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Kwik Fli IV

The tapered wing version

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Martyn K11/06/2015 22:42:56
4541 forum posts
3169 photos

I need to thank Martin and Graham Fox for this one, they loaned me a very elderly (1969) RCM&E that had an interesting 2 page article. The article only details the revised wing, the fuselage is basically the more well known Kwik Fli III but with a different wing seat to cater for the 12" root chord and 20% thick root wing section.


I am also going to have a bash at cutting my own foam wing cores - the last ones I did was in 1980ish and they weren't very good, but I have 2 more models in the pipeline that need foam wings so its going to be a skill worth re-learning. If I really make a muck up of it I am sure that Elation models will happily oblige wont they Steve?

Power plant for this is an OS70FS Surpass.


This should be a good match - it will happily turn a 13x7 prop at 10k revs and the model should be quite light compared with models from the next 2 decades. However, it has a rear carb and I will need to move the firewall back about 10mm to keep the prop in the correct place. This means that the tank (10oz) will also need to go back a bit. However, on the plan its already seated up against the former that sits at the front of the wing so that will need to have a hole cut in it for the tank. No big deal... (I hope).

The Kwik Fli III plan was downloaded from the UKCAA Resources web page. Its from a Graupner kit but is fully detailed. Printed using Adobe tile printer and an hour in the kitchen taping it all together.

So - todays activities.

Cut out a set of formers.. (with a hole for a fuel tank)


and a firewall....


--which has been fuel proofed and a nylon mount fitted. The plan shows no right thrust, I cant imagine that it wont need any so I have offset the mount to the left slightly (about 3mm) and will build in a couple of degrees right thrust. I can always take it away if its too much.

Finally tonight, I have started laminating the fuselage sides with balsa doublers, weighed them down and left them to dry. 1/32 ply doublers need to be added as well and I have also started stripping balsa for longerons.


More to come


Steve Dunning11/06/2015 23:06:25
324 forum posts
186 photos

I had the same idea Martyn and produced CAD plans ready to machine parts and produce foam wings. Look forward to seeing your outcome.


IanN11/06/2015 23:18:56
1675 forum posts
119 photos

I have an original - although I believe incomplete (been a long time since I looked) - Graupner Kwik-Fli kit in the loft. This might give it fresh impetus

Martyn K12/06/2015 07:10:59
4541 forum posts
3169 photos

Its worthwhile building Ian, its a really fairly simple build and goes really well with modern engines.

Steve, I had a feeling that you were considering doing this one. After I built the Flea-Fli, I always fancied its bigger brother. The Mk IV version with taper wing screamed 'build me' and I couldn't resist..

Sorry, hope it doesn't stop you progressing yours..


Martyn K14/06/2015 22:16:41
4541 forum posts
3169 photos


Cracking on with this. One of the benefits of the treatment I am on is that my metabolism and body clock has gone haywire so I am waking at 05:30 am every morning. Which means lots of workshop time..

Friday evening - getting the formers into place..


Lots of batteries and squares keeping things vertical...


It looks odd - but that is wide angle distortion - honest

Saturday Evening after spending the day at the UKCAA event at Skegness..


When dry, the second side is attached. Additional cross pieces added top and bottom of each former to stop the formers from bowing.

I have made a couple of minor alterations to the design. I have used square section longerons at the top and slightly thinner doublers at the bottom of the fuselage. Phil Kraft also had an additional central longeron in the rear section. This has been omitted as I have extra wood in the upper longeron.


Lots of clamps and making sure that it is straight..


I have also added additional 3mm balsa doublers at the wing seat..


Check the fuel tank fits OK.

While this is drying, I started work on the tailplane. As usual, I have used Kevlar cloth for the hinges. I have talked about this several times on previous blogs, if you want more info have a look at my Flea-Fli blog


The outline is 3/16 sheet balsa, the TE is 2 laminations of 3/32 balsa with the Kevlar sandwiched between


Diagonals cut and added and allowed to dry

More to come

Martyn K14/06/2015 22:16:59
4541 forum posts
3169 photos

Sunday morning..


