This paper discusses two nonlinear phenomena of relevance in transonic flutter analysis and testing: (1) The possible appearance of more than one flutter mode (branch) in the aeroelastic response, for certain transonic Mach numbers at high mass ratios; and, (2) the possibility of bifurcations to subharmonic flutter at low mass ratios. In the first instance, the superposition principle breaks down and flutter may not necessarily occur when the first aeroelastic eigenvalue crosses into the right half-plane. In the second instance, the nonlinearities open up a new route to flutter, whereby the reduced frequency of the critical aeroelastic mode is lowered into the unstable range through a period-tripling bifurcation. Neither behavior can be understood within the framework of classical linear aeroelasticity.

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