Kwik Fli 40

[Martyn K]

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.

Thanks

Martyn

 

 

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

[Simon Feather]

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.

Simon

[Martyn K]

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

[Martyn K]

DRAFT plans can be downloaded from here:

KF40-ic-Fus

KF40-E-Fus

KF40-Wing-Tail

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

Martyn

[kc]

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 K]

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.

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.

[kc]

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

[kc]

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.

[IanN]

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 K]

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.

Martyn

[kc]

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 K]

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

[Martyn K]

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 Christy]

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....!

--

Pete

[IanN]

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.

Martyn

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

[flying pastor]

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 pastor]

Sorry that should be the Kwik Fli dance.

[Martyn K]

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.

Martyn

[kc]

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.)

[kc]

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.

[Martyn K]

Regarding undercarriage mods. Converting a trike to tail dragger is acceptable. I have seen a few done but to be honest , I don't think they look right. My thoughts are 'don't bother with a steerable nose wheel' . The model will have enough ground steering authority on rudder alone once it gets moving. It's lighter and easier to construct.

The use of plates for the undercarriage is Peter Millers method and is what was used on my big Ballerina. Works very well and IMHO is better than blocks when you have holes in ribs to contend with. The stabilising arm must point forward so that the normal torsional twist compresses the load rather trying to pull the saddles out. More than happy to steal the idea for this.

Martyn

[kc]

I am not querying the use of ply plates instead of beech blocks, I am commenting on the end which is restrained. Peter Millers Ballerina plan uses vertical blocks to restrain the end. These vertical blocks are glued to the liteply ribs R1a which are also the dowel supports so any landing force pushes the liteply ribs and the LE & dowel onto the bulkhead.

Your method as far as I read the plan means when the wheel touches the slightest bump on the ground it moves backward & rotates the torque rod part, the stub end being forward facing, in turn tries to lever the saddle downwards. Probably forcing the ply to break the rib out downwards. My view is that at least the stub end should face backwards not forwards. Facing backwards it would push into the rib. But vertical is much better in dealing with the force.

Of course if you have already used the forward facing idea then maybe you have proved it works despite my instinct that it would pull the rib apart!   But I think your photo of the Ballerina on 28th Jan  shows that the stub ends go up vertically into the blocks (much as Peters original.) and carry the force into the bulkhead  - that is what I am suggesting is best.

Edited By kc on 13/10/2016 19:12:22

[Martyn K]

Thanks. I'll take another look. I'll have to check my ballerina build blog. I knew it would be useful one day

[kc]

I think if you build as per your Ballerina it would be fine but as the u/c is further back then the ply " R1a " should extend further back to allow the vertical blocks to glue to something solid. Its also on Phil K's original he used a 1/8 ply rib support W11 which had a double thickness beech block at that point presuamably drilled vertically to take the stub end but that's not actually shown.

Whilst we are talking about the Ballerina my photo of my version using a Turnigy 3548/6 shows what I meant earlier - extended fus sides pulled in with thick doublers (instead of built up 1/2 inch balsa cowl )  Also shows the liteply Lipo support with holes to take connectors thro and slots for velcro straps.  It's a structural part.  So much easier to cut the slots & holes before installing it!  Multiple holes allows for whatever Lipo position eventually suits CG.   Any version of the Kwik Fli I make will be very similar to my Ballerina at the motor end

Edited By kc on 13/10/2016 19:49:02

[Martyn K]

Hi kc

I was out at the theatre so didn't get chance to look at the Ballerina undercarriage.

I am still adding detail although I hadn't planned on using hardware blocks. It should also be noted that the plan doesn't show the webs at the rear of the two front spars (yet). The plan does show the 3mm lite ply doublers that are full depth and are located between the front and rear spars

My intentions are/were.

1 Build the wing including webs

2 Add the Liteply doublers

3 Add the 3mm ply undercarriage plates. The front edge of the plate at the centre will butt up to the web

4 Add triangle reinforcement between the doublers and the plate

5 Add additional small pieces of 3mm ply where the saddle clamp screws will be fitted

I originally thought that would have been sufficient. Now I am intending to add a hardwood block in the area of the anti twist saddle clamp for additional security. I am not convinced that it is necessary though but perhaps edge on safety is not a bad idea

Regarding the cowl. The ic cowl will be built with the engine in situ. The engine is a sidewinder, the original was upright and there should be sufficient space to extract the engine if needed.

The electric cowl is a little more difficult. Built and carved from balsa block, my thoughts are there will be a 1/32 ply back plate to the cowl that will be located with 2 x 16g pins - one each side and magnets top and bottom. Simply remove the prop and spinner and pull hard.. Your idea is noted though. I just want to make sure that the motor can be extracted easily if there is a problem. Always scope for modifications though

Thanks for the ideas and comments. Keep them coming

Martyn