KF or Kline Fogleman airfoils are one of the most significant additions/innovations to flat wing park jets in my opinion. They can take a very simple flat wing and add stability, improved wind penetration, increased lift, increased speed and really help to give that "locked in" feeling if you want to fly your park jet with precision.
Much has been tested and written about Kline Fogleman airfoils, yet they still seem to be misunderstood or incorrectly applied in the park jet community. I am certainly not an expert by any means, but between Stephan and I we have done considerable field testing to dial in the correct KF configuration for our NAMC planes. I have also taken these findings across to other designer's planes and have had the same very good results, so it leads me to believe that for swept wing park jets we have a pretty good idea of what works and what doesn't backed up by thousands of flights worth of experience.
So how does a Kline Fogleman airfoil work? Well, essentially, here is a simple (simple is good for me) description from the Wiki link above.
"The purpose of the step, it is claimed, is to allow some of the displaced air to fall into a pocket behind the step and become part of the airfoil shape as a trapped vortex or vortex attachment. This purportedly prevents separation and maintains airflow over the surface of the airfoil."
This video hopefully gives you some visual idea of what is happening as air flows over a wing with KF airfoils.
Actually most park jets will fly quite well without any sort of KF airfoil, so if in doubt, cut them out, fly it without the KFs, then add them on and experiment later to get the feel and performance you seek. Without the KFs, you do lose some added lift, wind penetration and speed, so for some inexpensive foam, KFs can really add another level of performance to your park jet.
KF2 does seem to be a popular setup as some folks like their planes to feel "floaty" and have better slow speed characteristics. Fair enough, KF2 will give you that if that is what you seek. However, I know this flies (pardon the pun) in the face of what most folks say, I personally think and have found through hard earned experience that KF2 are not a good set up for beginners and here is why.
This is a bit of a repeat of what I have already said, but I'll drive it home anyway. KF2 causes the plane to "float", but also at times when slow have a mind of it's own. The first park jet I ever built was the F-35 V2 as you may have already seen in the intro post of this thread. I followed RC Powers advice and put KF2 on the plane as I didn't know any better and as I was losing control of my plane in turns, still didn't know any better as I put it down to me being such a new and not very good pilot. As my experience grew and I tested them further on our NAMC planes, I realized that compared to no KF or KF4, the KF2 made the plane feel very "mushy" and slow to respond in turns, in fact if I was to put a number to it, the response and crispness of the plane was reduced by about 20% over no KFs or KF4s.
So I ask myself and I guess I ask you, is that a good thing when learning? I personally don't think so, we all tend to panic and over control a lot when learning, especially if our plane doesn't respond when and how we want it to. I "over controlled" my F-35 V2 numerous times into the trees/ground, etc because it was very slow and sluggish to respond to inputs and would float/slide itself around turns. Then I would hammer harder on the controls, it would stall or go the opposite direction and normally a "walk of shame" would then ensue :/.
Although this video is using a flying wing, it is a swept wing like a park jet and I think shows how much more stable and precise the plane is with KF4 versus KF2.
I have flown planes with 30% KFs up to 60+% KFs and the results were not great. To keep it in context, from this point on I will be discussing KF4 as I think they are the best overall configuration for speed, balance, stability, wind penetration and precision for 99% of park jets. In all honesty, you are better to go smaller than larger. If you go smaller than 40%, you start to lose a little bit of the stability effect to the point if you make them too small, there really isn't any point in having them at all. What I have experimented primarily with in our development of the NAMC Mig-35B/Mig-FA as well as other designer's planes is when the KF gets too big and what happens to the plane. I found it easier to start off slightly large and then trim off foam as I went rather than start small and try to glue small strips back on. Obviously, I don't have wind tunnel data or video to back this up, but rather lots of hours spent at the ultimate park jet laboratory, the flying field.
What I have found happens is that the further the trailing edge of the KF goes in relation to the wing chord, flying straight and level it does become super stable, but as you start to maneuver the plane, this over stability starts to fight back with some very negative behaviors. Stephan and I have discussed it at length and our thoughts are is that as you manipulate the "bubble" of the KF further back, the center of gravity (CG) and center of lift (CL) start to work against each other and not in harmony. Back to something I mentioned in the last post, as you start to push a plane's setup too far in one direction aerodynamically, other aspects of the performance envelope are bound to suffer and this holds true with KF size as well.
Here are some characteristics I noted with KFs that are too large.
In turns, the plane almost fights back, becoming difficult to keep tracking straight, it can get "floaty" or "mushy" or just as quickly be too precise and twitchy. I know this sounds over simplified, but the plane almost acts like a toddler throwing a temper tantrum, just not a smooth or fun flight experience and very unpredictable at times.
Second and perhaps most significant is it can require much more effort to get the plane to pull out of dives or loops. Having the KFs too big really effects the ability of the plane to respond quickly in the pitch axis which if you aren't ready for it can lead to a walk of shame as your plane theoretically pulls out of the loop too low :/
A couple of other notes we found through testing KF4 airfoils. You can use up to the same thickness as the foam you built with, although slightly less seems to work best as it reduces the thickness of the wing and reduces drag. However, whatever you choose whether it be 6mm MPF, paperless dollar store foam (about 4.5mm thick) or even 3mm foam, use the same thickness top and bottom. Stephan and I have both tried what I have heard referred to as "KF 3.5" where the top layer is thicker than the bottom layer (6mm top and 3mm bottom for example). The claim is that it uses the best of both KF2 and KF4 properties. For how I like my planes to fly, I disagree as I find it is much closer to KF2 than a compromise of both and also causes even more imbalance between the top and bottom of the wing.
If you do decide to try out KF4 airfoils, another important note I have learned the hard way is to ensure that the trailing edge of the top and bottom airfoils are as close to even as possible. I know the angle of the picture below might show these as being off a tiny bit, but they are equal along the trailing edge. If either the top or bottom trailing edge is even a couple of mm further back than the other, it can cause your plane to unexpectedly dive or zoom as speed increases and the difference in the "bubbles" is amplified. If you do put KF4s on your plane and you find it has these tendencies as it accelerates, this would be one of the first things I would check.
So there is certainly lots to consider and to experiment with concerning KF airfoils on park jets. They are a very simple and inexpensive performance upgrade requiring some inexpensive foam glued in place and will transform your plane into a whole new "animal" at the field :) For testing purposes, you can even tape them on to try out, just don't affect the step along the back or you defeat the purpose of the airfoil. On the reverse side though as hopefully I have explained, they can also take your already decent flying plane and handcuff it, like putting a speed and handling governor on a high performance sports car, probably not something any of us wants to do :/
In the next and final installment, some final thoughts to wrap up this thread.