1/3rd Scale Mita Type 3 Production Notes
The eighth part of a twelve part series.
You may want to read the seventh part of this series before proceeding to this article. Also if you prefer, you can read this article in its original Japanese.
Fabrication Part 32: Seat Back and Shoulder Harness — Seats Completed
Making a wooden mold for FRP parts is a very time-consuming work. For a change of pace, I completed the seats by making the ‘backs’ that attach to the ‘seats’ previously made.
The ‘Back’ of the Mita
The ‘backs’ of the Mita seats are made of cloth. Their lower ends are attached to the seats, and the upper ends are wrapped around aluminum horizontal bars and inserted into two parallel slots in the upper fuselage as shown in drawing 45.
There are four holes in the slots that allow you to change the reclining angle by changing the position of the horizontal bars on the backs. Widths of the back cloths are trapezoidal with the lower parts slightly narrower than the upper parts. The back clothes are attached to the seats by a thin aluminum plate. I don’t know why, but the back of the rear seat is attached to it far forward than the front seat, and as a result, the back angle of the rear seat is looser than that of the front seat. Also, the front seat has four screws and the rear seat has five.
The Completed Seats
The seats were completed by attaching the “backs” made according to the drawing with the “seats” already made on the fuselage (Photo 161). For the horizontal bars, I attached short aluminum tubes to the ends of balsa rods, and attached acrylic tips to the ends of the tubes. I hammered brass nail heads into the aluminum tubes to give the appearance of rivets. The slots are aluminum channels that I found at a Home Depot.
Shoulder Harness of Mita
Naturally, the Mita gliders are equipped with shoulder harnesses, but the one on display at the Shizuoka Aviation Museum is not, as it was already missing when the aircraft was donated to the museum. Therefore, I don’t know what kind of shoulder harnesses were actually attached to the aircraft, but I could know how they were attached as Mr. Kimura, who was the original owner of the aircraft, drew me a diagram of how it was worn. Therefore, I decided to pursue the atmosphere of the harnesses, not the shape.
The Shoulder Harnesses I Made
The most troublesome part of making a harness is the buckle. I decided to buy ready-made buckles and ransacked the handicraft stores, but I couldn’t find any 1/3 scale harness buckles, so I decided to substitute the plastic parts that hang the belt on the case and close it. I dyed parts of these in silver to give them the feel of metal parts. For the belts, I got a strip of string that looks like that.
In Photo 162, you can see how the harnesses are threaded through both the front and rear seats. Namely, the belt passes through the U-shaped bracket at the rear of the seat while holding the left and right shoulders, then passes the brackets right and left at the rear edges of the seats and closes with a buckle in front of the belly. When the shoulder harnesses are attached, they look like real aircraft seats.
Fabrication Part 33: Covering the feet around the rear seat rudder pedals
Rear Seat Pedal Foot Cover
The Mita glider has a foot cover around the rear seat rudder pedals, which adds an accent to the cockpit. If you are not familiar with the Mita, you may not imagine what I mean by “a pedal foot cover”. So I show you the finished product first, although the order is reversed.
The rudder pedals of the rear seat (instructor’s seat) are mounted on both sides of the front seat (student’s seat). In other words, the instructor puts his feet on the pedals with his legs open and the front seat between them. To cover these rear seat rudder pedals, both sides of the front seats are covered with foot covers. These covers give the cockpit a sense of presence and accent the Mita cockpit. Therefore, I could not omit them in the model. The foot covers of the actual aircraft are supposed to be made of aluminum (presumably), but in the 1/3 model they are made of FRP to reduce radio interference.
Making a Wooden Mold
Since the cover is curved, a mold is needed to make it in FRP. The mold was made by cutting a block of thick paulownia wood and pasting it together.
Using this as a male mold, I made a shape out of cardboard and impregnated it with epoxy resin to make the covering.
Making Shapes with Cardboard
I softened the cardboard by wetting it with water and stuck it to the wooden mold. Leave it to dry for a day.
Resin Impregnation
Then I applied epoxy resin to both sides of the finished paper form, attached it to the wooden mold again, put it in a “Futon (Comforter) compression bag”, and vacuumed out the air. Then, the epoxy resin permeated the paper and impregnated it well. I left it for a day to let the resin harden before taking it out, but it was difficult to peel it off, even though I had applied many coats of release wax to the wooden mold. When I finally peeled it off, the resin layer on the surface was partially left on the mold and the surface was not clean. The side that touches the mold is not visible because it is inside, which is good, but it is not thick enough, so I applied resin to both sides again. However, because the resin was applied by brush, the surface was not very smooth after hardening. I had no choice but to polish it up with water-resistant paper.
This is how the foot covers were completed.
But there remains one question of what is the need for this foot cover? I don’t understand the need for it. I don’t want to think it’s to prevent scary instructors from kicking students with their feet.
