KFm-6 Flying Wing

I admit that I am a lazy builder. As such, I am always on the lookout for new and innovative ways to build models that are both durable and perform well…

KFm Airfoils

The KF family of airfoils are nothing new. Originally designed 50 years ago by Richard Kline and Floyd Fogelman for paper airplanes. It was tested in the late 70s by NASA for potential full-size application but was ultimately rejected for poor lift/drag performance. However, due to it’s ease of construction and inherent strength the KF and KFm airfoils remain a staple of model aircraft construction.

Apparently the whole thing works by trapping a bubble of air (the “laminar flow vortex”) behind each step in the wing creating a fluid airfoil shape and thereby producing lift.

For my new wing design therefore I decided to try out the KFm-6 airfoil. This has a symetrical double step at 25% and 50% of chord and 6-9% thickness. With a wingspan of around 1000mm each 5mm layer of foam board would therefore provide the correct step thickness.

A new wing is born…

It took very little time at all to draw up a 3D model in Sketchup and then convert this into some 2D plans. I decided to take the majority of dimensions from the successful FPV Wing, based on the even more successful FT Versa Wing.

Although I had no intention of making this an FPV model from the outset I did retain a space in the nose for the GoPro as I knew from experience that it was good for balance and also the in-flight footage might be fun! I also used an extra layer top and bottom of the center section to make sure there would be enough space in the middle to cut a bay for all the electronics.

My friendly local printers put the drawings onto some A0 paper and it was on with the cutting…

Lots of cutting…

It seemed to take forever to cut out all the parts for this wing. I must admit that my initial thoughts were that this was going to be a very quick model to build but I didn’t consider that with 5 layers there is actually a lot of cutting involved. The only good thing is it is all straight lines!

The biggest pain was actually cutting out the bays for not only the battery and electronics but also the two fiddly little bays for the servos. Although the result looked good and worked well it is not a performance I would like to repeat any time soon!

Out comes the glue gun – or does it?

For this build, given the nature of the construction, I decided to try something different. I had a can of spray adhesive in the cupboard and a quick test showed that it gave a very strong and quick bond when sprayed lightly over both surfaces (without any bubbling of the paper).

Unfortunately, because of the shapes involved I found I couldn’t spray both surfaces without masking off areas and risking overspray so had to resort to coating only one layer. This turned out to be a bit of a mistake as the bond wasn’t as good and some peeling occurred later on that had to be fixed with the glue gun. On the plus side, for the most part the spray gives a strong and very light bond that is much neater than hot glue.

I had purposely ensured that the center plate joined in different places to the other plates so that the joints were stepped. With each joint taped on one side I folded the halves back and a thin bead of hot glue gave a nice strong seam.

Even with the offset seams, once I had cut the center bays I felt the wing simply wasn’t strong enough. Fortunately a recent rummage in the old camping gear had rewarded me with a collection of fibreglass tent poles so I pressed one into service as a wing spar. This only spans a small portion of the wing but provides more than enough strength for this airframe.

The top center plate was hinged at the nose with some old pin hinges and strong magnets at the back produced a nice full-length hatch, making access a doddle.

Fitting out

The center bay worked out really well and provides more than enough room for all the gear. The nose bay is large enough for a 1800-2200mah battery to slot in nicely.

I wrapped reinforced tape around the stepped leading edge to form a nice rounded section. Love that cammo tape look!

For power I used a Turnigy 2836/8 1100kv outrunner that was spare. This is turning a 10×4.7 folding prop that I have been desperate to try in a wing for ages now.

All up, and including a GoPro in the nose the model is comparable in weight to my FPV Wing so no complaints there. Even better, it balanced perfectly with no added weight.

I used an OrangeRX receiver to get started as they are just so cheap and so easy to use!

Off to the field…

Not only did I get to test fly this model within 24 hours of finishing the build but I even remembered to take my stand-in GoPro weight. This means you lucky readers get a maiden video with both head-mount and onboard footage!

I was just as surprised as everyone else at how well it flew! To be honest I couldn’t have asked for a better maiden flight. It was pretty windy but the KFm-6 wing tracked well through the sky. The performance envelope was pretty good although after a few minutes I did notice a few minor quirks:

• That roll at 0:27 was full stick. More throws are required on the aileron input.
• More noticeable on the FPV footage than the ground-to-air is the “wobble” in the yaw access.
• It could be a little “twitchy” around center in pitch.
• That final “landing” in FPV was actually a crash. I tried to pull out of a dive and just pancaked into the ground.
• That landing also removed the motor mount and made my GoPro dirty!

On the plus side, it has a good turn of speed (although not a great speed range) and I couldn’t get it to stall at all.

So, what caused the problems and what can I do about them?

The yaw “wobble” is a flying wing thing. With no tail and therefore no tail moment arm for stability it is up to the vertical surfaces to keep it tracking straight. In this case it appears that larger wingtip plates might be in order to rectify the problem.

The twitchy feeling around center in pitch I think is due to the symmetrical KFm-6 airfoil. When you first nose up or down the wing acts almost like a flat plate and “catches” the air as a positive or negative angle of attack is achieved. I’m not sure this is really a problem, or something that can be rectified; I just need to be aware for future flights.

That final crash landing is an interesting one. I can only speculate at what happened but I think that after a long dive with the power off, pulling the nose hard up actually caused the “laminar flow vortex” to separate from the wing. This meant a lot of lift over the wing was lost. Because I was so low I just didn’t have enough time for the power of the motor to restore airspeed and get the wing “flying” again.

As for the motor mount and dirty GoPro. I think I might fit some nice Coroplast fences to the center section to act as skids for future flights.

Conclusion

I’m pretty please with how this one turned out. I don’t think I realised just how much work would be involved in planning out and cutting something like this and at two full A0 sheets of foam it is not the most economical design.

I do like the neatness of the central bay and the strength of the overall design but this is a high-speed wing and as such I don’t think I will be putting an APM in it any time soon to fly some autonomous “missions”.

If anyone is interested I do have plans available – although they are currently incomplete so I need to clean them up first!