High Crosswind-Tolerance Airplane by Adjusting Dihedral Angle and Vertical Tail Volume

The present authors proposed the “quasi-neutral dihedral-effect and directional-stability” (QNDD) airplane, which is a modified neutral dihedral-effect and directional-stability (NDD) airplane for its realization. The QNDD airplane provides a superior tolerance to crosswinds compared to a conventional one by modifying geometric properties: dihedral angle of a main wing and vertical tail volume to achieve small values of [Formula: see text] and [Formula: see text], which results in a small effect of crosswind gust on an airplane’s attitude. While the QNDD airplane yields small [Formula: see text] and [Formula: see text], the NDD airplane yields [Formula: see text] and [Formula: see text] equal to zero. When the changes of [Formula: see text] and [Formula: see text] due to the wing deformations are considered, the NDD airplane is ideal, while the QNDD airplane is more practical. The QNDD airplane had four real eigenvalues rather than classically two reals and one pair of complex conjugates. The eigenvalues seen in a conventional airplane as a Dutch roll mode were seen to be reals in a QNDD airplane, and the mode was overdamped, which is different from a set of eigenvalues of a conventional one. From the Bode diagrams and unsteady simulations with a discrete gust applied as a crosswind, it was confirmed that the QNDD airplane had a higher tolerance to the crosswind.