1/3rd Scale Mita Type 3 Production Notes
The seventh part of a twelve part series.
You may want to read the sixth part of this series before proceeding to this article. Also if you prefer, you can read this article in its original Japanese.
Fabrication Part 27: Seats and Cockpit Floors
Seats and Cockpit Floors of the Mita
The Mita Type3 is a tandem double-seater, so it has two seats, front and rear. The seats are made of steel tubes welded to form a frame and covered with boards, and the backs are made of cloth and can be adjusted in length and angle. This time I made the ‘seat’.
The cockpit floors are provided to the minimum extent in order to reduce weight. The front seat has an aluminum floor in front of the control stick, and the rear seat has two thin wooden floor panels on both sides between the rudder pedals and the seat. This is what the drawing looks like.
The seats are not rectangular when viewed from above, but have a trapezoidal shape with the back side narrower than the front. When viewed from the side, it is not flat, but concave downward. The front legs are longer than the back legs. This is because the steel tubes at the bottom of the fuselage, where the seats are attached, become higher the further back you go. Furthermore, the front legs of the front seat are tilted backward. This is probably because the fuselage structure that receives the front legs is not in an appropriate position.
The front seat simply has four short legs, but the rear seat has longer legs in order to look forward over the head of the front seat passenger. For this reason, diagonal braces are placed between the front and rear legs in order to secure rigidity.
It is not possible to drill holes in the truss structure of the fuselage steel tubes to install the seats directly. I was curious to know how the seats were attached, and found small lugs were attached to the end of the feet, and “lug holders” were attached to the corresponding position on the fuselage structures to fix the seats. The feet touch the steel tube of the structure to support the weight, and the seat is fixed with these lug brackets. I will follow this method in the model.
Fabrication
The steel tube parts of the seat cannot be made of carbon because of its curvature. So I used 4mm diameter bamboo rods. After soaking the bamboo strand in water overnight, I sandwiched it between the jig in photo 136 and bent it to dry.
For the seat boards, I soaked 1.6mm thick plywood in water, then tied it along a curved tube and let it dry to add the bend. The bamboo strips and plywood were placed on the assembly jig (photo 137) and glued together with epoxy adhesive to make a concave ‘seat’.
After attaching the legs to the seat, the lugs must be attached, but they need to be aligned with the ‘lug holders’ on the fuselage. Therefore, the ‘lug holders’ were attached first.
These have a 2mm claw nut attached to them from the underside. I placed only the lugs on them, then placed the seat with the feet on them, and temporarily attached them with CA. After that, I removed the lugs with the seat and applied epoxy adhesive to fix them. Finally, I painted the steel pipe equivalent part white and the seat part maple.
The floorboard of the front seat is made of aluminum in the actual machine as mentioned above, but in the model it is made of 1.6mm thick plywood to avoid radio interference. In the actual model, the aluminum panel is bent around the perimeter to ensure rigidity, but this was omitted in the model. The floorboard of the rear seat is made of 4mm thick balsa.
Completed Seat and Floorboards
These are the completed seats.
You can see that the seatboards are curved and the front legs of the front seat are tilted. When viewed from above, you can recognize the trapezoidal shape of the seats.
The white brackets right and left at the rear edges of the seats are the shoulder harness holders. I attached them to the fuselage together with the floor plate. When the rear seat is attached, the elevator servo is hidden and it looks more like the actual machine.
Fabrication Part 28: Wheel Covers and Body Access Door
Wheel Cover
On the actual model, the upper half of the main wheel has a cover. At first, I was going to omit it from the model, but I learned from the Shizuoka Aviation Museum that the lower end of the cloth cover that covers the main wheel is tied to this cover, so I decided to install it. Here is the drawing of the main landing gear area.
The top of the fabric wheel cover is screwed to the structure. The upper wheel cover of the actual model is supposed to be made of FRP, but in the model, it is made of 1.6mm thick plywood side panels and 1mm thick balsa. In the actual model, the side panels are located inside the front support plates of the main landing gear, but at the stage of making the main landing gear model, it was planned to omit this upper wheel cover, so the gap between the front support plates and the wheel was made too narrow for the side panels of the cover to fit. Therefore, in the model, side panels were attached to the outside of the support plates. The finished upper wheel cover is shown in Photo 142.
