1/3rd Scale Mita Type 3 Production Notes
The ninth part of a twelve part series.
You may want to read the eighth part of this series before proceeding to this article. Also if you prefer, you can read this article in its original Japanese.
Fabrication Part 39: FRP Molding for the Center Wing Fairing
Now that the plaster mold has dried, I move on to the fabrication of the FRP molding for the center wing fairing.
Preparation
When the plaster mold was first made, it was cool because the water evaporated and took away the heat of vaporization, but as days went by, the coolness faded. As the mold seemed to have dried sufficiently, I applied eight coats of the mold release wax (Bonlease wax) to the surface to be coated with resin. PVA was then applied on top.
Normally, the cut lines are traced on the plaster mold after it is finished, but I am not confident I could mark it properly, so I put masking tape along the cut lines.
The glass cloths and the mat were also cut to the specified size. In this way, I was ready to lay up the cloth.
Cloth Application
I used two #100 (100g per square meter) micro glass clothes and one 30g per square meter glass mat. The mat is sandwiched between the clothes and laminated like a sandwich. The thickness of the #100 glass cloth is just under 0.1 mm, and the thickness of the mat is about 0.15 mm.
The resin I used is Brainy Giken’s GM-6600, an epoxy resin for glass cloth.
First, I applied a thin coat of epoxy resin to the plaster mold with a brush, then placed the first ply of the glass cloth on top of the resin and pressed it down with a resin-impregnated brush. When the resin seems to have penetrated sufficiently and the air has been released, I placed the mat on top and repeated the process. However, because I rubbed the mat with the brush here, the mat fell apart and became quite fluffy. So the mat seems to have become quite thin. I covered it with the last layer of cloth in the same way.
The resin I used was 39 grams including hardener (30 grams of main agent and 9 grams of hardener). I mixed 10g of the main agent and 3g of the hardener for the first coat, and twice that amount for the second coat. However, with such a small amount, a large part of the mixture stuck to the brush and mixing container, and the amount actually used on the FRP was probably considerably less than that.
Mold Release
The less epoxy resin is used and the thinner it is spread, the longer it takes to cure. 30 minutes is the standard curing time for GM-6600, but the fairing was so thin and used so little that it was sticky even after two hours. So I left it for a day and it cured completely.
Finally, it was time to release the mold. The fairing was easy to pull because it protruded around the perimeter, but it did not release easily because it partially stuck to the mold. I carefully pulled it off from the four corners and it came off with a little force.
However, if you look closely at the plaster mold after demolding, you can see that some of the corners are missing, and the corresponding areas of the FRP molded product have plaster pieces stuck to them.
It’s a shame because I didn’t pull with that much force and it chipped. There is air in some parts of the inside, but the surface is relatively clean because it was adhered to the plaster mold. However, there is a little resin chipping at the corners, which will need to be repaired with putty later.
Cutting
I peeled off the adhered plaster and cut off the surrounding area along the masking tape to complete the molded product. It weighed 36g and was about 0.7mm thick. It had moderate rigidity, which was a good result for my first FRP molding. I put the center wing on the fuselage and put the fairing on top of it.
It fits well. It also looks good from above. After this, the fairing will be completed by attaching the fittings and a frame for installation.
Fabrication Part 40: FRP Molding of the Nose Cowling
Following the center wing fairing, I finally made the FRP molding for the nose cowling.
Preparation
After waiting for the plaster mold to dry, I applied several coats of the Bonlease wax and then PVA. Normally, PVA is applied only once lightly, but since it was unexpectedly difficult to release the center wing fairing, this time I applied a few coats where it looked thin. After that, I applied masking tape along the cut line, and cut out the necessary amount of #100 glass cloth and 30g glass mat per square meter, the same as for the center wing fairing.
The difference is that in the case of the fairing, there was no need to divide the cloth and mat into small pieces because the entire surface could be covered with one piece, but for the cone-shaped cowling, it was not possible to cover the entire circumference with one piece without wrinkles, so I divided it into four pieces. Photo 195 shows the prepared plaster mold.
Cloth Lamination
First, 40 grams of resin and 12 grams of hardener are taken into a container, stirred, and applied thinly to the inside of the plaster mold. Then I put a glass cloth on top of it and pressed it down with a brush. I rotated the mold and finished applying the four cloths. This is the first layer, which will be the outermost layer.
There was still some resin left, so I put a mat on top of it and held it down. The mats are placed so that the center of the mat is on top of the overlapping part of the cloth, so that the thickness becomes even. When I finished applying two mats, I ran out of resin, so I took the same amount of resin and hardener in a separate container and applied the remaining two mats.
