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Old Oct 27, 2010, 01:33 PM
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KF airfoils, never seen that before.

Hi

I just found a airfoil that looks verry od. Spend most of the night searching for more info on this airfoil. Do any of you know this airfoil, if so does it preforms as god as a well desiged "normal" wing.

Here is one link: http://rcfoamfighters.com/FileDownlo...rFoil_Info.pdf
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Old Oct 27, 2010, 02:11 PM
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Have a look in the Foamies (Scratchbuilt) forum where there is lots of discussion about this RANGE of airfoils including a huge sticky right at the top of the forum.

http://www.rcgroups.com/foamies-scratchbuilt-428/
http://www.rcgroups.com/forums/showthread.php?t=1117276

Steve
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Old Oct 27, 2010, 02:56 PM
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They produce much more drag than a properly designed conventional airfoil which is why they never found a use in full size aircraft.

In the document you attached there are 3 main parts by different authors:

Part 1
The first few pages written by Richard Kline and IMHO much of it just doesn't stack up. You need to bear in mind that Mr Kline is a film director not an aerodynamicist. Most of the claims he makes have no scientific basis and are unsupported by any reputable testing.

Part 2
The page extracted from 'Fluid mechanics' by Frank M White appears reasonable. He points out that the KF airfoils have high drag. About the only positive of (some) KF airfoils is their lack of distinct stall, mainly because they operate partly stalled all the time, this is shown on the graph.

Part3
The wind tunnel testing is potentially very misleading. The comparison was against a thick symmetrical airfoil which would perform very poorly in terms of max lift and L/D ratio. If any of the tests had been done against a halfway efficient airfoil, such as a Clark-Y then the KF would have looked very bad in comparison. Also notable is that the top located 50% wide step that is most popular in foamy models performed very bad in every test; the LD fell from 9 for the symmetrical to 6 for the KF! If included a Clark-Y would have been around 13 -14 in the same test, well over twice as efficient as the KF. Things are probably not so bad at the very low Re numbers small models fly at but the performance would still be much worse.


Basically the foamy flyers like the KF airfoils because they are easy to build from flat sheet foam and they can perform better and are structurally stronger than a pure flat plate. They are fine if you have plenty of power and don’t care about efficiency. Don’t even think about it if performance is your main goal.

Steve
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Old Oct 27, 2010, 05:14 PM
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I did a search on KF airfoils and found information on "vortex trapping airfoils" that use a similar principle.

http://staff.polito.it/luca.zannetti/nota_acc_sci.pdf

http://www.vortexcell2050.org/synopsis.php

http://spiral.imperial.ac.uk/bitstre...M98.HighRe.pdf

It seems that it is, in fact, impossible to confine a steady vortex to the airfoil surface without active vortex confining devices

http://www.wseas.us/e-library/confer...rs/530-341.pdf

http://ltces.dem.ist.utl.pt/lxlaser/...ers/11.3_4.pdf
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Old Oct 28, 2010, 01:59 PM
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Thanks for your replays.

I thought ti worked like a golf ball "tiny holes" this should give less drag. But now I got some reading stuff, thanks.
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Old Oct 29, 2010, 03:46 AM
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Quote:
Originally Posted by frank40 View Post
Thanks for your replays.

I thought it worked like a golf ball "tiny holes" this should give less drag. But now I got some reading stuff, thanks.
Dimples only work on a blunt body like a ball. On a streamline body such as a proper (non KF) airfoil they would add drag in most cases.

http://www.aerospaceweb.org/question...cs/q0215.shtml

Steve
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Old Nov 13, 2010, 11:02 AM
SlingWinger
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Quote:
Originally Posted by JetPlaneFlyer View Post
Dimples only work on a blunt body like a ball. On a streamline body such as a proper (non KF) airfoil they would add drag in most cases.

http://www.aerospaceweb.org/question...cs/q0215.shtml

Steve
I think you're sort of right about where dimples work and where they don't.
I googled "Dimple tape", which is a thick (.015") tape with holes in it that can be applied to wings and props and other airplane parts.

