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Build Log
Robin Fowler YAK-9
Greetings one and all!
Honest Guv I didn't mean to do it - It was an accident! Really... A fortnight ago I would have not even thought about it. But there were four 30" x 40" foam boards standing on their ends in the workshop gathering dust from years ago... and I do mean years. When I checked, the drawings were done (such as they are) in October 2015 and the template sheets completed in May 2016 so they were printed and stuck onto the boards soon after. One day I noticed that the spray glue was starting to lose the will to live and since I had just returned from a visit to the flying field and so enjoyed the cameraderie that I realised I had been in danger of losing the will to live just like that glue and that what I really needed was to build something. That should stop the rot! Before I really knew what was happening I was carrying those boards into the house with a happy smiling inside feeling. I knew perfectly well that there were too many models in the workshop to stand any chance of building in there, but now my lovely lady had gone home I could work in the house. There was already a recently repaired 12 foot span DeHaviland Heron sitting on the table in the dining room so I brought the jigsaw puzzle board downstairs and laid it on top of the coffee table in the middle of the living room and started cutting. I even put a new blade in the knife! This was serious! Cutting bits out is pretty boring so it is normal practice to encourage ones'self by looking at some pictures of what it might look like eventually. I have no idea where they came from apart from assuming they were downloaded from the internet - so I have no idea who deserves credit for them. Robin |
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The design done in 2015 was for a 72" span model which I would build using foam board. That's artist's board made up of 3mm of foam sandwiched between 1mm card faces. An interesting feature of this board is that although it is probably a little heavier than if using balsa construction, the foam is under tension so if one card face is stripped off, the remaining board adopts a slight double-curve shape which can be straightened in one direction so as to be suitable for wing surfaces.
For now though I had a couple of days of simply cutting out the parts needed from the four sheets of templates. Not the most enjoyable part of the build! Robin |
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This is going to be very entertaining! Excellent choice of subject. Cheers!
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Welcome aboard StangBanger! Should be plenty of entertainment ahead as I do tend to find I'm having to dig myself out of holes at fairly regular intervals. It's often because I dash ahead without checking my own drawings. Consequently there is a steady stream of work-arounds in hand.
One example is with the retract system I put together back in 2016. I thought it would be a good idea to gain some mechanical advantage by installing a lever arm so that the units were operated by about half the effort expended over twice the distance. The problem was that they didn't like being pulled in a direction even slightly off in-line, so the lever pivot had to slide sideways in a slot. That introduced a large amount of friction which negated any mechanical advantage I had hoped for. I took the lever arms off and gave up of the idea of using rack and pinion drive . The retract units are mechanical and cost about £18 back then. I also bought a pair of HK oleos and Dave Brown 4" dia. foam wheels all of which are already assembled. Robin |
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Once foam board starts sticking together it builds quickly. It comes from cutting the whole of a wing surface out as one piece. In this case I was a little optimistic as I was hoping to wrap the top sheet around the leading edge to meet the undersurface at the bottom spar. This meant a very sharp bend which I knew to be difficult to achieve as I had tried it before. Still I was going to try again with a backup plan in mind if it all went horribly wrong (again)... I would cut off the bit that wraps under and stick a balsa leading edge on. From there on it would be a case for lightweight filler.
I was using white PVA wood glue (from the local DIY) for the foam - because it takes about the same time to dry as Uhu POR and costs about a tenth of it. Robin |
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Next instalment...
I have yet to decide on the method for operating the retracts. I have a sail winch that saw service in my Short Stirling until I decided that there was so little benefit from having flaps on a model that light and took the system out saving about a pound of weight. It used a rack on either side of a pinion to push (or pull) in both directions but I have experienced difficulty betting the pinion not to slip on the shaft I have also ordered a motor and gearbox to which I can fix an operating arm which will be more reliable and has the benefit of applying maximum effort at the ends of travel where it is needed for locking. Robin ImagesView all Images in thread
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Once the control runs were in the wing of course I could get the upper surfaces on. I mentioned the fact that with one side stripped of its card face foam board takes on some double curvature due to the manufacturing process that leaves the foam layer under tension. This can be used to advantage but there are limits. The first pic is to show the natural curve the upper surface takes on, while the rest show the top sheet added. It's a big wing so I only glued the top sheet on between trailing edge and main spar. Once this was well dried (left overnight and some of next day) the front section can be glued up and bent over the sharper bends using a combination of indentations by pencil and heat gun.
Robin |
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With wings surfaced I had a bit of a head-scratching session trying to figure out how I had intended the parts labelled 'wheel covers' to go onto the wings. These form the angled inboard sections which include the leading edge air intakes and bend back to the wheel wells. The air intakes are very much needed with an otherwise totally enclosed engine bay and battery positioned well forward. Air from the intakes is directed forwards round both sides of the batteries and will be ushered out again via the under-belly radiator which will be arranged to extract it by venturi effect.
