SLOPING OFF...
...the Newsletter of Christchurch and District Model Flying Club for December 2021
SUKHOI 26 BY CLIVE SPENCER
Here (Now - Ed) is the winter of my discontent, as our bard put it. I have had this brain worm for many years, in fact since we all flew using seven or eight ni-cads, brushed motors & long lances sticking out of our trannies. It’s all Hewson’s fault? He built a beautiful Sukhoi which he used to fly at Kings Park. I loved this plane and coveted one as soon as I saw it. Trevor then decided to sell said plane & offered it to me at some outrageous price. I was very tempted but knew in my heart that I was not a good enough pilot in those days to fly it, so what to do? After all these years the only way to placate the worm was to bite the bullet & build said Sukhoi. I decided on a model that I could get into my car without dismantling it. This then dictated a max wing span of 1.4mtrs. Try as I might I could not find a plan of the correct size or quality. I also wanted to build the model mainly of white & blue foam, with a covering of 1mm balsa, of which I had a fairly large quantity. The only thing to do was to draw my own plan. I did some research regarding wing loading & aerofoil sections using Martin Simonds excellent book, “Model Aircraft Aerodynamics”. This resulted in a wing section of NACA 63018, a tailplane & fin section of NACA 009. The wing area is around 4 Sq/ft with the tail area of about 20% of this. I was aiming for an all up weight of around 4.5 Lbs giving a wing loading of around 18 ozs/ft.
I found a small outline plan on the internet & multiplied the dimensions X8 to give the required 1.4 Mtrs . Having drawn the outline I then added the 1mm skin & the internal structure to support the motor, battery & undercarriage. This is a flat sided box made from balsa & ply with the bulkheads added. The external shape being formed using a hot wired cut foam sections. Then came the tricky bit, making up templates for the external structure & wing & tail sections.
These were made from ply of varying thicknesses dependant on what I found in my supplies at the time. I hot wire cut the the foam sections & covered them with the 1mm balsa. I intended to make the cowling of fibre glass but then had a little piece of luck, I had in the workshop an old spare cowling from my Flair Sopwith Pup, which I offered up to the nose of the Sukhoi to find that it was a perfect fit, except it was about 12mm too short. So what to do? I either had to extend the fuz or live with the shorter nose. To extend the fuz at that stage would be a massive job so I decided to live with the shorter nose. This had consequences for the balance later on. The wings where hot wire cut from white foam block, not the Recticel you see here, after attaching the tip & root templates, not forgetting to add two degrees wash out to the tip template. Having cut a groove for the aileron servo wires & servos it was then a matter of covering with the 1mm balsa. Because the balsa planks are only about 100mm wide they had to be joined lengthwise to achieve the required width. To do this just tape the edges together, fold & add glue to the joint, then fold flat, weigh down on a flat surface & allow to dry overnight . The balsa sheets can then be cut to size to fit the curved surface of the wing & glued in place using white wood glue. This can be diluted with a little water if required but don’t overdo it. The wings are then reassembled into their foam blocks & weighted down. They will take a long time to dry so you have to be patient here. When completely dry I trimmed the panels & fitted the l/edge & servo attachment blocks. At this point the ailerons were cut out & the attachment blocks fitted. To fit the wing halves together I made up a wing brace of 3mm ply to cover about thirty percent of the span, an attachment brace for the front location dowel, & and a plate for the rear attachment point.
Here is my hot wire setup. It has .6mm dia Nichrome wire x 70 cm long, & will cut blue or white foam at 5amps x 12 volts. A word of warning here, do not under any circumstances use the Recticel foam shown here. It will not hotwire cut? I increased the power to the cutter up to 200watts. The wire was red hot & it would still not cut properly. I suggest you only use white or blue foam. (I tried Recticel and as Clive says, a waste of time! - Ed)
Having got to this point all the external blocks, the tailplane & fin were attached to the fuz & glued in place. I cut the slot for the wing spar & joined the wings together using epoxy glue, then fitted the front location dawel & the rear attachment plate. The whole lot was given a light sand & the wings assembled to check the fit. Up to this point I had ignored the canopy, I intended to make a block & form my own using a plastic bottle & a heat gun, but having looked at the the Vacform web site, I saw that they could supply a canopy to the exact size, & for £11.00 it was not worth the agro to make my own. I also found a cheap pilot on the internet. Having fitted all the controls the time had come to choose a colour scheme to copy. The one shown was found on the internet & Tim kindly supplied the covering & then the decals which were of his usual excellent standard.
So there we have it? Weight = 84oz, w/loading 20oz/ft2, 12 x 7 prop, 500watts on a 4cell lipo, Stall calc at 16.5mph. I had to add 4oz of lead to get the correct balance point , which the shorter cowl didn’t help.
Will it fly?
Well it does, the first flight was very fraught with a change of trousers required immediately afterwards. It resulted in a very hard landing, which resulted in a bent undercarriage & a loose motor. On reflection I had made the mistake of transferring the control movements from my other aerobatic model to this one which resulted in huge over control. Luckily the damage was soon repaired and the control throws reduced by fifty percent. Having no spare trousers with me I accepted a kind offer from Trevor to trim the Sukhoi for me which he duly did. It now flies very nicely although it is not a beginners model. That is the sum total to date as Covid grounded us for the best part of a year, and my flying days are severely limited at the moment. I hope this is of interest & perhaps encourages some of you to have a go yourselves.