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Old Dec 22, 2010, 01:40 PM
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Quote:
Originally Posted by gyrocptr View Post
For the KFm3-TGK:
"Thickness: 4.0%
"Camber: 4.0%"

Is this correct? %camber = %thickness ?
Sorry. I can't answer your question. You would have to go to the site and ask the people who posted those numbers. They were the ones who produced the results.
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Old Dec 22, 2010, 03:34 PM
Boffin
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Quote:
Originally Posted by gyrocptr View Post
For the KFm3-TGK:
"Thickness: 4.0%
"Camber: 4.0%"

Is this correct? %camber = %thickness ?
It looks like there are errors in the data. The 3 is much thicker than 4% (probably 10%). Could mean the polars are erroneous too.

Rick.
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Old Dec 22, 2010, 04:11 PM
just Some Useless Geek
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Yeah, I was a little disturbed by the "steps" in the lift curves, too. I wonder if their software is producing artifacts that have nothing to do with the actual surface curve? Does anybody else have some insight into this simulation stuff?
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Old Dec 22, 2010, 07:02 PM
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Quote:
Originally Posted by Dickeroo View Post
Bruce... if you can give me a profile of the KFm that you are talking about, I might be able to make something happen in terms of getting someone to produce data on it.

Thanks,
Dick
Dick, that would really be great; the image below is the version that I would like to see evaluated; it certainly penetrates winds well, handles cleanly and precisely, and glides very efficiently.

Here's wishing the best of everything to everyone this Christmas Holiday season, and hoping each and every one of you has a great 2011!!!

VIKING
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Old Dec 22, 2010, 08:14 PM
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Would be very interesting to see how Viking's variant checks using the sim program.

I just imported the kfm2 and kfm3 profiles into sketchup and sized for the 5" chord of the OSG, and it's scary how identical it works out. At a 5" span, the rectangular aft portion of the foil is 3/16", exactly what I had using the dollar store foam. total thickness comes to 7/16, which is perhaps a bit more than the 3/8" that a double thickness would be, but allowing for waves in the foam, it may be closer to what the wing came to. So it looks like a real world profile. Unfortunately that's the wing that was on the plane when i lost it, so can't measure.

Don't know how they calculated camber, but the kfm2 thickness in my sketchup tracing came to 8.75% vs the website's 8.8%. The kfm3, on the other hand came to 11% not 4%. i wonder if that value actually gets used in the simulation?
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Old Dec 22, 2010, 08:40 PM
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GYI, Here's the site where the algorithyms are that are used in the sim program: http://www.mh-aerotools.de/airfoils/javafoil.htm Hepperle does show some validation with real wind tunnel data on an existing airfoil and Xfoil. interesting stuff. Still don't know how to read a polar, but I'm looking.....
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Old Dec 23, 2010, 08:53 AM
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Good day fellow KF enthusiests....

I am finally home after another year abroad, working for the Natural Gas Industry. The holidays are now upon us and I would like to take this time to wish everyone here a warm and wonderful holiday season and a adventurous new year. I have a few designs in various stages and hope to again become a contributer to this area of flying contraptions. The KF airfoil has opened a whole new world in aeronautical design, allowing anyone with a dream to become a designer, builder and showman. Well, the grandchildren have just arrived but I felt I needed to say a big thunderous thankyou to Mr. Dick Kline and to all who have helped in the advancement of all things KF.

Ok, I am hopping off my soap box now and getting another eggnog....

cheers..........


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Old Dec 23, 2010, 09:36 AM
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Quote:
Originally Posted by Dickeroo View Post
..."The KFm's did really well. Examine the CL (coefficient of lift) and the Zero Lift angles. Very good. You'd have thought we knew what we were doing!"

Tony...
My late father-in-law was a big fan of old timers, and we shared a common love of well drawn model airplane plans. (I'd wallpaper with Frank Ziac if I could afford it) He and I spend many a pleasant hour going over them, and talking about their qualities.

One of the things I've noticed from this is how model airplane design has evolved with the hobby. For instance early free-flight designs started with parasol wings, airfoils on the horizontal stabilizers, and very short coupling on the nose. These were planes designed for heavy engines without much power, and were intended to self-stabilize to maximize flight time. They are large, heavy (for balsa, mind you) and draggy. They also are beautiful in the air, but that may be just my personal opinion.