On the actual model, a cloth wheel cover is tied to the bottom of the upper wheel cover and the string is screwed to the cover to prevent the cloth cover from shifting upward. In the model, there is no space for screwing, so I put bamboo string stoppers a little above the bottom edge of the cover. The lower part of the cloth cover is planned to have a rubber ring instead of a string.
In the process of making the upper wheel cover, I came to think that the cover is not only for fixing the cloth wheel cover. If it was just to hold the cloth cover in place, there would be no need to cover the circumference, and only the lower part of the side panels would be needed. It seems to me that it plays a role in preventing airflow from entering. If there is no cover, the airflow will flow to the rear of the fuselage through the gap between the wheel and the cover. This is because there are many gaps near the tail fins. This internal airflow will act as drag and degrade the performance of the glider. I don’t know the quantitative size of this drag, but I suspect that the glider is designed with the drag as minimum as possible, which is why this cover was installed. It may also be to prevent water droplets from wet tires from entering the fuselage.
Fuselage Access Door
Mita Type 3 has an access door on the left side of the fuselage, just below the center wing, to facilitate maintenance and inspection of the internal mechanism. The door is made of aluminum plate and hinged on the lower side to open downward. In the model, this door is made of 1.6 mm thick plywood to avoid radio interference. The door of the actual model covers the fuselage, so the thickness of the door juts out from the fuselage contour. In reality, to increase the rigidity of the thin aluminum plate, the door is slightly bent with a radius around it, so it protrudes more than the thickness of the plate. In the model, the door is aligned with the fuselage contour in order to minimize drag. Photo 143 shows the completed door.
I would really like the door to go down to the bottom, but since the hinge hardware I used only opens this far, I have no choice.
Actually, painting the flat door was quite tricky. In order to get the feeling of a smooth aluminum plate, I applied sanding sealer, dried it, polished it with sandpaper №400 three times, and then sprayed white surfacer before painting. Even so, the unevenness was still noticeable, so I had to fill in the putty, sand again, and then apply the surfacer and paint.
Attachment of a Dummy Aileron Servo
Operation of the Control Stick of the 1/3 Model
The gimbal mechanisms of the control sticks are connected to the elevator servo, so they move back and forth by the servo movement. However, the aileron direction is free because the servos are attached to the main wing. Therefore, the left-right position of the control sticks will not be fixed unless some support is provided. So, I installed a dummy aileron servo for the control sticks to make them tilt left and right along with the aileron operation.
Installation
I replaced the rear gimbal mounting shaft with a 50 mm long bolt and attached a horn to the end of the shaft to rotate the gimbal with a mini servo. This shows how they are installed.
The large servo next to the mini servo is the elevator servo. The control stick has not been fabricated yet, and the mounting pipe is visible. Since the mechanism is under the rear seat, it is hardly visible from the outside, so it does not spoil the feeling of the real aircraft. The mechanism was tested with a servo tester to make sure there was no problem. Now all three control systems work as in the real aircraft.
Fabrication Part 29: Release Mechanism for Winch Towing
Towing Cable Release Mechanism of Mita
The actual Mita Type 3 has two cable release mechanisms, one for winch towing and one for aerotowing. The former is installed at the lower part of the fuselage just before the main wheel, and the latter is installed just before the nose skid. Since I and my RC club do not own winches nor towing machines for gliders, I did not plan to install these release mechanisms at first. However, during the production process, I gradually wanted to try car towing if I had a chance. So I decided to make a cable release mechanism for winch towing. The cable release mechanism for winch towing of the actual machine is located at the lower part of the fuselage as stated above, but its inner mechanism is unknown because it is covered by a case. However, since the installation method is known, I asked the Shizuoka Aviation Museum to take photos of it. I decided to make it while imagining the internal mechanism based on the photos.
