It was a sad day when I crashed my first KFm6 FPV Wing into the sea, so it was never going to be long before I built another…
Katana X KFm6 FPV Wing
The Katana X KFm6 FPV Wing is an evolution of the original KFm6 FPV Wing project. It takes it’s name from the short run of EPP wings I cut last year for a few guys here in Australia. It borrows a lot of the design elements and ultimately the size and layout from that wing.
Now, I know you like watching videos more than reading my endless prose so watch this first and I’ll rattle on for a bit longer afterwards. If you scroll to the end you may even find some free plans to download…
Design elements of the KFm6 FPV Wing
So it flies, and pretty well at that. I guess I should tell you a bit more about what I did to this one and why I did it…
The pod was something that came from the original Katana EPP wing. For that wing I had used a design drawn up by Adam at Skunkworx Hobbies. This time though I wanted something a little more low profile as I didn’t intend to mount a full size GoPro or Xiaomi Yi on it. I included 4 degrees of downwards tilt on the camera plates and 2 degrees of downthrust in the firewall as experience tells me these are good things to have on a wing like this. The pod was CNC cut from G10 and is assembled using standoffs and screws rather than glue. A main spar towards the rear and short spar at the front help to align and tie the wing halves to the pod.
Foam leading edges
One of the biggest drawbacks of building KFm wings with foam board is the leading edges. It is difficult to sand them to shape as the paper burrs and the foam tears. I had tried stepping the leading edge plates and then wrapping it in tape on the MicroWing. This worked on a small model but on the last KFm6 FPV wing the result was less than perfect and didn’t give a smooth finish.
A suggestion from Andrew was to fillet the gaps with Depron foam and sand that smooth as it is easier to work with. That works well but it is fiddly gluing the fillets in and the Depron is very fragile so it has to be protected somehow to keep the shape.
For this KFm6 FPV Wing I decided to try a new approach. I build the LE square but cut short by 40mm. I then used my newly acquired hotwire skills to cut some leading edges from XPS foam. I could have used EPP but realized XPS is considerably cheaper, quite strong and also a bit heavier (not a bad thing in front of the CG on a wing). These were then glued directly to the front of the wing and sanded a little to make sure they sat flush to the wing plates.
I just couldn’t stomach sticking on another set of Really Big Bunny Ears (RBBEs) and I had some thought about the aerodynamics of the vertical surfaces. Yes the RBBEs add a lot of side area and reduce waggle but they also increase sensitivity to gusty winds (negating the use of a KFm6 airfoil in the first place) and are prone to damage.
After some thought I decided to go the other route and make the winglets longer rather than taller. The theory is that by increasing the vertical area behind the CG it increases the moment arm of effect on stability. This is kind of like have a traditional fuselage and fin. The further you extend these winglets, the more stability you get (to a point).
Time will tell if I have a winner but early indications are these winglets are equally as good as the RBBEs if not better.
For the first time in a long time I also abandoned my widened elevons, but for a good reason:
Another drawback of these KFm wings, and also the flat bottom folded designs such as the Versa Wing is that the section has no reflex in it. A “proper” flying wing airfoil has a slight upturn in the elevons that creates a downward pressure on the trailing edge of the wing. This is needed to counter the effect of removing the horizontal tail surface and keep the wing flying. In these simpler airfoils we recreate the effect by adding a bit of up trim to keep the wing flying very slightly nose up.
For this KFm6 FPV wing I wanted to try something different. The KFm “step” works by trapping a bubble of air behind it. This curves the air around the airfoil creating a traditional Bernoulli effect and keeping the wing flying. My thought then was that if I put a step on the bottom of the elevon I would create an inverted airfoil, thereby recreating the effect of reflex on the wing.
What I can say is that this seems to have worked as when trimmed for straight and level flight the elevons appear to be at 0/0 degrees to the wing chord line.
I’ve been thinking about power distribution for flying wings for some time now. It is good practice to reduce the power cable runs as much as possible whilst still moving the receiver and VTx out to the ends of the wings to reduce interference.
With this in mind I made sure the pod had the facility to mount things like flight controllers and on this build actually put a simple mini quad PDB in. The BEC output from the ESC is then soldered directly to the PDB (where the battery cables normally go in) and then power runs out to the receiver, servos and FPV camera from there. The VTx requires a higher voltage so a power feed was taken directly from the battery supply and passed through a filter before snaking out to the VTx in the wingtip.
The result has been pleasing with only tiny amounts of interference detected in the FPV signal even at extended range.
I wanted something that looked the part and would last a while so I took my time covering with colored packing tape and wrapped the leading edge in reinforced tape to strengthen it up a bit. This paraglider inspired scheme looks great both on the ground and in the air and I’m really pleased with the outcome.
Al wires were cut into the top surface and taped to keep the airflow smooth. The original Turnigy 2826/1100 motor has now been replaced by an Aerodrive 2836/1500 and folding 10×5(ish) carbon prop fed by a Plush 30A ESC and 2200 – 5200mah 3S LiPo.
AUW with a 3300mah 3S and the Foxeer Legend 1 HD camera is 1.4kg. This includes a little weight needed in the nose to achieve the 180mm CG.
Flying the KFm6 FPV Wing
Well you’ve seen the video so you know it flies okay. In fact it flies better than okay – it’s awesome!
From the easy launch through to the stable handling in strong winds and on to the gentle landing with no fear of stall or spiral dive, the Katana X is exactly what I wanted in a KFm6 FPV wing. I’ve yet to push the range as the original motor was a little gutless and the new Aerodrive is as yet untested but I am hopeful for some long and enjoyable flights with this wing over the rest of the year.
The shaped leading edges do improve stability in pitch although it is still a little twitchy. This can be said of a lot of wings though and is a lot to do with the lack of tail. The KFm6 airfoil penetrates brilliantly although the trade-off is a steep glide angle with the motor off.
At 1.4kg it is a bit portly but this actually helps with penetration and stability so I’m quite happy with it. In fact there is some research to suggest that the effect of the laminar flow vortex (the bubble thingy) actually extends out behind the trailing edge of a KFm6 wing increasing the virtual wing area and providing more lift than a true symmetrical airfoil. Maybe that is why KFm6 wings shrug off the extra weight – who knows?!
So should you build one yourself?
Quite frankly, YES! This is a great wing and the lack of a stall, tip stall or spiral-dive-of-death common to wings like the Zephyr II and my own original Katana makes it a great choice for the new FPV wing pilot.
Wot no plans?!
I’ve removed the plan download from the article for one simple reason – you guys deserve the best and this wing could be better!
After replacing the motor and prop I’ve discovered that I needed a LOT of nose weight to get this wing balanced. I’ve added around 200g to the nose now and it is still a little tail heavy. Because of this I’ve experience some flutter at high speed and stripped some servos.
The Katana X2 is already on the bench with these problems ironed out. Stay tuned for more info and some updated plans very soon…
Over the next couple of weeks I am going to make the fuselage pod used on this wing available in the Red20RC store. If you don’t want to use this then a bit of imagination will be needed to make a blunt nose center section to hold your gear.
Whatever you do. Take your time to build it right, balance it well and have yourself some FPV wing flying fun!