More work on the tail feathers

After an unforeseen break I started building again. Past few weeks I cut the ribs for the underside of the stabilizer en glued them in place. A lot of sanding, cutting and trial fitting as all of the wood apart from the ply core is hand cut and scratch built. Very happy with the result though:

Next step is cutting the inserts that go in-between the ribs. An easy but time-consuming job, as we need 28 pieces of spar, and 52 slanted blocks that go in the direction of the ribs. I like to build as straight as possible without using too much glue, so I’ll take my time for this step in the build:

The leading edge of the control surfaces will be sanded round later on, and the Robart hinges will sink into them so the hinge point lies between the two leading edges of the elevator.

By the way, I discovered a small error in the drawings, as the front part of the outer rib on the elevators shouldn’t be in line with the aft part of the rib. That way, the ribs of the stabilizer and elevator do not align. So I need to cut away some pieces of the elevators and glue in new ribs offset to the outside of the elevator. No big deal and certainly part of building a prototype. I plan to finish up all the work on the tail feathers before going back to the fuselage again (rounding all of the edges, preparing for the control horns and hinges etc.).

Upper stabilizer ribs done

Made some progress on the stabilizer. First I built the outer edges (1/8″ on the front, 1/2″ on the back) and then the crossbar. After that, I’ve cut all ribs to the right length and made the notches for the crossbar:

When the rib fitted nicely, I cut the slope in and glued it on with the stabilizer clamped to the table to ensure keeping the whole assembly as straight and true as possible:

All ribs are cut from balsa sheets. First using the balsa cutter to get the correct height, which is quick and easy. Then, mostly the skewed notches take some time, but it’s fun to do and so much nicer and stronger than straight notches of pre-cut ribs. I enjoy these scratch building parts of the build. When the notches are finished, I cut the slope in the rib using my scroll saw machine:

Finished all the ribs on the upper side of the stabilizer! I’m very happy with how straight and true it came out:

A few close ups of some nice details:

And for scale reference… could’ve been a flying wing too if you look at it like this:

Firing up the engine!

What a lovely evening! Tonight I started my ROTO for the first time (or any gas powered engine for that matter). First I checked all the settings on my transmitter, throttle travel, choke, spark switch.. fueled the test stand, flipped the prop once with choke, turned the choke off, flipped it once more.. and there it was, the lovely purring sound of a happy four-stroke two-in-line engine! It ran so smoothly… and it didn’t stop until I flicked the ignition switch on my remote. Loved it!

After running idle for a little while, it already started picking up very nicely when revving up. The video below isn’t completely idle, just a few clicks of throttle, but it does run like a charm. It was a bit louder than I expected it to be, but running it underneath the overhang of my garage roof didn’t help at all, and even more so because I was running it in the corner of my house and garage, I don’t think measuring the sound level was representative, so I’m still hoping it’ll be okay in the end, also because the exhaust has to build up a layer of carbon inside for the optimal muffling effect.

Starting stab & finishing engine

It’s been a little while since my last post but I haven’t been idle. I started building up the stabilizer. To ensure it is nice and flat, I clamped it onto the workbench while gluing the leading and trailing edges:

Working my way around the stabilizer:

After this, I can glue on the main ribs and by then it will not deform anymore and I can continue building up the rest of the structure without the guidance of the building table.

But first, let’s get back to the engine. I look forward to hearing it run, want to finish the firewall so I can continue building up the fuselage, and I’m curious to learn how my custom muffler holds up!

I finished the installation of the muffler by joining the front exhaust elbow to the muffler using the silicone tube provided by ROTO. I did however replace the tie wraps with spring clips, as they are way more sturdy, air tight, and easier to take on and off. I also think they look better than tie wraps too:

Finally, the propeller is on! I drilled the holes and attached it, what a pretty sight this is! Can’t wait to see this running…:

But before I can enjoy the sweet sound of a multi cylinder four stroke, I need to build up my test stand for this last and final phase of preparing the engine to go into the model. I’ve placed the throttle servo and fuel tank at the exact same height as they will be in the fuselage, though in there it will be a lot tighter:

Also, one of the things to tackle was building up a fueling station, as I am new to flying on gasoline, I didn’t have any field equipment either..

So now, I’m ready for breaking in the engine :-). To be continued!

Working on the elevators

The elevators are built with a solid piece of balsa on the inside to support the control horns. On the plans, the 3 mm outer edge continues all the way to the inner end of the elevator, but I decided to build this whole area out of one solid piece of balsa – this is easier to build and also makes it easier to get a smooth transition from the block to the edge.

I first laminated a piece of balsa out of two 1/4″ thick layers, sanded the perpendicular edges straight, and then I placed it on the elevator to trace the outlines and cut the piece to the right size, including the opening for the control horns:

Then, I sanded the piece to shape. It’s slightly oversized (by 0.5 mm) because I didn’t yet glue it on. I have to build four blocks and this way it’s easier to compare the result and modify them until I have four identical pieces. After gluing them on I will do the final sanding:

It’s a complex shape though, because the thickness should be 3 mm all along the outer edge:

While the inner side runs in a slope from 1/2″ to 1/8″:

I have developed a neat little trick for this, which I often use when sanding the more complex shapes. I just cover all the parts that need to stay untouched in blue tape. The blue tape is less sticky than the white one, it’s easier to bend around curves, and – most important – it creates a nice visible contrast with the color of the balsa wood. This is much easier to see than a pencil line, and you can never sand too far, as the tape prevents you from doing so:

Then I sand it roughly to the right shape with a coarse grit (80) on my simple self made sanding block. I usually prefer to move the piece over the block instead of the other way around, which is more precise and easier to do:

And I end the sanding process with some finer grit sandpaper. Because I need to make four blocks, I keep comparing the result while sanding, to make sure I end up with four identical blocks. The glue line of the lamination is an extra guidance here, because it indicates if the pieces are identical (\the glue line shape should be symmetrical). Here you can see I’m almost there:

And when finished you can unpack your gift and you’re treated with a very clean and straight edge as a result:

Before I will glue them on, I first will test and adjust the control horns, after which my elevators are almost finished!

