Kwik Fli 3 - 50th anniversary build

[Simon Feather]

Some of you may have realised, if you were following the Mass Build 2017 chat thread, that 2017 is the 50th anniversary of Phil Kraft's Kwik Fli 3 winning the 1967 world championships.

This seemed too good an opportunity to miss doing a tribute build, and since I was considering building a retro aerobat anyway, the decision was effectively made for me!

So, here is my blog showing progress so far; updates as and when I can. Hopefully I'll get this done reasonably quickly as I don't want to be too late starting the 2017 MB.

[Simon Feather]

Firstly, here's the power plant I plan to use:

The model will have been designed for 1960s 60-sized engines, so putting a modern OS 61FX up front should give it plenty of oompf, even with the extra-quiet silencer I shall have to use.

Here's a general view of my "shed" for anyone who's interested - actually, a vaulted roof cellar under the kitchen - with the tailplane on the bench.

[Simon Feather]

Now some shots of build progress so far. Most self explanatory, I'll add notes as necessary.

I have two plans for this model: a version redrawn in CAD by Richard Browning, and a Graupner version in German, which looks like it may have once been part of a kit. I'm building from Richard's plan, partly as it contains drawings for both wings (luxury!) but referring back to the Graupner plan for anything that isn't clear.

Tailplane part built and complete:

I've opted to use a wire "U" shaped joiner for the elevators, rather than the dowel specified on the plan but otherwise no changes.

Rib templates, cut out on the scroll saw:

and the resultant rack of ribs:

easy, since this is a constant chord wing.

[Simon Feather]

and a little bit of effort later, we have a wing kit:

... which goes together like this:

Tube for the aileron wires is a thin strip of brown paper wound round 15mm copper pipe in a helix, then glued and doped for stiffness, and I drilled a hole in the innermost ribs for this by clamping all the required ribs together and drilling through them with a 16mm spade bit in the pillar drill. Sacrificial wood top and bottom, don't go too fast, and the result is as neat a hole as you could require.

I decided to elongate the main spar dihedral brace for a bit of additional support: this is now almost twice as long as specified on the plan.

.. and here's a wing almost there. The spar webs also were not on the plan, but I thought for the minimal increase in weight they would add some useful stiffness.

Oh, and here's how I aligned the D-box half ribs: the square wouldn't fit in, but a quick raid of the toddler's toybox produced square guides that DID fit

More later: I've been sheeting and decided to start this blog whilst glue was drying. Let's go and see if it's ready for some more ...

[Tony Cook]

​ love the classic ones!

[Simon Feather]

Oh, btw - Richard's plan says the KF3 was the Nationals winner in '67 and won the World Championships in '68 - it still makes it a 50th anniversary build IMO

[Paul C.]

Looking good Simon 👍 I like the 'top tip' to use brown paper for the aileron wire tube bit to late to use it on my flea fli 😢. Saving myself for martyn's 40 size electric version.

Paul.

[McG 6969]

Nice work, Simon.

I see you found the way to the 'building blocks' as well...

Cheers

Chris

[Martyn K]

Hi Simon

Have you sheeted Top and Bottom of the tailplane?. If not I recommend that you add an additional brace. I folded the tailplane on my KF4 (full size)

Martyn

[Simon Feather]

yes, top and bottom are sheeted, and I also added some packing in the centre where it mounts in the fus. It seems very strong...

[Simon Feather]

So... the wing is done and sheeted, apart from building the servo boxes, wingtips, ailerons and final sanding (note the currently rather square leading edge! I *like* sanding leading edges, very satisfying):

TE sheeting is bevelled and goes right to the edge, which makes for a really tight and smooth TE - see closeups

Fin is also done - plan called for solid 1/4" sheet but to save a little weight I've used the same construction as the tailplane i.e. a sheeted frame with solid tapered rudder.

Onto the fus tonight

[john stones 1 - Moderator]

Nice tidy work Simon

[Martyn K]

Don't get too enthusiastic rounding that LE. The original had quite a sharp entry do it stalled properly.

[Simon Feather]

You're right, Martyn - just look at the LE on this photo from the MAN article:

[Simon Feather]

so, a little more progress but not as much as hoped... taking advantage of the weather at the weekend to go flying and doing childreny things. Fus kit is cut out, doublers glued to sides, and internal formers in place. Slow epoxy on the straight section:

Then into the jig to get the taper right. Firewall on tomorrow when the glue's dried then onto the internal trappings.

