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Top Flite B-25J Mitchell Project

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       The true heart of any model aircraft project is it’s powerplant. In some cases a modeler might choose an engine before he chooses a plane and just  adjust the planes size to match the engine. In my case the Saito FA-90T is just about the right size for the TF B-25. The kit was designed for two 46 two-stroke or two 70 four-stroke glow motors which is a little less power than what the FA-90T can produce. The big advantage of the FA-90T is that it has a lower head profile than a 70 four stroke and a much lower vibration level than a single cylinder engine.
       What made me choose the FA-90TS is the sound they make. I own a Saito 200-Ti twin in my Super Cub and I love the sound it makes. I couldn’t wait to hear two Saito twins in sync!!! The only misfortune I had was that the TF B-25 kit was really not set up for much more than the 70 four-stroke engines it was designed for. The fire walls are sized for a common plastic motor mounts that have small footprints compared to the metal Satio motor mount.

       To get the Saitos to fit on the nacelles I would need to devise a new firewall setup. I started out by finding the center of the motor mount placements that the manual suggested for reference. This is the same point I will use to center the Saito engines on. The paper templates supplied with the manual helped me plot out the engine crankshaft centers on the firewalls. This measurement is important to compensate for the thrust angles the firewalls are set at. Without this offset the engines drive washers would be out of center when they protrude trough the cowls.

       As you can see in the photo below the stock motor mounts will not work as they are. The mounting holes are too wide and too low on the firewall. The thrust angle offset also causes a problem with one of the motor mount rails being right over the nacelle side formers. This will make it hard to bolt the mounts to the firewall.

       To fix the mounting problem I swapped the inboard motor mounts from one engine to the other creating an offset on the mounting holes. This will make it possible to use the stock mounts with very little modification to the firewalls.

       Once the motor mounts were swapped I marked out the mounting locations of the engines on the firewalls.

       The mounting holes of the motor mounts ended up in odd locations on the firewall which motivated me to strengthen these locations with a plywood “plate”. In order to get the plates to fit on the firewalls I needed to remove the plywood tab at the bottom of the firewalls. I used a Dremel cutoff wheel and a sanding drum to flush out the tabs.

       I decided to make my firewall plates out of 1/8” birch plywood so I cut out two 113 mm X 65 mm pieces. These plates were oversized to also serve as the lower mounting tab for the cowls. The lower stock cowl mounting tabs bolt to the firewall where my new firewall plate will be so I added the mounting tabs to the firewall itself to eliminate the need for them. Notice the tab marked “A” in the lower right photo. I used this tab as a template to lay out the new cowl mounting tab in my firewall plate.

       Because the fiberglass nacelles were designed to slide over the stock firewall it will require that they be cut to accommodate this new cowl tab design.  Of course there may be a better way out there to secure the lower end of the cowl, I just liked this way for it’s simplicity.

       I cut out my final design and test fit them to the firewalls. After taking several measurements to double check my work I was ready to glue them in place. 30 minute epoxy was used to secure the new plates to the firewalls.

       Note that the plywood plates are epoxied in position with the “top” edge even with the vertical plywood tabs that protrude through the firewall.

       After going through the motions of the research and development of these firewall plates I decided to draw them up for future reference. The Diagram below depicts the exact measurements I used for my plates and can be printed out to use as templates to make your own firewall plates for the Saito FA-90TS. You can save the diagram below by right clicking on it and choose “Save Picture As”. Once you have it on your computer you can right click on the file and click on the “Preview” tab which will open the Windows Picture and Fax Viewer.
       Once the preview window opens you can then click the print button on the bottom of the window.  Follow the instructions in the Photo Printing Wizard and choose “Full page fax print” to get a scale printout. After the drawing has printed you must verify that the scale is correct by using a metric ruler and compare the measurements before using as a cutout template.

       There were still a couple of things I had to do to prepare the engines for mounting on the firewall plates. The fuel line barbs on the carburetors needed to be swiveled downward to allow the fuel line to clear the motor mounts. I loosened the jamb nut on the banjo fitting and turned the fuel barb downward, just clearing the intake manifold as seen below (both engines).

       The stock throttle arms also needed some modification to work with my firewall setup. As you see below the carburetor throttle arm is offset to the inside of the motor mount which would be difficult at best to connect to the throttle linkage as is. Saito makes a throttle arm adaptor to move the arm outward but unfortunately it moves the arm out too far for this application. I decided to modify the stock arm to suit my needs.

       Using a pair of needle nose pliers I carefully bent up the arm to clear the Saito’s motor mount as in the pictures below.

       When I reinstalled the throttle arm I positioned it so it was about 18 deg. to the left of top dead center (with the carb fully closed) as in the photo below. I used some blue Loctite on the set screw before tightening it in place.

       The next step was to install the supplied 4-40 blind nuts into the back of the firewall. I used a 9/64” drill bit to drill through the firewall where the motor mount holes lined up.

       I used a carbide rasp bit to hollow out where one of the blind nuts lined up with a reinforcing block. I cut a pocket just deep enough to fit the 4-40 blind nut as seen below.

       Using the supplied 4-40 cap screws and some washers I seated the blind nuts into the back of the firewall.

       I used a sanding drum to trim one of the blind nuts that encroached into the fuel tank stopper hole for clearance.

       I was now able to test mount the engines to the firewalls. All looked good up to this point.

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