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Kwik Fli 40

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Martyn K12/10/2016 16:50:14
5102 forum posts
3701 photos

I have started work on the 40 sized variant of a Kwik Fli III

Electric and ic fuselages.

There are subtle differences between the two and the original design.

1. The wing section has been thinned to 15% thickness to make it more hospitable to the UK climate

2. The front former (F1) on both models are in different places to cater for different motor mounts and engine/motor lengths

3. the undercarriage on the electric version is slightly longer as I imagine that a larger diameter prop (12" ) will be used

4, the fuel tank floor and the LiPo floor are at different heights. This makes F2 different on both variants

5. I haven't sorted out where a 60A ESC will be located

6. The E version need cooling slots adding

7. I'll make the full size plans available as PDFs later

8. I am going to do a similar exercise for the Kwik Fli IV (tapered wing)

9. The drawings probably need more comments and annotations, but its all obvious to me. Maybe some confusion around F2 on both versions


E Fus


i.c. Fus


Wing and tailplane

Please let me know if you are interested in building one and if so, which.





Edited By Martyn K on 12/10/2016 16:53:09

Simon Feather12/10/2016 17:19:04
242 forum posts
178 photos

Martyn - I'm definitely interested in building a KF3, whether or not it gets selected for the 2017 mass build. I have an SC40 and an OS61 currently homeless, so I was actually considering building the full size and using the 61 for it. But that's not yet cast in stone... what wingspan does your 40 size end up as? Mine would definitely be IC.


Martyn K12/10/2016 17:42:04
5102 forum posts
3701 photos

Hi Simon

The plans are metric but its really 54" span..

Build it light and a modern 35/40/46 will happily drag it around the sky. I am planning on putting an OS35ax in mine


Martyn K12/10/2016 17:45:21
5102 forum posts
3701 photos

DRAFT plans can be downloaded from here:




They will almost certainly get updated. I'll add version controls on subsequent drawings


kc12/10/2016 20:20:16
6732 forum posts
174 photos

That's very interesting Martyn.

I have already printed out the Flea Fli at 137 percent, this comes almost the same size at 53 inches. The reason for starting with the smaller and enlarging that ( rather than reducing the larger ) is that the Flea Fli already has the former in front of the wing and is designed for dowel and wing bolts rather than rubber bands to hold the wing. Also it has a slightly longer nose - about 1/2 inch so slightly better for electric.

As I would only build electric my version would have my usual construction - a 3mm liteply plate ( = tank bay floor) for the Lipo which also reinforces the motor bulkhead. Holes in several places to allow the connectors to pass thro into the compartment underneath for ESC. This has a simple hatch underneath to install ESC but connection is done through a top hatch which also is for removing the Lipo. So the lipo floor is a structural part which connects the ply doublers and both engine bulkhead and wing bulkhead. Worked well on other models for me.

Ply doubler. I reckon for electric the 1/32 ply doubler only need to be about half the fuselage height, so a diagonal line from top of the motor bulkhead to behind the wing. Half the normal size, half the weight and half the cost. In my case I want a fuselage mounted u/c so the ply doubler provides mounting for the torque rod ply u/c mounts.

Unlike the Flea Fli I would use 1/4 sq spruce spars ( actually 6mm square pine from B& Q probably) rather like a Kwik Fli3. Same wing section as Flea Fli about 16 percent. Why reduce it?

Martyn K12/10/2016 21:35:37
5102 forum posts
3701 photos

Hi kc

Apologies, I just realised I never responded to your email

The reason why I reduced rather than enlarged is that its easier to trace within lines. I wanted to get it into CAD just in case we needed to get the parts to a laser cutter. Enlarging tends to make tracking difficult as the lines become rather thick.

