Thursday, January 25, 2018

Which Park Jet for me? - Part 11 - KF (Kline Fogleman) airfoils on Park Jets

Hi Everyone -

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 (NAMC chief designer) 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."

There are numerous varieties of KF airfoils as shown in this picture below, I will be discussing KF2 and KF4 in this post as they are the most popular for park jets. I know it is difficult to read the description below each one, but looking around Google, you can probably find much more information about the more exotic or complicated setups.😊

This video hopefully gives you some visual idea of what is happening as air flows over a wing with KF airfoils.  

The video above shows a straight wing where the normal rule of thumb is that the KF extend to 50% of the total chord of the wing at the root and tip.  Through considerable testing, experimentation and some discussion with Scott Lott and Dave Powers (Scott Lott was the primary designer of all the RC Powers planes up to and including the V4 lineup), once you start to sweep the wing even slightly, this number decreases to about 40%.  So if you are adding KFs to a plane where the designer has not included them or you want to modify existing KFs on a plane, 40% is a very good starting point for most swept wing park jets, even a moderately swept wing like the RC Powers F-18 V3.  At about 0:55 in the video below, you can see/hear Dave Powers discuss the 40% starting point which just seems to work well on almost every wing configuration I have ever tested.  

With our NAMC planes, we actually took this a bit further as we fine tuned things at the field, our KFs are very close to 40% at the wing root, tapering down to about 35% at the wing tip, but if you are playing around with KFs on a plane, 40% of wing chord overall is a very good starting point.

A few things of note from Dave's video that my experience has taught me.  While I agree to a certain point that the addition of KF airfoils do help strengthen the wing, I would never use them in place of recommended carbon or other reinforcement in a wing, there will still be too much flex.  I have found that when really dialing in a plane for best precision and performance, that a few mm between trailing edges top and bottom when using KF4 do make a difference in how the plane performs.  Having said that, you can certainly trim KFs down to really optimize the plane for how you want it to feel as we all seek different feel and performance from our park jets.  Lastly, we found through testing that KF2 while popular with RC Powers and others degrades the precision and handling of your park jet by about 15-20% as the increased and unbalanced lift cause the plane to "float" and the controls to be "mushy" and very slow to respond in turns if you get too slow.  I'm not saying your plane will fly "horribly" with KF2 airfoils, I'm simply passing along what I have seen in extensive testing with many KF configurations.  More discussion on that later.

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 RC Powers F-35 V2 (no longer available) 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. 
So getting back to why starting the KF depth at 40% of total wing chord at the root and tip and why it just seems to work and why getting too much less or too much more than that doesn't.

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 or decelerates, this would be one of the first things I would check.
Picture
You may also note that the leading edge has been shaped so that it is symmetrical.  I have used a more "chisel" type of leading edge early on in my park jet career and found it to cause a lot more drag and instability, so having a symmetrical leading edge like in the picture seems to work the best for overall performance with KF4 airfoils. There are also arguments that having the leading edge rounded works best. Honestly I have not tried that, perhaps because the setup above has always worked for me and I don't want to mess with success😉

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. Not to "name drop", but when this article first appeared on the old NAMC blog, I was honored that Dick Kline himself (The "K" in KF) commented on that post, I have pasted our conversation below as a post script.

Park Jet noise...the "other" sound of freedom😎
Cheers,

​Scott

Chat with Dick Kline

Scott... Enjoyed reading about your research on the KF concept. KF airfoils are designed for power because it takes energy to create the vortex. With the vortex employed, you have excellent control over your aircraft. Thank you for exploring this subject. Did you ever complete your final installment?
~ Dick. The K in KF
Reply
3/8/2017 01:07:36 pm
Hello Dick -

Very honored that one of the inventors and innovators of such a simple yet amazing addition to model airplanes would find this article. Thank you so much for your kind words and response. Very interesting insight as to how the power affects the vortex, KF4 have certainly been one of my absolute favorite addition to foam park jets, so I like many others am in your and Floyd Fogelman's debt for this invention. I did write a wrap up article to this thread about "Which Park Jet for me", you can read it here http://www.migsrus.com/blog/which-park-jet-for-me-some-final-thoughts

All the best to you and Floyd and thanks again for taking the time to write to me.

Cheers,

Scott
Reply
3/8/2017 01:19:42 pm
Dick -

Sorry in my excitement I forgot to make a very important acknowledgement. Much of what I know and have learned about the proper thickness, depth in relation to wing chord, etc about KF airfoils on foam park jets has been the result of hard work, knowledge and testing by my NAMC partner Stephan Moran. He is the scientist and chief designer behind all our NAMC airplanes to date and I have learned an incredible amount from him and through lots of fun flying and testing at the field.

Again, all the best!

Cheers,

Scott
Reply
Dick Kline
7/29/2017 02:43:33 pm
Scott... it is great to acknowledge Stephan Moran for his contribution to your knowledge base. He also showed that he believes in you which gives you permission to believe in yourself. We ALL need that in our lives when we are young. And, we are all creative because creativity is all about problem solving. Always acknowledge your own creativity to yourself. It empowers you to be even more creative. 
~ Dick
Reply

No comments:

Post a Comment