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May 19, 2009, 02:53 PM
Slow is beautiful
Alex Slowfly's Avatar
I like the _very_ slow wings. Im used to look at profili 2 with quite sucess. Nevertheless, below Re 50'000 things get hairy... in this area it's more a TLAR aproach....
So, undercamberd Frog-like wings rule for very slow (rather high drag) and very predictible wings (something like a 8-40 to 12-40 undercamberd wing). Clark-Y is fine but still too fast.
Different KFm1 wings turned out to be too fast, rather near flat plate characteristics and to some degree a disapointment regarding the hype of feedback here in the forum.... (?)
So my research is heading towards good slow wing profiles like "Andrukov" or the like. For the ease of construction I stick to the "Frog-like-wings" right now. As I do AP and FPV as well I really stick on the very slow end of the range.... still a lot of interesting stuff to explore there...
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May 19, 2009, 02:58 PM
Registered User
Dickeroo's Avatar
[QUOTE=magic612]This isn't a poll, because I was looking more for a discussion. It stems from a comment I saw in another thread about the KF style airfoil. I don't want this to become a contentious debate; rather, I hope we can all be respectful of others' views. We all like different planes, flying styles, and are at different levels of accomplishment in our hobby.

Magic612...

You have posed a very interesting question on this thread.

Here is a puzzle that I can't figure out. A number of people have mentioned that the KFm airfoil produces more drag than conventional airfoils. And, that may be due to the height of the step. However, when Dave Powers tested the four variations KFm1, KFm2, KFm3 and KFm4 on his KF Airfoil Testing video, it turned out that the KFm4 flew the fastest. And, that was with two steps, one on top and one on the bottom. Why wouldn't the KFm4 produce more drag instead of producing more speed? It doesn't seem to follow.

Also, a guy from Missouri put the KFm airfoil on the tip of a prop and compared it to a conventional prop. He got almost 9% more thrust with the KFm over the conventional prop. How can this be?

If anyone has any thoughts on this matter, I would sure like to hear them.

I know that it is true for the people who have experienced the increase in drag. I have always felt that the KFm airfoils need thrust in order to capture a vortex and trap it. The thrust can come from a motor pushing the wing forward or from a strong wind moving over the airfoil. Hopefully some day it will be better understood.
May 19, 2009, 03:12 PM
Bernoulli+Newton=Lift
magic612's Avatar
Thread OP
Quote:
Originally Posted by Dickeroo
Magic612...

You have posed a very interesting question on this thread.
I have a bad habit of doing that.

Quote:
Originally Posted by Dickeroo
Here is a puzzle that I can't figure out. A number of people have mentioned that the KFm airfoil produces more drag than conventional airfoils. And, that may be due to the height of the step. However, when Dave Powers tested the four variations KFm1, KFm2, KFm3 and KFm4 on his KF Airfoil Testing video, it turned out that the KFm4 flew the fastest. And, that was with two steps, one on top and one on the bottom. Why wouldn't the KFm4 produce more drag instead of producing more speed? It doesn't seem to follow.

Also, a guy from Missouri put the KFm airfoil on the tip of a prop and compared it to a conventional prop. He got almost 9% more thrust with the KFm over the conventional prop. How can this be?

If anyone has any thoughts on this matter, I would sure like to hear them.

I know that it is true for the people who have experienced the increase in drag. I have always felt that the KFm airfoils need thrust in order to capture a vortex and trap it. The thrust can come from a motor pushing the wing forward or from a strong wind moving over the airfoil. Hopefully some day it will be better understood.
Well, I will say up front that I'm not the best person to answer that. Based on my 100% totally subjective opinion, the KFm airfoils I've flown "feel" like they've got more drag. Yes, I know, very objective data there, eh?

But perhaps it has to do with how the plane with a KFm responds differently in slower speed situations. Think about it: If you've flown one plane with a "standard" airfoil (Clark Y, whatever), and you know at what speed it stalls, or how it reacts when it is close to a stall, or even IN a stall, and then you fly the same plane with a KFm airfoil, and it doesn't stall, or it's stall is far more mild, does that make it "feel" like there's more drag, because we're seeing the plane fly slower than we think it should?

I don't know. Again, it's a "feeling" for me. It feels like there's an optical illusion at work, because the KFm can fly slower, we assume that the airfoil has more drag (since most slower flying airfoils do have more drag). Yet as you described, it can and has flown faster on the same aircraft.

I'm going with the "mind trick" thing. Our mind is tricking us into believing that it has more drag, because it will fly slower, and since we've always been taught that a slower flying airfoil is thicker, we believe it also has more drag.

Maybe???

I don't know.
May 19, 2009, 03:18 PM
Postcards From The Purple Edge
tuppertn's Avatar
Quote:
Originally Posted by Alex Slowfly
..below Re 50'000 things get hairy... undercamberd Frog-like wings rule for very slow (rather high drag) and very predictible wings (something like a 8-40 to 12-40 undercamberd wing).

