Top Flite B-25J Mitchell Project

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Page 3

       The next step was to install the throttle servos and linkages. I decided to use a set of Hitec HS-81 micro servos for the throttles on my B-25. I have used these servos in countless applications with excellent results however excessive vibration has proved to be the Achilles heel of the HS-81. Needless to say I do not recommend using these on large engines that vibrate a lot. The Saito FA-90T engines run very smooth so I have no concern using the HS-81 servos. The main advantage in using a “micro servo” is the weight savings it will have over a standard one (about an ounce or so) and every ounce I can save is important.
       Behind the upper firewall is an area that is perfect for a micro servo so I CA glued in some 3/8” square X 25 mm long basswood blocks to the back of the firewall as shown below in the photos. I installed the blocks at 37 mm to the left of the inside edge of the nacelle structure former on both nacelles. This block will hold the front of the HS-81 servos.

       To support the rear of the servos I installed two 3/8” square X 65 mm long basswood blocks at 30 mm behind the front blocks as shown in the photos below.

       Two 3/8” square X 20 mm blocks were glued onto the rear blocks so the servo could be screwed in. Notice that they align with the front blocks.

       I used a 1/16” drill bit to pilot the servo screws on both blocks. Notice that I drilled trough the side of the nacelle structure to access the rear blocks. This hole was widened to get a Phillips screwdriver to fit for the servo install.

       Notice below that I installed the servo bushings with the flange outward (backward). This is to reduce the servo from flexing in it’s mounts allowing more precise throttle control.

       During the construction of my B-25 I used my Futaba 8U radio to center and test all of my servos. Ultimately I planned to use my Futaba 14MZ radio for the plane but I felt more comfortable working with my “cheap” radio when dust and glue was present.

       To connect the servos to the throttle arms I used 2-56 rods connected by Dubro ball links (Stock #190) and Dubro E/Z connectors (Stock #121). Notice the slots in the firewalls, I cut them with a Dremel tool and a carbide rasp.

       I tested out both throttles with my radio to check for binding. Once satisfied I moved onto the next phase.

       The next step was to install the fuel tanks but before I could I would need to address the motor mount screws. The supplied 4-40 motor mount screws proved to be too long for my application and would end up hitting the fuel tanks so I cut them down to 12 mm long. Half inch long replacements would work just as well if you like.

       I could now set up my fuel tanks for installation so I rounded up my stoppers and fuel clunks for assembly.

       The fuel tanks come with several aluminum fuel tubes: two long and one short. I will be using only one long and one short for my fuel system. The short one will be the suction (clunk) and the long will serve as the vent line. I needed to bend the two tubes to clear the carburetor of the Satios so I used my tubing bender to bend a 45 deg. bend upward in both tubes.

       A second bend was needed on the inside of the stopper for the vent line to reach the top of the tank. Make sure you install the inside stopper backing plate before you bend this tube.

       I used a short piece of the supplied silicone tube to extend the vent line to reach the top of the tank. I then cut a piece of tubing at 105 mm long for the clunk tube and installed the clunk. Notice the angled cut on the vent tubing to prevent the blockage of the line.

       I test fit the fuel tanks to see if the tubes cleared the engine and then removed them to secure the nacelle structures to the wings.

       It was time to remount the nacelle structures to the wings so I installed a 12” servo extension to the throttle servos (not shown) and threaded them above the nacelles and into the wings. Since the fuel tanks will cover up the nacelle nylon mounting screws I decided to hold them in place with some high temp hot glue.

       I reinstalled the fuel tanks and inserted the supplied plywood keeper plates. I used small screws to hold in the keepers opposed to gluing them in for future tank serviceability.

       To prevent any unnecessary tank movement I applied some hot glue to the stopper area as well as the rear edges of the fuel tanks. Hot glue is an excellent way to secure things as it can be removed later for service.

       The fuel tanks were now complete and I could get into the retract system for the nacelles.

       I had originally had the Robart retracts for the Wing Manufacturing B-25 kit which included the Robart “156VRX” medium air control kit (discontinued). I decided to use the air kit I had for this build saving me some money on the suggested standard (small) air kit “188VRX”. Apparently the medium sized air tank has been discontinued from Robarts line since the completion of my kit.

       One of the first things I did was to rotate the hose barb on the main wheel retracts so it points upward into the wheel well. This is so the hose will clear the retract mounting rails. It’s pretty easy to do considering the hose barb will swivel in the cylinder, you just need to remove the cylinder mounting pin to get the barb moved into the right position.

       The next step was to install the tubing to the cylinders. I use the purple line for the “down” supply line and the pink for the “up” supply. Notice the air line retainers (Robart Stock #170), they keep the air lines attached to the barbs.

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