Aircraft-Engine-Accessory Vibration

1. The overhang-moment limit, or still better a limit on the moment of inertia of the accessory about a line through the center of the accessory mounting surface, would be helpful by limiting one of the two factors which control the natural frequency of the accessory-rear-cover combination. However, the natural frequency is the factor of prime importance, and, if control of the natural frequency is to be maintained, it should be applied directly. This control is not practical with the equipment now in use, because accessories have grown so large that sufficient stiffness cannot be built into their supporting structures to keep their natural frequencies out of the range of the engine- propeller natural frequencies. 2. A survey of accessory-vibration data obtained on recent and current aircraft power-plant installations indicates that chance is the most important factor in determining the suitability of an accessory installation. Practically all accessory installations have natural frequencies in the frequency range where engine-propeller resonances exist. If an engine-accessory natural frequency coincides with an engine-propeller resonance in which large amplitudes of vibration occur, failures will result in the accessory and/or the accessory mounting studs. To obtain remedy for these unfortunate installations, either some revision of the installations is required, which results in its being nonstandard, or some other accessory or accessories must be used. 3. Accessory development is facilitated greatly by the use of an accessory-vibration-testing machine. An aircraft-engine rear cover should be mounted on the machine for attachment of the accessories.