This is a slightly off beat subject both in appearance and model size, and I guess that was one of the reasons I was attracted to it. I hadn't gone out with the intention of buying a kit, but whilst browsing amongst the boxes and shelves in Spire Models I stumbled across this box labelled 'Kobuz'. The packaging was pretty plain, but there was a label showing the finished model and a few vital statistics. The manufacturer is Michel Clavier Modelliseme, no I hadn't heard of them either. I think it is a 'cottage industry' type of set up, definitely from France and judging from the stock in Spire they have a small range of slightly 'different' kits, not conforming to the standard Multiplex/Simprop fare. I opened up the box and was impressed with the contents, so money (plastic of course) changed hands.

Before starting on the kit building details just a quick word about the subject -

The full size Kobuz is a Polish design from the SZD stable, type number 21, this being the mark 3 version. It is a fully aerobatic design of 14m span built around 1961. The original has airbrakes and a retractable undercarriage, if space permits I will squeeze in a 3-view I picked up from The White Sheet. Browsing on the web produced a few photos of originals, all seemed to have a similar colour scheme and after all this was only going to be semi-scale. In Germany models of the Kobuz are popular in their aerobatic events, using aerotow for launching. They are always at a large scale, 1/3 or 1/2.5. This French kit is 1/6 scale and inevitably there are deviations made from scale, some I think to make manufacture and building easier. The main deviations appear to be increasing the aileron chord at the root end, making them tapered instead of constant chord, plus using an all moving tailplane.

So what is in the kit and how does it go together?

It must be realised that this is from a small manufacturer, so there are no glossy instruction booklets and not a lot of prefabricated extra parts, you get the basics and work from that, which personally I find more enjoyable.

The fuselage is a good quality epoxy glass moulding with white gelcoat. The seam isn't bad, needing a little sanding. The wings are white foam and veneer, the ailerons are pre routed out, attached at their tips and roots to hold them on the wing. The rudder is a foam/veneer part, the canopy is a clear moulding and there is a separate vac formed black ABS plastic base for it. The joiner is a 15mm round Dural tube (about which more anon) and there are a few bits of white epoxy board - about 3mm thick laminations of glass and epoxy, read on for more info on these. There is a tail crank and pivot rods and the tailplane is pre shaped, made from laminated balsa, complete with the joiner tubes installed - a nice touch. There are a couple of pages of English instructions, fairly well translated from the French ones I think, and a plan of the fuselage full size and a reduced size wing plan.

Putting it together.

The fuselage is pretty straightforward, the centre of the joiner is marked, it needs careful drilling and then trial fitting made with the wing panels to ensure all is square. Slight adjustments were required, so one hole was made slightly oversize and the joiner was coated with wax. It was then slid in place with cling film protecting the wing roots and the joiner holes on both sides filled with an epoxy and glass micro-fibre mix. I added an oval washer of 1/8 ply as additional reinforcement, there is no ply root rib used in the fuselage or outer joiner tube. A piece of ply is supplied cut to the external shape to fit inside the fuz, this just needs holes cutting in it for the servos (rudder and elevator) plus receiver location and switch.

At the rear of the fuselage the moulding is all open, both the fin rear and the platform above the tailplane. What happens here is that the tail crank and pushrod is installed and then the opening closed with a piece of the supplied epoxy board. The tail crank pivot is supported with more pieces of the epoxy board, and I'm quite sold on this method, gluing everything in place with 30-minute epoxy. The only modification I made was to cyano short strips of 1/8 square balsa inside to act as locating stops.

I used a carbon fibre push rod, well it was lying around unused, overkill maybe but it does give a light rigid pushrod. The tailplane halves are ready finished, the front actuating wire is located in plastic tube that grips the wire quite effectively whilst still allowing easy removal.

The fin is closed off with another bit of epoxy board, inset to give a shroud effect. The rudder is pivoted on a couple of 2mm bolts, top and bottom, the top one can be unscrewed to allow the rudder to be removed - oh yes, the fin top is capped with epoxy board, it extends over the rudder as per scale. The rudder is operated by closed loop, the wires enter the fuselage near the top, and this isn't scale but a practical solution to an odd layout caused by the scale design.

The wings are pretty straightforward, facing strips for the ailerons are glued in place (balsa strip supplied) and sanded to shape before cutting the ailerons free; this way the section and slight washout is maintained. The leading edge is obechi strip, pretty standard construction so far. The differences come next. The tip rib is ….. yes, epoxy board of course. The root rib is a bit different, epoxy board again, but it has moulded in a thickened (5mm) lump. This is shaped to fit between the wing skins after digging out the foam and acts as support for the incidence pins. These are glued in place and then the wings attached to the fuselage, the pins on the inside are located by more bits of epoxy board. It is certainly a different approach but works very well. The wings have grub screw set in to clamp onto the joiner, but so far I haven't used them, sealing the wings with tape instead. I installed a couple of the new JR 331 mini servos in the wings, these fitted flush in the pore cut holes, reinforced with a 1/64 ply base and a ply brace an inch or so wider than the cut out.

