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Old Nov 02, 2010, 10:00 PM
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Science please?

Isn't the basic question here: "what exactly is the air doing around this wing?"

The wind tunnel would be the first thing to try, but in absence of that, can't you just do the converse of that? Has anyone ever considered just picking a windless day and setting up thin wall of smoke and flying a plane through it, and using a high resolution, high framerate camera to video the results? Or some version of this?

The other obvious thing that strikes me... shouldn't you guys be putting pitot tubes... on the BACK side of the KF step, instead of the leading edge?

Or is all this besides the point?
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Old Nov 03, 2010, 01:37 AM
flyin' fool
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VIKING:

Changing the camber line, that's a game changer for sure. Now what I've done so far with KFs is starting to make some sense.

With no wind tunnel, a slope machine will give a better indication as to input of what changes you make, as long as you have a constant wind. For those here who rely on mother nature at the ridge, you'll know exactly what I mean, as it's a different way of flying with constant trim changes involved.

I need a new sloper for backup while I'm waiting for the mail man from all the way down under. So now I got some new ideas rattlin' around the noggin'. A comparison on the natural wind tunnel with no prop blast to effect it should prove valuable.

We'll see..........Frank
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Old Nov 03, 2010, 09:01 AM
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Frank

I started reading the SOARTECH journals back in the early 1980s, where there was a focus on designing the most efficient, most competitive sailplanes. Later, I also read the articles on specific airfoils & their development in S&E Modeler magazine (later renamed 'Silent Flyer'.) I researched airfoils which perform well at low Reynolds numbers (narrow RC model sized wings), scratch-built a lot of glider wings & sport aircraft wings, and studied why some wing shapes / airfoils performed better than others.

Slope flying is the place where you really see how an aircraft handles and responds to a wide spectrum of variables, and where you can easily see how it responds to your control inputs... it's all happening right at eyeball level, very close in front of you, across a wide speed range.

Slope flying taught me a lot about RC flying that I couldn't learn in any other flying environment, and allowed me to take those understandings and skills away with me to use in all of my RC flying. Slope Flying made me a better thermal flier, too. As you pointed out, on the slope you're constantly fine-tuning & optimizing the aircraft's trim, evaluating how a slight shift in balance will allow your plane to fly just a bit more cleanly, penetrate the winds a bit better, carry energy through the turns more efficiently.

And there's simply something elegant about pure soaring flight- just the wind, gravity, and your aircraft flying, not needing any 'power' to move through this environment. It absorbs all of your attention & focus, and so, for a while, the rest of the busy world out there ceases to be important.

It's a bit of magic in action, and we're the one at the controls!

VIKING
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Old Nov 03, 2010, 10:50 AM
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nicely put............
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Old Nov 03, 2010, 11:23 AM
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Goatzilla, I don't understand why you would want a pitot tube on the back side of the wing. The idea of the pitot is to let us know the airspeed at which our data is being taken, so that we can duplicate the test on other wings.

What did you think the pitot at the rear would tell us?

Roger
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Old Nov 30, 2010, 09:13 PM
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Sorry if this has already been talked about, but have any of you guys used xfoil? On a thread in modeling science forum, that I tripped over and posted my kf results ( http://www.rcgroups.com/forums/showthread.php?t=1329266 ). Later, a poster actually ran several airfoils through Xfoil, and posted results. KF didn't seem to show well, but looking at the airfoil he used, doesn't look much like the kfm's I do, so may be just the modeling, also an interesting comment on last page that the trapped vortex airfoils can't be modeled with current software (or something like that) anyway, so experiment is only way to develop. I downloaded the software and some cold, windy, snowy winter night I'll sit down and figure out how to use it!
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Old Dec 07, 2010, 02:13 PM
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There have been xfoil attempts reported here before. I didn't believe the results were particularly useful when I did it because the discontinuity in the airfoil can create variable results with only minor changes.
There's no aerodynamic evidence that there is a trapped vortex -- that was a speculation that some one came up with a long time ago. The step does trip the boundary layer and that is all that's necessary to explain the high drag and deep stall behaviour of the KFm. It has been modelled and studied in wind tunnels in the past.

Rick.
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Old Dec 07, 2010, 06:40 PM
Jack
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"..The step does trip the boundary layer and that is all that's necessary to explain the high drag and deep stall behaviour of the KFm..."

High drag as compared to what? If there is a chart or graph or something shows where it stands relative to other airfoils that would be interesting.