All sanded flat using a Perma Grit then the top and bottom sheet is added. The elevators are linked using a 14g piano wire bar rather than dowel as shown on the plan. Small recesses were cut to accommodate this and epoxied using slow set Araldite


3mm lite ply caps added to the elevators at the root. They add very little weight, but lots of warp resistance

Allowed to dry thoroughly - so onto the fuselage again..


Top deck added. Nice light 3/8" sheet balsa. Its not very clear, but the slot for the fin has been cut using a bandsaw


...which in hindsight was a mistake as it wasn't central enough and needed to be adjusted..


Tailplane and elevator all sanded and ready for fitting


The fin and rudder is laminated from 1/8" sheet with Kevlar hinge core at the join


The slot for the tailplane in the rear fuselage was opened up to allow the tailplane to fit and the fuselage placed in the jig. A mix of old and new technology used to get the alignment correct. The top fuselage deck is parallel with the datum so a free bubble app was used on my smartphone to level up the fuselage in X and Y axes.


A very slight side tilt but it is horizontal fore and aft


Use the phone on the tailplane and check. Make minor adjustments so that readings on the fuselage and tail are the same.

I also used a string triangulation to check that the tail was centred correctly and not skewed


Finally tonight, the fin was glued into place and a laser sight was used to check that it is vertical. That works quite well even though the photo is on its side

And the last job is to make sure there is plenty of glue and allowed to dry (overnight).


More to come..


Bob Cotsford15/06/2015 08:04:03
7162 forum posts
405 photos

That's fast work Martyn - whatever meds you are on - I want some! It's taken me about a month to put together an ARTF Maule and I've not finished yetfrown

Peter Christy15/06/2015 08:20:29
923 forum posts

Nice work, Martyn! You should enjoy flying it - these older designs just seem to fly so much "nicer" than their modern equivalents!



Paul Jefferies15/06/2015 08:25:17
252 forum posts
39 photos

Very nice Martyn, I really like the kevlar elevator hinge....... How will you cover/paint/fuel-proof it and ensure that it doesn't get all gummed up or restricted?


Martyn K15/06/2015 09:32:17
4541 forum posts
3169 photos

Thanks all.. Steroids Bob - and you really don't want the dosage that I am on..

Paul, the Kevlar is sailcloth, it's very flexible and fuel proof (or has been so far!). I use PVA to glue the wood parts together and liberally coat the sailcloth with Cyano (on both sides), sandwich with heavy weights and leave it to dry properly..

Covering is a bit tricky. Basically, I cover in one piece, forcing the film onto the hinge material then VERY carefully slit the film taking care not to slit the sailcloth hinge - it wont tear under any (normal - or in our world abnormal) circumstances, but it is quite thin and a scalpel will go straight through it.. Finally, I seal the edges with fuel proofer to stop seepage. I haven't had a problem so far - although I did manage to totally slice off the rudder on the Concept (by accident..)

So - this mornings efforts. Body clock alarm went off at 05:10, but I did manage to get back to sleep for another hour (thank you)..

So, this mornings work involved getting the firewall fitted. With the model still in the jig I started the alignment process.

3 degrees of Right Thrust is equivalent to 7.3mm deflection at 140mm out. (I forgot to mention that I had offset the mount on the Firewall by 7mm)

Extend the engine mount using a pointer


Adjust the side thrust so that at 140mm the (inner edge) pointer intersects with the extended centre line. Check we are at the correct height and also the firewall is at 90 degrees - Vertical - no downthrust.


Then take it all apart again - add slow setting Araldite - and reassemble


Add triangle reinforcement pieces - adjusting angles and cutting slots for a very good fit, glue and clamp in place - I use a liberal amount of PVA to attach these - making sure that the Firewall isn't disturbed in the process.

Finally, while waiting for the glue to dry, I cut and added the two parts for the fin extension on the top of the fuselage.


Not much I can do now until the Araldite dries - so probably tomorrow morning.

More to come.



Edited By Martyn K on 15/06/2015 09:56:30

IanN15/06/2015 09:45:41
1675 forum posts
119 photos
Posted by Bob Cotsford on 15/06/2015 08:04:03:

That's fast work Martyn - whatever meds you are on - I want some! It's taken me about a month to put together an ARTF Maule and I've not finished yetfrown

Seconded. That's absolutely awesome progress. Watching with interest!

