Top Flite B-25J Mitchell Project

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Page 5

       At this point the main gear doors were almost ready for installation on the nacelles but there was one more thing to do. To keep the strut doors from closing too far inward I needed to install a small plywood stop on the inside of the main door as seen below.

       The next step was to hinge the gear doors on the nacelles. The supplied hinges were prepared as the instruction manual suggested. I decided to go a step further and sand the bottom mating surfaces that will be glued to the nacelle. This will give the epoxy something to grab on to.

       I used a high speed Dremel cutter to remove the paint where the hinges would be glued to. I also used a sanding drum to scuff up where the hinges would meet the nacelle sides.

       When fitting the doors you need to position them as best you can and tape them into place without warping them to fit the nacelle. If you were to tape the doors tightly to the nacelle when gluing the hinges you will be disappointed with the action of the doors. This is because the doors will warp from the tape pressure and spring back when you remove the positioning tape causing the hinges to bind. I applied my positioning tape very lightly as to not warp the doors.

       To glue the hinges into place I used 5 minute epoxy and milled fiberglass (for added strength).

       The hinges were glued into the positions that the manual suggested with the exception of the front inboard hinges. These hinges were offset back to clear the strut doors as seen below.

       After the epoxy had cured I remounted the nacelles to the wing for a fit check. After some adjustments I got both sets of doors to work flawlessly.

       The next step and the most difficult was to figure out how I was going to actuate these doors. The kit has a system that closes the door when the strut is retracted but this will not work with a sequencing door system. I needed to devise a way to close the doors without interfering with the action of the main wheels. After messing with some ideas I came up with a bellcrank system that closes the doors from aft end of the nacelles.
       The first step was to replace the rear hinge screws with some 2-56 threaded ball links. I used a set of Great Plains ball links for this. (Cat #GPMQ3841)

       The ball links replaced the 2-56 stainless screws that were installed into the nylon ball link couplers on the rear hinges as seen below. This will provide a connection point for the gear door linkage.

       The next step was to create a bellcrank system that could actuate both doors simultaneously without interfering with the main wheels when retracted. I decided to use a set of Carl Goldberg aileron bellcranks (Cat #420) in conjunction with a K&S Engineering 1/8” brass tube (Stock #5127) to make my bellcrank system.

       I cut a section of the 1/8” brass tubing to 83 mm long. I then drilled out the bellcranks with an 1/8” drill bit to fit over the new axle. The bellcranks were then pushed on the axle leaving 12 mm of axle exposed on either end.

       To affix the bellcranks to the brass axle I needed to drill a couple of small holes to install the set screws. For this job I need to use my precision carbide drill index. I have several favorite drill bits that I use a lot with my Dremel tool. I use 1.15 mm as well as .035” for different jobs and even a carbide rasp bit to hollow out wood and cut fiberglass cowls (as seen below). These bits are an absolute must for the meticulous builder and can be found from a couple places but are becoming more scarce these days.
       I am told that these bits were made for the printed circuit board industry and are being phased out due to the ever growing use of laser cutting. This prompted me to stock up before they are no longer available and now I have several hundred :0) You can find them around the internet and I have seen them at
Harbor Freight’s website. I believe that Bob Violett (BVM Jets) carries a set of rasps as well.

       For the additional hardware I needed on this build I used Micro Fasteners as my source. They have an incredible selection of very small screws that work perfectly for my needs. In this case I used their #0 X 1/4” long pan head screws for the bellcrank set screws. This size is a must in your building arsenal of hardware.

       I used my 1.15 mm carbide drill to pilot hole where the screws would be installed into the brass tube as seen below.

       A 5/64” hole was drilled into both sides on one end of the bellcrank. This is to accommodate the 2-56 control rods that will be installed on them.

       The bellcrank axle sits at the aft end of the nacelle just below where the main wheel sits when retracted. I aligned the bellcrank so the forward arms were vertically under the ball links.

       To make a pivot point for the axle I needed to make some basswood blocks. I cut some 3/8” X 1/2” basswood blocks that were 30 mm long for the pivot points.

       To get the blocks to fit against the angled sides of the nacelle I needed to drill them to match the angle. Not an easy task but you can make a few extra blocks for practice until you get the right angle. You can also sand the block until you get a close fit to the nacelle.

       Once the pivot blocks were fitted to the axle I set them into the nacelle. At this point I made sure that the main wheel did not contact the axle when retracted.

       To prepare the pivot blocks for gluing I applied a small piece of masking tape to the back side of the block so the epoxy does not get onto the axle hole.

       To glue in the pivot blocks I used 5 minute epoxy and microballons. Once again making sure that the retracts did not contact the bellcrank system.

       To connect the bellcranks to the doors I used a set of 2-56 control rods fitted with the female ball link couplers. I used Z-Bend pliers to bend the ends of the rods at about 55 mm from coupler to bend.

       After the linkage was installed the couplers were adjusted to make the doors close evenly. When done the doors closed with very little effort.

       Now the only thing left to do is figure out how to actuate the doors. Other than that the system worked like a champ, closing the doors over the strut with ease.

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