My reading seems to indicate that it doesn't do much if any good on the thinner, high efficiency airfoils used on some wings and props, but does help on thicker sections used on high-lift wings and props.


http://www.oshkosh365.org/ok365_Disc...5&topicid=2346

http://www.backcountrypilot.org/foru...php?f=3&t=4309

For example, the guy flying a "Tundra" low speed, high lift 'bush' plane got positive results, while a guy with a Piper Comanche, which I believe has a laminar flow wing, got poor results. So did the guy with a "Cozy" high-performance canard plane.

In the field of aeronautical science there are few hard-and-fast rules. What works in one area of the performance envelope doesn't work in another.
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Old Nov 13, 2010, 11:09 AM
SlingWinger
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Regarding the above, another example for model planes would be that free-flight hand-launched gliders like relatively thin wings with a highly polished smooth surface, while the very slow rubber powered planes often perform better if you "crumple" the tissue covering before applying it.

Vortex generators of many various types seem to have similar effects as surface roughness.
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Old Nov 13, 2010, 11:24 PM
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KF airfoils are fine for sport models that have a limited speed range due to the drag they generate. They are not exactly the perfect solution to all airfoil needs, though....
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Old Nov 18, 2010, 11:04 AM
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Quote:
Originally Posted by Thomas B View Post
KF airfoils are fine for sport models that have a limited speed range due to the drag they generate. They are not exactly the perfect solution to all airfoil needs, though....
A very good point, and conversely, a " streamlined body proper non-KF airfoil" isn't the perfect solution for all airfoil needs either! I'm one of those in the Scratchbuilt foamies forum who play with KFM's a lot. If drag was the only really important characteristic of airfoils, then KMF's would not have the following they do. However it's only one of several characteristics important to getting a good flying plane.

In my experience, KFM's display extremely gentle stall characteristics, excellent glide performance (characterised by what I'd call great lift without too much speed - Cut power and the planes just seem to slow down without dropping into a stall - good glide ratio). Plenty of speed for anything I fly out over the 100acre field behind my house - they get small pretty fast. I'm flying 30 to 40" span planes in the 10 to 20oz weights with either 2s or 3s packs.

As an example, I built the One Sheet Glider - 60" span, 8oz typical flying weight http://www.rcgroups.com/forums/showt...=60+span+motor . Original wing was a Undercambered with bottom step (orginally did it to get the strength/stiffness - as noted by Jetplaneflyer- but liked the flight characteristics - it's actually a KFM5a airfoil). She flew nicely, but would tend to balloon in any wind, so mostly a calm/very light wind flyer. Decided to try a KFM2 (50%top step). That solved the ballooning issues, added better glide/penetration performance and became my favorite wing. One of my FB's and I had an ongoing discussion about whether the KFM2 was actually the best wing for the glider, so I made a "proper" wing approximating the 7035 airfoil (to the extent one can carve it by hand). We haven't flown it much, but so far neither of us like it as well as the KFM2. It's faster, but doesn't have as good "hang", which I'd characterise as lift in the glide/thermal mode. Penetration was better, marginally, in wind, but it seemed to take more "work" to keep it flying where we wanted to go. Anectdotal evidence, but added to many other folk's experiences, seems to place the KFM as a very viable airfoil for the size/speed planes most of us fly.
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Old Nov 18, 2010, 01:46 PM
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Springer,

first I've got to admit that I've never used a KF airfoil.. my comments are based on theory, testing by others and observation of other models..

I'm interested in your comments about your comparison on the KF against a 'sort of' SA 7035..