I lifted a few of the design ideas from the Flea Fli as you noted. The wing is designed to be bolted (2 bolts, 1 peg) and the formers around the wing have been relocated. I may redo F2 and make it 1 piece. I am not sure why I split it. It seemed a good idea at the time. indecision

I have used a Liteply plate for the LiPo. It actually passes down through F2 so you have plenty of scope to move the LiPo to get the correct CG and there is also plenty of space around the LiPo to get it out the top hatch. I was actually a bit worried the nose may be too long - especially for the ic version. I have found that scaling downwards tends to leave the model nose heavy as the engine mass is not reduced proportionally. As this is only a 90% scale I think we will be OK, but 80% caused big problems for my Curare 25. I think that I will make a lower hatch for the ESC under the LiPo but I need to make sure that I don't foul the noseleg steering.

Noted on the ply doubler. I think that I would rather keep it full depth (or up to the lower edge of the triangle stock to prevent the fus splitting lengthways on a heavy arrival (like my Aurora did). Reduce weight by adding some lightning panels or holes if you think it will make a significant difference

The wing section was reduced to make the model less susceptible to turbulence and wind gusts.


Here is the original from MAN, its not a vast difference but it was a section that was very similar and already available in Profili. When I scanned the original, there were too many lumps and bumps, The revised section is a NACA 63015A. I think it will be fine.

kc12/10/2016 22:26:39
6732 forum posts
174 photos

have you read what Phil Kraft said about the 15 percent airfoil in the original article? It's here on Outerzone

Basically he said he tried a 15 percent and it wasn't as good as the thick airfoil! ( sorry I cannot copy & paste for some reason )

When I looked into this model a couple of years back I found that  'Trainer 60'  in Profili  came out very similar to the Kwik Fli when printed ( not in the preview though )

Edited By kc on 12/10/2016 22:32:18

kc12/10/2016 22:50:30
6732 forum posts
174 photos

Another idea i have is to just use the proper Flea Fli enlarged airfoil which would allow the wing to be built flat on the board by using the Peter Miller method - putting the LE and TE sheeting down first allows just enough clearance. ( the cap strips are omitted until later ) Avoids propping the LE up with blocks.

IanN13/10/2016 10:55:40
1675 forum posts
119 photos

I've been promising myself a Flea Fli ever since Martyn's build, and will definitely get down to that asap. Only hanging on to see if the Kwik Fli options are selected for the Mass Build - if not I'll just crack on anyway

I do like the look of this size very much though, and with both a few red Irvines and a suitable electric motor currently unemployed I'm covered whichever option I plump for. Don't know if I'd fancy the original wing section or the reduced version. Very tempting ......

Martyn K13/10/2016 12:35:10
5102 forum posts
3701 photos

I have worked out that the original section used by Phil was a NACA 63-018A with the nose radius reduced to 2%. I will redraw with the correct wing section.. It doesn't look too bad.

Ian - I think this is a better size. I found the Flea Fli just a little too small so it gets overlooked. I have always liked 40 sized models, they fly well and are easier to build strong and light. Need to keep the AUW down to less that 2Kg for this if we want a reasonable aerobatic performance.


kc13/10/2016 14:24:42
6732 forum posts
174 photos

Certainly like the way Martyn's design is shaping up.......

So you read Phil K's article and changed your wing section!

When is a classic aerobatic design not a classic? Does resizing rule it out? Does changing the wing section still conform to classic regulations? Does changing a tricycle to a taildragger cause offence?

Just wondering if Martyn's design qualifies as a classic. Personally I just build models to fly well and not worry about conforming to classic or vintage classes but there may be those who want to take part in classic events.

Martyn K13/10/2016 14:31:45
5102 forum posts
3701 photos

Thanks kc

I was already aware of the article and also aware of the 'Fish tailing' that the design is susceptible to. (Anyone reading this and is not aware, its not worth losing sleep over - basically on fast and straight flight, the model can wag its tail visibly - but not very much - its called "character" - My KF4 does it and so do a couple of others I have).

There is a document on the UKCAA website where the KF3 model has been modelled in Reflex XTR. It's an interesting read

TBH, the difference in thickness between 15 and 18% is marginal and happy to go with the flow. No matter what, it is far more important that the model is built as light as is reasonably possible (without compromising its toughness).


Martyn K13/10/2016 14:44:05
5102 forum posts
3701 photos

To answer your questions about defining a Classic.