Different KFm1 wings turned out to be too fast...
I agree. I've found UC wings to be more favorable for slowfliers. The KFm's have good slow flight characteristics in that the stall is expectionally mild and they handle high alpha very well. But slow flying at high alpha is very different than what you'd want in a good Slowflyer.

kendall
May 19, 2009, 06:07 PM
Registered User
Dickeroo's Avatar

Magic612... it's a good habit to ask interesting questions.


You may be somewhere south of Chicago, but you're far north of Inquiry.

One thing I forgot to mention was that several people who first flew a flat plate aircraft, then added a KFm, found that the plane went faster. I also think that occurred when Dave Powers went from a flat plate to a KFm2. If a KFm2 would create more drag, then wouldn't that have slowed the plane down instead of speeding it up? Who knows? I'm only making an observation.

Never stop asking questions. As someone once said, "There are no dumb questions." He's right. Posing a question opens up things and eventually leads to answers.
May 19, 2009, 06:21 PM
treefinder
springer's Avatar
throwing more observations in here: my experience with KFM particularly the "simple" KFM2 laminated w/50% top step is that when I am flying slow with power on, then cut power, I get a slight increase in speed with the least little drop of nose. Now, I seem to like a little nose heavy cg location - (fear of tail heavy lack of control) so that might explain it, but I certainly don't get a slow down that I'd expect from a draggy airfoil.
May 19, 2009, 06:36 PM
Bernoulli+Newton=Lift
magic612's Avatar
Thread OP
Quote:
Originally Posted by Dickeroo
Never stop asking questions. As someone once said, "There are no dumb questions." He's right. Posing a question opens up things and eventually leads to answers.
That's how I learn. Ask questions, keep asking questions, make observations (even if they are wrong), and don't be afraid to admit when you're wrong. It's worked pretty well for me so far.

Okay, so here's a possible explanation for what you've posed (again, based on zero evidence, just me thinking "out loud"):

A flat plate isn't really an airfoil, as such - there's no curve, nothing to change the course of air over or under it save for the leading edge shape, which may or may not be flat or curved. So in my mind, the way for that kind of wing to get lift is more "Newton" and far less "Bernoulli" - give it enough angle of attack (via decalage, or thrust angle, etc.), and the air underneath the wing forces the plane up.

But that also creates drag, yes?

But with a KFm 'foil, the fluid motion is different. I've not delved into enough to really understand it well, but as you put it, the "captured vortex" is what may be creating the lift (more of the "Bernoulli" variety), therefore requiring less of the Newtonian "forced-air underneath" uplift. That would allow the wing to work in a more "straight on" orientation to the direction of flight, rather than forcing an angle of attack to create lift.

Less drag that way, right?

Again, I don't know, but that's where my mind is pointing me at the moment. Doesn't really explain the KFm4 airfoil, though, and the KFm1 does have an angle of attack to it as well, but if a vortex is created boosting lift, it could still be a plausible theory at least for that one. KFm4? Got me.

Any further thoughts on all of that?
May 19, 2009, 08:53 PM
Registered User
Dickeroo's Avatar
KFm4? Got me.

Any further thoughts on all of that?[/QUOTE]


Yes, Magic612, I do... as I understand it, only the first 25%-35% of an airfoil produces lift. The remaining portion of an airfoil needs to be there but doesn't do any lifting. The KFm3 airfoil quite possibly produces lift the length of the foil from leading edge to trailing edge because the two trapped vortexes are rotating faster than the air beneath thus creating lift the length of the foil. And, that is probably one of the reasons the COG needs to be further back. The KFm3 is a heavy lifter airfoil and it is perhaps because of the placement of the two vortexes as well.

On the KFm4, 50% of this airfoil does not produce any direct friction or drag because it is air against air. Thus the first 50% from the leading edge back is producing lift. There may well be a slight forward push from the vortexes both above and below because of the forward rotation of the air. The air pressure in these two pockets is moving faster than the air beneath the airfoil. It's like taking the negative force of drag and turning it into a positive.

One more thing. the KFm wings have a tendency to want to stabilize and fly level. I believe that is because the air pressure within each pocket needs to be equal at all times on both wings. So, if you bank the plane and let go it will return to level flight. That is one of the reasons for the resistance to stalling. This is just my opinion, not fact.

If others have different opinions, then please join in.
Last edited by Dickeroo; May 21, 2009 at 07:06 AM.
May 19, 2009, 09:32 PM
Registered User
lake flyer's Avatar
"I just maidened my Fantastic acro. 1m span, 540g, thin symmetrical 50% KFm profile. Absolutely stallproof. I couldnt believe - I cut throttle at about 30m above ground and pulled full up elevator. No wind. Plane landed to my feet like pancake, vertically fell from the sky . "

That is a stall.
Last edited by lake flyer; May 20, 2009 at 12:06 AM.
May 19, 2009, 09:54 PM
Bernoulli+Newton=Lift
magic612's Avatar
Thread OP
Quote:
Originally Posted by lake flyer
That is a stall.
What is?