And what exactly is a "deep stall"? I've found the stall on KFm wings to be more gentle then the wings it was used in lieu of. I usually see a gentle nose drop, maybe some wing fall off but not much.

On gliders and other very slow fliers with lightly loaded wings, if you don't pay attention you may miss it.

Jack
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Old Dec 07, 2010, 06:58 PM
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I know of a number of wind tunnel studies of KF airfoils done at full scale piston and jet Reynolds numbers. I have seen none done at model airplane Reynolds numbers. Rick do you know of any at model RN, that I can review?

Roger
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Old Dec 11, 2010, 12:44 PM
just Some Useless Geek
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Quote:
Originally Posted by jackerbes View Post
I've found the stall on KFm wings to be more gentle then the wings it was used in lieu of. I usually see a gentle nose drop, maybe some wing fall off but not much.
Second that. I've used KFm2s on a whole raft of planes and the universal truth about their performance is that they didn't stall. Typical "stall" performance for a KFm2 is the plane sorta mushes out when the AoA increases or airspeed decreases. The plane tends to fall with its nose still high. Think lifting body descent slope and you'll get it.
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Old Dec 22, 2010, 10:51 AM
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Interesting data on the KFm airfoils

I came across this site which compares many airfoils, including the KFm2 and KFm3 airfoils. The results are very interesting...

http://worldofkrauss.com/foils/1815

http://worldofkrauss.com/foils/1814

Some of the other airfoils...

http://worldofkrauss.com/foils/13

http://worldofkrauss.com/foils/7

http://worldofkrauss.com/foils/90


Tony65X55 thought that these were very good results for the KFm airfoils...

"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

And, here is a comparison between a Clark-Y and a KFm2 airfoil...

http://worldofkrauss.com/foils/show_...mit=Compare&id[]=483&id[]=1815

And here is the Clark-YH and KFm3 airfoil comparison...

http://worldofkrauss.com/foils/show_...mit=Compare&id[]=1814&id[]=175
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Old Dec 22, 2010, 11:39 AM
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I notice that Krauss has the maximum airfoil thickness on his KFm3 drawing at 50% of chord, and at ~42% of chord on the KFm2. I'm finding that having the maximum thickness at about 30% of cord is working very well on my KFm2 & 3 variant wing designs.

I'm finding that dropping the trailing edge thickness to ~1mm also produces a wing which flies cleaner & responds more precisely to control inputs, and should reduce drag significantly..... And, of course, the more recent KF3P build approach, with the (~2mm thick) angled panel behind the shallower primary step running back to the low depth secondary step is very promising... It would be really neat to have someone with the software & the capability to do so run that airfoil profile through some good analysis at the appropriate Reynolds numbers!

Just offered as food for thought...

Here's wishing everyone a great Christmas holiday season!!

VIKING
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Old Dec 22, 2010, 12:42 PM
treefinder
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Wow, Dick, that's a cool site! just spent the last 20 minutes pouring over it (on dial up that's not much pouring, tho....) not only a ton of airfoils, but makes templates and cand do comparisons. I did this one, and find it may explain how the OSG can "hang" in the air.


The high lift at low angle of attack versus other foils would seem to be what I see when I cut power and glide, then pull up just a bit and she looks like she stops and just hangs in the air. Obviously moving a bit, but not moving and dropping even in low lift conditions. Also explains how KFMs fly so well at low angles of attack compared to other foils.

Not sure how to interpret the moment coefficient anyone have a good explanation of interepretations?
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Old Dec 22, 2010, 01:18 PM
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Quote:
Originally Posted by viking60 View Post
I notice that Krauss has the maximum airfoil thickness on his KFm3 drawing at 50% of chord, and at ~42% of chord on the KFm2. I'm finding that having the maximum thickness at about 30% of cord is working very well on my KFm2 & 3 variant wing designs.

I'm finding that dropping the trailing edge thickness to ~1mm also produces a wing which flies cleaner & responds more precisely to control inputs, and should reduce drag significantly..... And, of course, the more recent KF3P build approach, with the (~2mm thick) angled panel behind the shallower primary step running back to the low depth secondary step is very promising... It would be really neat to have someone with the software & the capability to do so run that airfoil profile through some good analysis at the appropriate Reynolds numbers!

Just offered as food for thought...

Here's wishing everyone a great Christmas holiday season!!

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

Is this correct? %camber = %thickness ?
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