Energy approach to flutter suppression and aeroelastic control

Flutter is only possible in regions of the system phase space where the net aerodynamic work is positive over a suitably defined time interval, for example, one period of oscillation. In regions where the aerodynamic work is negative, no aeroelastic instabilities can occur, and the system is totally stable. In most cases, the unstable region is a finite subspace bounded by a closed flutter surface corresponding to neutral stability. The flutter subspace can be defined by a nonlinear aerodynamic work functional, whose magnitude represents the strength of the aeroelastic instability. A new flutter control concept based on aeroelastic energy principles is proposed, in which the main objective is to move the system out of the unstable region of phase space, by altering the critical aeroelastic mode, The classical picture of frequency coalescence flutter portrayed in aeroelasticity texts may be misleading and not a reliable guide in the unstable region. The more typical behavior appears to be one of frequency coalescence-divergence, where the mode frequencies tend to merge as the flutter boundary is approached from below, but then reverse direction and start to diverge after the system has crossed the flutter boundary and penetrated about 10-20 % into the unstable region. This observation is of significance in the design of aeroelastic control laws for flutter suppression.