A very important thing to consider whenever building a giant scale airplane is to set the engine’s down- and side-thrust according to the instructions. With my current project, a 1/3-scale Fokker Triplane that I am building from the new Balsa USA kit, the instructions state to include 2 degrees of down and two degrees of right thrust. Nothing new there, but really, there’s no set way to install a firewall with the proper offsets.
Here’s how I did it.
There are a couple things you need to take into account whenever building a model and that is, the engine length (prop thrust plate to mount tabs, the depth of your engine cowling. Other things that affect engine and firewall measurements is whether or not you use a hard mount or a soft mount. For my triplane I installed soft mount vibration isolators.
Of course you need to have the engine and mufflers, to make sure your firewall position is correct. Here’s my Zenoah GT-80 twin cylinder gas engine and a pair of “Smoke” mufflers from Slimline Products. They make great smoke because of their pre-heat inserts.
(Above) these are CAD drawings I based on the 14 1/8 inch diameter cowling I got from Arizona Model Aircrafters.
The next thing I do is to jot down the important measurements of the cowling and then make rough sketches of how things need to be laid out. From here you can then figure out the distances that need to be measured from the front of the fuselage to the firewall to determine the correct thrust angle.
Without going into the math, for my firewall, which is 9 5/8 inch wide, I needed to make the right side of the firewall 0.336 inches further back than the left side when measured from the front fuselage frames. Yes that’s pretty specific but using a digital depth gauge makes it pretty easy to be accurate. My engine is a Zenoah GT-80 twin cylinder gas burner so to check the rough sketches and measurements are important. The next thing to figure out is the depth of the firewall so your engine, and its muffler(s), will fit within the cowling. My cowl depth is 4.5 inches from the front of the fuselage so it is a pretty easy thing to set the engine on top of a piece of paper and sketch out its outer dimensions including your mount setup.
Next, trace the mounting pattern for your mounting bolts on a piece of paper and transfer them to the firewall along with the horizontal and vertical centerlines. For my soft mounts, this required 5/8-inch holes for the rubber isolators to fit into, and some plywood blocks glued to the back of each attachment point to make the overall firewall thickness 3/4 inch.
I purchased the Precision Aviation mounts some time ago from Ziroli Plans but similar mounts are available from other sources as well.
Note: to make the front of the engine remain centered in the cowling faceplate, you have to offset your engine attachment points up and to the left of the model’s thrust centerlines. (See below.) For the GT-80 this offset is 1/4 inch. The length of your engine will vary the amount of offset needed.
To make the installation easier, I first attach the engine to the firewall and then I fit the entire assembly into place. Also, placing the fuselage upside down on the workbench makes it easier to block up the engine for precise measurements. The top of the fuselage frame is set flat on the workbench and is a 0 degrees reference point to measure from.
So, now slide the firewall and engine into the front of the fuselage and set the firewall vertical by blocking up the front of the engine. Use a square to check the vertical alignment of the firewall. After you get the depth correct for your engine and engine cowling, angle the engine to the right side of the fuselage, and measure the difference from left and right sides of the firewall to the front of the fuselage frame to establish the right thrust (2-degrees). You may notice I have installed an extended prop hub on the engine. I got this from Horizon Hobby Dist.
Next to set the 2 degrees of down thrust required, I installed a small square in the prop hub and paced a digital “Angle Pro” incidence gauge on top. I first set the gauge on the workbench top and zeroed the reading. I then placed the gauge on the square and tilted the engine until it indicated 2 degrees. Again very easy to be accurate.
Now, with everything set, use a sharp tipped pencil and mark the back edges of the firewall where they meet up against the fuselage. Also it is a good idea to mark the top of the fuselage and the top of the firewall so you install everything correctly later on. Once you have the lines drawn, remove the engine and firewall.
Using ½-inch balsa triangle stock, glue the firewall reinforcements into place along the lines you just drew. Be very accurate and as exact as you can be. I use thick ZAP CA and kicker.
To maximize the gluing surface, I mitered the joints for a tight fit. (See above)
Once all the tri-stock has been glued in place, unbolt the engine from the firewall. Now mix up a batch of 15- or 20-minute epoxy (I use Zap Z-poxy) and apply the adhesive to both the firewall edges and the fronts of the tri-stock reinforcement strips.
A good way to hold the firewall in place is to use the old “rubberbands and sticks” trick. (see photos). You can also place the fuselage on its tail facing straight up and apply weight to the firewall to press it firmly into place until the adhesive cures.
Once everything has set up. You can go ahead and install the front tri-stock reinforcement strips to the face of the firewall. Again, I use epoxy here.
That’s it, once the epoxy is all cured, you can go ahead and finish the rest of the construction and install your engine cowl. Having a secure firewall installation with the proper amount of right and down thrust ensures your model will perform as its designer intended.
This may not be the quickest way to install a firewall and engine, but it is the most accurate way I could come up with. You should be able to use this technique with other fully built-up stick planes with recessed firewall construction. Have fun!