There’s nothing new about building a flying wing with a KFm airfoil and stuffing it full of FPV gear. I did however have some very specific reasons for doing so this time…

The KFm airfoils have been proven time and time again as solid performing sections in RC aircraft and flying wings in particular. The FPV49 by FPVTrond has used the KFm airfoils successfully for some time. Even the original Sensefly drone – the Swinglet Cam – used a KFm airfoil in its construction. Sure, they have some issues but in general they are fast building, high performing wings.

Okay smarty pants, why did you build a KFm6 FPV Wing then?

In a world where the (surely now legendary) blunt nosed Versa Wing is the perfect choice for any scratch builder looking for an FPV design, I chose the KFm airfoil for one reason initially – it’s always f#$*ing windy around here!

I chose the KFm airfoil then because I knew two things:

1. I didn’t mind if it wasn’t that lightweight
2. I know the KFm airfoils can fly fast and penetrate well in windy conditions without too much of the dreaded yaw “waggle” that can ruin otherwise great flight videos

Pulling the design together

I used the original KFm6 wing project as a starting point for this one. I still don’t know why I didn’t pursue this design in the first place but hey-ho. I’m guessing those pesky miniquads got in the way somewhere along the line!

I stuck with the KFm6 airfoil with steps at 25% and 50% chord as I knew firstly that it works and secondly that it would give me enough thickness in the wing to hold a decent size battery without it sticking out too much and causing drag.

It seems that a wingspan in the region of 1100-1200mm is optimal for these wings as that gives the correct thickness airfoil if using 5mm foam board. It’s also a nice size for transport and launching whilst still being beefy enough to handle full size FPV gear and a GoPro style camera for recording.

I widened the center of the wing to accommodate a large flight pack and also configured the nose section to fit my own CNC cut GoPro/FPV camera bay. I included a short fiberglass spar for some added strength in the middle but left the rest pretty much open to on-the-spot modification so FPV gear could be slotted in wherever.

This then is what I ended up with…

That’s probably all you need to know except for the fact that it is entirely made from 5mm foam board and there are 5 wing layers plus an extra two in the center (top and bottom) making it 35mm thick in the center and 25mm thick at the leading edge.

Building the KFm6 FPV Wing

The great thing about building a KFm wing is that it is all straight lines and no fancy folding. The parts were marked out on a couple of large sheets of foam board in no time and the major components cut shortly afterwards. One thing I did do that you can’t see on the picture above or the plans is to cut the leading edge of each layer 5mm shorter than the layer below giving a stepped profile to the section.

I knew I was on to a winner as soon as I laid the components out on the bench to get a feel for the size.

The next thing I wanted to do was give it a bit of color and protection. I’d painted foam board before but it takes too long and is messy and smelly. I’d seen lots of people using tape and it just so happened that I had a number of different rolls of cheap packing tape in the cupboard. It turns out this was really easy to do using a rubber tiling trowel to smooth the strips into place. I did each layer separately with colors that took my fancy and soon it was looking great…

The next job was to measure out the gear and make a few holes for the bigger items. The battery bay goes through all but the last layer (and has a thin ply plate at the bottom to strengthen the area). The Rx goes through two layers, whilst the servos and VTx only one.

Gluing KFm style airfoils together has always been a stressful time for me. My first wings were always hot-glued but this was heavy and could leave gaps in the layers if you didn’t get it on thin enough. For a more recent project I tried spray adhesive. This was no good though as it is messy and really needs to go on both surfaces to get a good bond. With this one then I tried something different – glue sticks!

Does it work? Does it ever! It’s easy to apply and sticks like, well, glue! There were only one or two places where I didn’t make it to the edge and a little hot glue here sorted things out. The result was a solid wing, glued together with minimal mess and maximum strength. Just remember not to put the tape over the areas to be glued!

