Topic: engine

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.

Flying season 2010

Here’s an update about what’s going on in the 2010 season. The integral is pictured here with a new undercarriage which was violently torn off minutes after the picture was taken.

The Integral is the best flying plane I’ve seen. I just love how it presents itself in the air and the color scheme is brilliant. The CDI engine is really easy to operate and is very fuel efficient. It’s also one less thing to charge before I go flying since there’s no glow driver involved.. I typically charge about 600 mAh after four flights in the combined RX and ignition battery pack. At the annual Løten F3A cup I experimented with my own blend of fuel with 10% oil. That definitely did not work well, it overheated in both rounds and stopped. After a forced landing the undercarriage mount was badly damaged and is currently being repaired. I have changed to the taller ZN landing gear that is used in their electric models like the Xigris. It looks really nice and gives better prop clearance so that I can run 19 inch props if I want to. I have had some hysteresis problems with the throttle servo which gives an unreliable idle so I’m changing the throttle servo position and will be using the Futaba 9650 servo.

150 km/h into a wall of grass. Integral does not approve.

While the Integral is undergoing undercarriage surgery I have flown and trimmed the Lorenz Laser. It hasn’t flown since I broke an undercarriage leg last year and now has a stronger, lighter undercarriage and it’s converted to Futaba 2,4 ghz. Trimming it with the new radio and receiver should have been uneventful but it was rather terrifying when the elevator horn broke. Nothing raises your pulse like seeing your elevator flapping freely in an untrimmed plane. To top it off, a wheel fell just when I was touching down. I got some applause from rolling out perfectly with one wheel and one elevator. Every dog has it’s day! Upon inspection, the other elevator horn was fractured and broke off with a light twist. A light surgical procedure involving 5-minute epoxy later, the Laser flew two trim flights and one P-11 schedule successfully.

I have swapped the lovely little T8 Futaba radio for it’s bigger brother, the T12FG. It offers more programming ease with it’s larger screen and also has logical switches which I find very helpful for switching flight mode from normal to snap roll using stick positions. I also have a flight mode called “IGN ON” which powers on the ignition. It’s a non-standard flight mode which means the radio gives and alarm if switched on when the ignition switch is on. I have installed a 2800 mAh Robbe Li-Ion battery and updated it to firmware 2.1. It’s a superb radio but sadly it lacks the trim position reset feature and the superior ergonomics of the T8. And I really miss the backlit display, it’s almost impossible to see the screen in poor lighting conditions. It’s always nice to have an excuse to upgrade your radio if something better comes along…

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.
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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.
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Engines part two: pumped engines

This is a series of general advice on engines describing my experiences. These posts are intended for those who still think about engines as a complete mystery -things I wish someone had told me before I had to spend the time to find out for myself. (YS mechanics and engine doctors already know this.)

I thought I knew everything there was to know about glow engines, and then I got my hands on a pumped O.S. Hanno Special and a Webra .80 LS with pumps. My experiences with these engines helped me a lot when I installed perry pumps on my bigger pattern engines later.


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Engines part one: two strokes

I’ve gotten to a point where I feel comfortable giving general advice on engines so I’ll write a series of posts describing my experiences. This series is intended for those who still think about engines as a complete mystery. These are my experiences and they are all things I wish someone had told me before I had to spend the time to find out for myself. (YS mechanics and engine doctors already know this.)

Part one is about bog-standard two strokes: un-pumped and uncomplicated. Read More »

200 pumper

I have finished installing a Perry pump in the standard (unpumped) O.S 200 FS that will power my new Laser F3A model.  As you can see the pump is hung on to the engine mount, adding minimal weight and is a compact installation that I’m very happy with. 

The fuel line that pressure feed the pump is routed through the engine mount, as straight and short route as possible. The tap is a 4mm pressure tap. Tapping the crankcase is tricky- there is no clearing on the inside and the cast backplate is VERY thin. I did my best so I hope it’ll hold. The crankcase is in fact already tapped on this engine. Not visible, but just above my pressure tap is a cast hole that has a pressure feed going into the carburettor. No Idea why, but I hope there’s enough pressure left for the pump…

The whole assembly including pipe. The Hatori header requires some work to fit this engine. The exhaust port is deeper than on the 200 EFI so I needed to cut a big spacer from a plumbers copper joint in order to be able to tighten the header nut. I also had to drill and tap the top to accept the screw from the header fastener. Presumeable this fastener is to prevent the header from cracking from soft-mount vibration.

