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Jun 17, 2013, 12:30 PM
Registered User
Dickeroo's Avatar
Quote:
Originally Posted by A Useless Geek View Post
It seems to me that such a profile would require a minimum of changes to a mold to make it out of plastic, Dick. I wonder if any prop manufacturers have the fortitude to attempt a test of this shape? And would the prop benefit from a tip plate winglet, I further wonder?

-- Marty, scratching head and looking askance at APC and Windsor --
Marty...

As you know, there has always been a lot of resistance to the KF concept. It looks butt ugly. It shouldn't be able to fly like that. The drag is too high. It just doesn't look right. It isn't a true airfoil shape.

It was never meant to fly like a glider. It needs power in order to trap and utilize a vortex. The step takes some of the negative force of drag and turns it into a positive. The KFm4 is the best example since it is the fastest of the KF airfoils. And it does it with a step on top and on the bottom.

BUT, nothing is real until you experience it for yourself. This applies to everything. You have to see it for yourself or it just doesn't register. It also has to be done correctly or you will get results only justifying your position.

I am just thankful that Tony65x55 back in 2006 was willing to find out for himself whether the KF concept had any merit.

But, we stumble forward and have a hell of a lot of fun doing it.

~ Dick
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Jun 17, 2013, 12:45 PM
Jack
jackerbes's Avatar
I like the Great Planes Powerflow slow fly props that have little winglet or tiplet or something like that. It is on the end of the prop, at a not quite right angle to the blade and rises from the leading edge to form a sort of mini-wing or mini-prop or something.

The are pretty quiet blades, as strong as APC as far as RPM limits (based on experience, they don't publish an RPM limit), and they are about perfectly matched to my preferences for larger slower turning props over smaller faster turning props.

More info on them here:

http://www.electrifly.com/miscproducts/gpmq6610.html

And Dr Kiwi did some testing on them for me:

https://www.rcgroups.com/forums/show....php?t=1248148

Jack
Jun 17, 2013, 08:41 PM
Registered User
poormansairforce's Avatar
Quote:
Originally Posted by Dickeroo View Post
Marty...

As you know, there has always been a lot of resistance to the KF concept. It looks butt ugly. It shouldn't be able to fly like that. The drag is too high. It just doesn't look right. It isn't a true airfoil shape.

It was never meant to fly like a glider. It needs power in order to trap and utilize a vortex. The step takes some of the negative force of drag and turns it into a positive. The KFm4 is the best example since it is the fastest of the KF airfoils. And it does it with a step on top and on the bottom.

BUT, nothing is real until you experience it for yourself. This applies to everything. You have to see it for yourself or it just doesn't register. It also has to be done correctly or you will get results only justifying your position.

I am just thankful that Tony65x55 back in 2006 was willing to find out for himself whether the KF concept had any merit.

But, we stumble forward and have a hell of a lot of fun doing it.

~ Dick
No doubt on the experiencing part! I was testing my C6 again tonight and it is a streamlined plane to begin with but when I'm overshooting the zone by my feet by 50' consistently(4 go arounds on my first landing attempt) when gliding in then its "flying" not plowing air like a sheet wing!
Aug 17, 2013, 03:28 PM
Registered User
Michael V's Avatar
Hello guys,

First post in this thread, so I'll start with an introduction.

I have much to say (not sure if that's good or bad) so I'm starting easy so as not to blast too much stuff.
I have been reading all (I'm back on page 3 right now, so a bit to go, but now it's flying time) this thread and all ancillary links (lots of stuff, all great). That required the last 3 weeks to do BTW.

It's a fascinating subject to me and I love all the testing that has been done, more on that later.

I am an engineer by trade, so I have some knowledge of the subject, but nowhere as fine tuned as some others here (clearly aerodynamicists, familiar with wind tunnels procedures and software), yet I hope I know enough to hopefully have some valid insights.
On the other hand however I cannot contribute much in the empirical realm as I'm a beginner flyer and builder, but there are others here that clearly can do that too. Incidentally, I fly slope.

My stable now consist of:
1- an MPF OSG platform with:
- 48" KFm2 wing with slight undercamberd LE 9.4% thickness (the whole section made to match an AG09 airfoil, see below) and strong dihedral
- 64" TP96 thin wing (6.25%) flat with winglets
These were not designed to test the airfoil and I don't expect it to be a reliable test platform for it for reasons I can go into later. The idea here was to test designs (and building concepts) more than airfoils specifically.
I like this plane in KFm2 config and like to fly it. It flies well and is very gentle, and in particular it can fly in 15mph wind (not bad for a 7.5 oz plane) but it seems less "floating" than it was expected. In other words it requires more wind to stay afloat than I expected ( KF drag maybe, I haven't measured glide ratio). But again remember pilot is not experienced and other experienced pilot around are not too vocal, good or bad., so it's hard to have any valid statement, even qualitative or subjective.