The release mechanism is activated by the spherical knobs located on the left side of the front and rear seats that can be pulled to open the hook. The knobs are connected to the release mechanism by a wire. The outline of the mechanism as seen in the photos are as follows.
- The pulley at the top of the mechanism bends the wire 90 degrees downward to enter the mechanism.
- When the wire is pulled, the hook rotates to the front side to open the lock.
- A circular ring is attached below the hook to prevent excessive side force from being applied to the hook when the tow rope comes off the axis.
- Two towing rods extending from the fuselage structure are attached to the rear of the mechanism to transmit the towing load to the fuselage. The towing rods form a ‘inverted V’ shape and are attached to the lower main longerons of the fuselage.
Interestingly, the mechanism is not located on the center axis of the fuselage, but about 100mm to the left. This is to avoid interference with the rear seat control stick, aileron and elevator control rods, which are located on the central axis of the lower front fuselage where the mechanism is installed. According to Mr. Kimura, the former owner of this glider, it is needed to step on the left rudder pedal significantly at the start of takeoff because of this offset installation.
Designing the Release Mechanism
Initially, I didn’t think too much about it. So I cut a 2mm thick steel plate and made the release mechanism. When the upper lever is pulled up by a wire, the lower hook is opened by a cam mechanism.
This prototype helped me to understand the main points of the design. The prototype has the following problems:
- When the tow rope is attached to the hook and tension is applied, the load is applied in the direction of opening the hook.
- The wire can pull up the lever, but it cannot close the hook.
The solutions for these problems were as follows:
- The rotation axis of the hook should be placed on the line connecting the position where the tow rope hangs on the hook and the center of the circumferential hook.
- The lever or the hook should be equipped with a spring and they must be connected so that when one of them returns, the other also returns.
I reconsidered the mechanism with the above conditions in mind. The first thing I thought of was a release mechanism using the same cam system as the prototype.
The cam is geared to meet the requirements of problem 2, above. This way, if you attach a spring to the lever, the hook will return with it. However, a small cam requires a certain level of precision.
It would be difficult to make this by hand. So the next idea was to use a link-type release mechanism.
The lever and hook are connected by a small link. The lever is pulled upward by a wire through a pulley at the top, and it opens the hook by rotating the hook clockwise through the link. A small spring is attached to the lever to return it to its original position when the wire tension is lost.
These mechanical parts are assembled between the two side panels left and right. The side plates enclose these parts and also transmit the tension of the tow rope from the hook to the fuselage. For this purpose, the rear part of the side plates are like a lug, and two tow rods extending from the fuselage side are connected to it. Between the rotating shaft of the hook and the lug, two reinforcement plates in the shape of a band are attached right and left to prepare for the towing load. A ring is attached to the underside of the side panels. This design seems to work more accurately than the cam type, even if it is made by hand. So I decided to make the mechanism with this design.
Fabrication
First, I cut out the parts:
The lever and hook are cut out of a 3mm hard aluminum plate. The link that connects them is made of 2mm aluminum. DURACON bearings are inserted into these mounting holes. The pulley is a 9mm diameter one that I had on hand. The side plates are made of 1.5mm aluminum plates, and since the two plates are assembled with a 5.5mm gap between them, I made spacers with MDF plates of the same thickness. For the ring, I used a steel ring with a diameter of 25mm, which I found at Home Depot attached to the end of a chain.
Assembly Completed
I assembled above parts, but since the parts themselves were cut out with a hand saw and file, and the holes were drilled with a hand drill, the accuracy was limited. As a result, the lever and hook hardly moved at all during the first assembly. It took quite a bit of work to carefully find and fix the points that were hit and file them little by little, but finally the hook started to open and close smoothly. Photo 147 shows the release mechanism that was finally assembled.
You can see the pulley and spring on the rear side. The two ends of the spring are parallel, but for installation reasons, the spring was twisted 90 degrees, which caused the spring to twist slightly and the lever to lean to the left. I’ll fix this later. This is the open/close test of the hook.