Now contrast that kind of design with a flat plate foamy made for 3D flying. Every single aspect about the plane has changed, from wing location, to proportion of wing area to stabilizer area, to power source. All these changes came about because the knowledge of model airplane designers increased, along with the abilities of the pilots to control them.

The one thing that hasn't evolved much is airfoil design. I guess on average airfoils have gotten slightly thiner, but by and large they still mimic 1930s airfoils designed for airplanes with Re numbers in the millions. There are good reasons for this, mostly because it wasn't cost effective for a governments to research airfoils below a certain Re number, until recently that is. And yes, a lot of amazing work has come from private sources over the last 20 years of so. This is especially true of glider airfoils, coupled with the technology to glass bag wings.

But in all of model airplane airfoil history, I can think of only two major evolutionary steps, steps that radically affect how modelers build and fly their planes. The first I already mentioned above; that is the flat-plate airfoil, which technically is a very thin, fully-symmetrical airfoil. The other is of course the KFm series of airfoils.

Mind you, I may be all wet on this, as my knowledge of airfoils and model airplane design is admittedly limited. But that being said, I thing the FKm airfoils represent the largest evolutionary step from the past. A step that I think will eventually revolutionize the hobby just as much as cheap brushless motors (more accurately, cheap brushless motor controllers) cell-phone (li-poly) batteries, and cheap, easily available sheet foam, have done for the hobby.

After such a long winded ramble, here's my point. It is easy to assume that what we did in the past was the right thing to do because reasonable men, some even really smart, all agreed on a particular design. The problem with this logic is the assumption that those smart reasonable men of the past knew what we know today. They didn't. They built very good planes based upon their knowledge, and limitations to the hobby at that time. Today's model airplane designer has so much more knowledge, so many more materials to chose from, way more methods of controlling the plane, and a plethora of power sources to move them along. And best of all, so many more airfoils to choose from.

As the song says, "the future's so bright, I gotta wear shades." Thanks Dick for helping us move along, and think new things.
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Old Dec 23, 2010, 10:44 AM
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Well spoke!
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Old Dec 23, 2010, 04:52 PM
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Tolladay wrote: ..."It is easy to assume that what we did in the past was the right thing to do because reasonable men, some even really smart, all agreed on a particular design. The problem with this logic is the assumption that those smart reasonable men of the past knew what we know today. They didn't. They built very good planes based upon their knowledge, and limitations to the hobby at that time. Today's model airplane designer has so much more knowledge, so many more materials to chose from, way more methods of controlling the plane, and a plethora of power sources to move them along. And best of all, so many more airfoils to choose from.

As the song says, "the future's so bright, I gotta wear shades." Thanks Dick for helping us move along, and think new things."


Tolladay,

Yes, very well said indeed! Thanks!

VIKING
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Old Dec 23, 2010, 11:06 PM
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Quote:
Originally Posted by springer View Post
GYI, Here's the site where the algorithyms are that are used in the sim program: http://www.mh-aerotools.de/airfoils/javafoil.htm Hepperle does show some validation with real wind tunnel data on an existing airfoil and Xfoil. interesting stuff. Still don't know how to read a polar, but I'm looking.....
Martin identifies the key limitation:
Quote:
LimitationsAs said above, JavaFoil is a relatively simple program, with some limitations. As with all engineering computer codes, it is up to the user to judge and to decide how far he wants to trust a program. Because JavaFoil does not model laminar separation bubbles and flow separation, the results will be incorrect if either of these occur.
The whole point of the step is to cause the flow to separate so the results of the simulation are not reliable.

Rick.
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Old Dec 24, 2010, 09:08 AM
ApachePilot
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Here is a simple thought that I had when I began designing aircraft using the KF airfoil.




Could the reason behing the KF airfoils unearthly ability to create lift where many others cannot be due to this? We all by now have some idea how a standard more conventional airfoil works along with their coefficiencies.
I have gathered that a KF airfoil has not one, but two or more moments of lift, sometimes perhap double the lift of a conventional airfoil. ( I am most likely wrong, as I am not a aeronautical engineer, or highly skilled in this arena, but this assumption works for me.)

