Top Flite B-25J Mitchell Project

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Page 9

       The next step was to paint the inside edges of the cowl hatches to clean up the appearance of the gaps created by the cutting of the cowl. For this I used a olive drab Tamiya spray paint #AS-6 which is a close match to the B-25’s olive drab but not perfect.

       To apply the paint to the cowl I sprayed the paint into a epoxy mixing cup and let the paint stand for a few minutes to stabilize. I then used a hobby paint brush to apply it to the plywood strips on the cowl.

       To match the medium gray on the bottom of the cowls I chose to use simple automotive gray primer. This Dupli-Color paint (Cat # FP101) is a perfect match for the gray on the B-25.

       Since I was in painting mode I decided to touch up the gear doors with a little black paint. For this I used a flat black from Testors (Cat #1249) which will be used for all of the black painting on this model. I used the same epoxy cup method to brush the paint on.

       The next step was to paint the chrome valve covers to match the cowl hatches. This will really clean up the appearance of the cowls and preserve the scale appearance.

       To get the paint to stick to the valve covers I decided to lightly sand blast the covers in a blasting cabinet with silica sand. The covers can also be tumbled in sand to roughen up the surface for the paint to stick to. If you should decide to sand blast the covers be sure not to blast too aggressively or the chrome plating will come off of the aluminum cover.

       This surface change in the valve covers finish will help the paint stay on the metal.

       To help the color coat stick to the bare metal I used an automotive primer in two thin coats.

       I used the same olive drab paint as before to put a final color coat on the valve covers.

       Since I had the blasting cabinet out I decided to “treat” the propeller tips for painting as well. You can see the subtle difference that the blasting makes on the epoxy filled glass. Masking tape was used to protect the rest of the propeller while in the blasting cabinet (not shown).

       A coat of white automotive primer was used under a coat of yellow LustreKote model paint (Cat #TOPR7504).

       Now that the valve covers were dry they could be mounted to the engines for final assembly.

       The final outcome was well worth the effort. The cowl hatches fit beautifully and also serve as a valuable access point to the engines.

       At this point I was very pleased with the outcome of the engine nacelles and it was time to move on. I wanted to finish up the wing panels and move on the the fuselage so I still needed to install the flap and aileron servos.

       Because of the added weight of the larger engines as well as the glow driver system some consideration must be given to saving weight somewhere on the model. Unfortunately there aren’t too many things you can lighten on a ARF model. One thing you can do is find lighter servos to install as I did with the throttle servos earlier. The flaps call out for four standard servos weighing in at an average of 48 grams each (for a total of 6.75 ounces). I decided to use a Mini Hitec HS-5245MG digital servo for each flap that weighs 32 grams which will save me just over two ounces in added weight to the model.

       The Hitec “Mighty Mini” digitals produce 61 oz-in of torque at 4.8 volts and features a metal gear train that will help prevent stripped gears if the control surfaces should get bumped. The servo hatches will of course need to be modified to fit the smaller servo. For this I used a piece of basswood to shim up the servo block on the open side of the servo hatches as seen below.

       When building a model of this size I hesitate to use anything other than the best parts available. The supplied control linkages that came with the kit are of good quality and would probably do the job just fine. In this case I want to be absolutely sure there is no failure in the linkage so I opted to use all 4-40 control rods on this build. I like using Dubro’s Kwik Links (Cat #306) for all my big planes control rods. Along with their solder on Kwik-Links (Cat #604) these rods are indestructible. I have used these rods on everything from my Goldberg Ultimate to my $7200 Boomerang turbine jet and have never had a failure.

       The rods can be used full length to lay out the position of the control horns and then cut to length once the horn is in place.

       The key to using the Kwik Links is that you must silver solder the loose end to the rods completely. For this I use Stay-Bright silver solder (Cat #2000SBKIT) and a propane torch. I use a bit of sand paper to clean off the rod end and then slip the link on the end. I then apply the supplied acid flux to the link (at the rod end) and then heat up to add the silver solder. (Be careful of the acid flux, it can burn your eyes and skin easily)

       I set up my flaps to max out at about 45 degrees. This is the maximum flap deflection I would suggest for this model.

       The pictures below demonstrate where the servo horn needs to be in the flap down position (top picture) and the flap up position (last picture).

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