Category Archives: Glow engines

Killer Kaos restomod part 3

The finished radio bay in the fuselage

My favourite part of any build is the pushrod mechanicals, the electronics and engine installation and so I take my time getting it as perfect as I can. I ground out a huge cutout for the Tettra 400cc tank in the fuselage former. The tank would fit in front in the tank bay, but I wanted it further back in order to not have the CG shift in flight due to the tank being emptied.

The Tettra 400 cc Crank Tank fitted. 

The servos were moved forward from their original position and the battery sits behind them, in order to fix a CG issue that the plane had back in the day. Because of that, the pushrods had to be lengthened. I wanted to stick with the original Carbon Dubro pushrods that were already in there, so I simply added a longer threaded rod that extends a bit into the pushrod inner and outer tubes. It adds a little bit of weight, but it adds it behind the CG where I need that weight, and as a bonus it stiffens everything up a little more. BLS451 servos were bought very cheap from a chopper guy switching to HV gear. They are great servos, so I get them whenever I see them up for sale. The throttleservo is a Dymond metal gear digital midi.

To top everything off, I fuel proofed everything using flat black enamel paint, which also looks cool. I’m very happy with how the interior bits and pieces turned out.

The radio bay. Note Tettra switch mount bracket

The aileron linkages. I used a pair of trusty old 9154s that have over 400 flights but are still pretty solid. I added plywood inserts to the ailerons so that the screws have something to bite in. I shortened some Futaba servo screws to fit, as I didn’t want anything visible on the upper side of the ailerons.


The O.S .61 SF engine was fitted in order to get the throttle linkage hooked up. The intake is finished. It’s a lot of motor for so small a nose. 


Supra Fly 25 resurrection

I built an old Supra Fly 25 from the OK model kit in 2002, flew it with an O.S 40 LA, and stored it away when the engine was sold. I came upon a new-in-box O.S 32 SX engine in the summer of 2013, and I put two and two together, literally. What a match! The airframe was still as solid as when it was new after all that time in storage, it just needed a good cleaning. I treated it to some more color on the wings and a dozen stickers to liven it up a bit.

Inside, it got a new CG-mounted fuel tank, a whole lot of stuff removed with the dremel, and new digital servos snugly fitted to a carbon tray. But there was no cowl! I had the old one, but it was A) not original, being taken from a Supra Star, and B) had all the cut outs for the engine and muffler in all the wrong places. I was unsuccessful at moulding a new one and so ordered a Great Planes Escapade 40 cowl. It was too deep and too high but I managed to cut it in half, join it again and shorten it, before repainting it yellow with red trim. It fit perfectly!

An underslung Hatori tuned muffler finished it off nicely, and I look forward to flying my old friend with all new equipment this winter.
This is the way it looked in 2002, and this is what it looks like now.

Curare update

The 2013 contest winning Curare ARF was badly treated. In order to win, I practiced intensely two days before the contest and ignored signs of worn bearings. The engine quit and I landed nose first into tall grass. Broken header, bad bearings, broken nose gear. With no time to spare I managed to patch the cracked header using silicone tubing, screwed the nose gear in the lowered position and tried not to think about the bearings. It held together for 2,5 flights, enough to win my first contest.

Now for the overhaul. A new setup is costly and I decided to retire the engine and buy a new one. A new header was ordered as well. The nose gear was not that easy, I could only buy a trike set and not a single replacement nose gear motor, so to save on my budget I bought a cheap Hobbyking set. I should have known better, they fit poorly and I had to shim the mount to get sufficient height. Anyway, It’s on and it works.

With the removable cowl the overhaul was easy. However I noticed that the firewall had also come loose. All the glue joints were cracked and it is a testament to the build quality, the notches was all that held the engine for three flights, and I never noticed a thing. It was simply a matter of gluing the joints, and there it was – completely overhauled and ready for a new season that I very much look forward to.

Curare wins F3A retro contest

The Curare Trio: Me on the left with my Schweighofer Curare ARF (glow), Ingmar Svensson (middle, third place) with his scratchbuilt Curare, and Ola Maltesson on the right (5th place) with his new Schweighofer electric Curare.

