Postcritical Analysis of PrandtlPlane Joined-Wing Configurations

The postbuckling behavior of joined-wing configurations has not been fully addressed in the past. This topic is extensively discussed in this work. Starting from a baseline configuration, geometrical parameters as well as wings’ load repartition are varied to assess their influence on buckling occurrence. The snap-buckling phenomenon and postcritical pattern are investigated with the adoption of the arc-length technique. The complex load transferring through the joint induces a deformation shape that can no longer carry additional load after a critical point is reached. An abrupt snap to a configuration that is not continuously adjacent to the previous one is then observed. Comparison with the instability state obtained via eigenvalue analysis demonstrates that buckling prediction through linear-buckling analysis is inadequate, and often, the actual critical load is overestimated. This study shows that, for PrandtlPlane joined-wing configurations, increasing the height-to-wingspan ratio is beneficial as far as structural response is concerned. This provides an important design guideline because it is well known from previous studies that high height-to-wingspan ratio decreases the induced drag and fuel consumption. The lift repartition and its effects on buckling show that an improvement of the structural stability of the joined-wing airplane can then be achieved by increasing the amount of aerodynamic load on the upper wing while reducing the lift on the lower wing. The strong bending–torsion coupling typical of highly swept joined wings and its influence on the postcritical response are also investigated and discussed.