Later, before installing the mechanism, I found the ring was easily detached, so I modified it. Since the side plates of the mechanism are made of aluminum, the ring cannot be soldered. I bought some metal glue to attach the ring to the side plates, but once it was attached, the mechanism could not be disassembled. Since the inside is a linkage mechanism, I have to be prepared to disassemble it in case of failure. To solve this problem, I attached 1mm thick brass plates on the outside of the side plates (Photo 149) and soldered the ring to it. This will maintain the disassembly condition. It took a lot of work, but the mechanism is almost as expected.
Installation Drawing
The release mechanism is attached like this:
The mechanism is mounted on the truss where the rear stick gimbal is mounted, a little over 30mm to the left of the centerline. The tip of the mechanism sticks out of the plane. The rear of the mechanism is attached to a carbon mounting bracket. From the mounting bracket, two rods extend in an “inverted V” shape and are attached to the left and right longitudinal members of the lower fuselage. These rods transmit the traction force of the winch. The upper part of the mechanism is fixed to another mounting bracket coming out from the truss bar where the gimbal is attached.
The servo that opens the hook is located under the rear of the front seat. A mini-turnbuckle, which is not found in the actual model, is attached in the middle of the wire for ease of installation and removal.
Minor Modifications to the Fuselage and Fabrication of the Attachment Structure
The fuselage was assembled without taking care of the release mechanism, so I made some modifications before building the mounting structure. First, I removed the parts that were in the way of the mechanism attachment area. For this purpose, I removed the diagonal part toward the rear gimbal mounting axis and the part extending backward from the bottom of that part as shown in photo 150.
There, I assembled the mounting structure for the release mechanism:
The photo above is difficult to understand, so I turned the fuselage over and took a photo from the underside.
You can clearly see how the ‘inverted V’ rods that transmit the traction force are attached. On the lower side of the photo (nose side), there is another fitting that supports the upper part of the release mechanism.
Installing the Release Mechanism
With this preparation, the release mechanism was installed.
This is how the servo is positioned:
When the release mechanism is attached, it becomes more mechanical, which is quite nice.
The servo is the same as the one used for the spoiler, with the following specifications:
Fabrication Part 30: Fabric Wheel Cover
Mita’s Fabric Wheel Cover
Zuk (canvas) cover is attached around the main wheel of the Mita Type 3. The upper end of the cover is held by a thin aluminum plate and screwed to the fuselage structure, and the lower end is tied to the lower part of the wheel cover with a string. Drawing 38 shows how this is done.
Fabrication
The cover of the real aircraft is made of zuk, but it is too thick for a 1/3 model, so I cut up some old chinos and used them. The cover has a three-dimensional shape, with a rectangular top and a nine-sided base. Moreover, the base of the cover is not on the same plane. There are severe undulations. It is quite difficult to cut out a shape from a single piece of cloth that fits this shape perfectly.
First, I drew a development drawing, and then cut out the shape from the scrap cloth and tried to make it fit the fuselage. However, due to fabrication accuracy problems, the cloth did not fit perfectly. I took the actual dimensions from the real thing and corrected the development drawing before cutting out the cloth. Even so, the first piece of fabric I cut out was not long enough to form a shape.
This is the picture of the finished product after I revised it again.
The top is made into a bag around the perimeter, and an elastic cord is passed through the bag and tied to the bottom of the wheel cover. I folded and sewed the baggy part, but as I haven’t used a needle since elementary school, the needle marks were uneven and messy, so I decided to glue it. Around the hem area, I folded thin aluminum plates with a width of 5mm, sandwiched the cloth, and screwed them to the structure side. Photo 156 is what it looks like when viewed from above.
You can clearly see the overall shape. This is what it looks like from the back side (inside the fuselage).
Drawing of the Nose Cowling
I started to make the FRP cowling for the nose. This is the first experience for me to make FRP parts. I proceeded through the processes gathering various information on the internet.
Cowling fabrication requires following processes: 3D shape drafting → wooden mold fabrication → female mold fabrication → and FRP resin application. The first step is to draw the three-dimensional shape.
