Sukhoi Su-26

Shortly after getting into RC aeroplanes, and while learning to fly on a 'trainer' aeroplane, I decided to build one. I had looked at many models, but one in particular caught my attention: the Sukhoi Su-26. This project, although not yet 100% completed, was started in 2002, and was functional (i.e., flying) by 2004. Below is a image gallery of my Sukhoi in its current state.

After scouring the internet for Sukhoi plans, I came across some that were created in AutoCAD. I had already had a brief encounter with AutoCAD, seemingly making this the perfect project to continue working with the CAD package. However, the plans would create a '90 size' aeroplane; a bit big for what I wanted. The first task was to scale the airplane plans back to a '46 size'. I can assure you that this was not as simple as changing the drawing to printing scale!

Another modification that I decided at the time to be worth while was to make the wing one piece. However, in hindsight this is not always beneficial, particularly for mid-wing designs (such as the Sukhoi Su-26). Once I had altered the plans, I made up a one piece wing using the same construction ideas in the original plan - balsa ribs, spruce spars, and 1.5mm balsa sheeting. Unfortunately, this turned out to be a bit heavy for my liking. On my second go, I decided to change the construction technique. This time, I hotwire cut the airfoil core out of polystrene, and added balsa/spruce/plywood reinforcement. I also cut large chunks of foam out from the wing, and strengthened areas with 1.5mm sheet balsa.

The second wing was indeed lighter, but after gaining wisdom I recon I would have been better off making the wing entirely out of balsa with spruce and plywood reinforcement, but been more conservative on the balsa sheeting. I would have also made a better attempt at selecting the right density balsa for the particular job. Another weight saving method would be to use mostly CA for gluing the pieces of wood together.

Later on in the project, I started working on a mold for creating the cowl. Initially I planned on making it from a two piece vacuum formed plastic. For this, I made a wooden mold on a lathe. Considering the design of the cowl, it would not have been possible to incorporate an adequate relief angle to mold the plastic cowl as once piece. Thus to make the mold easily separable after I had finished machining it, the plug was created from layers of thick MDF sheets (with the laminates aligned to its centre line). What's the worst that could happen with this? Although I had put in measures to prevent it from parting, while working on it and one half hit the roof! It was lucky that no one was hurt - a 15kg rotating bit of lumber can carry a bit of kinetic energy. After fixing it up and gingerly continuing on, I finished the plug, separated it, vacuumed formed the two halves, and joined them together.

Unfortunately the plastic cowl turned out to be a bit heavy (I had to use thick plastic for it to hold its shape), and it was a bit ugly (its hard to nicely hide a join line on a mostly cylindrical shape). I changed tack, and tried fabricating a cowl out of fibreglass. For this, I created a polystrene plug, sanded it smooth, covered it with packing tape, and applied parting wax. I then fibgreglassed it using two layers of 2oz cloth and epoxy laminating resin. I had a hard time removing the piece from the mold. Fortunately, the plug was only made out of polystrene so I was able to remove it taking chucks out of it. The fibreglass part was a bit flimsy, so to stiffen it up, I added carbon tow to various areas. I also reinforced areas where I was planning on having mounting holes. Finally I had a cowl that I was happy with!

I also had to create a mold for the plane's canopy. Again, I tried a few different materials to make the plug from: MDF with varnish, modelling clay, and fibreglass covered foam. I can't recall which one I ended up using, but a canopy was made after was vacuum forming over the plug. I was not entirely happy with it canopy, as it had minor blemishes but also some wierd stripe pattern on it. By this stage, I was reasonably sick of this project, so I pressed on. The landing gear was made out of a sheet of stiff aluminium, and bent over a shaped wooden block. An image gallery of pictures during construction:

A few months later, the rest of the frame was complete and ready to be covered. This was the first time I had used RC covering film. The main skill as I recall was to tack both sides of model, and progressively shrink it using a covering iron (or in my case, mum's clothes iron!) and a heat gun. If you do one side at a time, you'll find that the piece will be warped by the tension of film. Also, remember that heat is what activates the glue, rather than pressure. Thus, to avoid marking your balsa model, don't press the iron too hard.

Specifications of my Sukhoi are:

  • Wing span of 1.3m
  • All up weight of 2.5kg.
  • Powered by a 52 ASP two stroke glow engine. It was propped with either a 11x5 or a 12x6. 
  • Five Hitec HS-422 servos (one for each aileron, and one for throttle, rudder, and elevator). 
  • Hitec 8 channel Supreme receiver.

A short clip of its maiden flight:

After knowing how much time and effort I spend making it I'm a bit scared of flying it. It is also a lot more inconvienient flying this size of model. Usually these size aircraft are flown at RC aeroplane club grounds, as it is (generally) illegal to fly in non-designated parks close to civilization. Glow powered models are also a pain. You need to first start the motor, and once that is going and you've landed it, they need cleaning as the exhaust sprays castor oil all over it. Every so often you also need to land dead stick, although this often makes things more fun. These days I fly small electric aeroplanes (sub 1m wingspan), and my Sukhoi Su-26 is back at home. It is currently stored in the living room back at home:

Maybe when I get an urge to fly it again, i'll finish it off - the cowl needs to be painted, it needs a pilot figurine, I need to make and apply some decals, and fix up odds and ends. If I were to build a Sukhoi again, I would have stuck with the original plans, or found ones that were of the right size. However, if I simply wanted to fly a Sukhoi, I would probably just buy an ARF kit as they are cheap, saves you so much time, and there is much less pressure while flying it (I could just buy another one if it crashed).

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