Side Stringers #2

Fabricated the side stringers tonight. After tracing out the stringer pattern from the original factory ones, I cut it out with my new shiny jigsaw cutter (bought a Bosch) and a new fine toothed blade for a smoother cut. It was a lot better than the first attempt with the cheaper ply, but no matter how hard I tried, I couldn’t seem to get a perfectly straight outside curve, and spent a heap of time sanding and filing down the outside edge to get it smooth enough. It still isn’t perfect, but I’m hopeful it’s good enough to get away with it, and that it wont show through the fabric.

I then used a new router bit with a tighter radius to put a bottom and top curve onto the outside edge, and used a file to debur the inner edges. Then I set about fitting them. I made the stringers all slightly longer, and had to manualyl fit an adjust the indentations at the end of the stringers to match the tubing layout I have. This is because my S-1S plans  show the old C tube layout. Strange. Anyway, after much trial fitting, I got them as close as I could, and rilled holes and attached them to the fuselage as a trial fitting.

This happened for all four stringers. The bottom ones I did slightly differently. Instead of just tracing out the stringer, I put a curve in them and clamped them to the bench and cut them out with a curve already built in. I had flexed the top ones, and they didn’t move, so I figured having less tension in the stringer would be a better bet – considering this is ply, and not straight wood.

This took a long time, made a lot of sawdust, and when I almost knocked the fuselage off it’s stands, realised I was tired and needed to call it a night. I’ll tackle the lower bulkheads and stringer tomorrow.

Time expended: 4.5hours. Total time: 612 hours.

Hoop Pine

Did a bit of research today on locally available woods. It seems Hoop Pine is the best option for me. It is readily avalable in both ply and plank in various thicknesses and length from a local special timber shop that brings it in from Australia.

Hoop pine is listed in Part 108 of the Australian Air Navigation Orders as an approved wood for propellor construction (and I know the Jabiru propellers are made of it) so it can’t be that bad. Checking the detailed tech specs shows that it weighs 15% more than Sitka Spruce, and is correspondingly stronger – so I wouldn’t use it on the wings, but for stringers – no problem!

According to Recreational Aviation Australia Inc, “Hoop pine is the preferred Australian timber but at an average 510 kg/m³ at 12% MC it is about 15% heavier than Sitka spruce, although stiffer and generally stronger and somewhat stabler…hoop pine is probably a superior timber for home builder construction – in weight and strength it is similar to Douglas fir but easier to work.” http://www.auf.asn.au/scratchbuilder/timber.html

That’s good enough for me!

Found this interesting table as well:

Note: the number preceding each strength value description is the table column number in ANC-18. The values for hoop pine are taken from ‘The Mechanical Properties of 174 Australian Timbers’ by E.Bolza and N.H. Kloot 1963.  http://www.auf.asn.au/scratchbuilder/wood_strength_values.html

Strength value	Sitka spruce	Douglas fir	Hoop pine
(4) Density at 12% moisture content [pounds per cubic foot]	27	32.5	32.5
(7) Static bending: fibre stress at proportional limit [pounds/sq. inch and MPa]	6100 42 MPa	6700 46 MPa	8160 56 MPa
(8) Static bending: modulus of rupture [pounds/sq. inch and MPa]	10 400 72 MPa	12 300 85 MPa	13100 90 MPa
(4) Static bending: modulus of elasticity [thousand pounds/sq. inch and GPa]	1460 k 10 GPa	1550 k 10.7 GPa	1880 k 13 GPa
(11) Compression parallel to grain: fibre stress at proportional limit [pounds/sq. inch and MPa]	4200 29 MPa	4850 33 MPa	4920 34 MPa
(12) Compression parallel to grain: maximum crushing strength [pounds/sq. inch and MPa]	5500 38 MPa	6500 44.5 MPa	7060 48.7 MPa
(13) Compression perpendicular to grain [pounds/sq. inch and MPa]	810 5.6 MPa	1100 7.7 MPa	?
(14) Shearing strength parallel to grain [pounds/sq. inch and MPa] [Higher value is better]	1030 7 MPa	1000 7 MPa	?
(15) Hardness [load required in pounds]	500	640	?
(16) Tension strength parallel to grain [= modulus of rupture ]	10 400 72 MPa	12 300 85 MPa	13100 90 MPa
(17) Tension strength perpendicular to grain [pounds/sq. inch]	131 0.9 MPa	130 0.9 MPa	?

So today I went off and found this place called PlyTech, and bought a sheet of 9mm hoop ply. It is the ply that meets specification AS/NZS 2272  and the British Standard specification BS 1088. I took a photo or two for fun too as a memory.

 

Side Stringers #1

Although I have stringers that I bought, they are designed for the S-1C fuselage, which has slightly different tubing at the tail, and as such does not fit perfectly. It’s not a big job to make a modification and glue some wood pieces in, but since I have to make lower stringers anyway, and have an abundance of this Birch plywood I bought, I decided to try it. This way, I can return the stringers to the guy who sold them to me, and I’m sure he’ll be happy.

