Organize your firewall: Correct engine installation practices

Organize your firewall: Correct engine installation practices
When you build a giant-scale airplane, many decisions will have an impact on the aircraft in one form or another. With respect to the engine, a few items you should consider include—but are not limited to—the size, weight, ignition, and fuel type. I’ll touch on a few fundamentals of engine selection as well as engine installation and related components. Whether you’re building from a kit, assembling an almost-ready-to-fly (ARF) plane, or modifying a second-hand aircraft, it helps to understand a few critical areas to ensure that your aircraft is structurally sound and will stand the test of time.

The Right Engine

Many pilots new to giant scale ask me whether they should use a large glow engine or a gasser with a magneto or electronic ignition. That depends. I continue to use all types of engines for various reasons. For a small giant, like the Hangar 9 Ultra Stick 30cc ARF, I might prefer a larger glow two-stroke to save on engine weight and the additional weight that an electronic ignition, switch, and battery add to the airframe. Fuel costs are a consideration, however, with larger glow-powered engines as they are generally thirsty.

Another option is a gas engine with a magneto-driven capacitor-discharge ignition (CDI) system. I own a Sopwith Camel that has a Zenoah G-38 with a magneto-driven CDI system and a manual kill switch for safety. Often, you’ll need an aftermarket spring starter or an electric starter—like the Dynatron from Sullivan, which can operate
on 12 or 24 volts—to start the engine at the field. These types of engines are simple as they do not require a separate ignition module, battery, and switch. They are typically heavier compared to their electronic-ignition counterparts, but weight is sometimes needed to achieve the correct center of gravity.

Another type of gas engine features an electronic ignition. I power my 1/3-scale Pitts Special with a Desert Aircraft 50cc engine based on displacement, power curve, weight, and ease in hand starting without an electric or spring starter. This engine features an electronic ignition, so a few items are required, such as a battery, switch, and (recommended) an optical kill switch.

Engine-Installation Fundamentals

Always begin by looking at the engine and equipment recommended by the manu­facturer. For the 1/3-scale Pitts, I compared the three-views provided by the engine manufacturer to the firewall and cowl dimen­sions to confirm that the engine would fit with only a portion of the cylinder and exhaust exposed.

Measuring the distance from the firewall to the front of the cowl, I added approximately 1/8 inch to allow a sufficient gap between the cowl and the spinner backplate, and realized that I would need to use standoffs. Typically, engines are mounted directly to the firewall or to standoffs. Exceeding recommended standoff lengths suggested from the engine manufacturer may result in damage to the engine due to vibration. Desert Aircraft recommends using standoffs no longer than 3 inches for the DA-50R. Because I needed additional distance, I laminated multiple layers of oversize 1/8-inch aircraft-grade plywood until I achieved the required distance. I always use aircraft-grade plywood because it contains more plies when compared to light ply, and having more plies results in greater strength and stability. If you use light ply, you may find that the wood compresses when you tighten the engine bolts.

Some aircraft manufacturers supply templates for a wide variety of popular engines. At a minimum, most ARF models have horizontal and vertical reference lines on the firewall that you can measure from, and you can use the plans if you’re building from a kit. While drilling into a 1/4-inch firewall is straightforward with a cordless
drill, I recommend using a drill press when­ever you have to drill into multiple layers of plywood.

I prefer to use 1/4×20 SAE mounting bolts and washers to secure the engine in place. Because the standoff is threaded, you need to bolt the engine in place from behind the firewall. Simply insert the bolts and washers from the rear side of the firewall, through the plywood spacers, and into the rear face of each standoff. If you aren’t using standoffs, use a washer and a locknut to secure each mounting bolt in place. Unless you use locknuts, thread-lock should be used on all engine mounting bolts.

Two unrelated items are important to note before proceeding forward. First, you need sufficient space near the inlet of the carburetor for unobstructed airflow. If the carburetor is in front of the firewall, which it is in this case, you need at least 1 inch of clearance from the carburetor inlet. Second, if you find that you need to change the thrust angle of the engine after the flight tests, you must be careful to ensure that the standoff and each engine mounting tab are in the same plane. Simply adding a shim behind two of the four mounting tabs can break an engine mounting tab over time.

Ignition components and wiring should be separated, to the maximum extent possible, from the receiver, receiver battery, servos, and so forth. A mounting location was chosen based on the length of the spark-plug lead and surrounding items (i.e., the engine mount, cowl, etc.). To install the ignition module, I made a plate that was the same size as the perimeter of the module out of 1/8-inch aircraft-grade plywood. I then glued two 1/4-inch-square spruce rails at each end of the plate to allow sufficient spacing between the firewall and the plate. As far as gluing the mount to the firewall, the mounting surface was prepared, cleaned with rubbing alcohol and a paper towel, and epoxied in place using 15-minute epoxy from Zap. The ignition cannot be rigidly installed to the airframe. To soft-mount the ignition module, I used 1/4-inch foam rubber from Du-Bro. I secured the foam rubber with hook-and-loop material and also held the module in place with hook-and-loop material with the addition of a hook-and-loop strap around the entire assembly.

Hangar Membership - Membership | Organizing your firewall: Install your engine correctly

A scale is beneficial to ensure that the proper amount of resin and hardener is used when mixing epoxy.

Hangar Membership - Membership | Organizing your firewall: Install your engine correctly

The throttle servo has been connected to the carburetor with a 4-40 ball link and 4-40 flexible pushrod.