Fabrication Part 34: Wooden Mold for the Center Wing Fairing
Fairing for Center Wing of Mita
There is a fairing made of FRP on the upper part of the center wing of Mita Type 3. The fairing connects the rear end of the cockpit canopy and the upper overhang structure of the rear fuselage, and covers the upper half of the center wing. The side view is as shown in drawing 46.
The blue area in the drawing is the fairing. In the plan view, it looks like drawing 47.
The cross-sectional shape of the fairing is semicircular at the tip because it is connected to the canopy, and the trailing edge is the shape of loof because it covers the upper overhang structure of the rear fuselage. The lower surface of the fairing touches the upper surface of the center wing, so it has an airfoil shape. In other words, it is a structure with a very complex three-dimensional shape change. A wooden mold was made in preparation for the fabrication of this fairing.
The Wooden Mold I Made
The wooden mold was made from balsa following the same process as the mold for the nose cowling. I drew a drawing of the three-dimensional shape, cut out several ribs, assembled them, and then filled the space between them with thick balsa boards before shaping.
Photo 167 shows the finished wooden mold. The sides are wider and straighter than the actual dimensions because of the need to make plaster molds. I planned to cut out this part after the FRP molded part was made.
The front side that connects to the canopy and the rear side that covers the top of the rear fuselage are shaped as shown in photo 168.
It took a lot of time to shape and kill the grain, but I managed to create the shape.
By using this wooden mold to make the center wing fairing before making the nose cowling, I hoped to acquire the know-how of making plaster molds and FRP parts, which was my first experience. However, after some research, I found out that in order to use it as a wooden mold, it is necessary to apply fine water-resistant paper up to about 2000 grit and polish it with compound before applying release wax to improve the release of the plaster mold. In addition, if it was the first time you use the mold, you need to apply the release wax about 8 times, so it was a long way to go.
Fabrication Part 35: Plaster Mold for the Center Wing Fairing
Waxing the Wooden Molds
I smoothed the completed wooden molds for the nose cowling and center wing fairing with 2000-grade water-resistant paper and polished them with compound. After that, I applied 8 coats of the Bonlease Wax for release. Photo 169 is a photo of the waxing process.
There is another mold in the picture along with the cowling and fairing. This is the mold for making the foot covers around the rear seat pedals as described in Part 31. Waxing each of these three molds eight times was a tough job.
Applying PVA
When the waxing was finished, the next was to apply PVA (polyvinyl alcohol), a liquid for releasing the plaster, on top of it. I applied the blue PVA on the sponge by stroking it in one direction as per the instructions. I was afraid that the liquid would pop off when applied on top of the wax, but my fears were unfounded. A thin film of PVA was formed.
Plastering
Finally, it was the phase of the real plastering. To prevent the plaster from flowing out, I covered the wooden mold with a wooden frame. A thin sheet of plastic was attached to the inside of the wooden frame with double-sided tape to prevent the plaster from sticking.
Photo 171 shows the plaster I used. It is the SAKURA mark Class A of Yoshino Gypsum.
I prepared 720cc of water to 1kg of plaster as per the instructions. I didn’t know what amount was appropriate, so I tempolary used 1 kg for the first time. Sift the gypsum into the water in the bucket, leave it for about a minute, and then slowly stir it with a round wooden stick. Be careful not to let air get in. After 200 times of stirring, the mixture became a little sticky, so I scooped it up with a ladle and poured it onto the wooden mold. By the time I finished pouring the entire 1 kg of plaster, the remaining plaster in the bucket started to harden. It seems that 1 kg of plaster is the maximum for me to work with at one time, and I should work a little faster.
When I finished pouring 1 kg of plaster, it seemed a little insufficient, so I dissolved another 500 g of plaster in water and poured it on. At this time, I was worried about the strength of the plaster, so I put some gauze in between as reinforcement. When I finished pouring 500g of plaster, I was worried that the thickness of the leading and trailing high parts of the fairing might be a little insufficient, so I poured another 500g of plaster over those areas. In the end, I used a total of 2 kg of plaster. This is what the plaster looked like after it was applied.
Removing the Wooden Frame
Plaster begins to harden quickly as it generates heat. It seems to be a good idea to remove the wooden mold when the heat reaches the maximum temperature, so I immediately removed the wooden frame. It was easy to remove the wooden frame because it has plastic plates on the inside.
Wooden Mold Removal
The climax was the removal of the wooden mold, which came off easily, either because of the shape or the preparation. I was very worried about it, but it was a relief. Photo 174 is a plaster mold with the wooden mold removed.
PVA is easy to dissolve in water, so it dissolves in the moisture of the plaster, giving it a blue color. There was no “soot”, which I was worried about, and the mold came out very clean. The edge surface is also beautiful. After this, it will take at least a week to dry, so it will still be a while before I can apply the FRP.
