Engine-mount basics

Model Airplane News, Mar 1999 by Wilson, George Jr

An engine mount is a device that attaches an engine to a model, and several of their features bear consideration: mounts can be scratch-built or purchased; each approach has its good points.

One of the best features of electric-powered models is the simplicity of their motor mounts. Electric motors are, in themselves, relatively vibration free, but do remember to balance the propeller. This article will not address electric motor mounts.

TYPES OF MOUNT

Beam mounts. These classic engine mounts date back to the 1930s and consist of two beams (typically, 3/sx/2-inch maple or similar hardwood) that are spaced to accept the engine lugs. The beams pass through and are glued to the firewall, rear tank compartment bulkhead and the bulkhead under the leading edge of the wing. They are usually mounted at a negative angle to provide downthrust when it is needed. The beams unite the fuselage's front end assembly into a strong structure.

A later development was to increase the beam spacing (usually to the full fuselage width just inside the side sheeting) and install a shear plate across the top, front ends of the beams. The shear plate is cut out to accept the engine crankcase and to provide horizontal engine offset (upthrust and downthrust). The shear plate is intended to shear off during a crash and thus minimize damage to the engine.

The shear plate is made of thin aircraft plywood or plastic, such as Micarta, and mounted on the beams and engine lugs with machine screws and nylon aircraft locknuts. Nose weight, if needed, can easily be added by installing sheet lead (flashing) strips between the screws that hold down the front of the shear plate. Use additional locknuts to secure the lead.

The engine and tank compartments should be thoroughly fuelproofed with thinned epoxy or a polyurethane varnish. Note that the structure can be made using an aliphatic-resin glue, such as Titebond* or Sig-Bond*. These are strong, fuelproof and easy to use.

Beam and shear mounts are strong, and they are easily modified for different offsets and even different engines.

Firewall mounts. Molded, glass-filled, plastic and cast-aluminum mounts that are attached to the firewall with blind nuts are very popular. Most are one-piece, but some are two-piece so the spacing can be changed to accommodate different engines. Some engines come with custom mounts attached to their crankcases and are attached to the firewall with screws and blind nuts. These are frequently called "radial mounts" because they are similar to the struts used to mount fullscale radial engines.

Firewall mounts are made and distributed by several companies including Dave Brown*, J'Tec*, Sig and CB Tatone*. They come in various lengths and sizes to fit many different cowls and engines.

Offsets are best added by making tapered hardwood shims that fit between the mounts and the firewall. Offsets can also be made by inserting washers under the engine lugs and/or between the mount and the firewall.

Firewall mounts usually have to be drilled and tapped to fit the engine. J'Tec markets pre-drilled mounts for a number of popular engines. I have been known to file flats on the bottom of the mounts and use machine screws and locknuts to hold the engine. Du-Bro* recently introduced an adjustable engine mount that includes a drill guide/marker and other goodies. Drilling and tapping the mounts takes some "hold your mouth right" type work, but these mounts are strong and do the job well.

VIBRATION ISOLATION

Vibration caused by the engine and propeller can be isolated (not eliminated) by using soft rubber mounts between the engine and the mount, or between the mount and the firewall. Isolation reduces the vibration transmitted to the airframe and helps preserve the airframe and the electronic components, including the battery. Soft mounts (typically by Sullivan*) are frequently made of rubber with screws attached to their ends. Others are rubber grommets that encircle the screws that are attached the engine or the mount. Vibration isolation also helps to reduce noise by reducing the vibration, but the noise reduction is minimal in .60-size and smaller models.

Single-cylinder engines are balanced internally to minimize vibration, but complete balancing is inherently difficult, if not impossible, at all engine speeds. Multi-cylinder opposed engines lend themselves to much better internal balancing. Propellers can and should be carefully balanced to minimize vibration.

CONCLUSION

I prefer to use beam mounts with shear plates. The mounts are rigid and easily adapted to other engines. Offsets are easy to make (and change), and they are inexpensive.

*Addresses are listed alphabetically in the Index of Manufacturers on page 134.