A conventional airfoil achieves lift because of a low pressure area on the the top side where airflow must travel faster in order for both the top and bottom to meet at the rear at the same time. This low pressure area caused by the curvature of airflow causes lift. (Laymens terms here.....)

KF airfoils with one or more steps act differently in many ways.. First we have the low pressure area created behind the leading edge. As the wing travels forward, this low pressure area begins to create the lift we need for flight. As this laminate airflow travels rearward it is met with a KF step where another low pressure area is created as a result of the vortex created by the first area and travels rearward.

This step is crutial in that it needs to be a precise distance from the leadiing edge and that it's height should not exceed a certain percentage of the wings overall chord.




I may edit this later on, just woke up and still working on my first cup o joe


I just hope it made sense....


Happy Holidays everyone.




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Old Dec 24, 2010, 11:14 AM
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Rick wrote: ..."The whole point of the step is to cause the flow to separate so the results of the simulation are not reliable."


I'd word it differently- (we're not dealing with laminar flow airfoils here...)

The ideal dynamic function of these KF variant stepped discontinuities & related structures (as they are implemented on the upper wing surface) is to trap a recirculating vortex and/ or turbulate the boundary airflow layer, thereby preventing massive airflow separations in the region aft of the stepped discontinuity through a wider range of angles of attack.

Placement and depth (and the shape to a certain extent) of these stepped discontinuities are the variables which can be fine- tuned; some implementations increase drag.... but we also have indications that some experimental implementations actually reduce drag, improve control surface response, and improve wind penetration capabilities.

Dickaroo forwarded to me the results of an Xfoil simulation of the MH32/KF3P airfoil which was shown in my diagram, with comparisons against the standard MH32 airfoil and a couple of others. The results of the simulation run indicated that, at lower angles of attack, the MH32/KF3P had lower drag than the standard (smooth unmodified upper surface profile) MH32 airfoil.

There seems to be a possible Xfoil simulation anomaly when trying to analyze the affects of the stepped discontinuities at higher angles of attack, however, which does not correspond to the actual in-flight performance of the wing.

This is something that I noticed first while flying DANCER III in gusty 18 to 20 MPH wind conditions out on 'Antelope Flats' back in September, and again a bit later when slope flying on the KING slope in South Park in up to ~30 MPH slope winds. [NOTE: The DANCER III fuselage is a sleek low-drag design with a folding prop; minimizing fuselage drag is really essential, in my perspective, when trying to do these in-flight analysis of the performance and handling of these prototype wings.]