The front intakes will be made out with a couple of applications of lightweight filler sanded down. I still have a lot of decisions to make. Motor is one big question mark. I have a used AXi 42xx which needs a new prop holder. It might do the job - but I also have a pair of Tornado 5055 motors with 70A ESCs which I bought from a fellow modeller who was giving up due to deteriorating eyesight. They only did a few seconds in the air I believe. I didn't see the reportedly brief flight, but the AXi did several hours in my own design 94" DeHavilland Leopard Moth. Somehow I think the 5055 will be more likely to swing the 20" x 8" x 3 wooden prop I built for the model when I drew it up. I am going to have to do some testing to find out how many cells each motor would take to swing a prop like this. A 4.5" spinner will also be required for this model. I would have to spend about £35-£40 for either a carbon fibre or aluminium spinner - which exceeds the cost of the basic airframe. I have to admit I am reluctant to spend so much on a single item and am temptad to try carving one from blue foam, Anyone tried this? Robin |
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Well, I'm now a bit stuck on the wing section of the build . I would like to get on with building the nose on, but think it would be unwise without having the geared drive for the main retracts installed and working. I could do some damage due to having made access more difficult. So rather than faff about doing filler applications in preparation for the finish it is better to start putting the tail together.
As will have been realised, the fuselage is going to be in two pieces with the tail section joining on to the wings at the main spar and rear spar positions. I have done this on a lot of different models now and like the way they transport and the fact that the method offers free access to all the radio gear and mechanics without the need for hatches. Fixing is achieved by a single dowel at rear spar position and an 8-pin DIN plug and socket automatically plugging in the rudder, elevator and tail wheel retract servos plus the connections that feed back positive or zero voltage to the main retract system all in one operation. I have yet to decide on the mechanism I will use to stop the tail sliding back in service, but I often arrange a cam system that throws a bolt sideways into the sides of the nose close to the main spar. The problem with this model is that there is no handy radome that can use as the operating handle as I had with the Short Stirling. I may have to hide the handle underneath the wings somewhere as the radio antenna is rather distant from the operating cam. Plenty of time for an idea to settle though. Time to get started on the tail. Robin |
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First job on the tail is strip inside card faces off the sides and score (with a pencil so you can see where it was done) and bend. Tight bends need the application of the heat gun, when the foam becomes easier to bend and when it cools it holds its shape. It also can also get hard and brittle so the amount of heat applied needs careful judging.
Balsa stringers are 1/8" x 1/4" cut from medium sheet and are mainly there to provide something to pin to at board joints. I'm indecisive about the type of glue to use. UHU POR is a little better than PVA as it can act as a contact adhesive, but it is ridiculously expensive in comparison. I am also using both pins and masking tape to hold butt joints while drying (often left overnight) and wooden clothes pegs also come in handy at times - though their springing can compress the foam. I'm going to get through a lot of lightweight filler! Robin ImagesView all Images in thread
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Once the fuselage sides are glued onto the structure at the upper position I could get on with adding the horizontal stabiliser. I had projected this onto a template consisting of a single piece so a very sharp bend would be required at the leading edges. I had found this to be quite difficult - impossible with and degree of certainty - so decided on a 1/8" x 1/4" leading edge strip glued inside once the foam is removed in the strip where it will glue in. This gave a quick way of bending the board back on itself.
Since the leading edge is at quite a high sweep the continuous surface is glued onto the fuselage tail first and left to dry once the spar is in place. A couple of simple ribs can be inserted before the top surface is glued down. The natural curvature of the sheet without one card layer pretty much pulls the tips down as needed. Just a little sanding of the foam to get a good glue surface and a bit of filler required. The connection to the wings requires sorting out before I could install servos and pull-pull system to the tail surfaces, (my apologies for a couple of blurred photos at this stage). I tidied up the receiver and stabiliser installation at this point. The tail has three servos in it, very close to the face plate, one for the retracting tail wheel is a Futaba 3003 and the elevator and rudder servos are a more powerful metal geared type - none expensive! Robin ImagesView all Images in thread
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Hi fzenere, I'd like to see your model of the Yak1 so feel free to post a couple of photos here if you wish. A little information like size and weight would also be of interest.
The next job on my Yak-9 was the adding of the fin. Nothing at all complex or difficult in this, just shaping foam board and adding a rib then it was added to the rudder post. In the meantime the geared motor for the main retracts arrived so, relieved that it was here at last, I cut a plywood mounting plate for it. I also decided that the simplest way of attaching the output arm was to cut a slot in the shaft to take the bent back brass arm I made up for it. I had been concerned that operation time might be a little long as I had chosen an output speed of 10 rpm. In fact it seemed a little fast - and, something that may cause a problem later is that it ran on briefly after switching off power. I will have to allow for that when I get round to arranging the micro-switching for the end stops. I also decided that I would make up my own retract system for the tail wheel. Robin ImagesView all Images in thread |
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Yak-1
Hi Robin, thank's from inviting me to post some pictures of my Yak-1!
Its overall dimensions are: wingspan: 1445mm (57") length: 1215 mm (47.8") weight: 2700 gr (8.8 lb) So far I did only two flights, but it flies well ! regards, francesco |
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Your Yak looks neat Francesco. Thanks for posting your pics.
Russian camouflage seems to offer quite a good choice of schemes. I am wondering about doing mine in green and brown (or grey?) with patchy white overpaint for winter - if I can figure out how to make it look reasonably authentic. But I have weeks to mull it over. I've just got back from a week away on holiday and haven't got back to my build yet. I'll probably post the next stage tomorrow when I have gathered myself together! Robin |
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