Originally Posted by goldguy

Except when sloping where the airfoil is everything. There's no comparison between any KF variant and the PW51 on a slope wing. It's all about low drag and energy retention, both features the KF does not have. Yes, it will work in big wind with ballast to a point. How do I know? Tried it.

Power is a whole different story.

+1 Exactly my experience as well.
All my early slope gliders had Kf airfoils. they flew fine, were very easy to handle, no bad stall, but clearly draggy in retrospect. Planes that should fly in 5-10 MPH wind, needed 12-15 to not sink. It's fine as I would fly them in 12-15.
Since then my planes now have airfoils, they may not be as gentle, maybe, but they definitely need less wind to fly for a comparable design.
If you have a motor, then it doesn't matter so much.
That is definitely not to say that Kf airfoils don't have a place., they sure do. I haven't tried Kf4 (which I suspect is in a different league maybe than the rest of the family, it may be the one that is actually superior to a symmetrical traditional equivalent airfoil, I suspect so, but haven't tried or experimented with it yet)

Originally Posted by Michael V

<snip> I haven't tried Kf4 (which I suspect is in a different league maybe than the rest of the family, it may be the one that is actually superior to a symmetrical traditional equivalent airfoil, I suspect so, but haven't tried or experimented with it yet)

Unlikely, I would think.
I have a couple of planes with KFm4 wings, and they fly well. However it is noticeable that there is no type of model airplane competition where a KFm airfoil is used, never mind being considered essential to ensure success.
If real performance benefits could be had by using KFm wings I'm sure those competition guys would be fully utilizing them.

If real performance benefits could be had by using KFm wings I'm sure those competition guys would be fully utilizing them.

I'm sure you are right about that. But there is a level of flying in any flying venue where having a slower flying and more stable aircraft is a useful thing. Like when you are a newbie to that flying venue or developing you skills or if you simply enjoy flying with less than a fully competitive aircraft. And most especially, if you can't or don't want to buy the most competitive plane.

Some of the competition guys over on the glider forums have no tolerance at all for anything but the state of the art and most competitive airfoils and have nothing but sneers and disdainful remarks about things like the KF airfoils if you try to discuss them on the glider forums.

And the glider snobs generally don't have any appreciation at all for the concept of an entry level or scratchbuilt glider for a newbie that can be put in the air for a small fraction of the cost of buying a RTF or ARTF glider with a more competitive airfoil.

I learned that the hard way when I was playing with the Blu Sail II and Big Blue 96 gliders and simply kept the threads and discussion on them over here on the scratchbuilt forums.

Jack

Originally Posted by jackerbes

I'm sure you are right about that. But there is a level of flying in any flying venue where having a slower flying and more stable aircraft is a useful thing. Like when you are a newbie to that flying venue or developing you skills or if you simply enjoy flying with less than a fully competitive aircraft. And most especially, if you can't or don't want to buy the most competitive plane.

Some of the competition guys over on the glider forums have no tolerance at all for anything but the state of the art and most competitive airfoils and have nothing but sneers and disdainful remarks about things like the KF airfoils if you try to discuss them on the glider forums.

And the glider snobs generally don't have any appreciation at all for the concept of an entry level or scratchbuilt glider for a newbie that can be put in the air for a small fraction of the cost of buying a RTF or ARTF glider with a more competitive airfoil.

I learned that the hard way when I was playing with the Blu Sail II and Big Blue 96 gliders and simply kept the threads and discussion on them over here on the scratchbuilt forums.

Jack

Same for my Blu-Guppy design. Luckily people who frequent the scratch built forums are open to new ideas, and don't mind a bit of compromise.

Originally Posted by Whiskers

Unlikely, I would think.
I have a couple of planes with KFm4 wings, and they fly well. However it is noticeable that there is no type of model airplane competition where a KFm airfoil is used, never mind being considered essential to ensure success.
If real performance benefits could be had by using KFm wings I'm sure those competition guys would be fully utilizing them.

I hear you and that is absolutely correct. However, first "superior" is always a relative thing to start with, so it may not be superior in the particular need of that competition, but offer some other advantage. And second there is a level of skepticism about Kf airfoils, which prevents a lot of people from experimenting with them, that may not be actually happening, but it may also play a role.
I think Kf4 MAY be an adaptive symmetrical airfoil (because of the steps) and thus have a bit better envelope and lift curve that could be more favorable to certain kind of needed behavior (in particular aerobatic). How much of a drag penalty would that entail and how penalizing it would be (does the penalty substantially outweigh the benefit) , I'm not sure.
I've read of people's experience with it, and there MAY be something there.
Like I've said it could behave like a semi symmetrical, "changing shape" between normal and inverted flight and in transitions, which would fit certain narrative from what people experienced. The theory also seems to favor that. That MAY be superior in some way to full symmetrical.
It's just an educated curiosity so far on my part.

KFm1 works with a step on the bottom, KFm2 works with a step on the top, KFm4 combines those two airfoils into one and I think it stands to reason that it has to work better than either of them. And when you use thinner foam, like Dollar Tree instead of FFF, you get a pair of lower steps and an overall thinner wing and that can reduce the drag.

