A summary of Engine Mounts

A commonly overlooked part within an aircraft is the engine mount. It may be a simple structure, but it has many essential roles beyond holding the engine on the airframe. For example, the exact placement of the engine mount is important. Designers must consider the thrust line of the engine, how high or low it is in relation to the airframe, as well as gyroscopic precession. In this blog, we will discuss engine mounts in detail.

As the engine is usually the heaviest component of a small aircraft, its location is critical to the aircraft’s center of gravity. Turbine engines are lighter than piston engines, which is why aircraft that have been converted from piston to turbine have long engine mounts and pointy noses - the lighter engine must be farther forward to keep the center of gravity in the right place. Adverse to this, aircraft that carry their loads farther forward require short mounts to keep the engine as far aft as possible. In addition to holding the engine, the engine mount transmits power to the airframe. A large engine and propeller can deliver several hundred pounds of thrust. The engine mount must be able to transmit all thrust to the airframe without distorting or flexing.

Early engine mounts were built from many different materials, but designers quickly settled on trusses of welded steel tubing. Mounts made from this material have been standard in light aircraft ever since. Chrome-moly (chromium-molybdenum) alloys are particularly desirable for this job. They come in a wide range of sizes and are strong in both compression and tension, meaning small diameters and wall thicknesses are able to carry significant loads. In turn, this allows the mount to be lightweight, which is always beneficial. Such mounts are also easily bent, cut, and welded.

Following welding, a mount must be protected from corrosion. Chrome-moly steel rusts quickly, especially after the heat of welding burns away its protective oil. Historically, boiled linseed oil has been run through any tubes left vulnerable to the atmosphere. Tubes not exposed to the atmosphere will not be susceptible to moisture in the air and therefore will resist internal corrosion for a long time. A well designed, built, and protected engine mount can last for multiple decades. That said, like anything, they are not immune to degradation over time. The engine mount’s first line of defense is regular maintenance. Careful inspections should be carried out at least annually, or as needed if the aircraft experiences a rough landing or other such stresses.

Aside from normal wear and tear, vibration and corrosion are the most detrimental to things an engine mount can experience. Vibration and impact lead to cracks, especially around welds. If caught early enough, these can be fixed by a simple weld or patch. If the problem becomes too significant, the damaged tube will have to be cut away and replaced. Corrosion damage happens more slowly, but is also more dangerous. It often occurs in areas where it is impossible to spot. As with vibration, early detection is critical. Light corrosion can be sanded away, but if it goes too deep, replacement is your only option. The engine mount is a critical part of every aircraft and should be cared for as such.

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