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Flutter is a species of oscillatory instability affecting parts of aircraft, such as their wings. However, in as much as an aircraft is a single system, it would always be more accurate to say that flutter is an oscillatory instability affecting an aircraft as a whole. There arc, of course, many kinds of oscillatory diseases which afflict aeroplanes and other aircraft, but the flutter disease is characterized by the feature that both aerodynamic actions and distortional oscillations of the structure play an essential part. Thus, if an aeroplane could be made rigid it would not flutter. Needless to say, it is quite impossible to make an aeroplane, or indeed any other structure, strictly rigid but it is true that increasing the structural stiffness is one of the basic methods for avoiding flutter troubles, a fact duly insisted upon by the Airworthiness Authorities in all countries. Likewise, an aeroplane would not flutter if it were projected in a vacuum so that the aerodynamic actions were annulled, but then the sustaining force would also be absent. Both distortion and aerodynamic actions play a part in many kinds of oscillation other than flutter and several of these, such as ‘snaking’, fall'within what is regarded as the domain of ‘stability and control’. However, in such oscillations the structural distortions are not essential for the occurrence of instability.