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Kwik Fli 40

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TIM Shaw07/07/2017 14:07:32
166 forum posts
41 photos

Just for a cheap laugh you might notice the second pic does not have the U/C fitted. I prefer fuz mounted, tail draggers for ruggedness - probably overly cautious since our field has been improved so much in the last 30 years, but old habits and all that.

Originally, I made the legs too long and it looked daft - and I will admit, this model would look much better as a trike - so I removed the legs, cut the axles off, re- bent new ones and bolted the legs back on.

Only then did I realise I had made a small mistake....


The canopy is simply carved from blue foam (well, it was my fourth attempt, tbh) until it looked right, then covered with glass cloth and eazykote. Its got a bit marked by handling issues but it looks fine in the air.

TIM Shaw08/07/2017 22:05:58
166 forum posts
41 photos
Posted by Martyn K on 07/07/2017 13:03:29:

That looks lovely Tim. Very nice and your canopy looks far sleeker than mine.

Interesting about the stall turn. When the model is flying it should have no idea which way the wind is blowing..

I think there is plenty of rudder authority, its lack of fuselage depth is the problem. Try a 80 degree knife edge. You may be surprised

Thanks for building it, I really appreciate it.

Ok So now I know what you are saying about the side area.

Was a beautiful day today, and I had a great flight with it. Then my throttle servo died and my good friend Jim insisted I had a flight with his Sebart Nuance 110, on 6s electrics ( the work of beelzebub in my view)

Yes. Something of an eye opener tbh, but somehow, nowhere near as much fun as a classic..

But applying the sort of rudder I need to hold the KFs nose up in knife edge resulted in a fairly spectacular (totally unintended) knife edge loop blush

Know which I prefer tot fly though, - and it involves smoke,oil, mess and noise!

TIM Shaw10/07/2017 16:52:30
166 forum posts
41 photos

Oops - sorry - that Nuance is a BJ Craft model, as is the Fantasista 110 he made me fly yesterday smile d

Nigel R14/07/2017 10:38:56
1038 forum posts
251 photos

Tim, the KwikFli looks great - nice one. I like the colour scheme.

What's the AUW?

TIM Shaw14/07/2017 11:40:09
166 forum posts
41 photos

Hi Nigel

Its just under 4 lbs mate.

Wing is a little unual because I didn't have any big enough bits of foam, and previous attempt at sticking bits of foam together were not entirely successful as the glue tends to snag the wire..

So I built a 1/8" balsa framework of the planform, stuck strips of blue foam to either side of it for the D box, ribs and TE, then hot wired the section, and then sheeted that with 1/16" balsa.

Nigel R14/07/2017 12:12:17
1038 forum posts
251 photos

Must go nicely at that weight on the AX.

So you've got (sort of) blue foam ribs, and LE + TE sheet, plus caps I guess. Are there any spars or webs in there?

As a regular foam wing cutter (and here comes the usual question) how do you find yours compare? I'm thinking particularly with the foaming PU type glues around.

It always used to be accepted wisdom a foamie would turn out heavier than a built up, and the only reason I can every come to is the large surface area that needs glueing...

At one point I wrote a spreadsheet to calculate built-up structure weight, it always came out that the internal elements of a balsa wing weighed very close to the cut white foam; the sheeting was a no score draw, leaving only the glue to add a bit of extra weight to the foam wing.

TIM Shaw14/07/2017 22:50:55
166 forum posts
41 photos

So I tried to post a few pics showing the process - seem to have come out a bit backwards, and the editing features on her are not compatible with my thought processes...

Was hoping to take it to the Retford meet, but I am in the middle of a Radiotherapy course and not at my best, so that might not happen



So I hope you got the idea. Agree absolutely about the glue, even if when I cut a normal core I cut holes in it and avoid putting glue there to save weight

Edited By TIM Shaw on 14/07/2017 22:56:08

TIM Shaw16/07/2017 18:06:50
166 forum posts
41 photos

So I didn't fancy the drive to Retford today, but did make it up to the local field this afternoon.

With a new throttle servo fitted and the engine finally breaking in nicely I was really enjoying myself.

I had increased the rudder throw considerably and it will now hold true knife edge both ways very nicely, although you still have to watch any crosswind to get the stall turn correct.

Unfortunately, pulling out of a half cuban 8 there was a clearly audible crack, loud enough to be heard by everyone on the field, and although nothing was visibly amiss it seemed to have developed either a bit of up trim or a rearward CG, so I landed it before anything nasty happened.

Casual inspection showed nothing wrong, to the extent that one club member suggested it could have been a motorbike backfiring as it passed our gate, but it seems the Tailplane (which was 1/4" sheet balsa, sorry Martin) has cracked across the middle as a result of in-flight loads.

Not quite sure yet as surgery will be required, but anyone planning a similar build might like to consider a spruce spar in the tail.

Nigel R17/07/2017 10:42:40
1038 forum posts
251 photos

"but it seems the Tailplane (which was 1/4" sheet balsa, sorry Martin) has cracked across the middle as a result of in-flight loads."

Seems a surprising thing to happen during a regular loops & rolls type maneuver. Maybe the sheet was weak to begin with? I would have thought a plank of 1/4" was more than up to the job for a 4lb model.

kc17/07/2017 11:18:27
5433 forum posts
161 photos

I agree with Nigel that 1/4 is usually quite OK for 4 pound models or more and for many flights too.