The Swedish/Norwegian border cup is back, and our Swedish neighbours hosted a three day F3A event at Lilla Anrås airfield just outside Fjällbacka on the beautiful Swedish west coast. The event started with an informal retro-F3A meeting on Friday. Designs older than 20 years were allowed to enter the competition. Electrics were allowed, but got a 10 % penalty due to “the silly noise”…

Ola Maltesson getting ready for his third flight, Conny Åquist carries his Curare

The nostalgic event was hosted by Mikael Nabrink, Ingmar Svensson and Conny Åquist. The “Curare Cup Normal” schedule was flown, and it worked well. It’s easy to fly but hard to master, so there’s something in it for everyone.

Ingmar Svenssons immaculate Curare with Webra 61

Three pilots flew Curare. Two were Schweighofer ARFs. Ola Maltesson flew his brand new electric Curare, and former World Championship contestant (1989) Ingmar Svensson flew his immaculate model built from MK plans. He used an old Graupner 6014 radio from the 1980, converted to 2,4 Ghz, and a Webra Speed .61 engine. Truly nostalgic! Ingmar flew the most precise figures, but had fuel tank problems and had to land dead-stick after two short flights. He came in third place following his great third flight.

Ola Maltesson used his new electric Schweighofer Curare to win 5th place

I flew my glow-powered Curare, and it performed well for the first two rounds, despite having undergone hasty repairs to the cowling and having a cracked exhaust header. I also damaged the nose gear two days before, so I had to fly with it locked down (nobody noticed!). In the final round the engine quit with ruined ball bearings, and I had to land dead stick. It was a worthy end to its hard life with full throttle and much Nitromethane – I won the contest and could take home the fabulous Asano wooden prop signed by Hanno Prettner.

First price – Asano propeller signed by Hanno Prettner

More pictures here

Hanno Prettner Curare ARF part 3

The final part of the build is the engine and pipe setup. This is usually time consuming with a lot of alignment, but I found that not to be the case here. One evening, and you’re done!

The firewall and tank area is 100 % ready from the factory, fully fuelproofed. Even the blind nuts for the engine mount is installed. No epoxy has been allowed to ruin the threads either, they have been very carefully covered with tiny pieces of masking tape!

The O.S. .55 AX engine is mounted. (Note the nose gear electric motor protruding through the firewall). Just bolt on the engine mount, and drill holes in it. My only grief here is that I did not find any measurement in the manual regarding distance from firewall to the back of the spinner, so that I could position the engine correctly. Trial-mounting the cowling solves this. By removing the cylinder head (you have to do this later anyway), you can slide the engine in and out with the cowling on to determine the correct position. All hardware for mounting the engine is included, right down to the last washer. The throttle linkage fits in a ready made slot in the tank tray, you just have to drill a hole in the firewall, and the resulting linkage is nice and straight.

The fuel tank comes next. It is included, it’s decent quality, and it fits like a glove. It’s large, and it needs to be -a piped engine burns a lot of fuel. The tank tray is installed, and has cutouts for straps. These are not included, so I used spare helicopter battery velcro straps. Fit some soft rubber sheet under the tank (to prevent foaming) and strap it in place.

The straps are slung around the nose gear cable to hold it firmly in place.

Tuned pipe
The pipe is next. It turns out you can’t buy a decent header/pipe combo at just any hobby shop these days, so you need a specialty supplier. Many experienced modellers might still have suitable headers and pipes in their drawers. The days when Hatori produced 2-stroke headers for just about every plane in production are over, but you can still get hold of a #400-series 2-stroke pipe in certain places. I opted for the so called Purple Pipe combo made and sold by Just Engines in the UK:

This is the contents of the medium size Purple Pipe Combo, that fits 40-50 size engines. For a very reasonable price you get everything you need with a 1-click purchase. The three-baffle tuned pipe, a header, silicone coupler, straps, pressure tap, screws and a pipe mount. You can choose three different widths for the header. The wide header is the best suited for our Curare. While our engine is a .55, you do not need the big 60-pipe. It will be too large for the plane and too long for the engine. And clever readers will notice that Purple pipes are no longer purple in color…

The header is fitted. It clears the nose gear perfectly. I bent it ever so slightly outward by hand (it’s aluminium) to get the radius 100 % correct.