Turned out less successful that planned. My jigsaw is not the best quality, and te blade not the best for fine cuts in wood, but that was not the problems. The Birth ply that I had bought turned out to be a little low quality for the job. After tracing out onto the ply, I ut out with the jigsaw, and then used a router to smooth the outside facing edge. I just felt pretty flimsy when I was done, and my mind was firmly made up when I flexed it a bit by hand, and it split! Ok then.. no more of this stuff.. tossed into the corner. I’m actually glad. This way I can source good qualty Rimu or some local wood now that I am sure about wood strengths (read Basic Woodworking and all the Bengelis books which were excellent).

I will also buy a good quality jigsaw, and some fine cut Bosch blades.

Time expended: 2.0 hours. Total time: 607.50 hours.

Trim Control

Made the trim control today. I had to decide what aluminum to use. The plans say .090 thickness, and I have only .070 and .125. I decided to make both and see how strong they felt. I traced out and cut out both shapes, but felt the .125 one was light enough and left the other one uncut unfinished. THe major job was trying to get it looking smooth and neat. I found an interesting door knob at the local Mitre 10 which I like, and attached it. It came in a pack of two, so I added the other one on the other side, and ended up like it this way – reminds me of my old Yak 52. I may remove it and just keep the inner side, but I’ll decide that later.

At Mitre 10 I found Birch plyood sheets. I had a really good look at them as they looked pretty well compressed. I’ve not considered using local wood at all, but price for shipping spruce and plywood from ACS is a serious problem, so I need to be creative. I found out a local aviation shop buys local stuff of good quality (but that’s heresay, I’ll need to check that) so asked about the plywood. They confirmed it is hot water compressed. It’s not 1/4″ as per the plans, but 9mm (which is slightly bigger – just under 3/8″ so close enough). I bought a big sheet of it for dirt cheap and took it home for inspection. It has 6 sheets bonded, with two tiny “face sheets” on the outsides. I traced out the seat back as a trial, and cut it with a jigsaw to shape. It didn’t splinter the ply sheets, which is a good sign, but the face sheets did fray a little. I’m not sure whether I want a wooden seat back – was favoring an aluminum one like the seat pan, but I’ll make the wood one to practice working in wood, and maybe I’ll keep it – I’ll weigh it against an aluminum one, and see). I didn’t finish the seat back, and will use a router to clean up the edges tomorrow, before varnishing with an epoxy resin.

Time expended: 2.0 hours. Total time: 605.50 hours.

Throttle Control

Whilst waiting for more parts to arrive from ACS I decided to finish the throttle control. I’ve been eyeing the various options that ACS sells, but I’m not not sure I like any of them, to be honest. They look bulky, and like an “add-on”. I want my throttle to integrate into the cockpit and look like it was designed from the start, and besides I spent all the time making a shiny new throttle panel.

With this in mind, and my nephew Ben as a helper, I traced out the stock throttle from the plans onto cardboard, transferred to a steel plate and started cutting out with a grinder. I decided to go with .100 4130 plate instead of the .125 on the plans. It seems rather heavy otherwise and besides I have run out of the thicker plate. A lot of the other throttles are made of Aluminum too, so it can’t need to be so string, surely?

After cutting out and trimming with the grinder, I set about filing it down and round as I could manage, put the drill holes in, and folded it as per the plans in my bench folder. I then had to decide about whether to weld on a bolt for a knob, or something else. Decided on a piece of 7/8″ .049 tubing, cut slightly long for trimming later, and shaped that with a mallet to fit around the throttle plate. I welded it together, which was quite hard as the plate and the tube are so different that getting a consistent pool was troublesome, so threw a lot of filler rod in. I ended up smoothing the welds down with a grinder and sander to clean it all up, but am happy with the welds that they are good and strong.

 

I was pretty happy with the rersult considering I haven’t welded in quite a while. Since Dean, I bought my own TIG welding rig and a much better autodarkening helmet (my 3rd – don’t buy the chinese stuff!) and this made life so much easier. Admittedly it wasn’t very pretty, and I piled a lot of filler in since the two metals were quite difficult to get to pool together being such differences in width, but with a dremel to clean up the excess material, it looks okay in the end.

I then did a trial fitting, of course. Not having cork/buna gasket matetrial, I just cut out some Butynol which worked quite well, but I’ll still get the gasket stuff as it will be heaps better. Good new, it fitted perfectly in the throttle panel with only slight adjustment, and I sat in the cockpit and vroom noises with my new throttle control – yay!

Time expended: 2.50 hours. Total time: 603.50 hours.

