|Nov 05, 2009, 07:50 AM|
I think of the KF steps / notches which we use in building these sctatch-built foamie aircraft as 'wing structure elements' or as 'airfoil elements' - as one of many wing design elements which we can optionally implement when designing / building a wing structure. I personally do not think of these step / notch discontinuities as 'airfoils' per se, because there is so much else which is also going on aerodynamicly *in front of, and behind* these stepped discontinuities. The shaping of the rest of the wing's airfoil exerts major dynamic influences on the way that air moves over a wing. Vortex-trapping discontinuities, when implemented properly, can be useful elements in achieving the performance we might want to end up with from a given aircraft design.
That's my understanding of why we now commonly refer to all of the wide variety of step-discontinuity-modified-airfoil wings & aircraft as "KFm" - "Kline Fogleman modified", giving recognition to them. We say THANKS!
A stepped / notch discontinuity appears to act as a type of vortex-trapping element. (It can be desirable to trap vortexes for a variety of purposes.) Foamie scratch-builders are free to build these in a wide variety of ways and in various relative dimmensions / proportions, located in many positions on an aircraft structure (including using stepped discontinuities on the fuselage and elsewhere, as may prove useful- not just on the wing surfaces.)
When building wing structures with layers of foam, it's very easy to implement KF steps / notches. That's why those of us who participate on the 'Foamies- Scratchbuilt' discussion forum have experimented more to date with implementing stepped discontinuities than anyone else. But we can sometimes fall into the trap of letting the materials dictate to us, as to how we use them.
I continue to build prototype wings and aircraft to investigate and confirm my own concepts, just because I've always had this life-long fascination with things which fly, and with designing examples that do it very well. It's a fascination and an obscesion which I suspect many of us share to some degree.
|Nov 05, 2009, 08:09 AM|
Joined Jun 2003
My wing is a success! My spar is ½ tall in the middle to 0 at the tips. Fanfold is ¼ thick so actual root thickness is 1 inch and tip is ½ inch. Step is ¼ tall at the tips and ¾ tall in at the root. Wing chord 13 root 6 tip no sweep 31.75 span.
Cg 27.7% 2.75” from LE CG 40% = 4” from root le
My CG is around 53% or 5.25 “ from the LE. I glued the wing on too far forward. Anyway it does fly fine like this. Elevator is twitchy but only down but this is on high rates. May need more expo. The flat plate would not fly with a cg this far back. It was all over the place. This wing flies very stable. It does tend to pitch up and requires a tad of down elevator to fly level.
Overall I am impressed with this KFM2 wing. Roll rate is great rolls are nice and axial. Flies inverted well. Glide is good too. CG could be moved forward.
|Nov 05, 2009, 11:14 AM|
grant ...youve tried a deeper step than I would have used..my max so far is about 1/2" that was using 1/4" balsa spar and a 6mm layer of depron...
well done good info
|Nov 05, 2009, 11:25 AM|
You may want to figure out how to do some thinning of your thicker foam material for use in certain areas, in order to build an aircraft with a more efficient glide. (I even use a sharp fillet knife for some of my foam carving jobs, then do more sanding.)
I first started building foam wings with KFm type steps / notches for the Slim Beagle / Dancer series wings by using the RG15 and RG14 airfoils as their basis back in early 2007 or before. I designed and built the wings in that series, experimenting with wings from just over 7% thick to over 9% thick. The one KFm3 wing that I built that was over 10% thick (The "Hot Dog" wing for the Slim Beagle) had noticably poorer gliding abilities.
In late 2008 / early 2009, I designed the airfoil and wing for the Me163-e Komet starting with the MH32 airfoil. I refer to it as a MH32/KFm2. It actually uses a modest sized secondary step panel just behind the higher sections of the primary upper wing panel's aft edge for roughly the inboard third of each wing. The web pages for all of these designs provide more photos, build details, airfoil structure diagrams, and flight evaluations:
ImagesView all Images in thread
|Nov 05, 2009, 12:38 PM|
Some analysis of the KFm step concept.
San Le was kind enough to run an computer analysis of the KFm airfoil concept. Here is his email to me along with links to the study.
I'm finished with my analysis. You can see the video for
and the velocity magnitude at:
I have also set up a page of still images at:
I hope this will inspire someone to do a better analysis.
Here are some impressions I have which you can see on the
still images page.
1) There is no region of low pressure behind the step. This
is very surprising and good. As you can see behind the
tail of the airfoil, there is a very big low pressure region
but the step seems to have the same pressure as its surroundings.
So the step does not add to the drag. We may have hoped for
a positive pressure there, but that may be optimistic.
2) For the Mach Number, which is very similar to the velocity
magnitude, you can see the blue region which seems to mirror
the shape of lower part of the airfoil. This seems
to be the stagnant air we expected. So this may
act as a region of still air which acts as a virtual upper
airfoil that has less friction than an actual solid upper
region. This may have the effect of reducing drag.
On the other hand, if you look at the analysis I did for a traditional
you would see this same stagnant air at the back-top, so
I'm not sure how significant this is.
1) I don't see lower pressure above the airfoil. The top and
bottom look the same. This could be due to crudeness of my
meshes, or that my software isn't solving an appropriate version
of the equations for airfoils.
2) I tried to do a streamline plot hoping that there would be circulation
at the back of the step. But this compressible flow doesn't capture
it well, like my incompressible code.
Again, I am not very confident about these results. At best, they
are a very preliminary starting point for someone who can do a real
calculation. Someone should also do an analysis with a similarly
shaped airfoil without the step for comparison.
|Nov 05, 2009, 03:44 PM|
Nice going Grant and thanks for your input. Keep at it. The KF approach is very worth while. Please post pictures and video if you can.