Martyn K15/06/2015 09:54:27
4541 forum posts
3169 photos

Its not called Kwik Fli by accident

Its also a very simple build so far... I think that it would make an ideal first Classic model for someone who has never ventured into this area before. I spoke to Steve Dunning on Saturday and he is putting together a (part) kit for this design. I suspect that it will take him longer to prep the kit parts (CAD etc) than it will take to build.


Martyn K15/06/2015 22:49:46
4541 forum posts
3169 photos

By about 7:00pm tonight, the epoxy had set hard enough to risk taking the fuselage out of the jig. A gentle sand down outside in the early evening getting the top deck to shape was a real treat and then back to work. Its slowing up a bit now..


I have added the SLEC snakes and push rods


At the rear end, there is a small spacer to hold the outer tubes rigidly and epoxied into place. In this shot you can see the 1/8 balsa doublers. On the plan these are full width and the sides butt up to them. Much more difficult to fabricate.

With the tubes and pushrods in place, I added the lower sheeting - grain runs crosswise..


I trim these back to size when the glue has dried.

Finally tonight, the nose leg has been added and tubes fitted for the throttle and the steerable nosewheel.


The nose leg tiller is at a funny angle so that I don't have to cut a slot in the firewall. It means none linear steering but I am not worried about that. Note the tank is back is to make sure that nothing binds and it can be installed and removed with comparative ease.

Things will definitely start to slow up now, the next job is to fit the lower front sheeting and then fuel proof the tank bay and then make a cowling. I am tempted to make one out of Fibreglass using a simple pink foam plug. I'll settle for polyester resin and sand it smooth when it sets.

More to come.


Peter Christy16/06/2015 09:22:17
923 forum posts

Wow! Laser pointers, bubble apps - that's some hi-tech build going on there! My KingPin had to rely on a Mk1 eyeball, with occasional reference to a set-square!!! surprise

Yes, the nosewheel tiller on my KingPin is exactly the same - requiring an offset to avoid hitting the bulkhead. In theory it makes it non-linear, but in practice, you won't notice it. I used thin piano-wire to drive mine, with a loop in it to provide some protection for the servo. If you make the nosewheel link too rigid you run the risk of stripping the rudder servo gears if you have a hard landing, or hit a bump on take-off. Don't worry too much about any "springiness" in the link - its only effective are very low speed on a smooth surface anyway. As soon as the model gathers any speed, the rudder takes over. From the pix it looks like you are using a snake of some kind to control the nosewheel. If so, I would recommend fitting a servo-saver - like the car boys use - to protect the rudder servo.




Martyn K16/06/2015 09:38:42
4541 forum posts
3169 photos

Its the first time I have used these hi-tech tools Peter, they do make life easier though and I'll certainly use them again. I have more confidence that this model is straight than any other I have built - unless someone points out a flaw in my methods..

So, this mornings activities

Removed all the pins and sanded down the rear decking


and then added the lower thick balsa floor in front of the wing. This is somewhat thicker than shown on the plan as I am planning to fair the fuselage into the lower wing..


The nose wheel spring is totally recessed into the floor - keeps it quite clean aerodynamically - although the engine will be hanging out in mid air.. Lots of clamps while the glue dries


.And from the inside, thick cheeks at the floor to side junction add a certain amount of additional strength.

Next jobs are to fuel prof this area, build the upper hatch and then start work on the cowl..

More to come


Martyn K18/06/2015 09:10:14
4541 forum posts
3169 photos

As planned, I have progressed with the cowl and the fuel tank hatch.

The hatch was quite simple. I have added 1/4 x 1/8 spruce rails to the fuselage top. Added a 3mm lite ply rear former on the fuselage and then simply assembled the hatch from strip and a 1/2" upper deck with play formers front and rear


Pretty basic stuff and mainly following the plan

The gussets have been added because the wood is rather thin at the junction between the top deck and the sides.

The slot is for the nose wheel leg mount - which is slightly offset to cater for the right thrust


Note the blood - that is to make sure it flies OK.