In no time at all I had a wing ready to be kitted out…

Fitting out the KFm6 FPV wing

Whilst I’m not an extreme long-range FPM pilot I did want this wing to maybe stretch the limits a little bit. The only other restriction I did have was that I was going to be using equipment that I already had in the stockpile rather than buying expensive new gear. With that in mind I ended up with the following parts list:

Propulsion

• Turnigy Park 480/1320KV motor
• Turnigy Plush 30A ESC
• 10×6 folding prop
• 3S LiPo – anything from 3300mah to 5200mah

Control

• FrSky L9R long range 2.4ghz receiver
• TGY-S712G wing servos

FPV

• Turnigy IC120-NH Micro CCD camera
• ImmersionRC 600mw 5.8ghz VTx
• ImmersionRC Spironet antenna
• HobbyKing G-OSD3 on-screen-display with GPS
• FatShark power supply filter

CNC cut components

One of the nice things about owning your own CNC is that you can solve many problems with the odd custom cut component. For this build I identified early on that I wanted somewhere secure for my FPV camera and GoPro along with a nice strong mount for the oversized Park 480 motor.

The camera mounting box was something I had worked on previously. It followed the lines of the old TBS camera bay but swaps the GoPro and FPV camera around. The reasons for doing this are that by doing it this way the two camera lenses are closer together and both are as close as possible to the centerline of the wing. This improves not only the FPV experience but also gives a slight improvement in recorded footage as well. I had previously built these boxes in 3mm ply but stepped it up to G10 fiberglass for this wing as I knew I would need the weight up front (and the extra strength can’t hurt!).

The motor mount was custom designed to fit this wing. I measured it up so that the drive shaft would sit directly on the wing centerline both laterally and vertically. The two horizontal plates sandwich the foam board structure and are locked into place with hot glue. This is a very strong mount and has really helped to reduce vibration.

In both cases, the tolerances of the CNC were so good that the components are a push fit. It only takes a drop of cyano here and there to create solid, accurate parts.

I did also cut some 1mm carbon prop-stoppers although these haven’t worked quite as well. Of course, carbon is very stiff and doesn’t really flex so it has cracked already under stress.

Fully fitted out and with the GoPro and LiPo in place the balance point is at around 152mm from the leading edge of the camera bay. The flying weight is 1.3KG, which came as a surprise as I really did think this one would be a lot heavier!

Finishing off

Given that this was a prototype I didn’t want to spend too much time making things neat. Some reinforced tape here and there secured the wiring and more tape wrapped around the leading edge gave a smooth(ish) section to the front of the airfoil.

I’ll be going back in the future and cutting some channels in the foam to clean things up. I’d also like to rebuild the leading edges with sanded depron fillets as used on the MicroWing UAV.

Into the air!

A video speaks a thousand words here so take a look and we’ll talk more afterwards…

I think it might be love!

From the very first launch through to landing the KFm6 FPV wing tracked perfectly. There is no discernible yaw “waggle” and the HD footage was awesome without any post-flight stabilization.

It’s only vice is the same as that of any KFm airfoil I have ever built – it just doesn’t glide very well. The stall is non-existent with no wing-drop I noticed but power off and the wing will loose altitude faster than you might expect. This is most apparent in low turns and on landing where careful throttle management is required to maintain altitude and bring it in safely.

But does the KFm6 wing perform in those windy conditions? You bet it does! The very first FPV flight was conducted in strong gusty winds and it handled them with ease. Flying at altitude into a headwind I recorded a ground speed on the OSD of only 25kph. After several minutes I turned around and was back to the field in under 30 seconds, traveling at a whopping 100kph (at only 40% throttle!).

That Park 480 motor and huge prop are power hungry but with careful throttle management, 20 minutes of cruising on a 5000mah pack is realistic. Given the wing was lighter than I expected it would even be possible to widen the battery bay to the rear and load two 5000mah packs in parallel.

So, can anyone build one of these then?

I’m not the best at drawing up plans but I do want to make the drawings for this one available! 

KFm6V2.skp

If you do want to give building one of these a go then it really is simple. The dimensions aren’t really that important but as long as it turns out around 1100mm span you should be okay. I’m not entirely convinced that the spar was necessary. If you do want to use one then just cut a slot for a 5mm fiberglass or carbon rod in such a way that the layer two panels will cover it completely.

Conclusion

The KFm6 FPV Wing V2 is everything I had hoped for and more! It’s performance can be compared with any other large wing I’ve flown and it seems to be tough and reliable enough for some extended FPV missions.

You do need to be aware of the glide issues as mentioned above. I think because of this the KFm6 wing may never be suited to extreme long-range flights but for the reason I built it – getting out and about in windy conditions – it is just perfect.