If you know the KF3, the eagle eyed may notice the front looks a bit short: the original had an integral cowl built as an extension to the fuz sides, but I'd like a bit more access to the engine than that gives, so my fuselage ends at the firewall, and I plan to build a removable cowl either out of balsa or fibreglass,

[Simon Feather]

at least I know this model should fly well, since it's just drawn first blood.... stabbed myself with the end of the razor saw! more pics later hopefully...

[Simon Feather]

coming together now; f/w fixed engine and nosewheel mountings sorted; nosewheel fixed temporarily just in order to sort cable runs; snakes in and servo mounts in the fus sorted; and the wing mountings fore and aft sorted on the fus:

Here's the wing mounting plate and the subsequent packing - doing this the same way Peter Miller did on the Ballerina, as that worked very well.

and here's a not-very-good picture of the wing after sanding the leading edge profile: tried to show the pointiness of it but it hasn't come out very well. Hopefully this meets with Martyn's approval!!

not much more to do now on the fus, then I'll go back to the wing to sort out ailerons and wingtips and servos, then I can start putting it all together.

[Martyn K]

Looks superb Simon.

How did you clamp the trailing edge halved together while the glue dried? Mine always ten to waver slightly - possibly because I use PVA

For the record, Phil Kraft won the 1967 World Aerobatic Championships - Held at Corsica

Here is a link

1967 w/c report

So it is a genuine 50th anniversary link

[Simon Feather]

Nothing special, Martyn! just normal spring clamps and scraps of wood to protect the TE; I don't remember precisely which scraps I used, they may have been bits of ply rather than balsa. I think the key was making sure the bevel was even and a good fit before gluing. I use Evo-stik green woodworking adhesive, it's not the cheapest but it has a quick grab and good joint strength once dry, and I've only once had an adverse effect with it (I learned the hard way, on the Ballerina build, that it is NOT the best stuff for gluing ply doublers to fus sides, unless you want nice curved ones as it wets the wood too much used over large areas. Contact adhesive is the stuff for that).

thanks for the link, that's good news and very interesting!

[Simon Feather]

Hit a minor snag on the fus as I realised my M2 threaded pushrods won't screw into my Sullivan 2-56 snake inners, so there will now be a slight pause on the fus whilst I sort out appropriate supplies to deal with the metric/imperial mismatch. At least I didn't miss Mars.

So plan is tonight, after children are abed, I'll move back to the wing and ailerons. The ailerons are top-hinged, so it seems obvious to build them upside down, which of course means putting the top skin down first... which of course means getting the bevel right, or at least nearly right, first. I've also been giving some thought to hinges. I don't fancy relying on the covering on this model, so my current plan is to use barbie hinges set into the wing and ailerons at an angle (so I can strengthen the joint) and go from there.

[Simon Feather]

Long story short... IT'S FINISHED! I'll upload more build pictures when I get chance, but in the meantime here are some pictures of the finished article. The sun was rather bright down at the field yesterday so the orange covering has come out rather yellow in some pictures...

Here's what I was aiming for:

And here's mine. Not maidened yet, because the weather was a bit blustery and crosswind to the strip, but I did take it down for engine running trials and hopeful for aviation soon.

[Simon Feather]

Dry weight came out at 6 1/4 lbs, which I'm quite pleased with - with a modern OS61 on the front it ought to perform quite well....

[Simon Feather]

Today's been much better weather-wise so I've had the opportunity to maiden the KF3.

It’s always very satisfying when a maiden is uneventful… .... the ground handling is superb, it turns on a sixpence; and after a longish run to make sure I'd got enough speed slight pull back and off; it needed a few clicks of up and that was all, straight and level flight at mid throttle straight off. Full power climbs a little as you'd expect.

I did 3 flights today, in the 2nd and 3rd I was confident enough in the model to do rolls, loops, Cuban 8s and humpty bumps, can't fault the model at all! All three landings were beauts, it practically lands itself! Vertical performance is great, as you'd expect with on OS61 hauling just over 6lbs of balsa and metal upwards. It just keeps going very very happy with that.

I've now made a start on the Skywriter for this year's mass build, but I'll add a few more kistorical KF3 build pics here shortly.

[Simon Feather]

Here are a couple of post-maiden pictures, the sun was kinder so the colours have come out a bit better.

[Paul C.]

Looking great Simon, I have just finished the flea fli had its first few flights but in the process of finishing off the trim etc. Went for the fixed front wheel but rudder does not have much authority for ground handling, flyes well for a small model but need to keep it close as it gets small very quickly.

Was going to do the 40 size o e next but got side tracked by a Loaded Dice 30E

Paul.