Fabrication Part 36: Plaster Mold for the Nose Cowling
Making a wooden frame for pouring plaster
Now that the wooden mold for the nose cowling has been coated with release wax, it’s time to pour the plaster over it. The first step was to make a frame out of 4 mm plywood so that the wooden mold would stand vertically and plaster would not flow out even if it was poured from above. Photo 175 shows the finished wooden frame and the wooden mold that has been coated with PVA and is ready for plastering.
Plastic plates are attached to the inside of the wooden frame to prevent the plaster from sticking. The bottom of the wooden frame is bent into a folding screen in two places to match the bottom of the wooden mold. The mold is placed on the crate like this. Plaster is then poured over the top.
Plastering is Finished
A total of 4kg of plaster was poured. I dissolved 1kg of plaster in 720cc of water and poured it four times. Since I had experience with the center wing fairing, this time the work went smoothly.
I wrapped gauze around the second and third coat to reinforce them, and after four coats, the first coat of plaster had already hardened enough, so I immediately removed the wooden frame.
Removing the Wooden Frame
The sides of the wooden frame are screwed to the bottom. When the screws were removed, the plaster came off easily without sticking due to the effect of the plastic plates inside of the sides. After that, when I lifted up the plaster, the bottom surface also came off easily.
However, the plaster soaked through the gap between the wooden mold and the bottom of the wooden frame, and the plaster stuck a little to the periphery of the bottom of the wooden mold.
Removing the Wooden Mold
The next step is to remove the wooden mold. The first step is to attach a “handle” to the bottom of the mold for pulling. I held the handle and pulled, but it was not easy to pull out because of the plaster stuck around the bottom of the mold. You can see the plaster stuck around the bottom of the mold in picture 179. After removing the plaster from the bottom of the mold, it was easy to remove the mold.
Completion of the Plaster Mold
Photo 180 shows the finished plaster mold. The inside corner is a little dirty because it was scraped off when the plaster was peeled off, but there is no problem because the regular border is located 2mm from the bottom of the mold. There is no “soot” inside, so I can say it’s a good job. The large plaster mold that I was worried about is now complete!
Fabrication Part 37: Failure of the Plaster Mold for the Vertical Tail Fairing
The plaster mold making has been going relatively smoothly so far, but this time I made a mistake in making the plaster mold for the vertical tail fairing.
Vertical Tail Fin Fairing
The bottom of the leading edge of the vertical tail fin of the Mita is notched in a V-shape for installation and removal of the horizontal tail. The fairing made of FRP is attached there. The red line in drawing 48 shows the fairing.
It covers the notched portion of the vertical tail fin and most of the lower surface covers the upper surface of the horizontal tail fin. The rear part goes through the inside of the elevator and sandwiches the fuselage. I tried to make a plaster mold from a wooden mold to make this model, but failed.
Wooden Mold
The first step was to make a wooden mold. Photo 181 is the finished wooden mold.
I was actually working on this mold at the same time as the center wing fairing mold, but it took a lot of time and effort and the completion was delayed. For the first time, I filled the balsa skeleton with clay and shaped it. When I thought the clay was dry, I polished the surface with sandpaper and put lacquer surfacer on it, and when I was correcting the small parts, I found a gap between the balsa skeleton and the clay. As I filled the gap with poly putty, the same phenomenon appeared in other parts.
I repeated this several times, but I wondered if the clay, which I thought was dry, hadn’t dried out completely. And when I pressed hard on the nicely shaped area, it dented. It turned out that the clay that was covered by the balsa skeleton would only dry on the surface that appeared on the outside, but would not dry on the inside because it was blocked from the air. I had no choice but to dig out the clay I had shaped and reshape it by filling it with balsa boards. However, the thin clay in the front part was already dry and could not be removed, so I left it as it was. That was also the source of the problem.
After shaping the balsa, I sprayed lacquer surfacer on it and polished it with water-resistant paper, but the surface skin of the clay part was partially missing. I had no choice but to fill the area with poly putty and start over again. However, the same thing happened every time I polished it with water-resistant paper. It seemed to be an endless process, but I managed to complete the work as shown in photo 181
Initially, I thought I would make the wooden mold of the nose cowling by filling it with clay, and even bought clay, but I was worried about its unknown characteristics, so I changed to balsa. In hindsight, this change was the right one. I learned that clay is unsuitable for molds that are not easily exposed to air.
Lessons Learned 6: Don’t use clay for shaping wooden molds. The parts that are not exposed to air will not harden forever.
Making the Plaster Mold
I covered the wooden mold with a frame and poured the plaster. This time I omitted the gauze for reinforcement because it was a small item, but I think that was the main reason for the failure. Unfortunately, I forgot to take a picture of the plaster being poured.
As the plaster began to harden, I removed the wooden frame.