The pipe mount is ready. This is a very simple Dave Brown pipe mount. I shortened it a bit, used the offset mounting option, and it fits beautifully. To secure the mount to the wing, I carefully drilled a hole in the center spar, threaded the hole, saturated it with thin CA, and screwed it in.

Note: Engine trials done after the pictures were taken proved the header to be too long, so I sawed off about 3,5 cm. using a metal saw.

And that’s it! Nothing to it. When the engine and pipe are in, you must Dremel a large opening in the cowling to clear the header, carb, cylinder head and needle valve. It seems brutal to cut out almost a third of the beautifully painted cowl, but when it’s done you can enjoy the sight of the beautiful O.S engine hanging out. It tops off a very nice looking plane.

Weight and equipment used
The ready to fly weight of just 2.93 kg for my model is nothing short of incredible. A decent Curare could weigh in at over 4 kilos in back in the day. The target weight of 3.1 kg is within reach for all builders of this ARF, regardless of equipment used. I built this plane like a would a 2-meter contest model, saving weight whenever possible. For an ARF, that means using glue sparingly, and fitting strong 40-gram digital servos. Using mini servos for flaps and throttle saved further precious grams. I used a Futaba 9650 for the flaps, Futaba S-3151s for ailerons, and Futaba S-3050 and 3172 HV for elevator and rudder respectively. Ideally, I would have used S3050s or even BLS-551’s all over, but I had all these lying around. The electric retracts has an operating voltage of 4,8 – 6 volts, so that excludes the use of HV-equipment. My 2200 mAh NiMH RX battery could be exchanged for a lighter 4-cell 1000 mAh pack, but the CG was spot on with the heavier pack. Higher voltage also means faster servo response and less current draw.

Ready to fly – almost. There’s still all the servo programming to do, and we’ll get to that in part 4 – I’ll show you a few videos of the plane on the workbench, demonstrating undercarriage operation and the different control surface mixing.

Hanno Prettner Curare ARF

Photo by Karlheinz Gatschnig, www.

I am doing a review of the new Hanno Prettner Curare ARF for our magazine, and since that magazine is in Norwegian, I’ll also post it here for a broader audience. The Model is produced by Modellsport Schweighofer in Austria. (It’s made in China exclusively for Schweighofer from their specifications). It will be available in two colors: Blue/orange and green/orange.

Me on the left, Hanno, and Conny Åquist in Klagenfurt with the new Curare ARF. Photo by Hanno Prettner Jr.

The new ARF is endorsed by Hanno himself, and he has been involved during development. It is based on the MK Curare 60 kit drawings, and modified for ARF construction, and also changed significantly in the nose section to accomodate electric power (It has a detachable cowl and canopy, and a battery tray). The same model can be powered either with glow or electric, parts are included for both.
Schweighofer have built and extensively tested at least three prototypes, and Hanno has flown both glow and electric versions. He has one himself, an electric one that he flies regularly. I have been fortunate enough to visit his field in Austria and testfly his model, and it flies great – it’s not like a 2 meter model, but surprisingly close. It is the ultimate sport and retro F3A model.

Note: I have a pre-production model, the shipment of production models (600 of them!) is coming soon.

Kit contents
The standard size ARF box holds no surprises. Wing halves, fuse, stab halves, rudder, etc, all nicely packaged. It’s when you start to look at the manual that you see how complete it is – there’s hardly any work left to be done by the builder! Noteable prefabricated things are wheel bays in the wings, servo mounts ready (even the covering is removed), every surface is pre-hinged (you just have to add thin CA), the canopy is cut, fitted and painted with the proper trim, the cowling is ready to mount (for electrics – you have to cut an opening for a glow engine), pushrod guides installed, firewall and tank area fully fuelproofed, firewall blind nuts installed -the list goes on. You’ll also notice the intricate trim scheme – it faithfully reproduces the paint scheme of Hannos original Curare in many colors and dozens of stripes and fancy trim. (He had both blue and green versions so both are “correct”). All hardware required to complete the model is included – as well as a tank, and mounts for both glow and electric motors.

