I started the thread on KFm sections and high power. So far it is looking like an area that few (if any) have explored.


If it proves a fruitful area (it may not, of course), then my recent experiments would suggest that your initial estimate (of 8 to 10 pounds weight and 7 to 8 feet wingspan) could be out by as much as a factor of two for a 1000 Watt KFm4 section aerobatic plane. In other words, perhaps 4 pounds weight and just 4 feet wingspan might be nearer the mark - outrageous though that may seem.


Anyway, I would not encourage anyone to try such high wing and power loadings unless the airframe has been properly designed to cope structurally with the consequential stresses.

Dave
Last edited by pardshaw; Apr 04, 2017 at 10:52 AM.
May 18, 2017, 10:31 AM
Registered User
Dickeroo's Avatar

San Diego Science Fair 2017


This study took first place this year at the San Diego Science Fair. Results are shown at about 3:14. It appears that the KF airfoil outperformed a conventional airfoil in this particular test.

San Diego Science Fair 2017 KF vs Standard Airfoil (3 min 16 sec)


https://www.ljcds.org/page/news-detail-?pk=1105391

https://bbk12e1-cdn.myschoolcdn.com/...isc_183124.pdf

Based on my research on the Kline-Fogleman wing, I believe this wing will perform better than the standard airfoil wing because of it’s increased lift capability at low speeds.
The objective of this project will be to build a prototype airfoil to compare the lift capabilities between the Kline-Fogleman vs Standard airfoil. The Kline-Fogleman design have been introduced over 50 years ago but was not implemented due to lower speed applications in aviation. With recent popularity of drones, the performance characteristics of the Kline-Fogleman willed be reviewed to see if there’s improvements can be made with existing airfoils.
An adjustable proto type can be used to test both the Standard and Kline-Fogleman (KF) design since the KF airfoil is basically a subset of the standard airfoil. The top rear section of the wing is removed to create a step down version of the Kline-Fogleman. The design can be created with simple available 3D design CAD programs and 3D printed on a commercial printer. I have used the online program Tinker CAD to create the frames of both wing designs and 3D printed the frames with holes to support pressure tubes. In addition to the test wings, we will need a platform to measure pressure so that we can see lift performance (measured in PSI). The test procedure will be to measure the pressure reading at 5 points on the top of the surface and 5 points on the bottom of the surface with the airfoils basically in the horizontal position (zero degree angle of attack). I have constructed a platform that will hold 6 manometers (5 points + one extra to record atmospheric pressure) and this will be measured at different airspeeds using an automobile. Normally, this would be done in a wind-tunnel for better control of conditions but this can be done easily by making the test platform mobile. In addition, I was able to design a frame that will mount the airfoil on top of the pressure test platform.
Finding the optimum speed is the next step and I have used the standard airfoil to determine the highest pressure readings or lift. Driving the test platform from 0-70 mph on a level track, I found the lift is maximixed at 55 mph. I have recorded the data for each point on the wing(s) for a total of 20 trials and charted the average pressure.
The readings with the highest average pressure difference between top and bottom of the airfoil will indicate which airfoil preforms the best. The data shows that the Kline-Fogleman airfoil outperforms the standard airfoil under low speeds.
Last edited by Dickeroo; May 18, 2017 at 11:44 AM. Reason: Additional info
May 18, 2017, 05:14 PM
Build straight - Fly twisty
Whiskers's Avatar
I wondered why the, " lift is maximixed at 55 mph" (maximized we assume)
The lift should continue to rise with velocity.
Then I looked at the video and I understood perfectly.
Positioning the test wing over the roof of a car is really, really bad science.
The venturi effect between the curved roof and the test wing would be considerable as would the airflow off the windshield.
Sorry to be so negative, but this is how science works, Sloppy methodology must be seen for what it is.
May 18, 2017, 10:47 PM
flyin' fool
goldguy's Avatar
Standard airfoil? Kinda pudgy to be practical.

I would think a 'proper' KFm2 and a flat bottomed (7-9 %) ClarkY 'type' would have been a better matchup.

I would like to see more comparisons of step height and placement.

