From Fluttering Wings to Flapping Flight: the Energy Connection

The papers presents a new paradigm for understanding ßutter and ßapping ßight. The paradigm integrates aeroelastic interactions with applied and induced propulsive effects. The framework is based on conservation of energy. Energy produced, energy lost and/or work done due to structural vibration, aerodynamic wake and propulsion are taken into account. It is conclusively shown that there exist three types of modes in an aeroelastic system, viz., i) unstable (ßutter) mode producing drag, ii) stable mode producing drag, and, iii) stable mode producing thrust. It is also shown that the ßutter region (in terms of the relative phase of the pitch and plunge) is separate from the thrust-producing region. For the case of a two-dimensional airfoil these regions are tangential to each other and have a common “neutral” point. The efficiency of ßapping ßight and occurence of ßutter is further investigated. Finally, the possibility of a limit-cycle oscillation due to constant thrust ßight is presented.