The vibrations experienced by the various engine components are extremely complex and variable in nature. Their research requires a very thorough discrimination of all elements.

It is, in particular:

• Study of vibrations due to lack of balancing of moving parts;
• That of operating vibrations, vibrations to which are submitted for example the valve springs;
• Torsional vibrations of crankshafts and propeller shafts;
• Gear vibrations, such as propeller reducers, etc.

It should also be noted that the vibrations of the aircraft, often have their origin in the vibrations of the engine and vice versa, which requires a special study of the vibratory isolation of the engine.

The study of vibrations and their origins is of paramount importance in aeronautics, the effects that they can have on the holding of the pieces, these in motion can have serious and unexpected consequences for the airframe - engine assembly - propeller.

The vibrations often unavoidable in the multiple organs of an aviation engine are added and "resonance" between them, creating vibratory regimes that must be kept as far as possible from the use regimes.

It is often difficult to determine by calculation the elements involved in a complex vibration (rigidity of parts and mechanical connections, damping of the vibrating system).

That is why it is preferable, whenever possible, to use the experimental method.

Vibrographer recorder

The SOCIÉTÉ NATIONALE DE CONSTRUCTION DE MOTEURS uses for this purpose the photographic recording of the instantaneous accelerations of the vibrating system. The apparatus comprises a piezoelectric detector, which is nothing other than an assembly formed of two cut quartz disks on which an inert mass rests. The pressure of the inert mass on the quartz varies according to the accelerations transmitted to it by the vibrating member, which has the effect of developing proportional electric charges. These suitably amplified and filtered charges operate a magnetic mirror oscillograph whose light spot is part of the photographic strip. The photograph on the opposite page represents all the equipment used to measure the vibrations of a grinding machine. The upper figure represents the natural vibrations of a motor mounted on a test bench. A specially developed similar apparatus allows the recording of all the vibrations of engines running on aircraft. The lower figure shows the torque accelerations of a 12-cylinder engine at 2,710 rpm. On such a recording it is possible to analyze combustion irregularities such as self-ignition.

Measurement of the natural frequency and the damping of a motor fixed on its test bench. Torque vibration of a 12-cylinder engine at 2,710 rpm.