Since there was still enough resin left, the third layer of cloth was applied. This was cut to be orthogonal to the first cloth. As I was applying the second layer of cloth, the viscosity of the resin increased rapidly, and the cloth did not stretch well. The remaining resin in the container is also starting to harden. The amount of hardener might have been a little different, although the working time was not longer than the first time. I had no choice but to stop using this resin at this point, and for the third time I took 10g of resin and 3g of hardener in a separate container and stirred it. The brush also started to harden, so I changed it to a new one.
Despite these hiccups, I managed to finish putting up the clothes and mat. Photo 196 is the state where the clothes have been applied. Leave it for a day and wait for it to harden.
Mold Release
Finally, it was time to remove the cowling from the mold. Based on my experience with the center wing fairing, I was prepared for some difficulty in the demolding process, but this time it was much more difficult than expected. Even the edges would not come off with a little force. I managed to peel off the straight edge with some effort, but I couldn’t peel off the curved part of the upper surface that connects to the canopy. I inserted a plastic spatula and finally peeled off a little.
As the edge peeled off, I inserted a plastic spatula further inward to peel it off, but it stuck so tightly that I had to use a lot of force to get it off. While I was trying to widen the gap by inserting my fingers in between the peels, I pulled too hard inward, and the beautifully formed FRP began to have broken lines. I still haven’t peeled off even 1/3 of the total depth.
It seemed that the FRP would break if I kept doing the same thing, so I decided to break the plaster mold. I placed the mold on the stone and hit it with a hammer, but it was not easy to break because the plaster was reinforced with gauze. Even so, after several rounds of hammering, the FRP began to pop out before the plaster mold was broken too much. It seems that the FRP will come off when vibrations are applied. Photo 197 shows the FRP product that was painstakingly removed from the mold and the plaster mold. The plaster mold is not completely broken.
Cutting
The next step was to cut off the unnecessary part along the edge of the masking tape. This is the cowling after cutting.
It is very neatly done. The black line is the marking line for cross cutting, and the blue part is the attached PVA. The thickness was 0.7 mm at the thin end and 1.2 mm at the thick end. This is due to the fact that each layer was divided into four pieces of cloth or mat, resulting in overlapping areas. The overlapping areas acted as reinforcing materials, and the result was a very rigid product.
Fit Check
The first thing I did was to check the fit to the body. This was my biggest concern. The result was perfect as shown in the picture below.
The weight is 114g. After this, I would repair the damaged area by forcibly removing the cowling from the mold, and attach the wooden frame to the structure side.
Fabrication Part 41: Installation of Nose Cowling
Mounting Method
Initially, the nose cowling was intended to be glued to the fuselage structure, just like the 1/5 model, so that it could not be removed. Therefore, the motors and rudder servo located inside the cowling are designed to be installed and removed from inside the cockpit. However, after building this far, I changed my mind to add the release mechanism for aerotowing, although I and my club don’t have any towing machines…
The release mechanism for aerotowing of the Mita is installed in the lower part of the cowling in front of the nose skid. This position can’t be accessed from the cockpit. Therefore, I decided to change the nose cowling to be removable.
Preparatory Work
Drawing 49 shows the cowling installation method.
First, fill the gap between the rear end of the cowling and the fuselage structure with wooden frames right and left. The frames are attached on the fuselage structure side. Similarly, a wooden frame is installed on the cowling side to border the canopy. The cowling is secured by two screws on each side and the nose skid. The nose skid is attached to the fuselage structure with four screws across the cowling, so it is necessary to prepare a skid and its attachment structure to the fuselage side. Photo 200 shows the fuselage structure after the preparation work.
You can see the wooden frame attached to the fuselage structure, the two mounting screw receptacles on the upper left and right sides, and the skid attachment structure. If you turn the fuselage over and look at the skid attachment structure, you will see Photo 201.
The wooden frame that will be the boundary between the cowling and the canopy was attached to the cowling (Photo 202).
Installation
With the above preparation, the cowling was installed. You can see the two mounting bolts on the left and right.
Photo 204 shows the installation of the nose skid as seen from below.
This is a shot taken from the cockpit side.
This may sound like a smooth installation, but in fact it took a lot of time to adjust. In particular, when I installed the wooden frame on the cowling, it was not easy to fit it into the fuselage. After repeated fine adjustments with sandpaper, the wooden frame became to fit easily.
Trial Installation of the Folding Propeller
I installed the propeller to check the folded state. The propeller is a Graupner 18x9. The hub is the smallest size (42mm), but it matches the nose shape and folds up nicely.
Fabrication Part 42: Attaching the Center Wing Fairing
Next, let’s install the center wing fairing.
How to Install the Fairing
The center wing fairing of the actual model is installed as follows,
- Attach the fairing to the upper surface of the center wing using the front two fittings.
