Extension of flutter analysis of the preceding chapter to three-dimen­sional structure is straightforward. In § 7.1, a practical approach, based on generalized coordinates, will be outlined in greater detail. The underlying principles for the selection of generalized coordinates is discussed in § 7.2. The limitations of the theory are then pointed out in § 7.3. Some general remarks on the control and prevention of flutter are presented in § 7.4.

Flutter analysis should be comprehensive so that no factor is overlooked. We know many instances in which the crucial factors that caused com­mercial disaster or loss of lives were so hidden that they were forgotten. One fine turboprop plane had several disastrous flutter failures because of an ignorance of the propeller yawing effect (oscillatory aerodynamic force acting on the propeller in a direction perpendicular to the axis of the propeller). One high performance fighter development lost many millions of dollars because of the use of a rubbing block (for dry friction) in the control system. A famous company which made very extensive flutter analysis in the course of development of an airplane was met with flutter failure because the engineers employed the same simplifying assumptions in building the mathematical model for analysis and the wind tunnel model for testing; in which case the confidence derived from the agreement between theory and experiment was meaningless with regard to the prototype.

On the other hand, the varied conditions in the service of the aircraft must be considered. Icing, fuel displacement, minor structural damages, may cause flutter. One small private airplane crashed because of accumu­lation of dust in the wing.

Safety can be purchased only with exhaustive care!

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