Hangar Membership - Membership | Organizing your firewall: Install your engine correctly

For the throttle servo, I decided to use a 4-40 flexible pushrod from Sullivan Products. The blue tape shows where the sheath must be cut to allow proper move­ment of the inner nylon pushrod, threaded stud, and ball link.

Hangar Membership - Membership | Organizing your firewall: Install your engine correctly

Because the throttle pushrod is quite flexible and long, it needed a plywood pushrod support to secure the black sheath to the plywood servo plate.

Geometry Matters

The geometry between the servo and the throttle arm on the carburetor is extremely important. To begin, I measured the distance from the center of the throttle shaft on the carburetor and the center of the attachment point on the arm that is secured to this shaft. This distance is what you want to have from the center of the throttle-servo arm to the center of the pivot point on the arm itself. Far too often, I see folks use a 1.25-inch servo arm on the throttle where the distance is much smaller on the car­buretor. This results in low adjustable travel volume numbers, extremely poor resolution, and often a nonlinear feel when you move the throttle stick on the radio and watch the response from the servo.

On the Pitts, I mounted my throttle servo on the same servo tray that I used for the rudder and elevator servos. I did this so that I could easily access the servo since it was directly under the cockpit floor. With the throttle-servo arm and throttle arm on the carburetor at different heights, I opted to use a 4-40 flexible pushrod from Sullivan Products. The flexible pushrod features a 4-40 black/gold threaded stud that screws into the inner rod, and typically, a clevis that’s attached to the other end of the stud. I used a 4-40 ball link at both the servo and carburetor locations, and added an 1/8-inch plywood support to brace the pushrod so that it wouldn’t flex through the full range of motion from the servo. I used Zap Goo to secure the sheath to the plywood support as this adhesive is perfect to use for bonding wood to plastic.

To find the proper drilling location on the plywood spacers and firewall for the pushrod tubing, I observed the full range of travel from the carburetor throttle arm and marked each endpoint as well as the center by aligning a pushrod, with paint on the end, with the carburetor attachment point while the pushrod was parallel to the engine standoffs. I then moved the pushrod so that the paint would contact the plywood spacers. With the spacer plate being marked, I could now drill through the spacers and into the firewall to insert the pushrod assembly.

Like the throttle linkage, a choke linkage is mandatory for gassers that have a Walbro carburetor, like the DA-50R. You do have options, however. On the choke, for example, you can use servo control or a mechanical linkage that can be manually operated. Regardless of the method, it is important that the choke is allowed to fully open and close. If it does not close, it will be difficult to prime the engine. On the other hand, if it does not fully open, the engine will run but will be unable to run at maximum power.

For the Pitts, I made a support out of G-10 Garolite, a fiberglass-laminate com­posite material. Because it’s difficult to cut and requires the use of a rotary tool and belt sander, it is important to use a proper respirator to avoid inhaling glass and epoxy dust. In fact, it’s a good practice to always use a respirator for certain adhesives, composite materials, and so forth. The G-10 support plate has two holes for the engine mounting bolt to pass through as well as a third for the choke linkage, where I could access the pushrod through the left engine-inlet opening on the cowl.

With the engine mounted, the throttle servo and linkage connected, and the manual choke pushrod secured, it was time to install the ignition switch and battery. The battery plugs into the switch, and the switch is then plugged directly into the ignition module. I opted to secure a Spektrum three-wire switch harness to the fiberglass cowl. The wire length was long enough that the ignition battery could be installed within the fuselage and easily accessible for charging purposes. I prefer to disconnect my ignition and flight batteries after a given flying session for safety. If you prefer to use a higher-voltage battery source, use a voltage regulator and simply connect the switch to the voltage regulator, which is then plugged into the ignition module. On this particular engine, the power source for the module must be between 4.8 and 6 volts. I decided to use a Spektrum 6.6V 1450mAh LiFe battery. While the factory recommends a 180mAh or larger capacity pack, which should be sufficient for five or more flights, I estimate that on a given outing I would not be flying this aircraft as much and decided to use a slightly smaller battery.

Hangar Membership - Membership | Organizing your firewall: Install your engine correctly

The ignition plate was built using 1/8-inch aircraft-grade plywood and two 1/4-inch-square spruce rails.

Hangar Membership - Membership | Organizing your firewall: Install your engine correctly

I made the G-10 choke-linkage support in such a way that it tied into two of the four engine mounting bolts.

Hangar Membership - Membership | Organizing your firewall: Install your engine correctly

As you can see, the choke linkage is easily accessible through the cowl of the 1/3-scale Pitts Special.

Hangar Membership - Membership | Organizing your firewall: Install your engine correctly

On another scale aircraft, I decided to hide the ignition switch under the instrument panel.

Hangar Membership - Membership | Organizing your firewall: Install your engine correctly

While we discussed using a manual choke linkage, you can also use a servo to operate the choke.

Final Thoughts

We’ve discussed a few different engine options, their applications, and a few fun­damental build practices to implement in proper installation techniques. This column is meant to serve as a guide and improve your understanding of general setup practices. Seek the advice of a fellow experienced builder, and enjoy this fine sport.


Updated: June 15, 2023 — 2:55 PM

1 Comment

  1. Excelent article!! Cogratulations!!
    I am a m/a builder since 1960, mechanical engineer.
    If I may make a a couple sugestions, I also use aluminum in bigger engines for the chocke push rod, and glue a plastic sleeve to prevent the wire to cut the support and it slides easier.

    Best regards and keep up the good work!!!

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