EDIT: Thanks for clearing that up, lake flyer.
Last edited by magic612; May 20, 2009 at 08:01 AM.
May 20, 2009, 12:09 AM
Registered User
lake flyer's Avatar
Sorry Magic ,

there were so many posts when I was posting , it didn't make sense .

I should have quoted the post I was referring to .
May 20, 2009, 12:17 AM
Registered Plan-a-holic
Hepdog's Avatar
Kfm on wings seems to work wonders. I built a largeish (70") tail less wonder that could only fly due to the unique balancing provided by the Kfm drag....BUT I don't think KFm's are draggy at all. In fact, I think they are the least draggy airfoils.

I'm not an aerodynamisist but a big drag factor comes from skin friction. The KFM step separates the flow and seems to eliminate that. Area ruled fuses and super skinny planes like Nemesis prove that the least wetted area is the least draggy. Perhaps at our Reynolds numbers the separation KFM provides is a really good thing.

IMHO if you can't have full and true laminar flow - the next best bet is to separate it as soon as is possible. I've played around with KFM's and traditional Clark Y's to go KFM wherever possible....scale planes not withstanding of course.
May 20, 2009, 03:00 AM
it WILL fly! someday....
Richard_s's Avatar
Quote:
Originally Posted by lake flyer
That is a stall.
stall with full aileron control? anyway, interesting . Guys with onboard cameras, could tape short threads to the wing and record whats going on there...
Last edited by Richard_s; May 20, 2009 at 03:28 AM.
May 20, 2009, 06:38 AM
Jack
jackerbes's Avatar
lake flyer wrote:

"..That is a stall..."

I consider a stall to be when a plane is falling from the air without any useful lift forces involved and in an uncontrolled or uncontrollable manner.

The situation you describe as a stall had the plane in a controlled descent (sometimes called the "parachute" or "elevator" manuever) so I don't consider it a stall.

If I get my KFm winged aircraft close to the ground, use up all or almost all of the air speed, and get the plane more or less vertical with the nose or one wing down, it can fall to the ground and I have no control over it at that point. That is what I consider a stall. Fortunately, low speed, a short falling distance, and the cushion of deep grass I fly over usually prevents any damage.

I build the more mellow flying foamies, the Blu-Baby trainer/sport planes and KFm flying wings (Zagnutz and Divinity). If I have a reasonable amount of altitude, chopping the throttle and letting the sticks center themselves will usually set me up for recovering from a situation where I am "out of control" (or is that just another kind of "senior moment")?

I was aghast when I first learned that some (or nearly all?) flat plate fliers have little or no glide capability and generally want to fall out of the air when the power is cut (intentionally or otherwise). That they don't "plane" or glide without power pretty much removed flat plate fliers from my sphere of interest.

The Kline-Fogelman designs of Tony65x55, ApachePilot, and others here have kept me in electric RC and brought me a lot of enjoyment. The fast and inexpensive builds are nothing less than wonderful. I think there will always be an interest in flight as a hobby and this is the prefect entry level for people of all ages.

So thank you Mr. Richard Kline (or Dickeroo as he calls himself here), Tony65x55, ApachePilot, and all the others who make flying such great fun and this forum such a wonderful to visit and hang around.

Jack
May 20, 2009, 07:26 AM
gpw
gpw
“There’s no place like Foam”
gpw's Avatar
Airfoil choice depends on what kind of plane you're flying and what conditions you expect to fly in...
Having built & flown many many planes in the past, we've found the simple under cambered airfoil (foamie) has been the best for slow floaty type of flying .... best suited to calm or low wind situations or Indoor flying... Lightly loaded UC airfoils can fly really SLOW.... We use the BendFoam jig to make simple UC airfoils... GeneBonds 4-40 airfoil seems to work the best for us..
For general outdoor or windy conditions, the KF airfoils are really hard to beat... We've Hot wired/laminated/stick and ribbed/vacuum bagged /etc. many different airfoils and while they 're very nice and airplane "looking" (full scale comparison), The simplest KF airfoil works BETTER in our "scale" air... We MUST consider "scale effect" when designing/building our smaller Foamie models ... What works in the real world ,doesn't seem to translate well to our size range... Small vs large bird thing!!!
I bought "The Book" many years ago , but never translated it well till we got into Foamies, where the material itself was the attraction...and the KF not only gave us a simple ,build able , airfoil with good efficiency, but also the KF addition provided extra support to the wing structure, eliminating the need for heavy spars/rods/skewers.... A much appreciated construction benefit.... Once we tried it we never looked back.... IMHO , the KF concept IS the ULTIMATE Foamie airfoil!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.... Strong words, but I assure you, well tested.....
Anyone who has not tried it should certainly consider doing so!!!!!!!!!
Being a builder of "disposable Foamies" (just stuff for FUN) we've always used just two variations of the KF, the simplest too...
For stall resistance we like the 40% on the bottom ... for best glide we like the 50% on the top... we don't use spars /etc, just stick one flat piece of foam onto another...The absolute easiest /quickest way to make a wing .... best of all , it works GREAT!!!! Thanks Dickeroo!!!!!
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