Lee, I always enjoy your inputs and comments and your tutorial video on construction of the Roswell has done so much to improve and simplify my building techniques with the soldering iron recesses for servos and other gear and the razor knife slits for wiring. My planes are looking much better now and the build time is actually lower, if you can believe that!
Bruce, I'll check out your links when I get back from Tagbilaran. Things that work for you at over 10,000 feet should be even better for me at sea-level as long as I stay out of the water. May even try to curve the wing tips like you have done in lieu of flat plate tips.
Any of you using the Chewing Gum cam please post pictures and/or more video. I'm trying to order two of them from a seller in HK. I used to be into the spy cameras years ago. I had a few of the Minox cams in my collection before I moved to the Philippines. Has anyone tried this Turnigy camera for AP?
6:30am and no wind so I'm off to the field for a bit with the new Pagan. Wish me luck. I want to see how she does with a bit of crank and bank.
|Nov 05, 2009, 06:34 PM|
A search on YouTube for "gum pack camera" will bring up bunches of hits in numerous uses and environments including a lot of RC. Here is one of mine, you can see the others I've posted from the "More From JackErbes" link:
I don't think the Turnigy camera is any better than the gum pack was at $20 or less. It looks like a bigger package and maybe a better protected lens but I'm not sure.
I've not flown in almost a week because of chores and the weather. We got our first visible snow today. It was an 18" to 24" snow for the most part. This early in the season we measure the distance between flakes in the air as nothing measurable can be found on the ground.
You remember snow, right? Do you know what a snowblower is? Here is a photo of mine:
You'll have to come up and visit sometime, bring your snorkel and fins too:
|Nov 05, 2009, 08:39 PM|
I remember snow. It's white. Right? But then sometimes it's yellow and you don't mess with it if it's that color! It's coming back to me... Slowly! I think I like the way we snorkel here better and I can see why the early Indians called us "White" men!
Flew the Buzzard in wind yesterday and it performed fine. Here's an example.
Then this morning I got in a few flights with the Pagan to test the performance and agility. It's OK in my book. A good sport plane but not 3D .
Overall a good flier. I'd like to try it with a KFm4 wing just to see the difference.
|Nov 05, 2009, 08:44 PM|
The maiden flight of the Jupiter Project.
Paul of RCFoamFighters has posted a video of the maiden flight of his cargo plane, which will be testing the lifting capacity of the KFm2, KFm3 and Clark-Y airfoils. Needless to say, the maiden flight went extremely well. He clocked the Jupiter at 74 mph on his initial flight. The outcome of these test should be very interesting...
|Nov 07, 2009, 06:38 PM|
Voyager ready for test flights
The new prototype 'Voyager' is ready for it's test flights.
This aircraft implements a KFm3 variant (double step on top) lifting airfoil. The steps on this wing build are limited to 6mm in height or less. Part of the objectives of this experimental prototype are to evaluate the lifting characteristics of this airfoil build, as well as to evaluate this wing's glide efficiency (lift versus drag) in power-off glide mode. This is the lowest thickness KFm3 build I've done to date, as far as the % of chord, so it will be interesting to see how it actually performs.
Wingspan is 36", built with minimal wing tip plates. Average chord of the tapered wing is 10", for 2.5 square feet of wing. Flying weight, as shown in the photos, when carrying a 3S 1000 mAH battery pack is 16.5 ounces right now, for a wing loading of 6.6 Oz./ Ft^2.
The Voyager also implements KF step structures on the sides of the upper fuselage- square steps at the rear edges of the forward fuselage doublers. Servos are buried flush with the surface of the three-layer FFF forward fuselage for minimal drag. 1mm solid CF rods are used in several places, inset flush with the surface of the foam, to provide stiffness and structural strength; they are inexpensive and very light weight, and very effective when installed in opposing pairs.
For initial test flights, the flight battery is temporarily mounted with velcro on the right side of the upper fuselage, until final ideal flight CG for this particular KFm wing build / airfoil is defined. Then the fuselage will be modified to carry the battery within the fuselage on the lateral centerline for better lateral balance and best handling across a wide speed range.
Since the later mounting of the set of full floats (with their added area in front of the CG) does require additional vertical stabilizer area for good stabile handling, the vertical stabilizer and rudder are quite large on this prototype for now. The nice thing about designing with FFF is that it can be easily modified & morphed!
There are 29 photos and more information in the build log on the Voyager's web page, for those who might be interested:
UPDATE: The Voyager is flying very well! I'll post details to the Voyager's web page soon. (The first videos did not turn out, so I'll see about shooting more soon.)
|Nov 08, 2009, 01:35 AM|
Joined Jun 2009
SkyFly on KFm3
Thought you might get a kick out of this video. I'm using the pod and boom from the SkyFly and the rest is custom. I decided to use this bird to test out the different KF airfoils.
This one is flying with ailerons. I had built one wing without ailerons and did not like the way it flew. It felt like I was flying a sailplane.
I bet it would carry that little gum pack camera around without any problems.
Anyway, I could only get video of one flight. Lots more to come...
Drawing showing wing build.
|Nov 08, 2009, 10:12 AM|
That little bugger really moves out! You gave your camera person quit a workout there.
That is pretty far removed from the starting point but when I did the same kinds of things it really helped with the learning curve on both building and flying.
My Sky Fly's pod finally got to the point where it was beyond practical repair and I moved on to other things. And at that point the pod was about the only thing that was original.
Thanks for sharing that.
|Nov 08, 2009, 01:19 PM|
North Kent, England UK
Joined Jan 2005
Steve, re the Turnigy mini cam, have you checked out the reviews on that page?
I ran this
and the quality looked ok though I don't know much about mini cams. I presume that it would lose resolution if blown to full screen, perhaps someone with experience could advise.
Just realised I could view full screen, a little loss of resolution but ok.
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