We had an informal gathering of pilots to fly, have fun and informally compete in a vintage pattern meet at Jarlsberg airport. The name honors the Norwegian pattern icon Tore Paulsen. I came 5th with my Curare. Enjoy the pictures here.
Topic: Curare ARF
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.
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”…
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.
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.
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 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 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.
This is part 2 of a field and bench review. In part 1, we installed the main gear, ailerons, and joined the wing halves. When the wing halves are joined, we move on to the flaps. The instructions are different for electric and glow power – for electric power, the flap servo is to be installed in the back half of the wing, presumeably to give space inside the finished fuselage for the battery box. For the glow version, you are instructed to put the flap servo where it fits most naturally, in the center of the wing, using the supplied pre-cut servo mount. It fits all std. size servos, but I modified it slightly to fit a Futaba 9650 mini-servo. The flaps are not big, so you do not need a powerful servo, but centering is important. The linkage is built into the wing, and all you have to do is add the two horns, and cut the supplied pushrods to length.
Why the flaps, you might wonder?
At the Las Vegas TOC in 1976, square manoeuvres were flown for the first time and the corners had to be flown sharp, crisp and tight. So Hanno invented a Flap system which operated with the elevator controls and called it Snap-Flaps. It was THE sensation at the TOC and the year after even more Snap- and square manoeuvres had to be flown, and most of the pilots copied Hanno’s Snap-Flaps. If you want to have fun, just couple the Flaps opposite with Elevator and you can draw square figures in the sky. And the stopping for entering into spin is even quicker. For smooth and rolling manoeuvres the flaps are not engaged.
Finished flap linkage. You’ll notice the mass of servo cables extending from the wing. There are five in total… Even if you fit mechanical retracts you will have four wires to plug into the receiver when assembling the model. I decided to spend an hour soldering an MPX 6-pin plug. Two plugs for +/-, and 4 for the different signal wires. (The main gear only needs a single signal).
Here’s the finished plug – five wires from the wing to the receiver via one 6-pin plug.
That completes the wing!
First thing to add is the nose gear. This is steerable (optional) and linkages are included. After cutting the nose gear leg to proper lenght (specified in millimetres in the manual) you simply screw it in using the supplied screws. If you want, you can leave it fixed by simply tightening the set screws. I chose not to make it steerable. If needed, you can add this later by simply loosening the set screws and hooking up a pushrod to the rudder servo horn.
The nose gear is installed. The nose gear and main gear on my plane are plugged into different channel ports in the receiver, and both are set as “Gear” channel with the same switch.
The fin is an integral part of the fuselage. To complete it, simply glue in the pre-hinged rudder.
The stabs have a negative angle, a central part of the curare design. It means they have to be glued to the fuselage separately with a central joiner, and there is much alignment that the builder must pay attention to. On my model, the fit was perfect, so after removing the covering (you attach the stab and draw a pattern around the base rib to see the area to be removed) I was able to glue both halves in one go using 30 min. Epoxy, and the alignment was much easier than I had anticipated. The stabs are fully sheeted, which makes them strong and light, and the whole assembly could be made removeable if cunning modellers would wish to make make such a modification to their Curare.
The stab root is traced, covering removed, and one carbon guiding pin and a negative angle plywood joiner glued in place after every measurement is checked. Then the stabs are glued in place with Epoxy.
The elevators are then glued in place with thin CA on the hinges, and you are ready to install the pushrods. Once again I differed slightly from the manual and used threaded rod as control horn, and substituted the plastic clevises for metal ones. The rudder has the supplied horn as you can see in this picture:
The rudder was too thin to use the threaded rod control horn technique.
Pushrod guides are pre-installed in the fuselage, but the supplied 1,5 mm pushrods are not threaded so that you have to solder on threads for the links. While they are supplied with the kit, and of good quality, I would have preferred not to have to solder, as this might be intimidating for inexperienced builders. It’s still an understandable compromise. Threaded 2 mm rods would be heavier.
The servo mount is installed the fuselage, but you have to add two spacers that put one servo higher than the other so the servo horns will not interfere. These spacers are supplied. To connect the two elevator pushrods, you are meant to use the supplied 3 mm wheel collars. I differed from the manual here, since I had a spare coupler lying around. It’s the same principle, but it just looks a bit tidier:
Elevator and rudder servo. The coupler can be seen on the right. I’m sorry I don’t know where I got this lovely specialty item from, but you can use wheel collars (supplied), that will work fine.
I used a metal gear digital mini servo from Schweighofers own brand for the throttle, which brings me to the final part of the assembly – the engine, which we’ll deal with in part 3.
Photo by Karlheinz Gatschnig, www.http://gatschnig.at
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.
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.
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 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.