(I'm re-posting excerpts here from the DANCER discussion thread which are relevant to this topic

~~~~~~~~~~

..."THE FLIGHT TEST" (From post 32, Sept 27, 2010)

Winds were gusty and irregular, and what thermals were moving through the flying site were quick & dirty, with turbulent air prevalent and gusts running up to ~18-20 MPH. But the recent modifications to the wing tips, and having the new rear 2mm filler panels installed as they are has resulted in an aircraft which ignores the rough air and is now flying very nicely! After launch, I can now pull it up into a full vertical climb and hold it there easily as it climbs smoothly like the proverbial 'homesick angel'. Rolls are smooth & axial, inside & outside loops are equally easy, and inverted flight is very manageable. So some mild 'hot-liner hot-dogging' is also well within the capabilities of DANCER III. This one is Definitely a KEEPER!!"

[Post #35, also posted September 27th, 2010:]

"Dancer III slope flying report
Friends,

With due west winds forecast for this afternoon, I took some time away from work to take Dancer III out to the KING Slope Flying Site. When I arrived, I checked the wind speed with my wind meter, and saw one peak at 29 MPH. Time to fly!!

Dancer III is flying with a Rhino3S 1050 battery now, for a total weight of 19-1/4 ounces. That comes out to a wing loading of only 5.7 ounces per square foot. Most slope fliers would consider this a bit on the light side for penetrating winds up to around 30 MPH, but after the flying I had been doing out on the flats, I wasn't too worried. I waited for the winds to drop off a bit, and launched without turning on the motor at all.

Wow- this sailplane LOVES slope flying conditions! it penetrated well & climbed out & up in front of the slope nicely. The stability added by the up-swept wing tips was apparent right away, and while it handled as if lightly loaded, it dealt with the cycles of building and dieing slope winds very comfortably. (Flying with a heavier battery pack for an increase in wing loading is always an option; I did not try that today, as I wanted to evaluate this wing / aircraft's ability to penetrate the strong slope winds without ever turning on the motor. And Dancer III was easily able to do that!

Turbulent thermal-generated winds were cycling into the face of the slope; between high wind cycles, the wind speeds would die down & the air would go cool, with a noticeable lack of lift, so that I needed to fly a bit more conservatively until the next wave of warmer air & stronger slope winds would cycle in. No problem- it handles light air & soft lift conditions very nicely, too.

When flying back & forth across a ~400 yard section of this slope, I played with turning just with the rudder at times (with a bit of elevator as needed) and then turning just with the ailerons. I was happy to find that it responded well in the slope winds to either the ailerons or the rudder. And while loops just in front of the slope can be a dangerous maneuver for some lightly loaded slope fliers, the Dancer III would come through an inside loop quickly and cleanly without loosing control responsiveness... how well it came through the loops surprised me and impressed me too. At this light of a wing loading, however, inverted flight in slope winds is not it's strong suit... no need to keep it inverted too long- that's best left for a slope ship with a higher wing loading or a load of ballast on-board.

I did have to turn on the motor one time during ~45 minutes of flight for a few seconds. It wasn't because I was loosing altitude, or because I was being carried back behind the ridge- those were never issues during today's flying. No, the reason I had to kick on the power was because one of the local hawks came over to check out the new fancy looking flier on HIS slope. After looking my plane over from several angles, it moved in close behind the tail- within 4 or 5 feet. That's when I gave it full throttle and climbed vertically a hundred feet in a couple of seconds, then cut the motor again. The hawk immediately lost interest after that quick maneuver, and moved back out of the area.

I only made two landings during this flying session, landing on the shelf where the first photo below was taken so that I could grab my camera out of the car. I then launched again, sat on the slope, and started to take some photos of Dancer III cruising in the slope winds. (It's a fairly stable slope flier that will let you fool around with your camera while still flying it as much as necessary in slope winds like I enjoyed today...) I'm REALLY getting to like this aircraft!!"...

~~~~~~~~~~

The highlighted / bold section of the flight report above is of particular interest, as it is a dramatic demonstration that this particular MH32/KF3P wing build continues to out-perform conventional airfoils at high angles of attack... my SD6060 airfoiled EPP slope ships do not carry the same precise control authority through inside loops on the slope anywhere near as well as the DANCER III does so easily at it's light wing loading.

So this is fine food for thought... Xfoil may not be up to the task of analyzing the dynamic function of shallow stepped discontinuities as they are implemented on this series of MH32/KF3P wings on DANCER III.

(DANCER discussion thread: http://www.rcgroups.com/forums/showt...=860461&page=3

VIKING
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Old Dec 24, 2010, 12:13 PM
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Dick Kline has recently sent me a copy of another document which deals with the use of vortex pockets on laminar flow airfoils at very high Reynolds numbers.

I'm way rusty on my Calculus these days, so I didn't look too closely at those aspects when running through the document. They do seem to be focused on very high speed applications- (Re numbers approaching infinity) in this paper, so there may be a lot of difference at very low Reynolds numbers on which they don't comment.

A major point of interest is that they do comment on the benefit of the smooth flow contours of a vortex trapping pocket. This is something which I've also given some thought to over the last 4 years. Since we want to have a smooth re-circulating vortex trapped behind the step, and disregarding how quick & simple it is to stack foam sheets to form the square-bottomed steps, an improvement in vortex airflow dynamics would be achieved by rounding the front end of the vortex pocket just behind the step.

This might not be significant on slow flying foamie scratchbuilt fun fliers, but if we want to explore the advanced limits of what's possible, the improved shaping could be a significant contribution- especially as air speeds increase. It would be relatively easy to accomplish this shaping using a rounded cutting tool on a low speed battery powered Dremel tool (without melting the foam!)

I've revised the MH32/KF3P airfoil structure diagram to illustrate what I'm talking about, and have posted it here as 'food for thought'.

Have a Very Merry Christmas!!!

VIKING
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Old Dec 24, 2010, 01:27 PM
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Bruce...

I hope you don't mind, but I created a silhouette of your design which clearly shows the curvature you are talking about.

Dick
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