Top Flite B-25J Mitchell Project

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Page 6

       To actuate the main gear doors I chose to use a Hitec HS-81MG servo (which has been recently replaced with the HS-82MG). I mounted the servo in the existing hole on the outboard side of the nacelle structure as seen below. The use of a metal gear servo for this purpose will help reduce the risk of breaking a gear set when the doors get accidentally bumped or jarred.

       I am a big fan of using Dubro’s aftermarket glass filled servo horns over the stock ones that come with the servos, I have yet to break one. They fit nice and snug on the shaft splines and can be modified easily with a Dremel tool. I used Dubro’s standard “Hobbico/Hitec” arms (part #675) for the gear door actuator servos.

       The key to making a servo system work with gear doors is that when the gear doors are in the closed position the door servo must not be continually “fighting” to keep them closed. Not only will this drain your receiver battery faster it could burn out your servo. The best way to eliminate this is to orient your door linkage so that when the doors are closed that the control rod is parallel with the servo arm (as seen below). This will keep the weight of the doors from pulling on the servo and also keep the doors closed when the model is not turned on. Closed gear doors makes for a better static showing :0)

       I used a 2-56 control rod and fixed it to the bellcrank system at the aft of the nacelle as seen below. Notice that the control rod is parallel with the servo arm while in the closed position (doors are open so you can see the install).

       Once the linkage was complete a test cycle was in order. You can see how crowded the linkage is when the wheel is in the nacelle. Notice that the servo horn is in the “down” swing position on this nacelle.

       Once the right wing was dialed in I set up the left wing nacelle. The only difference is that the servo horn on the left wing servo was set up on the “up” swing position so that the same servo channel can be used for all of the gear door servos (they will all rotate the same direction to actuate the doors so the servo horns must be attached accordingly).

       Now the gear doors and the retracts were basically set up and I could move on to finishing the engine installation.

       To keep the engines running smoothly at idle I decided to add a glow driver system to the plane (more details later). I wanted to use a centrally driven system that would drive all four plugs from one battery. To do this I would need to run a wire harness through the wings and into the fuselage. To connect the system to the glow plugs I used a set of Magnum remote glow plug leads (Hobby People item #279953). These leads are the best ones I have found to date.      

       To connect the leads to the central glow driver in the fuse I used 14 gauge automotive wire. Heavy gauge wire is a must for glow driver systems that are not current regulated. Excessive resistance in the plug wiring will keep the glow plugs from glowing hot so make your leads as short as possible.

       A 1/4” hole was drilled into the firewall to fit the glow plug wires.

       Both glow plug leads were soldered onto the red lead and the black lead was soldered to a “ring terminal” to be attached to the motor mount of the engine. Notice below that the wires have been twisted together, this is mostly for neatness and appearance. This can be done by pairing the wires together and putting one end in a vise. The other end is put in a cordless drill chuck and the wire is pulled taught while the drill twists the wires together, works great.

       The wires were pulled through the wing and out of the wing root for later attachment to the glow driver system. Hot glue was used to seal the wires as they exited the fire wall.

       The next big step was to install the cowl. This was not going to be an easy job because the valve covers on the Saito will slightly stick out of both sides of the cowl making a one-piece installation impossible (more on that later). To get started I installed the blind nuts that hold the “B” type cowl mounting tabs that are to be located on both sides of each firewall assembly. I used the mounting screws to help seat the nuts into place. Thin CA glue can be used under the back of the nuts to help hold them in place.      

       Once all of the blind nuts were installed I decided to fuel proof the firewalls with thin CA glue and an epoxy brush. I use an epoxy mixing cup to put a small amount of thin CA glue to dip a brush into. I then brush the glue into the wood sealing it permanently and quickly, taking only a few minutes to dry. The brush will last only a short time so work quickly and always use safety glasses when using CA!!!!! One drop of glue in your eye can cause serious damage. It is also advisable to have good cross ventilation or the fumes can get annoying.

       Because of where I mounted the throttle servo earlier I decided not to use the factory blind nuts to install the upper “A” mounting tabs. A set of #4 X 1/2” screws were used to mount the “A” tabs on the top of the firewalls. CA glue was used on the screw threads to help hold them in (vibration). Notice below that this “A”  tab has a blind nut installed into it to attach the cowl ring later on.

       The “B” tabs were attached shortly thereafter using the stock mounting screws. Blue threadlocker was used on the machine screws to prevent vibration from backing them out.

       It was time to test fit the cowl mounting ring in place. Three blind nuts were installed into each ring as seen below. Notice that the only screw in the ring to point forward is the “top” (shown as bottom) “A” tab which is blocked on the backside by the dummy carburetor scoop on the top of the wing.

       I decided to mount a couple of Dubro fuel filters to the firewalls using one of the mounting tab screws to bolt down a fabricated aluminum strap. I used a couple of 4-40 X 1/2” cap screws to mount the fuel filters on the blind nuts.

Notice that the filter sits vertically, this will prevent air bubbles from collecting in the filter housing.

       The next step was to figure out how to fit the cowl over a two cylinder engine without cutting it in half. The dummy radial engine will also be difficult when added to the equation so careful measurement was in order. I removed the valve covers to improve cowl clearance for measurement.

       After studying the cowls a bit I decided to use the panel lines that were already in the cowl to use as hatch cut out lines for each cylinder head. This way I could access the engine without removing the cowl. To cut out my hatches I used a Dremel cut off wheel and carefully followed the existing recessed panel lines on either side (horizontally) of the cowl (be sure to orient the cowl so the olive drab paint is at the top/vertical). The Dremel cut off wheels that I used are super thin at only 0.024” thick (Dremel cat #409) which take out only a small amount of fiberglass.
       The key is to use the same piece that you cut out of the cowl to use as your hatch so precise and straight cuts are a must if appearance is important.
Safety glasses are a must for this procedure, these disks shatter frequently!

       Because each hatch is unique to where it fits it is important to number them to where they fit in the cowl. This will save some frustration later.

       It was now time to set up the cowls on the nacelles so they could be glued onto the cowl rings. The only real way to do this is to have the wings set up on the fuselage as well as the carburetor scoops taped into place. You will need to eyeball how the cowl sits on the engine and nacelle before you commit to where to glue it on the cowl ring.

       The engines are set up at angles to counter act the thrust and torque produced by the engines so some alignment compromises will be necessary to make the cowls look right. Installing the propellers on the engines as well as the dummy radials will help you get an idea of how to align your cowls. The props I chose to use with my Saitos are Master Airscrew 13x8 3-blade props along with a set of Higley 7x1 mm spinner nuts (cat #SPN007).

       I temporarily installed the dummy radial engines in the cowls to get an idea of where the engine’s drive washer will protrude through the cowl. I then set up both cowls so that the drive washer was 8 mm in front of the face of the cowl (measured with a straight edge). After a some adjusting and looking at the plane from many angles I settled on where I wanted to glue on the cowls. To temporarily fix the cowls in place I used some thick CA glue with accelerator spray. I then carefully removed the cowls so I could epoxy them in place on the cowl rings permanently.

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