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Top Flite B-25J Mitchell Project

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Page 4

       A common problem with long air lines is that they have the tendency to “charge up” with air before they have enough pressure to move the air cylinders in the retracts. This may cause the retracts to abruptly “jump” which ruins the scale appearance you may be looking for. To solve this problem I like to install Robart air line restrictors (Cat #189) as close to the air cylinders as possible to keep the air flow as smooth and steady.

       As a precaution I like to use .015” stainless steel tie wire to secure all of my intermediate air line connections. You can get a 1/4 spool of the tie wire relatively cheap at McMaster-Carr (Cat #8860K11). I am using a spool of BVM tie wire in the photo below.

       Notice the double wrap on the tie wire. This helps make a positive seal on the tubing barbs and prevents the lines from popping off when they are in warm weather.

       The next step was to install the air line into the inboard wing panels and set the retract mechanism in the nacelle. I drilled out the #6 screw holes with a 3/32” drill bit and installed the mounting screws. Notice that the retract mechanism is as far aft in the mounting inset as it can go. This helps the nacelle cover clear the main struts when in the down position.

       The B-25 kit came with a set of foam rubber tires which were probably intended to reduce the weight of the model. Unfortunately foam tires don’t last long and can deform when left to sit on the ground in one position for too long. I chose to use a set of Robart scale wheels for my B-25 which look a whole lot better and don’t deform. For the main wheels I used a Robart 4” scale wheel (Cat #137) and for the nosewheel I used a 2-3/4” wheel (Cat #132)

       To prepare the main wheels for installation I glued in the 3/16” wheel bushings into the wheels with some Zap-A-Dap-Goo. (Cat #PT12)

       I used a 3/16” drill bit to drill out the rear hub cap using the bushings as a guide. Use slow speed as to not damage the plastic wheel bushings.

       Using my Dremel tool I flattened out a spot on the axles for the set screws. This will prevent them from shifting in the struts.

       To keep the wheels from hitting the strut mechanism a 3/16” X 1/8” wide spacer was added to the shaft. I used a couple of small spinner shaft adaptors as my spacers but washers would work just as well.

       Once I was happy with the fit I applied some light grease to the axles and installed the main wheels. I then used blue thread locker on the set screws and tightened the axles in place.

       The next step was to fit and install the nacelle cover. My modified firewall design requires that you cut the top front part of the nacelle cover to fit it over the firewall. This is easily done and allows you to open up the fiberglass piece and install it over the firewall cowl mounting tab.

       After the cuts were made in the nacelle covers they were trimmed to fit the firewalls and also clear the remaining cowl mounting tab locations.

       It was now time to install the nacelle cover mounting hardware. I positioned the nacelles so the rear nacelle centers were set at 112 mm from the outboard edge of the inboard wing panels. The front of the nacelles are basically guided by the firewall so there is no need to align the front of the nacelle.

       Blue painters tape was used to hold the nacelle in place while the hardware was installed.

       A Great Planes dead center hole locator (Stock #GPMR8130) was used to mark where the recessed mounting holes meet the wing panel. The nacelle covers were then removed and a 5/64” drill bit was used to pilot hole the mounting screws. The nacelles were then reinstalled and the mounting screws were installed using a magnetic screwdriver.

       To secure the nacelle covers where the cuts were made I fabricated a small 1/8” thick plywood plate that has two 4-40 blind nuts glued into it.

       The nacelle covers were then drilled to match the plywood plate and 4-40 screws were installed to secure the nacelles.

       Once the carburetor intakes are installed it will totally conceal the hardware.

       At this point I was ready for my gear doors. Unfortunately this kit does not provide an option for sequencing doors which really adds scale appeal to a B-25 model. To solve this problem I decided to fabricate my own gear door sequencing linkage that will bring this scale model to where it belongs.

       To start out I needed to figure out how to add hinged doors to the main landing gear doors. This will allow the main struts to be extended while the doors are shut.

       The inboard doors are the ones I needed to modify so I prepared them by grinding out flat spots on the inside of the forward end of the doors as in the photo below. Not that this is the left wing inboard gear door shown below. The right wing inboard gear door was done the same way but inversely.

       Using a Dremel tool equipped with a cutting wheel (Dremel Cat #409 only!) I cut out two sections in the doors measuring 26 X 36 mm. I made sure that I preserved the cut out pieces as best I could to reuse them as the new strut doors.

       To hinge the new doors I chose to use a Dubro nylon hinge. (Cat #116) To install the hinges I would need some 2-56 screws, washers and a small spring.

       I acquired the 3.5 mm wide spring from a Harbor Freight spring assortment (Cat #93323-4VGA) but any similar spring will do. This spring will close the door when the strut is not in the “down” position. I used a pair of pliers to make a set of shorter springs out of the long one for the doors.

       I installed the hinge into place with one of the 2-56 screws and installed the spring under the nut on the inside of the door. The hinge was then glued into place with thick CA. Notice that I had to trim one end of the hinge to clear the reinforcing rib on the door. Note that this is the right wing inboard gear door.

       The door pieces that were cut out of the main doors were then installed on the hinges in the same way with the spring under the mounting screw. Some trimming was neccesary to get the right fit.

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