Cutting out the canopy finishes off the model, fitting it to the base, this was easy, the mould marks on the canopy were exactly right. I painted up a 1/6 scale pilots head and glued this inside before fixing the canopy - well I think it would look odd flying around empty.

The covering was Profilm, basically white with red ailerons and trim. If you have read the review in this issue of the Solution 2 you will know that I have come across an article about using Profilm. I used this again to (I think) good effect, including hinging the ailerons using the covering film. A little tricky on the first one, but it worked well and avoids a gap or the use of tape.

The model was balanced as per the instructions, casting the required lead and fixing it with silicone sealant. A 1400mah battery pack is fitted in the nose, with the fuselage shape of the Kobuz it sits on edge. I located the canopy with a couple of elastic bands, basic technology that works. The radio was programmed with control throws as per the notes, it all looked like it could be lively, so rates were set at 60% just in case. Up aileron was programmed in for braking effect.

So to the flying -

This took place on the same day as I flew the Solution 2, hardly ideal with such a light breeze. Certainly from the model appearance and size I think the sensible approach would have been to wait for a day with more wind and better lift. Given the current weather pattern this could be a long wait - so of course I was going to try!

We started with a hand launch from a long way back on Oxo. This was most promising, the resultant glide was very flat and trim was looking spot on - then brain fade followed and instead of doing the sensible thing (i.e. following Glyn's advice) and landing before it went out of sight, I did let it go over the slight hump. What a plonker! I pushed in some down to ensure it landed and went to see the result. Well all was Ok at first sight, but the heavy arrival had actually allowed the wings to flex forward and a slight bend was put in the Dural joiner tube - I wouldn't have believed it possible looking at the joiner. Of course a contributing factor was the lack of an anti crush brace in the fuselage between the wing roots - if I had put one in place the wings couldn't have moved and all would have been Ok. Live and learn (but I had learnt this years ago - no comment please). I have now added a dowel brace across the fuselage.

Well a this wasn't going to deter me flying again, I pushed the wings back together again - not checking closely of course - and with a short hesitation and brief discussion Glyn gave an excellent launch of the slope. That's when slight anhedral was obvious, talk about not checking things!

I must say I think we were both surprised with the result - the Kobuz soared really well in the light lift, that Ritz section is pretty good. Controls were really positive but not twitchy and I was soon able to start chandelles at the end of the runs across the slope. This was another thing that surprised us, on the basis of the test glide it was thought that maybe the model was actually too light - it wouldn't retain any energy to allow aerobatics, well so far this was disproved to my delight. The lift improved a fraction and with Glyn flying his Flamingo I was pleased that I wasn't doing all that badly against a far more efficient and bigger soarer. I didn't have enough height I thought to try a loop, but I went for a roll. Now I'm lousy at aerobatics, I'll admit that now. My rolls always go 'barrel shaped' or worse. This time I followed my usual practice dive for a bit of extra speed, pull the nose up and then over with nearly full right aileron - hello, what's this? A pretty axial roll going upwards! It certainly caught Glyn's eye, and quite frankly I didn't recover from the shock! Some more general flying around and it was obvious that the lift was dropping, so I thought discretion had better take over and a landing was called for. This was uneventful, a good flat glide angle but the up aileron was effective and landing no problem at all.

All in all I was very pleased, at home I finished off the model by applying trim colours, fin badge and lettering - all home made, nothing is supplied in the kit. A final word on the joiner - I couldn't see an easy safe way of straightening this, so on a visit to Spire Models I pickled up a brass tube of the right diameter. I then fished out a spare length of glass fibre joiner rod from an old model; it was about 4 mm too small in diameter. I rolled around the perimeter uni-directional carbon fibre soaked in epoxy, added a layer of glass and popped it in a vacuum bag overnight to force out the excess epoxy. A fair amount of sanding was required to remove the flash and reduce the diameter to fit inside the brass tube (yes, I overestimated things), and then with the new rod inside the brass tube I ran in thin cyano. The weight isn't a lot more, and shouldn't harm things, and I can't see it bending.

In conclusion I must say that so far I am very pleased with this model, I'm sure that given a bit more lift I will be cavorting around the sky in an untidy fashion and grinning from ear to ear. The kit is not of the pre fabricated standards found with the big German manufacturers, but it is different, certainly a distinctive shape in the sky, perhaps not to everyone's taste. It was not so cheap, about £200 I think, but with short run production that is probably about right. On the basis of this kit I think it would be worth looking at some other models in their range - pop into Spire Models and ask.

A few key parameters -

Model - SZD 21 Kobuz 3. 1/6 scale. Span 2.33m, length 1.22m, weight approx . 4.lbs. Wing section Ritz 2.30.12