THE reduction of vibration in aircraft structures is becoming a major problem in aircraft design. The increasing emphasis placed on vibration studies is the result of three important influences: (1) the increased loading of the structures, (2) the broadening of the operating-speed range obtained with controllable propellers, and (3) an increase in demand for more passenger comfort.
In the past structural vibrations have been given attention mainly because of the danger of failure. At present to obtain the required comfort, the vibration studies must be carried out to a much greater degree of refinement.
The vibrations of the aircraft engine as a whole may set up vibrations of the whole airplane, but the vibrations of parts of the engine with respect to each other are not noticeable in the cabin; in fact, they are difficult to detect except by the use of special instruments.
A study of the vibration characteristics of an engine-propeller installation involves (1) a determination of the vibration spectrum of the installation, that is, a determination of the frequencies of all of the modes of vibration, (2) a study of the exciting forces set up within this combination, and (3) the operating conditions, that is, the operating speeds and the power required at these various operating speeds. From these data the engineer can predict the vibration characteristics of the installation.