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Kwik Fli 40

Plans and discussion

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McG 696927/12/2016 19:31:34
2076 forum posts
887 photos

Quite an achievement in just a few weeks, Martyn.

Really well done.



john stones 127/12/2016 19:53:42
9365 forum posts
1428 photos

Very good Martyn, pleased all went well for you, though never doubted it yes


Lucas Hofman27/12/2016 21:26:35
594 forum posts
316 photos

Congrats Martyn,

It is always a great feeling when many hours building lead to a successful first flight!


Martyn K28/12/2016 10:02:10
4559 forum posts
3169 photos

Thanks all

I was pretty comfortable with these two. They are quite a simple re-scale of a proven original. Any disasters would have been of my creation.

I ordered a couple of new props last night - an 11x8.5 and an 11x10 for the IV. Hopefully they will make it a bit more lively. Its not massively out but it will get better. As its is at the moment it would make an excellent Aileron and Aerobatic Trainer.

Ideally, the elevator and aileron horns need to be bigger than the ones that I made - I'll revise the plans accordingly and may get them out later this week. I cant change my aileron horns as they are built in so I'll have to play with the servo horns. I realy wish that servo manufacturers would provide a reduced travel = higher torque servos - 45deg each way would be more than enough for most of my models

That's basically it though.

Any changes, I'll update here


minty morton29/12/2016 08:40:57
99 forum posts
2 photos

Very very well done Martyn. Looking forward to seeing them next year.


Martyn K11/01/2017 13:58:52
4559 forum posts
3169 photos

Hi all

Kwik Fli 4-40 Plans now available for download (as a zip file).


Laser cut parts can be ordered from Dylan at LaserCraft. PM him from this website please

(Edit - Laser cut parts are Wing ribs, Fuselage ply formers and Wing ply parts - not ply fuselage doublers)

Dylan Reynolds LaserCraft Services

DXF files are available on request

KF3-40 plans to follow soon.

Best wishes




Edited By Martyn K on 11/01/2017 14:04:06

Braddock, VC11/01/2017 17:43:49
1464 forum posts
21 photos

Martyn, a quick query. I looked at the ic fuselage and see you "used" an OS 35AX and fitted the slec 9 ounce tank.

Is that tank a bit on the large size, I ask because I'm using an Irvine 36 in a wot 4 which would seem to be a similar source of drag etc and had 3, 5 minute flights [fingers get cold this time of year] with a third of a tank left from the slec 6 ounce tank.

I have another irv 36 and thought about building a kwik-fli around that.

Martyn K11/01/2017 18:28:28
4559 forum posts
3169 photos

Very good point. I had no experience of the 35AX before this model was unsure just how big a tank I needed. As it stands and without the engine being fully run in it will give a comfortable 15mins flight time.

You could comfortably drop down a tank size if you wished. I certainly would if I did it again. I wish my OS61RV was that economical - even pro rata

Martyn K13/01/2017 10:02:03
4559 forum posts
3169 photos

Apologies all. Just realised that the cut parts drawings were missing from the original zipped plans set I linked to above

Try this Set

Kwik Fli IV - 40 Plans v2a ZIP





Edited By Martyn K on 13/01/2017 10:03:27

Braddock, VC13/01/2017 10:22:32
1464 forum posts
21 photos

Thanks, martyn. I run my 36s on 15% nitro so I guess if I dropped down to 5 or 10% the fuel would go even further.

Martyn K27/01/2017 16:22:02
4559 forum posts
3169 photos

Belatedly, here are the Kwik Fli III 40 plans

Please let me know if you find any problems or have any difficulties with either of the plans

Dylan at Lasercraft has the DXF files if you want a parts kit cutting. The embedded file: kwik-fli-3-40-laser-parts.pdf shows the parts that comprise a parts kit. You will need to let Dylan know if you want the Electric or IC version.

Good luck


Martyn K02/02/2017 13:20:28
4559 forum posts
3169 photos

A brief update

I went out with the KF4-40 (E) at the weekend. I had 3 flights with the revised 11x8 prop (was 12x6) and it has transformed the model. Not unlimited vertical or anything uncharacteristically silly but comfortable 100m diameter loops and large sky filling cubans. An absolute delight to fly. Still getting the rates/EPA settings correct but its nearly there.

With the power setup I have used I am getting 6 minutes on a %age throttle setting - about 9-10 minutes of real flight time. If I was buying again, I would opt for a 3500 or 4000 4S LiPo. It would help the CG as well which may be a bit back for some.

Only problem I have found so far is a reluctance to stall turn away from the wind.. Maybe a KF peculiarity,,


Lucas Hofman02/02/2017 13:50:57
594 forum posts
316 photos

Hi Martyn,

Where do you get an 11x8 propeller for electro. I use often an APC 11x7E with 4S setups and a 800kv motor. Works fine but I would like to try a bit higher pitch. Regrettably APC does not do 11x8E, only 13x8.

Regards, Lucas

Martyn K02/02/2017 13:55:25
4559 forum posts
3169 photos

Hi Lucas

I bought them off an eBay Seller. I also have an 11x10 to test as well but I probably wont bother as I don't want to stress the system. It still easily out performs my 60 sized Kwik Fli 4


Lucas Hofman02/02/2017 21:47:50
594 forum posts
316 photos

Thanks Martyn. APC does make 11x8E (if these are not copies...). I ordered one. Lucas

TIM Shaw07/07/2017 12:49:34
166 forum posts
41 photos

Just so you know your work is appreciated Martyn, heres a couple of pics of my KF 4 40, powered by OS35 AX and 10x7 APC prop. Flies really nicely, although I've also noticed it is reluctant to stall turn away from the wind and the rudder is a bit lacking in authority in knife edge.

Built from your drawings and a real pleasure to do. Added the wing fillets in a bid to avoid the tail wag and I am pleased to say she hows no sign of it.

kf4 40.jpg20170330_111225.jpg

Martyn K07/07/2017 13:03:29
4559 forum posts
3169 photos

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.

Lucas Hofman07/07/2017 13:24:18
594 forum posts
316 photos


Reacting on your comment that the model does not know which way the wind is blowing, which is true. However:

- on a vertical with cross wind the fuselage is already pointing slightly into the wind, making it more difficult to stall turn away from the wind compared to into it.

- many models turn into the wind on verticals or when entering loops. I think that is because of the wind gradient (the wind blowing harder with increasing height). Thereby the model is accelerated sideways by wind and that will blow away the tail, making the nose turning towards the wind.

regards, Lucas.

ps. they look lovely. Maybe I should build one next vinter!

Martyn K07/07/2017 13:35:01
4559 forum posts
3169 photos

Fair point Lucas. I was assuming that the upline track was vertical and not leaning, however, you are correct that its a likely reason for the problem. I'll check mine again and watch a bit more closely


TIM Shaw07/07/2017 14:01:15
166 forum posts
41 photos

All good points.

If I fly straight into wind there is no issue with the stall turn, but very rarely is this actually the case - particularly if trying a figure M. And if there is any component of side wind, as Lucas says, it tends to blow the tail away and the model weathercocks into wind - don't forget, at the point of a stall turn it has no forward movement either, so has no idea where it is!

We shouldn't be surprised at that really, as surely this is the function of the fin?

I know exactly what you mean about an 80 degree "cheat" knife - I can do a very nice version of my Wall of Death ( christened on the slope, as it looks so wrong....) with it, which is a knife edge, outside, 360 degree outside turn.

But in pure knife edge I feel a bit more rudder would be a good thing, I may be able to solve it with upping the movement a bit but have yet to get into fine tuning - I like flying it so much!

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