KFm4 MicroWing

This project was a spin-off from the KFm6 Flying Wing project. The plan was to design, build and fly a small foam board flying wing based on a KFm airfoil…

Dubbed the “MicroWing” (because I’m not going to keep typing “KFm4 Mini Flying Wing”), I wanted to make sure that the whole thing fit easily on a single sheet of foam board and could be cut and thrown together in next to no time whilst still be relatively flexible on gear selection.

Because of the small size a KFm6 airfoil would have been too thick so I opted instead for the KFm4 airfoil. This has a single step at 50% chord top and bottom to give a symmetrical wing. I used what seems to be a standard 30° sweep but raked the trailing edge in so that both leading and trailing edge are almost parallel. A third layer top and bottom in the center only was added for some extra thickness to support the gear.

You can download the plans for this one for free at Red20RC - RED20RC.com.au

Get Cutting and Gluing

I’ll be honest, there isn’t a lot to say here. There aren’t that many parts and they are all straight lines! I’ve been trying out these foam board cutting tools and they really are turning out to be worth the money.

I tried the spray adhesive method on the panels the same as I had tried on the KFm6 Wing. It worked even worse than before and I found I eventually had to peel them off and re-stick with smears of hot glue instead. The problem is the spray adhesive needs to be applied to both sides of the joint for a permanent fix. Without spending hours masking off sections of the wider panel though it’s just going to get messy.

I haven’t marked cutouts in the center section as I didn’t really have a clue where to put them! I just laid everything out and drew round the components. In the end I stuck the third panel on the top of the wing but not the bottom. I cut a hole right through for the Turnigy 800mah Nanotech battery. With the receiver and ESC I only cut down two layers to make sure there was still some strength in the center of the wing. Finally I stuck the bottom panel in place to form a floor for the battery compartment.

The servos again were recessed into cutouts in the upper wing panels and fixed in place with a drop of hot glue.

Finishing Up

Stick all the gear in and go fly the thing!

Apart from sticking the servos in the only other thing to do is put in a motor. I am using a Turnigy Park 300 1600kv with a 7×5 prop. I cut a small ply firewall and stuck it directly to the back of the wing. Apparently this setup with the 3 cell lipo will give 420g of thrust.

The last thing I did was to wrap some tape around the leading edge the same as the KFm6 Wing to give a nice smooth leading edge.

The All Up Weight came out at around 350g. It should have been a lot lighter but just like many other wings I ended up putting 75g of lead in the nose just to balance it out. Ho Hum, the things we do to make these things look awesome (I hate sticky-out fuselages on wings).

The balance point I started with is 164mm from the tip of the nose. This seems to be pretty good all round.

Up and Away!

I had to wait a long time for the first flight as the weather is closing in around here and it rained for two weeks solid. Eventually though the skies cleared, the wind dropped and the ground dried up enough to head for the field.

I’ll let you watch the video first and then wax lyrical afterwards…

So, this thing is bloody AWESOME! I mean, seriously, how many wings fly out of the hand like that on the maiden flight? No trimming, no fussing, just full throttle straight into the fun.

The KFm4 airfoil shows no bad tendencies and is rock solid in the air. I’m finding with the symmetrical KFm wings they don’t glide forever but they do slow up really well. If you look closely at around 1:30 to 2:00 I was trying really hard to stall the wing and it just slowed up and kept nodding its nose.

Conclusion

The KFm airfoil may not be ideal for full size aviation but it just keeps proving itself in model design, especially flying wings.

This small wing is that sort of model that can be built quickly and cheaply. It can be thrown on the back seat of the car and flown in a small area without too much fuss.

If you’re looking for a great little wing for some sunny afternoon fun then you can look no further.

Specifications

• Wingspan:  860mm
• Flying Weight:  350g (with the listed setup)
• Motor:  Turnigy Park 300 1600kv
• ESC:  Turnigy Plush 18A
• Battery:  Turnigy Nanotech 800mah 11.1v
• Prop:  7 x 5 plastic/slowfly
• Radio: OrangeRX DSM2 (those cool little $6 Rx units)