Review on Sukhoi SU26 MX By Peter Hawtin

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Wingspan 81” Length 86” Wing Area 1241Sq in Motor:Zenoah 62 32oz Petrol Tank 24oz Smoke Tank Fitted weight 17.6Lbs. (8kg) (Includes 2 x 2000mha 6v Nicads & battery backer and smoke pump etc.) The fuselage is a glass composite construction using thin glass cloth over Kevlar honeycomb. This makes it incredibly strong, and Very light. For those of you who have not used honeycomb composites before will find they cause some interesting challenges to the builder. You can not just bolt through the sandwich as this will crush and break. Non-structural items like switches are fine, just don’t over tighten the screws. Structural areas have to be worked on before the bolt goes through. The simplest way is to drill the hole then clear the honeycomb with a small “L” shaped piece of wire before filling the space between the glass skins with epoxy. The wings are standard foam veneer construction with a VERY LARGE alloy scaffold tube used for strength and mounting the wings. The tube is inserted into the fuselage and connects with a smaller link tube fixed inside the airframe. The wings are held together by two large wing bands and this gives a very strong and stiff assembly. The plans give the option of scale wide cord or narrow cord ailerons. I chose the wide cord option, but I may regret this after seeing the finished article!! The tail is similar to the wing, with foam veneer construction. The tail-planes are fixed to the fuselage and with hindsight I wished I had made these removable as they suffer from considerable hanger rash moving the model around. Both the Elevator and the ailerons use the profilm as the hinge. This initially worried me as it sounded very weak, but the process makes for a very strong hinge reducing the risk of flutter. The fin is part of the fuselage and only has to have a strip of hard balsa glued into the rear edge to support the rudder hinges. The rudder follows the same foam veneer construction as the wings etc. The rudder differs from the other flying surfaces in that it uses heavy duty Robart hinges. The rudder is powered by a 9kg forward-mounted servo with closed loop connection. I was sad to find that this close scale model did not have any thingypit detail, the canopy designed to be painted black instead. I decided I would attempt to add some detail and polished the inside of the canopy to give some view of the inside, and constructed a thingypit resembling the original. Due to the positioning of the radio gear under this area, it was not possible to get full depth and have a pilot sitting in there. The model had all the flight surfaces covered in profilm and the fuselage painted in Cellulose. As this was the first time I had sprayed anything since rebuilding a Mini in my teens it was nerve racking to say the least. The glass was rubbed down and primed before several coats of topcoat applied. With hind sight I would of changed the primer to one of the thicker types as this would of improved the cover over some imperfections. When it came to choosing the power for the Sukhoi, I went in search of the “experts” again but you know, sometimes its best to go with your own best guess! The manufacture suggested anything from a 45 to a 62 (that’s cc not Cu) A friend who had this actual kit with a Zenoa 62 suggested the 42 is on the small side and would require additional weight in the nose. The radio is somewhat more substantial that my other models! and after getting much advice from the “experts” I finally spoke to some very helpful guys from the Large Model Association. They had a wealth of information on what is the legal requirements are as well as what is just good practise. Any model over 7KG must have a fail-safe device fitted, I chose to go with a PCM receiver that gave me the full 9 channels the Sukhoi requires and the fail-safe. The receivers sockets are filled up by the two channels for the ailerons, two for the elevators, one for throttle, one for engine kill, one for rudder and finally one for the smoke system, the ninth is where the battery checker/backer plugs in! The power to feed this hungry beasts radio came from two 2000mha 6v battery packs connected to the receiver through a battery backer from S.M Services. This has caused some problems, firstly it is supplied with screw terminals and secondly the diode causes a 0.8v drop from the batteries. This may seem insignificant, but on large aerobatics aircraft they need every bit of speed and power from the batteries they can get. One are of concern on the S M Services backer is their use of screw terminals. I am sure they have done it this way with no problems, but I prefer to have direct connection where ever possible. The screw terminals on the unit were simply removed and cables were directly soldered to the unit.