Nose of the Mita
The nose of the actual Mita Type 3 Revision 1 is covered by an FRP cowling. The rear part of the cowling that connects to the fuselage structure has a cross-sectional shape of the semicircular canopy on top of the octagonal fuselage structure, but the tip is oval. In other words, the cowling is a three-dimensional structure whose cross-sectional shape deforms in complex ways. A 3D drawing of the structure is required for fabrication.
The fuselage structures I have built so far are octagonal, and I was able to create the cross-sectional shape from the plan and side view, but this is not possible for the cowling. If I could use 3D CAD, this work would be relatively easy, but since I can only use 2D CAD, I must design the 3D outline shape of the cowling with it.
Creating the Primary Cross-Sectional Shape
First, I cut the cowling vertically in several places in the side view and drew the cross-sectional shape of the area while imagining it. From the plan view and the side view, I picked up the parts where I could see the width and height of the structure, and drew lines imagining the space between them. I was careful not to touch the motor or mounts that are wrapped in the cowling. The rear end of the cowling is not flat, but bent like a folding screen, so draw a cross-sectional view (cross-section A-A) of that part as well.
Checking and Correcting the Undulation of the Cross-Sectional Shape
There is no guarantee that the cross-sectional shapes drawn in such a way are smoothly connected. Therefore, I piled them up and made radial lines every 30 degrees from the front, and drew seven longitudinal section drawings by finding the intersection points with the cross section drawing (Drawing 43). If there was any undulation in the longitudinal section drawings, the cross-sectional drawings were modified. In this way, I found the cross-sectional shapes that are smoothly connected.
Completed Cowling Cross-Sectional Shape Drawing
In this way, the cross-sectional shape of the cowling was finally completed:
Fabrication Part 31: Wooden Mold of the Nose Cowling
Making the Wooden Frames
Using the finished 3D shape drawing of the nose cowling, I cut out the parts that would become the wooden frames from balsa wood. There are 11 cross sections and 4 cross sections at 90 degree intervals cut lengthwise. The skeleton was divided into two longitudinal sections, each of which was assembled and then pasted together. This is to make it easier to stand the cross-sectional parts at right angles. A skeleton was made by assembling these parts.
Pasting and Shaping the Outer Surface
There are various ways to make the outer surface, such as filling the spaces between the skeletons with styrofoam or clay. At first, I thought styrofoam would be easy, so I went to buy some styrofoam, but I couldn’t find anything suitable, so I bought some clay. However, I needed a lot of clay to fill the spaces between the skeletons. I thought about filling the center of the skeleton with dummy stuffing and then filling it with clay, but since I had no experience with clay, I didn’t know how much it would shrink when it dried, I began to feel uneasy. So I decided to use balsa, which I was familiar with, to fill the spaces between the skeletons. After roughing the surface, I filled the large holes with putty and shaped them.
So far, so good, but from then the hard work had begun. In order to be used as a wooden mold, the surface must be smooth and free of any dents. Since the final shape was created from a polygon with many balsa boards between the skeletons, I had to file, fill in with putty, and apply a sanding sealer over and over again. When I applied the lacquer surfacer after I thought I had completed much of the work, the small scratches, dents, and undulations on the surface came out clearly. I had to start the process all over again from sanding.
This is what I managed to get close to completion:
There were still a few minor dents, so I had to fill them in and then polish it up with some 1000-grit water-resistant paper.
After this, I was going to make a female mold from plaster, but I changed my mind to gain experience with the simpler center wing fairing first, and then start on the nose cowling.
By doing some research, I found that there were many cases where the wooden mold did not come out of the plaster mold properly, and the mold was broken.
If the FRP cowling does not fit well with the fuselage, it may be necessary to go back to the wood mold and fix it. In fact, I wanted to make the mold to match the actual dimensions of the fuselage structure, but the motor mount was in the way and I could not measure the actual dimensions well. So I had no choice but to make the wooden mold according to the drawing. If the accuracy of the fuselage structure or the mold is not good, I will have to modify the cowling or the structure.
This is the seventh 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.
