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Top Flite B-25J Mitchell Project

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Page 11

       The next step was to build the tail section so I proceeded to glue in the hinge points into the rudders using 15 minute epoxy.

       Before the epoxy cures you want to make sure you have about 45 degrees of travel on the rudders. If they are glued too tight they will not move as much.

       The elevator control wire and supports are the next to be installed. I epoxied the supports into place and secured the wire to the stab (I removed the covering from the stab beforehand at the support locations). Notice the notch in the upper (shown bottom) side of the stab to allow more motion of the control horn.

       The hinge points were also glued into the horizontal stabilizer at this time. The elevators were cycled to make sure they can move up and down at least 1-1/2”

       At the time of building this model there were no Hitec digital micro servos available for the rudders on this aircraft so a “substitute” was needed. In this case I needed a set of micro sized servos that could produce 50+ ounces of torque for the rudders. Futaba had such a servo that fit the bill that only weighs .8 ounces each so I picked up a set of the S3150 digitals for the rudders.

       Futaba servos are looked upon as the Rolls-Royce of servos in some RC communities but for the most part they are relatively pricey. I chose to use Hitec servos mostly for the price/value factor. If money were no object I would probably use all Futaba equipment (Maybe a sponsorship opportunity for Futaba, ehh?? :0).

       The Hitec HS-5085MG (which is now available) is a little cheaper than the S3150 and has the option of being programmed in the field with an optional programming unit..

       The single most important servo on this B-25 is the elevator servo. This is the servo you don’t want to fail on you at any point of your flight. Because the elevator is such an important control surface some model kit manufacturers allow for the use of two elevator halves controlled by two independent servos. If one servo should fail the other control surface can (hopefully) still control the plane and keep it from crashing.
       Unfortunately the Top Flite kit does not have independent elevator halves so one servo is used for the entire surface. Now I guess you could modify the plane to allow for two servos or just  make sure your servo system is rock solid for the application. I chose to use a single Hitec HS-5645MG servo for my $4200 B-25 model. The servo produces 143 ounces of torque @ 0.23 sec/60 deg. which is fantastic for a $50 servo. If you plan on using standard servos on your B-25 kit I would suggest upgrading at least your elevator servo to a high quality servo capable of producing at least 80 ounces of torque minimum.

       Before installing the servo in the stab I subjected it to a torque test where I hung a 5 pound weight off the servo arm (both directions) and cycled the servo for a period of 5 minutes to see how it would handle the load. The servo handled it beautifully and received my seal of approval :0)

       To make absolutely sure that noting can go wrong with the elevator linkage I decided to build a 4-40 rod with ball links. This setup is virtually bulletproof and works like a charm. I used Dubro (Cat #2161) 4-40 ball links as well as a Dubro 4-40 thread coupler (Cat #336) and 4-40 rod. The thread coupler was soldered onto the 4-40 rod as seen earlier with the wing linkages.

       The overall length of the rod was made to about 102 mm ball hole to ball hole (leaving room on the threads for adjustment). 4-40 cap screws were used to mount the linkage to the servo and elevator horns. Notice the “nylock” nuts used as to prevent them from coming loose in flight.

       The second to the last hole in the servo arm was drilled out to accommodate the 4-40 cap screw. The elevator control horn was also drilled out at the top hole as seen below.

       Now the rudders were next to be worked on. I mounted the control horn to the rudders at about 8 mm down from the bottom edge of the stab to the center of the horn. The horn was also set 5 mm back from the hinge center of the rudder as seen below.

       Short 4-40 linkages were made for the rudders out of habit only. 2-56 linkages would probably do the job just fine.

       The tail is now complete at this point and I am ready to move on to the fuselage!!!

       To help move the fuse around on the workbench I added some Velcro patches (the fuzzy side) to the bottom of the Styrofoam cradle. These patches act as skids and allow the fuse to be swung around with little effort, useful when working on large airframes.

       The next step in my grand adventure was to install the nose gear assembly including the gear door mechanism. To start off I needed to patch the larger hinge pockets that were intended for the spring hinges supplied with the kit. My doors will be servo controlled so springs will not be needed.

       Here is a photo of the stock kit hinges opposed to the new Great Planes medium pivot point hinges (Cat #GPMQ4002). They are the same size as the kit provided ones less the spring assembly.

       To patch up the pocket holes I used Northeast Hobby Products Micro-Fill to fill up most of the hinge pocket on both sides and let it cure for a few hours.

       Once cured I Dremeled out the pockets just big enough for the new hinges. They were also sanded flush with the fuse as seen below.

       Some Top Flite flat dove gray Monokote (Cat #TOPQ0511) was used to patch up the exposed filler as seen below (or barely seen).

       I decided to install the gear before I finish the door setup. I installed a Hitec HS-625MG for the nose wheel servo with a Dubro standard servo arm. The stock pull-pull system was used as was suggested in the manual.

       I installed the nose gear air lines with the restrictor fittings just as I did with the main gear.

       The nose gear mounted with very little effort, However I did have to notch the former for the air line as seen below.

       The next step was to prepare the nose strut by fitting the pivot axle.

       The pivot axle needed to be ground to secure the set screw positions as seen in the photos below. Blue threadlocker was used to keep the set screws in place. There are two axles being shown here. The long one was used full length for the pivot axle. The short one was used for the nose wheel.

       Note the use of the Dremel tool with cutoff wheel. It works great for a grinding wheel in this case.

       I adjusted the nose wheel strut so it was 4 mm “lower” on the shaft as seen below. This is to help increase ride height and allow for adjustability later.

       When I installed the nose wheel pivot bushing into the trunnion of the retract mechanism I also set it to 4 mm out of the trunnion to help with the ride height. This gives the gear an 8 mm raise at the nose. Ultimately the nose will ride at 133 mm from the fuse to the bottom of the wheel (this is of course if the main wheels are 4” in diameter as in this project). Once the plane is built and the the full weight of the aircraft is on the wheels you can make your final ride height adjustments.

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