Next, I tried to remove the wooden mold, but the back of the mold would not come off. This is because the plaster oozed out from the gap between the mold and the wooden frame, and hardened to cover the mold. The back of the mold has a lot of contact with the wooden frame, so it inevitably becomes like that. When I tried to remove the mold from the front, which is easier to remove, the plaster broke from the center when I put a little force on it. When I tried to remove the rear part, it broke into two pieces. That’s three disassemblies in total. In the end, it ended up in the state shown in photo 183.
This was the third time I made a plaster mold, and since it was a relatively small one, I underestimated the risks. I’m now wondering whether I should repair the broken plaster mold, remake it, or give up on the female mold and make an FRP product using the wooden mold as the male mold.
Plaster Mold Making Know-How
Through making three plaster molds, one for the center wing fairing, one for the nose cowling, and one for the vertical tail fairing, I have a vague idea of the procedure. I want to summarize the know-how for the future.
- Can the mold be pulled out? If the shape is upward open, most of the molds can be pulled out if the pre-treatment of the mold and the production of the wooden frame are done properly.
- Treatment: The wooden mold should be sanded neatly and finally polished with water-resistant paper of about 2000 grit, and then with compound. Then rub with Bonlease wax about 8 times, and finally apply PVA.
- Wooden frame: A thin sheet of plastic should be attached to the inside of the wooden frame with double-sided tape to prevent the plaster from sticking.
- Notes on setting the mold on the wooden frame: The most important thing is to eliminate the gap where the mold meets the wooden frame. If there is a gap, plaster will soak in and harden around the wooden mold, which is the biggest hindrance to mold removal. It may be a good idea to use double-sided tape to prevent gaps where the two sides meet.
- Plaster handling: Do not handle too much at once. The plaster hardens too quickly, and it will harden before all the work is done. In my case, the upper limit is about 1kg at a time.
- Reinforcing the plaster: Plaster is easily broken, so it’s a good idea to wrap a cloth around any areas of concern.
- Timing of mold removal: Plaster hardens quickly, so remove the wooden frame immediately after pouring. It is best to remove the wooden mold while the plaster is still hot. At this point, any plaster that has seeped into the gaps between the wooden frame and the mold will break down relatively easily.
Fabrication Part 38: Control Sticks and Related Structure
While waiting for the plaster molds to dry, I made the control sticks, spoiler lever and tow rope release knobs.
Sticks and Related Structure of Mita
Mita is a tandem type double seater, so there are two control sticks, spoiler levers and tow rope release knobs. The control sticks are located in front of the center of the seats, and the spoiler levers and tow rope release knobs are located on the left side of the cockpit.
Finished Control Sticks and Related Structure
This is the completed control sticks and related structure.
The upper part of the control sticks are bent like a crank and the grips are close to the pilot. The control sticks are inserted into the aluminum tubes of the gimbal mechanisms. The gimbal mechanisms are driven by the elevator servo and the dummy servo for the aileron, so the control sticks will fall back and forth, left and right, as they are steered.
The spoiler is connected to the steel tube on the left side of the fuselage via a link. In the 1/3 model, the spoiler is moved in and out by a servo located in the center wing, so this lever is a dummy. The other levers in front of the levers are the brake levers for the main wheel. The brake levers have a wire attached to them, and pulling the lever applies the brake connected to the wire. In this model, the wire is not yet attached. Of course, the brake levers are also dummy.
When the yellow ball is pulled, the wire connected to it opens the hook of the release mechanism, and the tow rope is released. The 1/3 model uses the servo for the rope release to open the hook, and the wire extending from these knobs were connected to the servo through a pulley. A weak spring is installed in the middle of the wire so that the knob moves as if it is pulled backward when unlocked.
This is an enlarged photo of the front seat. The orange ball is the towline release knob.
This is the rear seat. The two dark blue levers standing on the left side are the spoiler (rear) and brake (front) levers.
The grip of the real machine has a rubber cover with vertical stripes to prevent slipping. In order to create that feeling, the grip is made by attaching several thin wires vertically.
This is a photo of the spoiler operating mechanism from the left side of the fuselage.
The steel pipe is made of 4mm diameter bamboo string. The steel pipe is curved because the side of the body is curved. The steel tube is supported in three places. In the real model, the supports are screwed in place and can be removed, but in the 1/3 model, the supports are too small to be screwed in place, so the bamboo string is split in two at the center and connected with a short brass pipe. The brass pipe is attached in such a way that it can be pulled out and inserted, so the bamboo string can be removed from the fuselage.
The white bar with a yellowish-green ball on the end of it in the back is the elevator trim lever. This is also a dummy. With these equipment installed, the inside of the cockpit is almost completed except for the instrument panel.
This is the eighth part in this series. Read the next article in this issue, return to the previous article in this issue or go to the table of contents. A PDF version of this article, or the entire issue, is available upon request.
