3 metrowy YAK 55SP od Composite-Arf

Carefully packed and everything as it should be, as always – this is a reliable firm.

We start work on the Yak by attaching the engine hood. As always, we use 3mm blind nuts, glued in back-to-front.

I ordered a second undercarriage set. The red one will be for practice, and the yellow one for displays and competitions.

The place where the engine is to be attached is marked and drilled. The tools needed for this are a wife and a ladder.

The plexiglass can be initially adjusted by positioning it on the outside of the cabin.

This time, for gluing in the cabin I wanted to try out the American colourless E6000 silicone, which is recommended for this type of work. But before I had spread glue around the edge of the cabin, fastened the cabin to the fuselage (four screws) and put the plexiglass inside the cabin, the glue had dried.

Sadly I scraped off the silicone and did it again using my tried and tested method as used when I built my Extra 260.

I always glue in all switches using silicone (without using any bolts or screws to fasten them). This way the fastening is flexible, and there is also good sealing in case of sudden showers.

Fastening the servos was horrific. To obtain the desired displacement of 80 mm up and down, I had to battle for almost two days. Comp-Arf had provided the servo attachment too low down (on the pictures you can hardly see the servo arms, they are so low). I could have taken the easy route and used two-inch servo arms, but this would have risked possible flutter with only two servos per aileron. So I used 1.5-inch SWBs and had to go to a good deal of trouble to match the length of the pushers with millimetre accuracy. But in the end I succeeded in achieving the displacement I wanted.

The engine part was ready. The Yak is a fair-sized plane, with plenty of space inside. It is therefore not a problem to position the various devices so as not to touch each other. Unfortunately it wasn’t possible to fit in my favourite Zimmerman silencers – they were too thick. I bought slimmer MTW silencers, with a diameter of 70 mm.

Now comes the decision about the internal equipment. We’ll use the latest Power Box product, Power Box Royal, which has five matchbox channels and a display like in a Boeing. There have to be two receivers with synthesis and lithium ion batteries.

The inside is ready, so........

I’m not too happy with the fact that the horizontal stabilizer can take only one servo – it isn’t possible to mount two. There now exist such powerful servos as the JR8711 (25kg), but nevertheless it would be much safer with two.

..time for a trial start-up of the completely assembled model (without the hood, for better cooling) on a stand specially adapted for that purpose. While the engine runs for about 20 minutes I have time to set the transmitter, regulate the engine revolutions for flight and landing, and check the range with the antenna down.

The model is now ready for its first flight!

On its test flight, the Yak proves to fly beautifully, straight and with good stability. Razor flight is ideal, without any mixer. For ordinary aerobatics I think it flies a bit better than the Extra 260 – of course these are just nuances which someone who has flown both models will notice. It does snaps very accurately and easily both horizontally and vertically, but a bit too fast during descending flight – I’ll make some kind of mixer and it will be OK. But I also noticed that its flight is extremely parallel – in spite of an oblique wind of about 5m/s, the model kept flying within a zone ideally parallel to me. With an Extra I would have had to sweat some to keep it on the proper line of flight.

I don’t know how it will be with 3D – I haven’t tried yet, because the engine is still too “fresh”.

Ends.