A small electric airplane such as a small airplane with its span length of 10 in is subject to crosswind in the take-off and landing phases because the flight velocity and the gravitational force are not large enough with compared to the velocity of crosswind and the aerodynamic force due to crosswind, respectively. The NDD (Neutral Dihedral effect and Directional stability) airplane identified as C_1_β=0 and C_n_β = 0 was proposed by Katayanagi so that the airplane has a tolerance to crosswind. However, one of the roots obtained from the lateral/directional characteristic equation for the NDD airplane is zero, which means that the corresponding mode is neutral stable. In the present analysis, an airplane with small values of C_1_β and C_n_β, which led to all stable non-oscillating modes in the lateral/directional motions, was investigated, and the configuration with all stable subsidence modes was named as "Quasi-NDD" (QNDD) airplane for a small airplane with a high tolerance to crosswind by the authors. It is found that all of the eigenvalues of the ONDD airplane are negative real values, then it has two representative subsidence modes: one is a well-known Roll mode and the other is a subsidence mode with respect to a sideslip angle. By plotting Bode diagrams and conducting unsteady simulations in a discrete gust and continuous gust as crosswind, it is found that the QNDD airplane has a high tolerance to crosswind.
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