Top Flite B-25J Mitchell Project

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Page 13

       The next step was to install the linkage for the sequencing nose wheel gear door.

       To actuate the gear door I used a Hitec HS-81MG servo (which has been since replaced with the HS-82MG). I cut out a couple of 1/2” x 3/8” x 12 mm long basswood blocks to be used as servo mounts for the HS-81MG.

       The servo blocks were pre-installed on the servo and 5 minute epoxy was applied to the blocks as seen below. The servo can now be glued into the fuse as one unit but still be removable for service later.  The blue tape will protect the servo from being glued permanently in place.

       The servo assembly was then placed into the gear door bay as seen below. I positioned the servo so the servo’s top edge (the part with the Hitec label) is flush with the door opening.

       A small Great Planes control horn was attached to the door with #0 x 1/4” screws and epoxy as seen below. Notice that the top hole of the horn has been trimmed off to clear the gear.

       A 2-56 rod linkage was added to a Dubro standard servo horn on the 81 servo. The ball link on the door horn is important as it allows the pivot motion needed to close the door.

       I set up the servo linkage to be parallel with the servo horn when closed as seen below. This will prevent a constant load on the servo when the door is closed and also keep the door closed when the plane is in static display.

       As a final touch I painted the hinges and horn with some flat black Testors paint.

       The next step was to install the nose wheel. For this I used a Robart 2-3/4” scale treaded nose wheel (Cat #132).

       The nose wheel was installed on the strut with the supplied axle.

       The B-25 was now on it’s feet and beginning to look like a plane!!!

       I could now move on to the internal wiring and plumbing of the airframe. I decided to start off by mounting my retract air tank. The tank I used was a Robart medium air tank which has since been discontinued. I will still show you how I mounted the tank for reference or just in case you can find a similar air vessel to replace the Robart tank.

       I cut out a pair of plywood cradles to hold the tank in the top of the fuse behind the cockpit. I cut them to fit the tank snugly to not allow any vibration clearance around the tank.

       The plywood cradles were epoxied into the fuse on the formers as seen below. Make sure the tank/s will clear the main spar tube.

       The reason I decided to not use a smaller tank for the retract system is that I feel the one suggested for the kit is way too small for the application. You should have at least six full cycles of air supply for your gear as insurance. I personally like to retract my gear and extend them in flight to watch their operation, especially with sequencing doors. Two or three cycles of air reserve is cutting it too close especially if you develop an air leak in flight.
A sufficient substitute for this medium air tank would be to either use the Robart Large air tank (Cat #192) or two of the small air tanks in tandem (Cat #172)

       Below is an example of a tandem small air tank installation (Cat #172).

       A Hitec HS-81 servo was installed for the retract valve servo in the plywood deck under the cockpit. The plywood was notched to accommodate the servo as seen below.

       The Robart retract valve was installed just as the manual suggested.

       I added a Futaba S3114 micro servo alongside the retract valve servo to control the landing lights. A EMS-Jomar E-Switch (Cat #ESWJ) could be used as a substitute for the servo controlled one if you prefer solid state :o)

       The micro switch supplied with the landing lights was installed near the switch servo as seen below.

       A Robart air pressure gauge and filler fitting was added to the left side of the fuse in the pre drilled holes. Thin CA glue was used to stiffen up the balsa sheeting before installing the fittings into the wood as to prevent the fittings from crushing the delicate outside sheeting.

       The air line system was plumbed at this time. I used .015” stainless tie wire to secure all of the air tees and fittings.

       I installed the Robart quick couplers (Cat #190) to the air lines for the wings with the tie wire as well. Notice below how I threaded the wire inside of the hose keeper collar.

       I installed a male and female coupler on opposing air lines so they cannot be switched around. I use the pink line for gear up and the purple for gear down.

       I installed the voltage regulator for the landing lights and soldered the switch wires in place. I added some new wire to the voltage regulator to reach the wings so I repackaged the regulator with some new shrink wrap (in case yo noticed).

       To connect the landing lights to the regulator I needed a series of Deans micro plugs (Cat #2R) as to allow the wings to be disconnected from the fuse. I made sure that the polarity of the LED lamps were correct as to not burn out the regulator or lamps. Luckily the Deans 2R plugs won’t allow a reverse polarity connection by accident.

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