An Investigation of the Conditions for the Occurrence of Flutter in Aircraft and the Development of Criteria for the Prediction and Elimination of Such Flutter

A review is presented of the work done by other investigators on the effects of inertia couplings in producing flutter in control surfaces that are not mass balanced. The conclusion is reached that for the prevention of such flutter complete dynamic balance should always maintain. Flexural-torsional flutter is investigated in considerable detail from the consideration of the dynamical equations for steady state forced oscillations of the two dimensional case. A complete set of response curves for two typical cases are included to show the types of responses that should be observed in flight with vibration pick-up equipment. The important fact is brought out that the response and behavior of the wing at its natural bending frequency has little or no correlation with the behavior of the wing at the stability limit of flutter. Curves are presented to show that, for normal airplanes, the most important parameters which determine flutter in this mode are (a) the position of the inertia axis, (b) the torsional frequency, end (c) the radius of gyration of the wing mass about the inertia axis. The dynamical equations are set up for the cases of flexural-aileron, torsional-aileron, and flexural-torsional-aileron flutter in the two-dimensional case and an example is given of the determination of the stability limit of a specific example of the first of these modes. An extension of the two-dimensional case to the three-dimensional case is presented with particular reference to determining the flexural-torsional flutter speed of a tail surface with vertical surfaces on the tips of the horizontals. The method of attack is outlined for the calculation of natural frequencies at zero airspeed to use in determining the flutter speed. Statistical data in a graphical form show the variations of natural frequencies of the various components of airplanes with the size of such airplanes. The conclusion is reached that the speed of airplanes should be restricted to two-thirds of the critical speed for any mode of flutter, divergence, or aileron reversal