The latest model I've flown with the 50% KFm2 worked amazing, rock solid with full blown aerobatics. It makes me wonder the advantages of the KFm4, other than a higher wing loading for windy days. And, having built a few with the KFm1 I found it to have less performance than the 2. Although, step differences could tell the real story.
Last edited by goldguy; May 19, 2017 at 09:11 AM.
May 19, 2017, 05:35 AM
gpw
gpw
“There’s no place like Foam”
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QUOTE: “ having built a few with the KFm1 found it have less performance than the 2. ” ... For us, the performance most noticed on the KF1 was better low speed performance , felt like it was flying on the edge of a bubble and resisted the stall at very Low speeds … That and the added bonus of adding necessary reinforcement ( functional reinforcement) to many thin foam wings … The KF2 for me didn’t double my glide or anything so miraculous , but it did a lot to SMOOTH out the glide , and with less rollercoasting , the apparent glide “seemed” flatter … 75% of my fleet use the KF1 & KF2 airfoil in one form or another.. been doing that for many years now … I think of them as an Airfoil Enhancement … used for a purpose … JMHO …


Note: And , for those who’ve been looking for a really GOOD adhesive to join KF strips to flat foamie wings … We now use Glidden Gripper ( it’s a primer) , but it sticks like the devil to foam and laminates the pieces forever … ( takes several days to dry thoroughly on really large pieces , so plan ahead … ) … And it’s Thrifty because it goes a long way ...
Last edited by gpw; May 19, 2017 at 05:51 AM.
May 19, 2017, 04:11 PM
Registered User
Michael V's Avatar
Quote:
Originally Posted by Whiskers
I wondered why the, " lift is maximixed at 55 mph" (maximized we assume)
The lift should continue to rise with velocity.
Then I looked at the video and I understood perfectly.
Positioning the test wing over the roof of a car is really, really bad science.
The venturi effect between the curved roof and the test wing would be considerable as would the airflow off the windshield.
Sorry to be so negative, but this is how science works, Sloppy methodology must be seen for what it is.
Yep I'm with whiskers on this. This is a great attempt at a good approach to a scientific study, and it shows good understanding of some basic things. However it falls definitely short in details, which makes it pseudo science unfortunately. So close though!
1- Indeed the test field (roof of a moving car) is certainly completely messing up any valid result. It's not an isobaric (same pressure), isoclinal (same direction), straight on and "isotachic" (all same speed) field. All four necessary components to produce valid measures, and in this case in particular.
2- An airfoil is not a "lift machine", it's a lift/drag machine, so while measuring pressure is useful, it certainly cannot by itself be used to qualify "outperformed". In such limited context qualifying "performance" is not valid, and furthermore misleading.
3- The airfoil shown is not very close to being what for all intents and purposes is a KF airfoil. While a certain characteristic of a KF airfoil is the presence of a step, the more common characteristic is that it is made out of a superposition of plates. This airfoil is more like a standard airfoil shape , furthermore with a step that in relative thickness percentage is small compared to what most typically is present is what people make as KF airfoil. That however is the least important critic, and in itself would not disqualify the study, just limit its qualification.
The airfoil shown has a thick and quite rounded "belly" and that has influence. The small step has relatively smaller importance in this case. Caution as to what is exactly being quantified.

A KF airfoil definitely has interesting characteristics, and I personally am a definite advocate for it, but let's not fall into making it to be the greatest thing ever, especially based on limited or erroneous fact finding.

