AIRPLANE vibration produces many undesirable conditions during flight, such as fatigue of structural members, a bad effect on the nervous systems of the occupants and the like. Excessive vibration leads to premature deterioration or to erroneous indications of instruments. Vibrations can be analyzed from a mathematical viewpoint with gratifying results, but such analysis is sometimes difficult and often is applicable only to selected conditions.
A serious mathematical analysis was carried out in the investigation of resonance conditions between engine and engine mount. Then the problem was approached from a rather empirical viewpoint to give vibration relations, not, as heretofore, to bodily sensation, but to such terms as amplitude, frequency, the relation between the two, form and the like. As no instruments are commercially available for recording these values under flight conditions, an apparatus of the same size as that of any standard aircraft instrument-a photographic recorder-was developed. It operates on the principle of the seismograph and embodies many interesting and novel features. A complete mathematical analysis, description and method of calibrating it is given.
Vibration test-stands used for duplicating airplane vibration in the laboratory are also described. The analysis of records obtained on them seems to indicate that the duplication of vibration in the laboratory has not been fully accomplished. A total of 216 records from actual flight tests and from the laboratory tests was obtained. A limited number of records are reproduced, and the results from flight-test records are plotted in groups on four charts, which show that some airplanes vibrate excessively as compared with others and that, on some, the resonance points are within or very near the operating range. A table is presented which gives the arbitrary relation of bodily sensation to amplitude, and eight conclusions are stated which throw some light on the peculiar problem of vibration in airplanes.