Started preparing the test stand

I’ve once more adjusted the test stand for the engine, now for the last phase: I’ve drilled some more holes for the fuel lines and throttle pushrod, cut away the muffler template section completely so the muffler can vibrate freely while running in the engine, and I’ve fuel proved the front and side by coating it in PU varnish:

Now it’s time to plan installation of all the components. I’ll try to mimic the actual available space in the airplane as much as possible to prepare for the actual installation in the airframe. This is my starting point:

I’ve attached the engine again to start the installation of all components. Couldn’t resist sharing some more pics of the engine and exhaust from different angles. Here’s one from the top with the fuse side attached:

A side view, only about half of the muffler actually sticks out:

And, as a first, one from the front. I love those angles! It’s a very minimal setup, as efficient as possible, I hope it works as intended:

Custom muffler part 4

This week we also soldered the last parts of the muffler. The pipe for the connection to the front exhaust elbow now is attached to the muffler:

And we also soldered the interior of the muffler in place:

The muffler feels very rigid with the tight fitting interior installed, but very light at the same time, as it is built up from 0.5 mm stainless steel completely.

Now that the construction is finished, it’s time again for some cleaning:

Took me a whole night of sanding and polishing… starting at 120 grit all the way up to 1200 grit. But it’s a fun job to do and it certainly pays off:

The front pipe connection looks the part:

As does the end of the muffler, all nice and shiny. The inner pipe sticks out of the muffler just enough to be effective, without being visible from the side. I really like this detail:

And… it’s finished! This part only weighs 144 grams in total (without the front exhaust elbow), which is quite good to be honest. I’m really happy with how it turned out:

Basic elevator structure done

Made some more progress on both elevators. They are not finished yet, but the basic structure is done on both sides. I need to add some reinforcements and blocks for the hinges, but I want to first build up the stab in the same manner, so I can add the reinforcements on all parts in one go. But the most important work on the elevators is done and they came out nice and straight:

Also, the trailing edges need to be sanded to a round shape. I want to build a sanding jig for that, to get a uniform shape:

I also made some progress on the muffler, but I will share that next time, as I need to do some polishing first :-).

Elevator construction started

Meanwhile I’m already well underway with the elevators. This is the same construction principle as the rudder, so I will not document this into detail, but still share some pictures of the progress now and then. Built the outer edge from strips of balsa:

And finished the first basic rib frame of one of the elevators:

Now I need to repeat this three times and add the reinforcements and blocks for hinges and control horns.

Fitting front exhaust elbow

Back to the custom muffler. The front elbow still has to be fit onto the muffler. I can’t build this as a single piece, as the elbows are attached to the cylinder heads with thread in different directions. My idea is to keep the front elbow as a separate piece and connect it to the muffler using teflon tubing. The front elbow was already bent in the right direction, now I also cut it to the right length, could be a little shorter still, but this at least gives me room enough to work on the connection:

I had to come up with a way to mark the exact shape of the hole to cut for the angled connection to make between the two pipes. Again, angled in all directions, so it will not be a circular hole. And it has to be in the exact right location, as there’s no room for play. Also, the muffler was quite a lot of work up until this point, so I didn’t want to make a mistake here. Only one chance to get it right. After a bit of thinkering I came up with a solution using toothpicks. I put tape with the sticky side outwards on the exhaust pipe and stuck toothpicks to it so they just touched the muffler:

Secured it with another tape on the outside when finished, took it off and got my template:

Transfered to paper, then to a piece of pipe, and cut it in the exact right angle. This is a temporary piece solely used to mark the hole to cut:

I used a piece of wood to connect the two pieces, so I could extend it to the right length and get it in a straight line with the exhaust pipe:

Then I used pieces of tape to tape off the contour of the exhaust pipe:

Removed the temporary piece and got the exact shape and location of the intersection of the two shapes in 3D:

Traced it with a marker, ready to start cutting:

Cut the marked hole and gradually grinded it bigger, adjusting the shape bit by bit, until it exactly fit the new pipe in the right angle. Went rather quick due to the experience I gained with the first hole on the other side:

And stuck things together, not soldered yet, no glue or tape, it just fits and holds like this. Quite happy again with the fit:

I expect this connection to be easier to solder. Last time we had to connect 1 mm copper to 0.32 stainless steel. This extra piece of pipe I will use now is a 0.5 mm stainless steel one, which matches the material of the muffler way better, so it will be easier to heat them evenly. The pipe used will be a little bit shorter in the end by the way, as I want some play between the two pipes underneath the teflon connection. Too bad it won’t stay all metal, because I think that’s prettier, but it is what it is, I need to make it out of two separate parts.