Whirl Flutter Stability of Two‐Bladed Proprotor/Pylon Systems in High Speed Flight

The lack of polar symmetry in two‐bladed rotors leads to equations of motion with periodic coefficients in axial flight, in contrast to three or more bladed rotors that result in constant coefficient equations. Although numerous studies on whirl flutter of three‐bladed rotors are available in the literature, very few studies have been directed towards the analysis of two‐bladed rotors. In this paper, the aeroelastic stability of two‐bladed proprotor/pylon/wing systems is examined in high‐speed axial flight. Several parametric studies are carried out to illustrate the special nature of two‐bladed proprotors and to better understand the mechanism of whirl‐flutter in such rotors. The wing beam bending mode for two‐bladed rotors is found to be stable over the range of parameters examined, a behavior very different from that of three‐bladed rotors. Also, the wing torsion mode exhibits a new type of instability, similar to a wing torsional divergence, but occuring at 1/rev frequency. This type of behavior is not seen in three and more bladed rotors. The interaction between the wing chordwise bending and torsion modes is found to be much greater in the case of two‐bladed rotors and, over the range of parameters considered, these two modes govern the stability of the system.