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KFm-7 Stepped Airfoil 165cm span Delta Wing + Maiden Flight

Foam board construction

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Chuck Plains12/06/2016 14:40:30
1096 forum posts
244 photos

This really flies easily!

Maiden Flight FKm-7 55deg Delta Wing Dollar Tree type Foam

The design was copied and +60% from Dutch RC Adventures on Youtube.


I was dubious about the stepped airfoil idea as soon as I read about it and saw some pics. Having set my sights on gliding etc, I could see all the reasons for arguing about drag and lack of efficiency and didn't listen to the praises that were being sung by more than just hundreds of scratch builders who just wanted to get flying as quickly as possible.

Yes, there's more drag than a 'proper' airfoil, but for just having a nice time and stooging about, without having to first worry about the niceties of a build, I promise you, this really does fly easily.

KFm-7 55deg 165cm delta wing.jpg


It's quite heavy, at 1.3 kilos without the battery. Bu it ignores that even with a merely sufficient motor and prop.

I haven't made a flight video before and only have a phone attached to my hat, so bear with the low quality please.

The construction is relatively simple, but I made it a tad more complex by deciding to make it as bg as reasonably practicable. ( I first used "reasonably practicable" when I was a TGWU shop steward ).

The construction is 5mm art board and 6mm underfloor insulation. And although it does actually fit in my car, all passengers deleted, I spent a bit of time and made it a flat pack plane. It would fold at the time I made the vid, but the vertical stabilizers would not. Now they do.

The inclusive angle at the nose is around 55 degrees and the span ended up at 165cms. I don't have a drawing, but you really don't need one for this kind of malarkey.

Here's a screenshot of the CG calc dimensions.

55deg delta cg numbers.jpg

My first top step is approx 35% back from the leading edge and the second step is approx 60%. The step on the bottom is approx 40%

The vertical plates are just over 30cm from the center and just what was available from an offcut and what I thought might look nice at the time.

Believe me, it's as simple as it sounds. I glued everything with carpenter's polyurethane. The colour it packing tape and all edged apart from the trailing edge are protected with glass reinforced tape.


Info on the motor pod coming next.

Edited By Chuck Plains on 12/06/2016 15:15:59

Chuck Plains12/06/2016 15:08:56
1096 forum posts
244 photos

The Motor Pod.

3mm Liteply all round. The wood was glued with CA and the foam pieces glued to it with carpenter's ployurethane glue. I've started using Geocel Carpenter's Mate. It only expands about 1/1 instead of Gorilla's claimed 3/1

KFm-7 wing motor pod.jpg


The gap matches the thickness of the trailing edge at the center and the vertical 'key' engages with a matching slot in the foam. It may not seem particularly robust due to the foam, but it seems sufficient so far and all the motor does is push against the back after all.

KFm-7 pod alignment key.jpg



KFm-7 pod keyway.jpg

The foam 'spacer' matches the center of the trailing edge.

KFm-7pod internal angle.jpg

Edited By Chuck Plains on 12/06/2016 15:09:49

Essjay12/06/2016 19:00:02
578 forum posts
1 photos

Just an observation, and I may be talking complete botox, but when you were gliding (motor off) everything seemed to settle down. It only seemed to dive when power was applied.

Could it be that you need a little more 'downthrust' on the motor, bearing in mind that would mean shimming the 'lower' mounting screws as it's a pusher.

Just a suggestion, and as I say I may be talking rubbish.


Chuck Plains14/06/2016 20:52:10
1096 forum posts
244 photos

I thought so too and added a 2mm spacer between the motor plate and the ply mount, but it didn't seem to make much difference. Although, Sunday just gone was 20mph winds whereas the previous was about 5, plus I used a 9x7 prop instead of the 9x6 I used previously.

But I'm thinking now that I need some reflex in this airfoil as, when I finished flying this time I had dialed in nearly 15 degrees of up elevon. I does mean I'll have to remove the vertical fins temporarily, but it can be done, just with a slit on the underside and some glue to fill the gap when I bend the trailing edge up a little.

I've already decided that a 3x thicknesses of printer paper (0.3mm) will be my spacer to open the slit for the glue. That will be applied right across the span from about 10mm ahead of the elevators. So the last 70mm of the trailing edges will be angled up about 5deg.

Experimentation is fun anyway! laugh smile d

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