And onto the Cowl

I am going to try the method used by Chris Barlow in his 2015 triple mass build blog. This involves making a foam core then covering in Fibreglass tissue and then Fibreglass cloth. Chris has kindly provided me with some useful hints in a PM - thanks Chris.

Here is the cowl.. carved and sanded from pink foam






Looks OK at this stage - it can only get worse now..

The cowl was then wrapped in clink film then a single layer of packing tape.


This is the release agent.

Then a single layer of tissue with polyester resin.


and sanded down to get rid of excess lumps

And then, this morning - a second layer of tissue/polyester resin has been added


Its not quite as bad as it looks. I'll leave it until this evening to harden and then sand down and possible apply another layer, Eventually a top layer of 170gm cloth will be added secured with epoxy resin.

More to come


Jack Banner18/06/2015 09:48:23
332 forum posts
43 photos

Great Build Martyn,

I have been tempted by this design for a long while, if there does end up being a short kit with foam wings available then it would definitely be a contender for my next project.

Subscribed to see how the cowl goes!


Martyn K19/06/2015 23:05:12
4541 forum posts
3169 photos

A bit more on the cowl.

The cowl was sanded (fairly) smooth and the 170g cloth was added with epoxy resin and allowed to harden. Although not brilliant, its was considered worth continuing so early this morning after the Resin had hardened over night, the cowl was sanded down again (with a power sander) and then given a coat of finishing resin and carefully worked in until it started to harden then left for the day


The plug was dissolved using Petrol as suggested by Chris Barlow - that was VERY effective and the cling film and packing tape have subsequently been removed. The cowl is quite useable, It has a couple of high and low spots and unfortunately one bubble but its not as rough as I expected it would be. Tomorrow, it will get a reinforcement ring of 170g and epoxy around the inside edge and then trimmed to length and the bubble filled in.

After that, its another sand down, high build primer and filler (hopefully not much) then paint and fuel proofer. That will really show up what its like and whether it ends up in the bin

However, I am fairly pleased with it so far..

Which leads me nicely onto the wing and the foam cores. A couple of years ago someone posted on U Tube a method of cutting tapered foam wings with one end of the cutter held in a pivot. A friend at the flying club tried the idea and it seemed to work OK - so this is my attempt

Firstly, the geometry.

The pivot point has to be EXACTLY the same height as the centre of the foam core and also on the convergence of the projected leading and trailing edges.


This is a plan view of the arrangement with the wing panel on the right hand side.

The panel is 31" long, the root chord is 12" and the chord at the tip is 9". There is equal taper at the LE and TE.

The projected LE and TE converge at a point 93" from the tip, so the length of the cutting bow with be at least 93 + 31 + a few inches - say 130" - you need lots of space. Wings with a sharper taper will need less space.

I used stainless steel wire from the pivot (actually a cup hook screwed in the shed wall) - about 80" long then linked it to a length of 24 g Nichrome wire up to a handle


This is the arrangement. I used my SLEC building jig screwed to my mobile building board and adjusted the height until it was correct. The squares help with alignment as a guide for the heated wire to get it smoothly down to the LE where I start.


The handle is a short length of plastic conduit with the Nichrome wire fed through and secured. It needs to be secure - you are going to pull very hard!

I used an engineers square as a guide to get the heated wire lined up with the LE and the - while pulling like mad and moving very slowly I traced around the template in the root. AT the other end of the foam block, a 75% copy of my efforts at the root was being created.


And this what you get




I am really quite pleased. I cut 4 cores - 2 were unusable and 2 were better than just OK.

Added the false LE and TE and also assembled one of the skins (I only have space to do one at a time)




Really quite pleased with progress today...

More to come


Colin Leighfield19/06/2015 23:18:57
5381 forum posts
2207 photos

I really like the look of that, what a good idea.

Martyn K19/06/2015 23:23:36
4541 forum posts
3169 photos

I couldn't believe how easy it was compared with using a bow. The problem I had with the 2 rejects was that the wire was too hot or I was moving too slowly. It's a bit of palaver getting it all lined up but it was stable and repeatable when done.


Edited By Martyn K on 19/06/2015 23:24:47

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