The UKCAA official view is that:

1. A Classic must maintain the same outline and basic dimensional ratios and should be built from a plan that was published in the era in question - Eras are basically by decade, 60's, 70's etc The UKCAA Classic cut off date is currently 31/12/1985 but we have a motion tabled at the AGM to extend the date to 31/12/1995.

2. Scaling is permitted but 1. above takes precedence. i.e. you are scaling from a plan

3. Minor changes in wing sections are OK - they should be visibly similar by eyeball.

4. Changes in constructional materials are generally OK, for example substituting a built up deck with a foam deck, use of glass or carbon for local reinforcement, using built up to replace foam wings or vise versa.

5. The spirit is that the model should be representative and recognisable as a model of the era in question without having to resort to physical measurement.

6. The overarching rule is to enjoy yourself, no-one is really that bothered

Peter Christy13/10/2016 14:46:10
1910 forum posts

Re: Fish-tailing. This was a well known "feature" of the Kwik-Fli, back in the day. A fellow club member got so fed up with his doing it that he rebuilt the tail as a thin symmetrical section, rather than a flat plate. This completely cured it....!



IanN13/10/2016 15:14:36
1675 forum posts
119 photos
Posted by Martyn K on 13/10/2016 12:35:10:

Ian - I think this is a better size. I found the Flea Fli just a little too small so it gets overlooked. I have always liked 40 sized models, they fly well and are easier to build strong and light. Need to keep the AUW down to less that 2Kg for this if we want a reasonable aerobatic performance.


I know what you mean - and I did look at the Flea-Fli +10% - but I usually have at least one smallish model in the fleet, and being able to build the wings out of 36" sheets with no waste is a plus. So, a Flea-Fli it is

I may well go for this Kwik Fli 40 aswell though wink

flying pastor13/10/2016 15:22:56
25 forum posts

The "Kwik flip dance" is well documented. Cures seem to be by adding wing to fuselage fillets or by gluing an 1/8 inch square strip of hard balsa or spruce approx. 2ins long vertically on each side of the trailing edge of the rudder. Incidentally I think the size that has been drawn up will be right for many people.

All the best.... Chris N

flying pastor13/10/2016 15:24:14
25 forum posts

Sorry that should be the Kwik Fli dance.

Martyn K13/10/2016 16:05:58
5102 forum posts
3701 photos

Thanks Chris - doesn't auto correct drive you round the bend.

Back to the wing section debate..

The Bar Fli (which was Phils backup model at the '67 W/C) had a 15% thick wing section...<snip> got a thinner airfoil (15% instead of 19%) and a deep fuselage with much side area as well as a large rudder like that of the Kwik-Fli Mk III. Unexpectedly and despite being a bit small, it turned out to be a very good pattern ship, especially in high winds and turbulence. </snip> It was calm at Corsica so he chose to use the KF3

It was the UK "high winds and turbulence" that the mid-west US of A don't seem to suffer from as much..

Anyway - I am not changing it again. its redrawn with the original wing section.


kc13/10/2016 17:04:56
6732 forum posts
174 photos

Thanks for the clarification of classic 'rules'. As you didn't comment on taildragger I assume that probably means it's heresy to modify a tricycle! I will do it anyway.

I am not sure whether your cowl for the electric version is just screwed on or glued on. If glued on the nosering might need a gap to allow the motor shaft to go thro for installation or replacement.

My idea would be to have the top hatch extend over the motor giving access to the motor fixings. Then the cowl is formed from the fuselage sides leaving the bottom open. Fuselage sides can have thick balsa doublers to allow some shaping although they might pull in enough anyway. ( presumably fuselage needs a sheet join anyway as they would be well over a 36 inch sheet so an extra 2.25 inch for the cowl won't matter )

( although I am not involved in UKCAA I feel a cutoff of 1995 would be good as it would allow the Spirit, Graphik 40 and 60 etc.)

kc13/10/2016 17:22:55
6732 forum posts
174 photos

I am wondering whether the horizontal mounting of the u/c right angle bend on the bearer is a proven method? My view is that it would twist and just push the bottom of the rib out. In the usual vertical mounting it pushes against the spars & whole rib length rather than just the tiny narrow bit under the bearer.

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