Top Flite B-25J Mitchell Project

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Page 14

       The next step was to install the glow driver system. For this I used a Ram Models glow driver (Cat # RAM 50) that I modified to use with four glow plugs. Originally this glow driver was designed for a single plug but after doing some research I figured I could “Tim Taylor” this unit to meet my needs. Four OS Type “F” plugs require about 7.5 amps @ 1.2 volts so I needed to upgrade the circuit to meet the demand. Unfortunately I did not document the entire modification as I had worked on it several years before this build and did not know I would be using it in an article.
       I was able to remove the heat sink off the driver for a better look at the board and my modifications. However this may not be enough information to duplicate what I had modified so feel free to ask
questions if you like.

       The main advantage to using a solid state glow driver is that it will turn the glow driver off when the radio power is turned of. This is unlike a mechanical servo driven glow driver switch which can be left on after your flight and drain your glow battery dead. However there is no reason why you can’t just use a heavy duty microswitch and a servo to turn on your glow system, just be careful to make sure the system is off after each flight.
       You may notice that the driver has two heavy duty leads that I added on the left and right below. These are the glow driver circuit leads that connect the driver battery to the glow plugs. The LED at the top lets you know that the driver is operational (this is handy for setting up the activating point on your throttle channel). The lead just below that is the throttle channel lead which plugs into the receiver. The lead below that is for trickle charging the driver battery in the plane which is connected parallel to the “battery in” leads.

       The power switching transistor in the RAM 50 can handle up to 20 amps of current but will need a sufficient heat sink to cool it. In the original glow driver configuration (not shown) the transistor is mounted on top of the circuit board. I moved the transistor to the bottom of the board to allow me to mount a aluminum heat sink made out of 1” angle stock. Notice the three transistor leads next to the tie wrap below.

     I epoxied the soldered in transistor to the back of the board for added strength. A 15 amp automotive fuse was added to the board for circuit protection but is not necessary for proper operation.  I upgraded the transistor solder traces with more solder and a 12 gauge wire jumper as seen below.

       I folded over the aluminum angle edge to make the package more compact and bolted the angle directly to the transistor bottom (not seen). The angle also serves as a convenient way to secure the wires coming off of the board with cable ties. However it is important to not allow the angle touch any live parts of the circuit board. A piece of electrical tape can be used to guard the board.

       To attach the glow driver to the wing panels I used a set of Deans Ultra Plugs (Cat #1300). I used 14 gauge automotive wire to make the “wye” connectors that will extend out each side of the fuselage.

       To mount the glow driver to the plane I made a 1/8” plywood bracket that I glued into the fuse as seen below.

       I used cable ties to attach the driver to the bracket and threaded the wires into the fuse.

       It was time to add the servo extensions into the guide tube of the fuse. These servo extensions will power the elevator and rudder servos in the tail. I made a set of extensions out of four 600 mm Cirrus heavy duty leads (Hobby People Cat #CF600HD). I soldered two of the leads together (both sets) to make two 1200 mm extensions as seen below. Heat shrink tubing was used on the splices. I soldered the two leads instead of just plugging them together as the male pin connectors would not fit into the guide tube.
       You may notice that the leads have been twisted, this helps reduce any inductive transmission into the signal lead causing unwanted servo jitter. I twist all of my long servo leads for this reason.

I highly recommend the Cirrus HD leads as they are economical and built very well. I use the Cirrus HD leads on most of the planes I really “care” about.

       The rest of the servo extensions were run through the fuse at this time. I temporarily labeled the leads for future identification.

       The charge receptacles and radio switch were next. I used a set of Ernst charge receptacles (Cat #124) and a Futaba switch harness (Cat #SWH13).

       I installed the radio switch below the RX battery charge receptacle on the left side of the fuse. The glow driver charge receptacle was installed on the right side of the fuse as seen below.

       Since I decided not to install the profile pilot figures into the cockpit I needed to patch the slots used to mount them. I used some NHP Micro-Fill to patch the slots and sanded them flush. A coat of the Testors flat black spray paint was used to finish the job.

       I am a big fan of using on-board voltage meters for RX battery condition. I wanted to add a meter to the cockpit of the B-25 so I made a 1/16” plywood back to the cockpit to hold a Hobbico VoltWatch meter as well as the glow driver status LED.

       The VoltWatch unit shown here has been discontinued for some time. There is a newer model available (Cat #HCAP0332) called the VoltWatch II. I still like the old style meter over the new one and have several on hand in my personal stock.

       I used some masking tape to cover the LED indicators on the meter and spray painted the plywood black. This flat black will help the back plate blend into the cockpit.

       The instrument panel was next to go. I touched up the paint on the instrument cluster board before proceeding to glue it to the gauge decal.

       I applied 3M spray adhesive (Cat #77-7) to the back of the cluster board and adhered it to the decal as seen below.

       The bezel decal was then added over the gauges. I used a heavy book to press the decals together. Medium CA glue was then used on the edges of the cluster to permanently hold the decals on the board.

       To install the instrument cluster into the cockpit I used three #0 x 1/4” screws (Micro Fasteners Cat #SMPP0004).

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