The only thing required is a suitable undercarriage. No undercarriage, fixed or otherwise is in the box. Thankfully Schweighofer stocks a suitable electric retract kit of high quality. The model is retract-ready, so that you would have to modify it significantly to fit any fixed gear. The electric retract kit is is complete with a nice 3 – 1 cable, and extension cables are also supplied. Most pilots have an old set of retracts in their parts drawer. You can fit almost any type of mechanical or pneumatic retracts if you wish, and the common Supra mechanical retracts fit without modification.

The undercarriage legs have to be cut to length (specified in the manual). A Dremel disc cutter is prefect for this.

You do not have to file flat spots, as everything is held in place by double set screws.

I opted to build the glow powered version. It is designed around the O.S. .55 AX. You do not need to fit a tuned pipe, but I thought it was right to have one, both for the looks and for the extra power and low noise it provides. Schweighofer does not recommend the use of a bigger engine. The model is far lighter than the original and uses CAD-designed lightweight construction, so you could have CG problems if you fit a 60-size engine. The target weight is 3,1 kg for the glow version.

The build
The instruction manual starts with the joining of the wing. I opted to complete each wing half before joining. I found that easier, as each wing half is easier to handle. The retract simply slot in, just remember to add the extension chord first. When the aileron hinges are glued, servos hooked up, and the retracts are installed, then I joined the wing. The wing joiner is strong, light, and the fit is perfect.

The retract kit from Schweighofer as it is delivered.

Add a piece of heat shrink tube to secure the extension chord before installing the main gear

The main gear installed

I used a different approach for the ailerons than specified in the manual. While the supplied equipment will do the job adequately, I tend to stay away from plastic links so I used ball links that I had lying around. The supplied aileron and elevator horns are surprisingly good, not unlike MK horns, but through a force of habit (Old habits die hard!) I used my tried method of simply gluing in a 3 mm. threaded rod. I use this method on 2-meter models, and it holds up well. The Curare ailerons are not thick, so I beefed up the wood by saturating it with thin CA, before gluing in the rod with rubber-strengthened thick CA. I have previously used Epoxy, so we’ll see if this holds up. If it proves weak I can always substitute these for the supplied horns.

Joining the wings are uneventful – You just glue in the joiner, and then join it with 30 min. Epoxy. After that you add the two forward dowels and the plate shown in the picture. That usually completes the wing on a normal ARF, but the Curare has flaps, and we’ll tackle them next!

Click here for part two, when we install the flaps and nose gear.

Tiger 2 maiden flight

The Tiger 2 was flown for the first time today, and it was successful. I ran the engine for about a tank and a half on the ground, and after that it was well behaved. The O.S. .55 AX is the easiest engine I’ve come across. I just flipped the prop and it started just like that. It’s a bit noisy on an APC 11×7 prop, but it had great power after just a few tanks of fuel. Read More »

Engines part four: Why won’t it run?

Everytime I go to the field someone has a “faulty” engine – one that won’t start, or that dies in flight or on takeoff for some reason. Or at least they think they do. Fact is, it’s hardly ever the engine. People say the wildest things: “The whole setup is gone” (meaning cylinder and piston) “the bearings are busted”, “this damned engine is just crap”, “I heard [any brand engine]s are just crap”, “the engine was full of metal shavings when I opened it up for inspection” yada yada yada…

Hear this:

  • 8 times out of 10 it’s the fuel. Too much, too little or just not the right kind
  • 1 time out of 10 it’s the plug. (Which means it’s not an O.S. plug, or it’s a very old and worn O.S. plug)
  • 1 time out of 10 it’s the pilot (That would be you)

That’s 10 out of 10. My guess is, about 1 time in a hundred, something in the engine itself might actually be broken. In 20+ years of glow engine flying, that’s happened to me just once.
Read More »

Engines parts three: tuned pipes

A tuned pipe can increase engine power dramatically while at the same time reducing noise. Sounds too good to be true, but it requires a good setup and a pipe that matches your engine. You’ll also have to take into account that fuel consumption will be a lot higher.
Read More »

YS 170 CDi conversion

I’m rebuilding the YS 170 in the new Integral with the CDi and low-oil conversion kit. The CDi consists of a new backplate, ignition box, wiring and cable protectors, head and plug. Low oil parts are just the new crank and cam gear, all other parts will be reused. Getting the engine apart was not really that much work, putting it back together is. The crank ring in particular is very delicate.