- Screw the two metal fittings at the rear of the fairing to the fuselage overhang structure.
- The center part is screwed to four locations, two on each side of the center wing.
In the model, the fairing is relatively stiffer than the actual model, so I decided to simplify the installation method slightly. Drawing 50 shows the installation method.
Attach a wooden frame to the front of the fairing as in the actual model, and attach two L-shaped fittings to it. Screw the L-shaped fittings to the pillars that rise from the inside of the center wing. For the rear part, two metal fittings similar to the actual model are attached. These fittings are open at the rear by less than 1mm and they are fixed by sucking the L-shaped aluminum channel attached to the overhang structure at the top of the rear body.
Processing the Fairing and Center Wing
The wooden frame and fittings are attached to the FRP fairing according to the drawing.
The center wing has two pillars for screwing the L-shaped fitting of the fairing.
Installing the Fairing
To install the fairing, first place the wing cover to hide the spoiler servos.
Place the fairing on top of it and screw it in place to complete.
Fabrication Part 43: Cable Release Mechanism for Aerotowing
Outline of the Mita’s Cable Release Mechanism for Aerotowing
The cable release mechanism for aerotowing of the Mita Type 3 Rev 1 is attached to the lower part of the nose cowling in front of the nose skid, and is connected to the same knobs as the release mechanism for winche towing. It is opened by pulling the knobs. The actual structure of the mechanism is difficult to understand because it can’t be accessed, but I devised a simple mechanism for the model. Here is the drawing.
The simple mechanism consists of a hook sandwiched between two aluminum frames, and the hook is opened by pulling the wire attached to it. A weak spring is attached to the hook so that the hook closes when there is no wire tension. The wire is connected to the same servo as the mechanism for winch towing. This means that when the servo is activated, both release mechanisms are activated simultaneously.
Fabrication of the Mechanism
This is the cable release mechanism fabricated based on the drawing.
The hook is made of 3mm thick hard aluminum plate, and the frame is made of 2mm thick aluminum plate. The rotating shaft of the hook is a 3mm bolt, and the frame is fitted with a DURACON bearing. The hook has already been fitted with wires and springs. A 20mm ring is attached to the bottom of the hook.
Installation of the Mechanism
The mechanism was attached to the lower part of the nose.
Two 2mm thick carbon ear-rings are attached to the rotating shaft of the hook, which is then attached to the truss bar one behind the motor mount. On both sides of the earring, two carbon rods are passed between the truss bar and the motor mount, and a small earring is attached to it to support the front part of the mechanism to hold the rotation of the mechanism.
In the beginning, this cable release mechanism was not intended to be installed, so this part of the structure was as shown in Drawing 52.
Namely, there was a diagonal member running to receive the reaction torque of the motor. This interfered with the release mechanism, so I removed it and modified it as described above. To avoid any reduction in the anti-torque holding capacity, I also installed two diagonal members outside the mechanism. Photo 213 shows the entire mechanism in relation to the servo.
I put the cowling over it. The ring shows a little bit.
I think it gives a good feeling.
The cable release knobs on the left side of the cockpit and the servo are also connected with wires. There is a spring in the middle of the knobs as shown in Photo 215, so when the hook opens, the knobs also move backward as in the real model.
Although the fabrication of the cable release mechanisms were completed by this, I must confirm their operation thoroughly later.
Fabrication Part 44: Instruments Panel and Its Mounting Structure
Instrument Panel
The instruments panel of the Mita Type 3 glider looks like the one in drawing 53. I made it out of 1.6mm thick plywood (Photo 216). It will be painted matte black before the instruments are installed.
Location and Mounting Structure of the Instrument Panel
The instrument panel is located inside the nose cowling, in front of the control stick. The actual model has a curved steel tube welded to the fuselage structure along the radius of the cowling, and the instrument panel is screwed to it. However it is difficult to get a curved carbon tube in a model, so I decided to attach a wooden frame to the cowling.
The mounting method was decided, but when I put the instrument panel on the fuselage, it turned out to be a problem. It interfered with a lot of things. Drawing 54 shows the position of the instrument panel.
Initially, I thought I had confirmed that there would be no interference in the drawing, but it turned out that the horn of the rudder servo would hit the instrument panel when it was rotated to the maximum angle. I had confirmed that the servo itself did not interfere with the instruments panel in the drawing, but I had not checked the interference with the horn. Furthermore, I found that the neutral position of the control stick was slightly tilted forward compared to the drawing, so the stick still hit the instruments panel when it was tilted forward the most.
I had no choice but to move the rudder servo forward by 8 mm and adjust the neutral position of the control stick to a slight backward tilt. The forward movement of the servo and the backward tilt of the control stick were beyond the adjustment range of the links connected to them, so I had to rebuild them. Furthermore I found that the backs of the bottom two instruments would hit the top plate of the nose structure.