Auto 4C Glow-Pro

Have you ever primed the engine, connected the glow driver, flipped the prop only to have the &%¤%¤##%!! engine lock up, backfire, toss the glow clip, loosen the prop nut and generally misbehave? Of course you have. This is something we all live with. With 2-strokes ist just annoying but with 4-strokes it can be lethal. So when a friend of mine said he would make a glow driver that would end my troubles I was first in line. And what I received blew me away – this is THE mother of all glow drivers. I thought I was getting something homemade but this is a professional product. It’s called the Auto 4C Glow-Pro and is handmade by Audun Thinn and Sverre Johannesen, both seasoned pilots who know the demands of trying to start a psychotic 4-stroke while the judges are timing you.

To start with, it has an extremely rugged aluminum case that feels as solid as it is. On the inside theres a print plate with all the electronics (and lots of it!) plus tree LEDs, two dials for adjusting timing and current and the on/off switch. Standard power is a 3-cell 2400 NiCd battery but you can fit just about any battery in there, even up to four cells. Typically you can recycle your old receiver batteries should have have some lying around ( I know I do). It will run on anything from 2 to 6 volts and any type of battery including LiIon and A123. It has 4mm banana plug out for the glow clip, and a 2,1mm plug for charging. This is the same as the plug you use to charge your transmitter so there’s no need to make a new plug.

Here’s how it works: Connect the glow clip, and the yellow LED glows to signal a good plug connection (no need to light the plug just to check if it works!). Then press the start button, spin the engine with the starter and the glow driver will delay the power to the plug as indicated by the green and red LEDs, giving the engine a soft start AFTER it has started spinning. Presto! No more burned plugs, backfiring engines and shamefully missed rounds in F3A. The timing and glow heat can be adjusted to suit your engine using the potmeters that you access from the front (covered by rubber seals to prevent dirt inside the case). Once set, you do not need to adjust them again. There is also an option to have it fitted with a buzzer at extra cost. The buzzer signals the end of the timing, it beeps when you start, when the timer is ready and signals glow power is on with a constant tone. I do recommend that since you often watch the prop and not the glow driver when you start. The buzzer

And the best part is: This limited production item is for sale. Get yours today! You will not be sorry. Contact Audun Thinn at if you want to buy one. They have a limited number already produced but they will also make to order. The price is 1295,- NOK plus shipping. That includes a fully charged battery and the glow driver as shown in the picture.

The glow driver has been tested by all the top Norwegian and Swedish F3A pilots flying YS (and amateur-me with my OS 1.60) so we know it works – don’t take my word for it, buy one for yourself.

Laser test flight

I got 2 flights with the Laser yesterday. I spent some time tuning it again, only to find it was still rich at idle and now even lean at full throttle. After landing and making adjustments again I had two nice flights (one in pouring rain!) trying the N-09 schedule again. It really is a very nice schedule to fly and very well laid out so that the combinations are exciting to fly. I think the engine still has some potential and I suspect my pipe length is not optimal for the 18x10PN prop. I will try the 17×12 prop again to see what difference that makes in performance. I’m glad the plane is out of the workshop and ready for action. Sadly I won’t be competing until next season but now my A (laser) and B models (Saphir) are trimmed and ready.

160 with pump

Here’s the updated plumbing on the 160 with the Perry pump installed. Pretty straightforward, so with hindsight I guess there was nothing to worry about but I’m also glad I’ve got experience of running it without a pump. Since the picture was taken I have added a Sullivan Crap-trap fuel filter between the tank and the pump. I replaced some fuel lines and made them more accessible, plugged the pressure tap in the pipe and routed the vent line to come out of the belly just behind the cowl. The tank has been repositioned on a new tank tray and the throttle servo moved forward for a straighter linkage. 

It all worked very well, the engine ran happily at 7800 RPM with more to go on an APC 18×10 PN prop and idling at 2000 RPM (1500 possible but shaking a lot!). Initially I was not able to figure out the idle, I didn’t count on having to turn the low-speed needle a full two turns in, but that did the trick, the pump really does deliver great amounts of fuel and both needles required considerable adjustment to run with the new setup. So now all I have to do is fly.

Interestingly I had a bad case of hysteresis with the throttle setup. Idling at 1500 or so RPM, I ran it at full throttle and then it returned to 2500 when the throttle was retarded, giving a difference of 1000 RPM. I guess it’s a risk we take with soft-mounted engines. I have a digital servo on the throttle but that does not help – the servo will hold but the engine shakes in the other end, pulling the linkage and increasing the carb opening. I’ve even had a case of a plastic link jumping off an idling engine, it came to full power and there was nothing I could do to stop it. The plane roared into a fence, breaking a wing and barely missing my leg. Since then I respectfully and carefully install linkages in my models. But hysteresis is a big problem. I’d love to hear from readers who have experience (and perhaps solutions?) with this. Leave a comment and we can have a discussion.