I maintain, and haven't seen anything to the contrary yet, that a KF airfoil doesn't "increase lift". At best it allows for a more stable flight with higher AOA than a conventional airfoil (that we know why: stabilization of stall regime due to the "step or steps"). So while higher AOA implies higher lift in the absolute but also much higher drag and much higher power consumption, one could say "I achieved more lift", true, but with severe penalty, and with lesser lift than if a conventional airfoil wing would have attained the same AOA, not quite the same statement (the plane can achieve a more lifty regime, but the airfoil itself has less lift)
So while the absolute lift may be higher, it is more costly in terms of drag and power expended to do so. Certainly not an "outperformance". If I push the reasoning I use a plank, tons of power, fly at 45deg AOA and claim higher lift, the lift is in the prop airfoil really. The limit of this is an helicopter, no airfoil (in the body), great lift (lift/weight ratio >1, more than most airplanes).
For a given AOA and power, I seriously doubt that a KF airfoil would outperform a conventional one. But it still may offer an easier build process and lesser stall characteristics which may be useful if drag or power are not that critical (electric foamies for one)
And yes I'm aware of the "vacuum bubble" theory (too simplistic), the Kamm effect (separate issue, valid theory but not applied correctly) and the increased efficiency prop blade (that last one remains a question to me, but I have a feeling it may have to do with longitudinal flow and tip vortices reduction, I don't have any study to show anything definite though, so I just don't know)
Last edited by Michael V; May 20, 2017 at 12:45 AM.
May 19, 2017, 04:21 PM
Registered User
Michael V's Avatar
Quote:
Originally Posted by goldguy
The latest model I've flown with the 50% KFm2 worked amazing, rock solid with full blown aerobatics. It makes me wonder the advantages of the KFm4, other than a higher wing loading for windy days. And, having built a few with the KFm1 I found it to have less performance than the 2. Although, step differences could tell the real story.
Think of a KF4 as a symmetrical KF2, which it is so it's not a stretch. But joke aside what it probably does (I haven't tried it yet but go from people's reports and narrated experience) is offer a "less stallable" airfoil in all flight modes (regular and inverted) and thus would almost acts as a "flexible airfoil". So it's not impossible that it may offer better lift characteristics than a regular symmetrical airfoil (think of it as becoming semisymmetrical at will, almost flat bottom if you fly regular and same opposite when upside down) but with a drag penalty of course as always. It should at least offer higher "agility" and maneuvering for aerobatic purpose.
Anyway that's what I suspect so far, short of being proven or experienced.
May 19, 2017, 10:21 PM
flyin' fool
goldguy's Avatar
Maybe yes, maybe no.

Still, you have to factor in the pilots skills and how he might compensate without being aware he is. I've been flying long enough that if I have to think, even for an instant, about what I'm doing, it's screws me up. Please............. don't tell the wife.

What I know is, that compared to flat, the KF family make scratch building a good flying machine quick n' easy. I only cut cores only for my slopers, because with sloping, the airfoil is everything.
May 19, 2017, 11:43 PM
Build straight - Fly twisty
Whiskers's Avatar
If you go inverted with a KFm2 wing it then becomes a KFm1 so it would be just fine.
What amazes me is how well a Clark Y winged plane can fly inverted
Watching a demo video of an RTF Super Cub being put through a lot of negative G maneuvers I was left wondering, "How can it do this?"
May 20, 2017, 02:36 AM
flyin' fool
goldguy's Avatar
As long as the cord line is positive, it will generate lift.
May 20, 2017, 05:23 AM
Build straight - Fly twisty
Whiskers's Avatar
Quote:
Originally Posted by goldguy
As long as the cord line is positive, it will generate lift.
Yes indeed,
However seeing the plane preforming inverted so well was most thought provoking.
It trashed the old school theories, that's for sure.
May 20, 2017, 10:27 PM
Registered User
I'm still of the dual opinions that:
a) if the KFm airfoil was actually better than a conventional airfoil then we would see them on commercial aircraft
b) who cares if they aren't as good because any performance you lose is more than made up for on a homebuilt plane in ease, cost, and time of build
May 21, 2017, 12:17 AM
flyin' fool
goldguy's Avatar
Here's a test the shows the m2 wins the KF comparison test..................... http://blog.thehobbyguy.me/wp-conten...ison-study.pdf
May 21, 2017, 04:58 AM
Registered User
Blendedwing's Avatar

Fair comparison "flat plate" vs "KF foil"


Interesting, but ...

If , as described, most of the improvement in the lift coefficient is due to the thickness of the airfoil, a fair comparison with the flat plate would include... a thicker flat plate :-)

In addition: there was no measurement of the drag involved in creating this lift. At least measuring the length of the flight with the give accumulator, say from 90% full to 25% full. Or better, through current Amps measured via telemetry at the various speed: slow (How slow is slow?) fast(how fast is fast?) cruise (how fast is cruise?) . Which by the way should also be metered and be the same for all reported judgments.

That would probably show that the improvement in the lift coefficient is really mostly due to the KF typical vortexes, hence additional drag.
A fair comparison to the flat plate would then include equivalent drag augmenting lift devices for a plate , e.g. a split plate. For example a 25% chord flap deflected to 5° forming thereby a camber. Or any other combination of chord % and deflection that produces the same drag as the KF airfoil.

After all the title of this Discussion Blog is :"Advanced theory and science." and not "beauty pageant" or "beauty contest "


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