This top plate is not found on the actual model, but it was designed to hold the amplifier for the motor. The part where the instruments touch is solved by cutting out the top panel, but the problem is that the carbon square tube attached to the back end of the top platel is in the way. I had no choice but to cut it out as well.
Completion of the Instrument Panel Mounting Structure
As stated above, I encountered many problems, but I solved them one by one and finally completed the installation structure. After attaching the wooden frame to the cowling, the instrument panel was mounted and checked in the aircraft.
It looks pretty good. I think the actual instruments will look much better when they are mounted. After this, I asked a club member who has a mini lathe to cut the cases for the instruments. I will make scale boards and attach them to these instruments.
Drawing of the Canopy Wooden Mold
There is a big task left to make the canopy. I had no experience with the vacuum method of making the transparent part. When I was gathering information on how to make the canopy, I found out that Mr. Toyama, an expert in this field, is in Nagano Prefecture and he makes canopies for many people. I immediately contacted him and he agreed to make the canopy for me. Moreover, he will also make the necessary wooden mold. So I made a drawing of the canopy mold.
Canopy of the Mita Type 3 Revision 1
The canopy of the Mita Type 3 is divided into two parts, front and rear. The front canopy can be opened, passengers both front and rear seats use it to get in and out. The rear canopy is fixed to the fuselage so it can’t be opened. This is what the drawing looks like. The red area is the canopy.
The rear canopy has a complex shape across the center wing. Moreover, the bottom of the canopy is not on the same plane with the front canopy. I thought about how to make the wooden mold for this canopy, and decided to use the black line in the drawing, which matches the bottom surface of the front and rear canopies. This way, the shaped transparent part can be cut and divided into two parts.
Wooden Mold Drawing
Now that the scope of the wooden mold has been determined, I drew a drawing to define its 3D shape. The drawing method is the same as the nose cowling drawing.
Define the cross-sectional shapes of the four sections: the very front section that touches the nose cowling (section A-A), the section where the canopy sill kinks in the plan view (section B-B), the dividing line between the front and rear canopies (section C-C), and the last section that touches the center wing fairing (section E-E). Then draw the appropriate cross sections (A1, A2, B1, C1) between them.
Cut them at multiple planes of 20mm height each and draw contour lines. If the contour lines have a strange waviness, modify the cross-sectional drawing. However, there was nothing too strange, and I was able to define it in one shot.
This is a big canopy. The dimensions of the wooden mold are 670 mm in length, 210 mm in width, and 180 mm in height, and it seems that Mr. Toyama cannot cope with the vacuum equipment he has, so he will make a new large one.
I was a little worried about making the canopy, but with the support of a veteran, I was able to solve the problem.
Improvement of the Cable Release Mechanism for Aerotowing
When I carefully checked the condition of the cable release mechanisms for the winch and aero towing, the condition of the aerotowing was not so good, so I improved the design and modified it.
Cause of the Problem
Actually, the design of the mechanism for aerotowing was made in the same way as the mechanism for winch towing, without thinking it through. However, the situation when the rope is released is very different between the two mechanisms.
In winch towing, the cable is pulled by a very strong traction force, and is released when the aircraft is near the top of the winch. In other words, the cable is positioned at a very downward angle to the aircraft axis. On the other hand, in airplane towing, the tension applied to the cable is much lower than in winch towing, and the cable is at an angle parallel to the fuselage axis when it is released.
The release mechanism that I made the other day did not take this difference into consideration, and I designed the shape of the guide that receives the ring-shaped metal fittings attached to the towing cable to be the same for both release mechanisms. When the cable was actually attached to the mechanisms and the release condition was confirmed, it turned out that the cable for winch towing was released straightforwardly, but for aerotowing it was not released well.
Improved Design
I redesigned the guide for the aerotowing mechanism to take into account the release situation described above. Drawing 57 shows the comparison between before and after the improvement.
The side plates have a different shape near the exit.
Rechecking the Release Condition
I installed the redesigned mechanism and rechecked the release condition of both the winch and aero mechanisms.
Photo 219 is the ring attached to the cable.
Connect the servo tester to the servo for the release mechanism.
Then, the ring of the cable is hung on the hook of the aerotowing mechanism. In this state, I pulled the cable at an angle that seemed to be the aerotowing state, and turned the dial of the servo tester. This time, the ring came off very smoothly.
In the same way, I tested the winch towing mechanism at a deep angle to the fuselage axis, and reconfirmed that it would release straightforwardly.
Now I have confirmed there is no problem in the release condition of both mechanisms.
This is the ninth 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.