Fuel Tank Straps #3

Started on the top rear strap tonight. I immediately knew I was going to have a problem. The tank is such a tight fit up against the cabanes that it is already tight and flush against the tank, never mind fitting in a .050 strap + 1/16″ neoprene channel from Aircraft Spruce. I pushed and puleld and jiggled, and decided I was likely to put a dent in the tank, or at least put too much tension onto the cabane, so decided to make another plan.

I have previously bought a strip of butynol, probably only 1/32 thick, but it is very flexible and strong and highly resistent to temperature and chemicals. I did some research on the material and feel it is a good replacement for neoprene, and fit for the job, see the specification sheet (Butynol Specification). I spent the evening cutting strips out of the material and using contact adhesive to attach them to the strap. Unlike the channel, it has no lip, so I had to sand down the straps for grip, and leave a 1/4″ overcut on either side to provide some room for movement before metal to metal occurs. It take 24 hours to set properly, so clamped it up, and will try again tomorrow and see if this will fit in better. I will probably have to do all the straps this way, so that I can save space at the bottom as well!

Time expended: 1.5 hours. Total time: 601 hours.

Fuel Tank Straps #2

Did the forward top strap today. I cut and fitted the strap to be tight with a few washers between the straps at the attach bushings, so that I can tighten them further. This seemed to work well, and it feels plenty tight. I’m starting to think I don’t like these rubber straps from Aircraft Spruce, and may just buy some rubber that I can glue to the inside of the straps. May end up being thinner, easier to work with, and provide a firmer grip. Busy weekend, ran out of time, and left the hardest strap, top rear, for next time.

Time expended: 1.5 hours. Total time: 599.5 hours.

Fuel Tank Straps #1

Wanting to get the turtledeck complete I removed all the cleco’s and drilled some remaining holes, then realised that I do not have the right sized dimpling heads for my rivet squeezer (still enroute from Spruce), so I moved onto the fuel tank installation.

The fuel tank is a very VERY tight fit between the cabanes and diagonals. Ater playing with it and jiggling it this way and that, I’ve worked out that unless I physically mallet grooves into the fuel tank, it will need to sit at a 1 or 2 degree angle around the thrust line. Honestly, I don’t see any major with this. I worked out the best way, and then tackled the fuel straps. Getting the straps to sit inside the rubber sleeves is a major effort. At first I thought to glue them in – disaster, so after acetone to clean up with the mess, decided to leave them unstuck. After ages I managed to trim the bottom straps down to size, put the holes in and do a test fitting. I had to notch away at the rubber straps in two places to get a comfortable fit, but this is okay, I think. The removable cabane requires some downward pressure to get it into place to the fuel tank is being tightly squeezed. With the top straps on this, this will be even more noticeable. After 3 hours I gave up for the night, and will do the top ones tomorrow.

Time expended: 3.25 hours. Total time: 598 hours.

Turtledeck Skin #1

I’ve been itching to skin the turtledeck ever since I cut out the blank in the aluminum sheet. With the front bulkhead painted a few times, I felt confident that I could use it, and so committed it to the project and set about working out how to line up the pre-drilled bulkhead holes onto the skin. I attached the cardboard cutout onto the bulkheads as tightly as I could with clamps, and set the whole thing on the fuselage. Once I had it lined up, I had a cunning plan. I shone a torch from the inside of the turtledeck, and the holes lit up the cardboard, and so I could mark the holes on the cardboard with a felt pen. This worked well, except for 2 or 3 holes that didn’t shine through where the cardboard was overlapped onto 2 layers. I then simply transferred the holes onto the aluminum blank by tapping pilot indentations through the cardboard onto the blank to act as drill guides. Neato, it worked. It was then a simple case of drilling holes through the pilot indentations in the blank.

Feeling pretty chuffed I did a trial fitting. Wrapping the aluminum around the bulkheads was a bigger challenge than I expected, as it has springback. Clearly I got almost all the holes right, but one on the second bulkhead was slightly off alignment, and this is noticeable from the inside out. I may have to correct this with a washer, or tap the bulkhead flange outwards a bit to compensate. I think it will be okay. It looks great to have the real turtledeck taking shape. Next time will be cleco’ing on the rear bulkhead and splicing the end to curve inwards near the tail.

Time expended: 2.75 hours. Total time: 594.75 hours.

Painting Parts #2

Painted the parts with the topcoat, a 2 pack acrylic urethane enamel, in white. Remind me to hang up the parts in future, as even after waiting an hour for side one to dry, and turning over, when I came to check them the next day, they had stuck to the chipboard underneath and had little fragments of wood embedded in the parts. Irritating, but I knew better. Fortunately most of them rubbed off, but some didn’t. I packed the all away for another day.

I also drilled test holes for the headphone jacks. They have a little finger that juts out, presumably to prevent the connection from twising in the panel. The clearance is very tight, and even though I was careful with a tiny file, the gap still projected out behind the washer. You can see it in the photo. On the real panel, I will need to be very careful to hide this.

Time expended: 2 hours. Total time: 592 hours.