Melmoth 2 inherited its engine mount brackets and isolators-the rubber pads the engine is actually bolted to-from Melmoth 1, which had in turn acquired them, in 1971, from a Cessna Skymaster that was already far from new. It's surprising that any rubber compound can retain a whiff of elasticity after 35 years or more in the heat, gusty winds and general turmoil of an engine compartment, but those dampers still felt more like rubber than wood to the touch. Nevertheless, the engine vibration struck me as excessive. Now, the quality of the vibrations coming from an engine and propeller is to some degree a matter of personal perception. One pilot found the airplane quite smooth. But even after trying to convince myself that it was all in my mind, I still thought it felt harsh; and so I decided to replace the isolators.
Looking at a listing of Lord mount types, I was surprised to discover that there is a huge variety of them, and a bewildering number exists specifically for my Continental TSIO-360 engine. Naturally, Melmoth was not one of the airframe types mentioned in the list. I called Paul Snyder at Lord for advice about which mount type to select, and he recommended a J9613-31.
The old dampers resembled two thick discs stacked one on the other. The new ones were more like hollow cups. A complete assembly-there are four in all-consists of two of these cups clamped, face to face, to a thin steel plate that is part of the engine mount. A cylindrical spacer holds the two cups in a fixed relation, somewhat compressed, so that the engine is supported on the north and south poles of a more or less spherical rubber shell whose equator is secured to the airframe.
Curious about why so many different types are available for the same engine, I called Lord again, and this time spoke with Product Support Specialist Tom Law.
Law explained that isolators are tailored to different engine mounts. Mounts vary both in the detail geometry of the brackets that support the isolators, and also in their own susceptibilities to flexure and vibration. The function of an isolator is to stop engine vibration on its way to the airframe by being particularly flexible in the frequency range where the predominant vibrations occur. Its design is constrained, however, by the requirement that the isolators also support the weight and thrust of the engine during maneuvers and limit its freedom to move within the cowling, particularly, in the case of recips, during the shaky moments of starting and shutting down. The engine mount must be sufficiently rigid to support and restrain the engine, but sufficiently compliant to damp the characteristic frequencies that would occur, for example, at cruise rpm.
All modern engines have "focalized" mounting points; in Lycomings they are typically at the back of the engine, on Continentals typically on the bottom. Focalization, which came into use with big radial engines during World War Two, means tilting the axes of the isolators so that they aim at a single point. By tinkering with the shape and orientation of the isolators and the so-called "L value," which is the ratio of the isolator's stiffnesses in tension/compression and shear, this point, called the "elastic center," can be made to coincide with the center of gravity of the engine-propeller combination. The effect of focalization is to simplify the engine's response to its own internal impulses, removing rotational components and making it easier to isolate and damp vibrations.
