Making big engines fit

Model Airplane News, May 2000 by Yarrish, Gerry

I think it is a foregone conclusion that gasoline engines are the most popular powerplants for giant-scale models. Gas engines come in a wide range of sizes and, in general, are very well-suited to the tasks given them by giant-scale modelers. If the model is designed for a gas engine, then installing it is usually a simple task. But, what if you want to install an engine different from that called for in the plans?

When you build a giant-scale model, the plans usually show you where everything has to go. If you want to do things "your way" and modify the engine installation, this simple task becomes more complex.

MAKING IT FIT

When building a model, one thing is for sure: you simply cannot change just one thing! When you make a single change, you ultimately have to redesign other parts of the model to make that modification work. It's an example of the domino effect; for instance, change the length of the engine, and you must compensate by adjusting the engine-mounting system. A new mounting system could mean that you have to change the position of the firewall; this could affect the fuel tank installation, and ... you get the idea.

Engine position is especially important if you have a model with a spinner. Regardless of the length of your engine, the spinner's location relative to the rest of the model has to be maintained. To keep everything as simple as possible, I try to restrict my changes to one area of the model. Instead of moving the firewall forward or aft to accommodate a different-size engine, I prefer to add a box structure to support it. Here's how to do it.

To establish a fixed reference point from which to establish the correct engine position, attach the spinner to the engine and work with the entire assembly. You can then take the engine/spinner combo and place it on the plans to determine the correct distance between the firewall and the engine-mount plate. Sometimes, your engine mount will take up this space and you can simply bolt it to the firewall. If this isn't the case, the space left must be taken up by the engine box.

If you haven't already begun to build your model, you have several options; establishing the engine installation details before construction is definitely preferable. You can cut and glue the firewall before attaching it to the inside of the planked fuselage (see Figures 1 through 3).

But what if the firewall is already installed? I recently started a project before I had the engine in hand to measure. I built the fuselage, remembering that later I would have to work out the engine-mount location. With the fuselage already built, I placed shims between the spinner and the front of the fuselage. By doing this, I established a little leeway for any misalignment in my engine installation.

I first tack-glued several 1/8-inch balsa shims to the front of the fuselage's foremost bulkhead and slipped the engine and spinner assembly into place. I then butted the spinner's backplate against the shims and carefully aligned the spinner with the rest of the fuselage. Once the spinner was in the correct position, I tack-glued it to the shims with thin CA. As it is made of aluminum, the spinner is easy to break free of the shims later on.

With the spinner glued into place, the engine is suspended and at the exact distance it needs to be in front of the firewall. It is a relatively simple task now to build an extension box of the exact size and shape needed to support the engine.

For the Brison* 3.2 used in my Meister Scale* Bf-109, I made the engine-mount box from 1/2-inch-thick birch plywood for the box face and 1/4-inch ply for the four sides. I used 10-24 blind nuts to install the engine and 30-minute epoxy to glue the box to the firewall. Epoxy by itself is not strong enough to take the stress of engine vibration, so I used 1/2-inch, aluminum, L-angle stock to reinforce the glue joint all around the box. Use either large sheetmetal screws or 6-32 cap-head screws and blind nuts to attach the angle stock to the box and firewall.

I've used this engine-installation technique on several models and have never had a problem with the engine-mount box coming loose. Give it a try.

MACMINARELLI ENGINES

At the Toledo trade show a couple of years ago, I saw some really interesting multicylinder engines on display. Imported by Tower Hobbies*, the AvioMac 70cc, 85cc and 125cc engines come from the 20-year-- old Italian company, MacMinarelli. It has a lot of experience in manufacturing 2-stroke engines; the company's main product line is competition go-karting engines. Far from being redesigned go-kart engines, the AvioMac gas engines are designed with CAD/CAM 3D systems using state-of-the-art materials and modern CNC-machining techniques. These powerplants are beautifully manufactured and carry a three-year warranty.

All of the AvioMac engines come with two spark plugs, a coil with ignition leads, a control box and another little black box called a Quick-Start unit. By adjusting the timing electronically until the engine comes on line, this device makes it easy to start the engine. Once the engine starts running, the CD ignition system takes over and the Quick Start unit can be disconnected and removed.