You say that the 7035 was noticeably faster, which I'd expect due to the KF drag issue.. The comments you make about 'hang' are interesting.. Does this mean that the stall speed is slower so the model can be flown slower, or does it mean the glide slope is better.. I'd be amazed if the last point was true because increased drag will always result in a worse glide slope. Also every winning contest glider pretty much in every contest ever has NOT featured a FK airfoil.. Which surely says something about their glide performance?
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Old Nov 18, 2010, 10:14 PM
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I've attached a picture showing the tips of the three airfoils. The wings have a taper from 5" to 4" the outboard 15 inches. My observations are generally in the power off soaring mode. For me, being used to the KFM2, when traversing, I had more trouble slowing the 7035 down when I wanted to stay in a potential thermal. With the KFM, I can easily move about penetrating wind, cross wind or running with it and just pull a bit of up trim to slow down and get almost stationary, and if in a thermal, watch the plane rise. I haven't flown the other wing in really good thermals yet, so don't know if it will behave that way or not (in the weak lift the day we tried it out, it didn't). I haven't flown it enough to do a really good comparison as to glide slope, but I do think the kfm seems to generate way more lift at low speeds, which is where I mostly stay. The KFm may exhibit more drag, but it seems to be compensated by what I'd have to say is more lift. For me who's just out for fun looking for thermals, and cheating with a motor so I can get 30+ minute flights instead of 5 minute flights I hear about with a highstart, discus, etc, the KFM is more "pleasant" to fly than the UC or shaped airfoil so far. My FB and I can move about pretty much where ever we want in an essentially square area 3/8ths of a mile per side. That 60" wing gets pretty hard to see at those limits.
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Old Nov 19, 2010, 02:28 AM
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Springer,
Thanks for the photo. Are all the wings near enough the same weight?

The SA 7035 is not especially great if you are looking for a slow flying 'high lift' airfoil. Something with a little undercamber would be better if slow speed is what you want. I'd still be surprised though if (given similar weight) the 'top step KF' airfoil really was able to fly significantly slower than the 'sorta 7035'.

It's often very hard to accurately compare unless you can fly the two wings back to back on the same day. I would expect though that the 7035 might show a more pronounced stall than the top step KF.

Steve
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Old Nov 19, 2010, 11:23 PM
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The three wings are pretty close in weight, the UC wing is lightest, as the other two have coffee filter paper/wbpu covering to make them stiffer and "slicker". Going with a non KFM airfoil we weren't looking for floaty, but rather one more like the ones the "experts" use in their gliders and the 7035 seemed to come up relatively frequently in posts, and is a fairly simple foil, so I decided to try it. The day we first flew it, I flew all three wings, and my buddy who has a kfm2 OSG flew his, then the 7035. No Question, the KFM2 flew way slower than the 7035, It exhibited the typical "hang" - turn it into the wind and it just looks like it's stationary in the air (I'm sure it's sinking slowly, but up several hundred feet, it's slow enough I can't tell, and if there's any lift at all, it will stay put or climb slightly) The 7035 in our experience never achieved that. Slowing it down, it would stall, and the stall was a real stall, nose dropping and plane losing altitude as speed built up and it could pull out of it. The stalls on the KFM are mostly little porpoise-like movements, and it takes a lot of work to get it to stall. All this was in power off glide mode.

I shot this vid with an onboard gumpack cam and the version 3 KFM2 wing OSG:
One Sheet Glider 8-18-10 (5 min 8 sec)


The flight was about 15 min on a single powered climb (says exactly in the vid) and could have gone on all afternoon. I only brought it down to shut off the cam. I edited it down, of course, would have been boring to see the whole vid, but you can get a feel of how she flies.
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Old Nov 26, 2010, 02:47 PM
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I've got a 36" span Infinion with KF "airfoils only because I needed to stiffen the wings to stop wing flutter. The most noticeable flying attribute of this plane is it can slow down till it looks like it's stopped. It can land with a couple feet of rollout. Hanging in the air is a good description of how slow it will fly. So this is due to the KF "airfoil"?
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