Paul Fuhrman of the Lord Corporation (founded by Hugh Lord in 1924) gave us a simplified explanation of what is involved in designing a device that will effectively vibrationally isolate the airframe and engine of an airplane.
Step one is ground vibration testing of the airframe to find its natural vibrational frequency. Every object has a natural resonant frequency-think of a tuning fork. All engine isolation systems are designed to prevent the suspended mass-the engine and propeller-from exciting the resonant frequency of the airframe. Doing so will cause everything from discomfort to the occupants through damage to the airframe and components.
The next step is to determine the natural frequencies of the suspended mass-all suspended masses have six natural frequencies: fore/aft, vertical, lateral, roll, pitch and yaw. In an airplane they must be identified and isolated so that they do not go into the resonate frequency of the airframe. That becomes challenging because the engine operates at different speeds and frequencies during start, idle, taxi, takeoff, cruise, descent, landing and shutdown. Some of the frequencies are quite low.
Naturally, what is right for one airframe and engine combination isn’t going to be correct for another-in fact, going from a two- to three-blade propeller changes the frequencies of the engine/propeller combination enough that it may require different isolators.
Added to the challenge is that the engine may only be allowed limited motion within its mounting system because of the demands of maintaining a consistent thrust line.
The manufacturer must take all those factors and design a mount that is soft enough to isolate the engine/propeller combination vibrationally from the airframe and keep the occupants comfortable.
Additionally, to some extent, allow for such changes as may be made to the airplane downstream, such as aftermarket air conditioning, an add-on turbocharger or a modified exhaust system.
