Aeroelasticity of the PrandtlPlane: Body Freedom Flutter, Freeplay, and Limit Cycle Oscillation

Aeroelasticity of PrandtlPlane configurations is a yet unexplored field. The overconstrained structural system and the mutual aerodynamic interference of the wings enhance the complexity of the aeroelastic response. In this work the aeroelastic behavior of several models based on wing system of 250-seat PrandtlPlane design is studied. When an aluminum version of the structure is considered, flutter is associated with a coalescence of the first two elastic modes, the first being characterized by a classic upward bending of both wings, and the second one being associated with an out-of-phase bending of the two wings and tilting of the lateral joint. Analyses show that energy is injected into the structure mainly at the tip of the front wing, close to the aileron. Effects of freeplay of mobile surfaces are evaluated, showing how, in some cases, an increase in the flutter speed is observed. When flutter analyses are repeated considering the configuration free to pitch and plunge, flutter speed does increase due to a particular interaction between rigid-body pitching and elastic modes. Several of the above findings are demonstrated on more detailed structural models considering the local stiffness distribution, and taking into consideration compressibility effects. When composite materials are employed, flutter issues are completely overcome.