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Engine and Firewall Setups — Tips for Down and Side Thrust

Engine and Firewall Setups — Tips for Down and Side Thrust

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!


Updated: July 16, 2015 — 4:24 PM


Add a Comment
  1. Good article!!

  2. Gerry…suppose you don’t have the right and down thrust angles…how do you calculate them for the airframe and engine. Any good rule of thumb you can offer to me…I have a GR 57T going on a 80 inch Ugly Stik type airplane…tail dragger…about 20 lbs.


  3. hello
    please could give me the reference of the soft mount



  4. Can you tell me how to set these angles on a rear mounted engine or electric motom? Thanks!!

  5. I think the late great designer Chuck Cunningham believed no design could suffer from a “little” right and a little down thrust. I think 2-3 degrees would qualify as a little.

  6. 1. What about the sides of the firewall…they too should be mitered at the 2 degrees right and down… otherwise they are not “square” to the sides and top and bottom of the engine box…

    2. Why didn’t you miter the front triangle stock… looks bad…

    3. Why balsa triangle stock… the gt 80 is a heavy engine… at least basswood….

  7. Gerry, I noticed from your “Rough sketch” (which looks pretty darn good for rough) you measured the 2 deg off set from where the prop hub extension exits the cowl and not from the face plate of the prop hub. With the added distance this would result in the propeller actually being off centre, would it not? I know we are only talking about a small amount but with the larger scale aircraft I have worked with I have been caught by his in the past.

  8. if you move the entire engine to the right looking from the front ,can you achieve the side thrust?

    1. If you move the front of the engine to your right, while the airplane is upside down and facing you, yes, you are adding right thrust. as the photo shows… I know. confusing right?!

  9. @member1340161736: Hey thanks for writing about my sketch. That’s what I meant by rough. The drawing was used just to figure out the difference in the firewall side offsets and distances out the front of the fuselage sides. I did center the angle at the engine where it exits the cowl. As I did not include left horizontal offset of the engine placement. I also think, that when I install the cowling and attach the face plate, I can fudge the faceplate position slightly to center the engine output shaft without it looking bad.

  10. @Rick Davey: Hi Rick. I don’t really have any formula for the amount of offset. I am a firm believer of following the airplane designer’s suggestions. Most ARF and kit planes specifiy the amounts. If you don’t have this info. I think it is not a very critical issue. I know people who have flown planes set at 0/0 degrees and the planes fly fine. The difference being the amount of trim used in rudder and elevator. There are several model designers that say a little offset never hurt anything!
    take care

  11. @javier: Hey thanks for writing. The soft mounts you see here were purchased a while back from Nick Ziroli Plans. They are no longer available but, Nick does supply JTec softmounts as well today. Available in several sizes, they work just as well if not better.

  12. @Larry THanks for your comments, but really, there is a limit, for me anyway, that you have to have a certain amount of chaftsmanship while still actually completing the build. Two degrees is not enough to make a difference in the joint between the edges of the firewall and the sides of the fuselage. That’s another reason to use slow setting epoxy instead of thick CA glue for making the joint. As for the use of balsa as tristock. By weight, the balsa is actually stronger than basswood. Or at least that’s what I have been told. The true factor for joint strength is “Gluing area” so…. on with the build.

  13. Good Article Lots go into that for sure and its a very important aspect of mounting any engine big or small to a plane. I am not a big kit guy and just stick with the ARF but you still have to do this with most.

  14. Great advice.

  15. excellent article I’m building one myself in 11 months into it it is the third US balsa kit I built in 20 years excellent kit is fully covered and painted just a few more details then I’m going to mount 170 cc gas engine on this baby I wish I can put post photos of mine just give you some inspiration mine should weigh in at at least 40 pounds plus looks pretty sturdy though I like the construction design is excellent this will be my first Deckers

  16. just my opinion of course I’ve seen these aircraft flown with less than 100cc engines the performance was acceptable but not great to my opinion aircraft of this type with engines over 100 cc performed excellent without hesitation while flying also fully Aerobatic with this larger engine once again just my opinion you can find me on Facebook under my full name join me check up my pics I have many

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