2- A small (22" wingspan) ME 109 check glider like plane, transformed into a RC sloper. That one has been essentially retired. The airfoil was flat funky airfoil, so no KF relevance at all. Probably no airfoil relevance at all either.
Retrospectively I possibly would have been better of with a KFm2 than the flat bottom funky airfoil. (funky in the sense that it's eyeball airfoil and wing progression, not any reference, no washout, no dihedral, just plain jane)

3- An almost finished built custom design of an MPF SU 27 for slope. See below.
Plain KFm2 wing, Constant 50% step, tapered but thick trailing edge and slightly balanced leading edge (stagnation point 30%).
It's a test platform of many design decisions, KFm2 wasn't planned to be a test here however, but rather a reliable data point (that it can fly). After all the reading I'm ecstatic as to what in fact the KFm2 could contribute to this plane (It's a sloper remember, not a park jet, no power). I don't expect to have the skills to be flying it, let alone test different things on it, but I should find pilots that can do that for me. More to come. Of course any KFm2 conclusion are useless, because there is no reference comparison, and there are aero options that I put in place that compound the wing behavior, certainly enough to render any absolute statement irrelevant as to "pure" KFm2 observation.

4- A work in progress of a forward swept wing MPF 38" with airfoil evolution from KFm4 (center section) to KFm2 (wings).
Airfoil base is TP100 (a reflexed wing foil), and I used a 6.2% thickness at root evolving into 7.3% thickness at wingtip. It has slight virtual washout (even though the LE is horizontal, the stagnation point is dropped in % and the TE is horizontal). It also has many other aero implements (winglets and Vstabs acting as wing plates to isolate flows mostly) so again because of the compounding I don't expect to get much valid conclusion as to the KFm part only (any behavior is compounded from many features). LE will be smoothened, not stepped like drawn below.
This design was based on Ken's Shrike, modified for slope and 38" wingspan and others mods too.

Of to the slope for today!
Aug 18, 2013, 09:29 AM
Registered User
Hey Guys,

I'm getting pretty interested in KF airfoils and I work in a fluid mechanics lab so I'm starting to design an experiment around stepped wings, specifically the KFm-2 style wings.

Obviously when most people build a KF airfoil they just stack two pieces of foam but if you were going to describe the ideal airfoil (like the one shown in the diagram on wikipedia) without the step, what would the NACA numbers be?

Since I'm interested in doing some experiments on KFm-2 in a wind tunnel, I want to make sure my foil is as ideal as possible.

Basically, is there some formula/equation to describe a standard for these airfoils?

Thanks!
Aug 18, 2013, 10:12 AM
Jack
jackerbes's Avatar
Have you seen the figures in post #2 over on the building discussion thread?

** Kline-Fogleman (KFm) Airfoils - Building/Flying Discussion ** - www.rcgroups.com/forums/showthread.php?t=1117276

There is info there on the step locations and the wing thicknesses at the thickest point expressed as percentages of the chord. That is really as much guidance as we (the KF aficionados if you will) have ever wanted or needed to build with.

People will fine tune that as much as they want or need to in accommodating their material and desires. We are not in denial about drag and know that it can be further minimized in almost any KF wing build with thinning, tapering, etc., on leading edges and various things like that.

There are a number of regulars here that have been more focused than other on reducing drag for gliders and lower powered designs. Many of us just want the joy of quick and easy builds and to get it flying so we add as much power as we need to or want to to get it to fly to our conditions and to suit our tastes.

You can find more info on many different KF winged builds in this thread:

** Tony65x55 ** planes and plans. - www.rcgroups.com/forums/showthread.php?t=997484

I think the building and flying thread has the greatest amount of info on all the different type of planes, types of KF wings that have been and are being built. This thread was intended to explore the characteristics of the wing that are still somewhat of a mystery and point of discussion about the wing.

Jack
Aug 18, 2013, 12:06 PM
treefinder
springer's Avatar
Rick: great to hear you may be able to put some numbers on the kfm situation. I'd like to suggest 2 things: first, that you probably don't want an "idealized" airfoil based on a naca or other conventional airfoil with a step as your base kf airfoil. I say that because essentially no one on this forum uses that configuration. "Everyone" uses flat foam, rounded LE (from a simple radius to an approximation of the forward portion of a conventional foil - the latter being generally considered lower drag/ better, no taper at TE. I know not every one does that, but i'll bet 90% end up that way, cause it's easy to build and works.