Worth careful investigation and particularly looking to see if when trimming the covering film back that the balsa may have been slightly incised too causing a weak spot. ( Years ago I examined a clubmates Wot4 that had a tailplane break in half during flight. Some good flying by the instructor landed the model nicely despite only half tailplane so careful examination was possible - verdict the tailplane failed exactly where the film had been trimmed and the 'nick' in the balsa was still just about visible so was the cause of failure. )

Nigel R17/07/2017 11:38:03
1038 forum posts
251 photos

One thing I've seen done in the full size world is to laminate the solid section. That way, if a weak point causes a failure, it is restricted to a single lamination and the structure as a whole survives to get you down. Although the aim there is really to save your life more than anything else, as the spar structure is at that point defunct and needs replacing.

I wouldn't laminate a 1/4 plank tail, but my route would be a hard 1/4" x 1" wide strip along the hinge line, it does a decent job to provide a spar-like element to the tail without much weight gain over a med-soft plank.

TIM Shaw17/07/2017 15:48:52
166 forum posts
41 photos

Absolutely guys - I've used 1/4" sheet for years on anything up to 46 powered models with no problems - maybe even 61, can't quite remember what my old Gangster 63 was.

I used medium to hard sheet, made up of 3 pieces, one rectangle and two triangles for the LE, so if I had a bad joint between the two triangles there is the potential for a stress concentration there

Certainly I will be looking for stress concentrations due to careless trimming. It occurred to me I did have trouble with errant cyano locking my pushrods and fairly brutal surgery was required in the are below the TP to sort it - it maybe the mount has failed rather than the Tailplane itself - it's not loose exactly, it just seems that one side can be moved up and down a little rather more easily than I would expect.

Let you know what I find.

Nigel R17/07/2017 16:17:18
1038 forum posts
251 photos

Maybe you got into some odd resonance situation where the tailplane was able to buzz about?

Out of interest, I presume the crack was external to the fuselage?

Keep us posted anyway!

TIM Shaw17/07/2017 21:19:30
166 forum posts
41 photos

There was no hint of flutter or anything like that Nigel, just a sharp crack..

And there is no obvious crack external to the fuz either, although I have yet to cut into it - been a bit down from the Radiotherapy I'm going through, TBH, kind of drained and lacking in enthusiasm for anything.

Ends this Thursday though, so onwards and upwards....

TIM Shaw23/07/2017 22:52:30
166 forum posts
41 photos

So.. the plot thickens

I removed the rear fuselage floor and had a good look at the tailplane. The TP itself was sound, the problem seemed to be a crack in the fuz sides - so, amendment, anyone building one of these might like to consider fitting Tailplane seat doublers or a bit of reinforcement under the TP mount.

I flooded the area the area with cyano, then filled it solid with block balsa - nothing is moving down there now boys.

So, took it up to the field yesterday - beautiful afternoon, no wind, clear blue skies.

Took off fine, but t needed a lot of down trim, and at speed, flutter could clearly be heard, so landed and diagnosed a slightly loose elevator horn, nipped the bolts up and tried again.

Again, massive down trim was required, to the extent I ran out of trim and felt I needed to land as a matter of urgency, there being no wind I elected to come in on the short strip, having made my fellow flies aware, and commenced the approach. As I cleared the fence (as I thought...) I cut the engine, but missed the switch, looked down, and well, we all know what happened next.

The barbed wire caught it, stopped it , and flung it a couple of yards back into the adjacent field.

On recovery it looks fine from the top, but the underside of the wing looks like its been hit with a shot gun. The feeling was thats its a simple repair, but I don't like repairing...

I can now only think I had a problem with my elevator linkage, maybe the (metal) clevis jumping on the threads?

Let you know when I am sure.

To top off a perfect weekend I took my 80% Challenger out today, and managed to sieze the engine....

I guess I've had better weekends, but I'm sure I've had worse ones too.smiley

Martyn K24/07/2017 09:22:01
4518 forum posts
3117 photos


Regarding the Fus, a doubler is shown on the plan from just to the rear of F6 to the tailpost.. I agree - dont skip it

Regarding your clevis. Just check you havent put a UNC (Goldenrod) clevis on an M2 rod...


Nigel R24/07/2017 09:40:42
1038 forum posts
251 photos

re: doublers on tailplane; some vertical grain balsa was always my poison, usually dug out from the scrap box.


re: crack, was the crack in both fuse sides or only one?


re: flutter on the elevator, could be all sorts - let us know what you find...

Edited By Nigel R on 24/07/2017 09:42:30

TIM Shaw24/07/2017 11:07:03
166 forum posts
41 photos
Posted by Martyn K on 24/07/2017 09:22:01:


Regarding the Fus, a doubler is shown on the plan from just to the rear of F6 to the tailpost.. I agree - dont skip it

Regarding your clevis. Just check you havent put a UNC (Goldenrod) clevis on an M2 rod...


My mistake Martyn, don't know how I missed that - I guess I proved it's necessary!

My pushrod is a carbon rod with M2 threaded rods bound and cyanoed to both ends, M2 clevises and appears to be perfectly solid and slop free, but I did have an issue with one of these clevises slipping on the threads once before - thought I had dumped the entire batch of clevises and rods, but maybe not.

Once I have my models properly trimmed I usually lock the clevises with a drop of Cyano too, but I hadn't got round to that either.

Challenger is now sporting a nice, new Irvine 36 and the KF is being prepped for surgery after I do a bit more on my Bullet.

Trevor16/08/2017 20:02:17
235 forum posts
22 photos

It's gone a bit quiet here. Has anyone else got one of these on the board at the moment?

Ive just printed off the plans for the IV-40 and am aiming to build the electric version over the Winter. I'm getting the shopping list together at the moment but have a rule that Winter builds can't start before September so I'll have to restrain myself for a few more weeks!

The build will be documented on my website ( but I'll put a posting on here when the project gets underway.



Trevor03/10/2017 10:06:35
235 forum posts
22 photos

Okay, the Kwik-Fli build is now underway. As promised, here is a link to my build diary:

My thanks to Martyn for making this available.


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