We have not done the work to know the actual effects of step height on lift and drag nor have we gotten good measurements on the drag in each of the foils and of the drag with different foams.

But after building several KFm4 wings I would have to say that it is usually my first choice of the various KF foils for almost any build. Most of my KFm4 builds were Divinity flying wings of various sizes or similar designs and they were all lovely flyers.

Jack

Model aircraft with a flat plate airfoil have been around forever (and/or, since I was a wee child), worked great then and they still do. So yes, there's an application for all airfoils. I still use them where nothing else can do the job.

And ............. the KF for my small simple powered birds and cut my own cores for my slopers using the foil of the week.

Ever notice some designs call for very specific high tech airfoils for the wing and then use a flat plate for the tail feathers? HA!

And some of those flat plate wind up planes only cost 29 Cents or so!

One of the things that first attracted me to the KF foils was that the FFF, in a single layer, was not very strong and that limited the size of the builds so I had not built any flat plate builds yet. But I also was sort of stumped on building materials because of a lack of a nearby LHS and the high cost of balsa. I really wanted to find a cheaper material.

When I saw the KF wing and read the Blu Baby thread I realized that it opened the door to making the foam stronger with spars and the like and gave it a try. And not only was it stronger it flew very well. And by this time I was reading the Blu Baby thread regularly and getting all the reports on how well it worked and how versatile it would be.

So I broke down and bought a bundle of FFF and got started on a very enjoyable building hobby and discovering the RC flying hobby. That was in 2008 and I had not flown or built since the mid 60's or so and that was U-Control with fuel motors.

Thanks again to Misters Kline and Fogelman!

Jack

Originally Posted by jackerbes

And some of those flat plate wind up planes only cost 29 Cents or so!

One of the things that first attracted me to the KF foils was that the FFF, in a single layer, was not very strong and that limited the size of the builds so I had not built any flat plate builds yet. But I also was sort of stumped on building materials because of a lack of a nearby LHS and the high cost of balsa. I really wanted to find a cheaper material.

When I saw the KF wing and read the Blu Baby thread I realized that it opened the door to making the foam stronger with spars and the like and gave it a try. And not only was it stronger it flew very well. And by this time I was reading the Blu Baby thread regularly and getting all the reports on how well it worked and how versatile it would be.

So I broke down and bought a bundle of FFF and got started on a very enjoyable building hobby and discovering the RC flying hobby. That was in 2008 and I had not flown or built since the mid 60's or so and that was U-Control with fuel motors.

Thanks again to Misters Kline and Fogelman!

Jack

And, all the people said, "AMEN!"

I would just like to add to what Jack Erbes said. It took a village of highly motivated people who contributed so much to the development of the idea of a stepped airfoil. The list of these people is quite extensive. Their creativity and imagination gave the KF airfoil concept wings to be accepted by people from all around the world. Also, their willingness to share what they learned with others. Personally, I will never be able to thank you all enough. This idea started over 50 years ago with a piece of paper that I folded up and tossed out of my office window, which overlooked the New York Public library and Bryant Park in midtown Manhattan. What a trip it has been.

You should be proud.

We're grateful you shared it with us. The KF family of airfoils created a whole new dimension to the hobby for us, lending itself to building amazing models quickly and inexpensively, and ramping up the fun factor of the hobby. It seems some have rediscovered the hobby because of your wonderful idea.

My FB and I are building this KF wonder presently.................. https://www.rcgroups.com/forums/show...n#post36012642

After watching the vids, how could resist?

Lots of praise for the wonderful KF wing design! I learned to fly on the Russ 40 design, it utilized a 48" KF wing and I learned basic aerobatics on this plane!
I have a question that the KF #1 might resolve - I goofed and built a little negative incidence and dowthrust into a flat 3D foamie, and I was thinking the KF #1 design might help with the incidence with a couple of washers under the motor mount to correct the thrust angle??? this incidence is maybe a couple of degrees at most, but it made this 32" foamie very touchy! Would up thrust have a negative affect on this flat foamie in this regard?
I sue would appreciate some helpful advice!

Dennis, a couple of degrees is quite a lot of incidence. Is there any chance you can rebuild the plane to get rid of it? You really need to get the wing parallel to the centerline of the plane.

Motor thrust you can mess with at any time by using washers, as you mentioned. But the accuracy of the plane's airframe needs to be sorted out to achieve a baseline for all your other performance experiments.

It might be easier to change the horizontal stab and play with the motor angles after you test the results.

the actual measurement with a straight edge down the flat part of the fuse is more like a 32nd ", at the most, but it made that plane crazy!
To reconstruct the nose section might be a possibility, but probably not! I have a better jig system now and that should not happen again!
the wing looks to have slightly less on the incidence though. I thought that KF piece on the bottom of the wing might negate some of the problem??
the slight incidence combined with the thin washer amount of down thrust on this very responsive plane is the problem! ( I think ?? )
Thanks