The smoke system is an electric pumped system controlled from the transmitter. This gives the advantage of being able to adjust the flow to suit you requirements. It is a little more expensive than the old valve, servo and crankcase pressure method, but far more reliable and safer for your engine. Megasmoke manufactures the system and Simon will give you any help you need. Before the final system fit I decided to run the engine for its initial start up and run in. As this was to be done in my back garden, Julia my patient wife closed the windows and doors and left for the shops! I fuelled up the tank and primed the carb and with Zoë (my daughter) sitting on the tail, garden glove on my right hand I started to swing the prop. It took about 20 swings of the 22inch prop to get it too fire the first time, then all hell broke loose. The motor burst into life with Zoë hanging onto the tail. I had put some rope onto the wheels just in case, good job too as Zoë dug her heals in to hold back the bucking monster. This is where Dad happily oblivious to daughters struggles went power crazy opening the motor to full! That was when I realised the rope wasn’t holding, as the Sukhoi and Zoë started to move across the lawn! With the full fitting of the radio, fuel system and control systems complete, the next session was off to the field to finally set the carburation before the cowl was fitted. The best thing about these petrol motors is the carb, once set, they don’t change much. So they are almost set and forget. I had promised to show the model at the next Friday club meeting so no flying just yet! (Much to the disgust of all present!) With the help of another club member, who was brave enough to stand at the back to hold the Sukhoi the motor finally burst into life. At this point the people standing a safe but close distance took a step back!! 22 inch of screaming prop is enough to un-nerve even the bravest of soles! A note to others who plan to use petrol engines with the walbro carb; if it has not been used for a few months (years) the it will almost certainly have dried out and the rubber perished. The carb has a pump built in and this once it has dried out stops working. The fix is to get a service kit that costs £10 and takes 15 minutes to fit. I get my spares from Glens Models, who is very helpful with information on the Zenoa engine. The mixtures were checked and a range check with the motor running on full completed. The motor was run for around 8 minuets at half throttle to see what the fuel consumption was like. I had fitted a 32oz tank for Petrol and a 24oz tank for Diesel (smoke system). Based on the amount of fuel used, the Sukhoi’s tank should have enough petrol for a 45min-flight! Maybe a 24oz-petrol tank would be enough! Happy with the results the motor was switched off, and the silence that followed was wonderful. First major problem to overcome is the noise! The Zenoa is fitted with a cast Aluminium silencer specially ordered from the US. They obviously don’t have the same noise problems we do and the sound emanating from the twin drainpipes may be aircraft like, but not good for the neighbours. Further tests at home showed that the “silencer” is not the only problem and much of the noise was coming from the prop. The prop fitted is of the old square end design, full size modern turbo prop airliners dropped this design due to the lack of efficiency and excessive noise. The favoured design now is the “scimitar” shape that reduces tip vortices increasing the power and more importantly reducing the noise dramatically. Tests using a prop with this design reduced the noise to a more acceptable level. As this was my first venture into models of over 7KG it was several weeks before the weather and more so my courage allowed the Sukhoi to venture into the air. A quiet afternoon away from the general club members was chosen. A call to my patient friend Brian to see if he would help and we were ready. It has taken some time to get the hang of starting petrol motors. The main problem is not getting it “wet” enough. On this day the motor was warmed up in the pits, stopped and then taken out to the flight line. So with no more excuses it was lined up, throttle opened and the Sukhoi was airborne in 30feet. Brian acted as my conscience all through the first flight. I did several gentle circuits to check the basic trim, then spent the rest of the flight checking the overall flight characteristics. I am please to say it is one stunning aircraft to fly, it goes vertical at just over 1/3 throttle and is fairly stable at slow speeds. The subsequent flights have shown that my skill ( or lack of it) coupled to a small 40mtr strip can lead to some interesting moments, but in the air it will do anything you ask, and then some.