Second; as an attempt at a "typical" kfm airfoil for documentation purposes, use two .25" thick slabs: bottom one 8", top one 4" (50% step). Leading edge radius less than .125" (1/8-3/16" dowell)tangent to bottom surface. Arc from LE radius tangent to top surface tangent is 3-4". That gives you the KFM 2 airfoil many if not most are using. You should easily be able to digitize this and vary it in experiments. Obviously chord and step height are varied on larger planes. But i bet you capture the bulk of the planes documented on this forum.
Aug 18, 2013, 03:29 PM
Registered User
Dickeroo's Avatar
Rick...

It would also be great if you could get some numbers on the KFm4 airfoil.

The KFm4 is faster than the KFm2, which seems counter intuitive if the steps are supposed to induce more drag.

Also, is there any way to see if the vortexes exert a small amount of forward pressure due to the counter rotation of the vortexes.

Don't mean to load your plate. You do what you are comfortable with.

~ Dick
Aug 18, 2013, 04:17 PM
OpenTX University Staff
maguro's Avatar
Thread OP
Rick,

Viking60 and I have found that a folded foam KF airfoil (as opposed to a slab airfoil) approximates an MH32 airfoil nicely. Depending on how the wing is built, you can do a very close approximation. I have some links to earlier posts in this thread where I show this:
https://www.rcgroups.com/forums/show...&postcount=316
https://www.rcgroups.com/forums/show...&postcount=353
https://www.rcgroups.com/forums/show...1&postcount=33

These links are to the test plane I built to try to get some data. Only one series of tests was ever conducted.
https://www.rcgroups.com/forums/show...&postcount=229
https://www.rcgroups.com/forums/show...&postcount=463

I hope this is of some help.

Roger
Aug 19, 2013, 02:46 PM
"Use the foam, Luke!"
SpaceDrydock-22's Avatar

Mother Nature's "KFm"


Take a look what kind of wing profile mother nature is developt during millions of years ago. It looks like it wont lift up nothing in the air, but it do and very well.

http://jeb.biologists.org/content/203/20/3125.full.pdf

http://jeb.biologists.org/content/199/2/281.full.pdf

http://www.sciencedirect.com/science...22519309003282

http://www.public.iastate.edu/~huhui...-2007-0483.pdf

Dragonfly airfoil has multiple "steps" (angles) which generate wortexes all around the wing, like the KFm profiles steps does.

This is not my video, I found it from Youtube.
Dragonfly corrugated Airfoil (0 min 19 sec)


I've been doing some field research around about these "wrinkled" profiles, in other words, made some models which have various of different "wrinkles" on their wings. Here is one example which works great. Almost symmetric cross corrucated profile in a simple free flight glider.

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It flies very good and I think there is no doubt about that these profiles will work in bigger scale also.
Last edited by SpaceDrydock-22; Aug 19, 2013 at 02:52 PM.
Aug 19, 2013, 03:01 PM
"Use the foam, Luke!"
SpaceDrydock-22's Avatar
Quote:
Originally Posted by Dickeroo View Post
The KFm4 is faster than the KFm2, which seems counter intuitive if the steps are supposed to induce more drag.
"From the computational results, lift coefficients are increased by the wing corrugation at all Reynolds number. However, the corrugation has little influence on the drag coefficients"

http://www.sciencedirect.com/science...22519309003282
Aug 20, 2013, 09:05 AM
just Some Useless Geek
Crap glide ratio. Oh, well.
Aug 21, 2013, 03:28 PM
No Hangar Queens!
Willsonman's Avatar
This an interesting subject for sure. I've recently come across something called project falcon. The best part is I can create my own 3D mesh and import to see the effects of flow. It would be interesting to do this for the various kfm airfoils. It does not really have the power of analysis but more a visual representation of what is happening under flow. Creating the mesh for import would be super easy in Wings3D and can be easily exported for use. I may play with this tonight, if I have some free time.
Aug 21, 2013, 10:20 PM
Registered User
Michael V's Avatar
Quote:
Originally Posted by Willsonman View Post
This an interesting subject for sure. I've recently come across something called project falcon. The best part is I can create my own 3D mesh and import to see the effects of flow. It would be interesting to do this for the various kfm airfoils. It does not really have the power of analysis but more a visual representation of what is happening under flow. Creating the mesh for import would be super easy in Wings3D and can be easily exported for use. I may play with this tonight, if I have some free time.
Nice, this is great, downloading as we speak, I have a few plane designs in Sketchup that I want to try out. Too bad it ends August 31.
Aug 22, 2013, 02:13 AM
Boffin
rpage53's Avatar
Quote:
Originally Posted by SanctusSanctorum View Post
"From the computational results, lift coefficients are increased by the wing corrugation at all Reynolds number. However, the corrugation has little influence on the drag coefficients"

http://www.sciencedirect.com/science...22519309003282
Remember that drag increases as the square of the speed. Their conclusion is only because of the very, very low Re and has nothing to do with the wing profile.
Insect studies don't mean much if your wing is bigger than 6".
Rick


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