Besides lift and drag coefficients, the moment coefficient c m is of importance for the behaviour of an airplane - it has a big impact on the longitudinal stability. While a conventional airplane can compensate the moment of the wing with its horizontal tail, a tailless plane obviously can't. It is possible to divide tailless airplanes in three groups, depending on how they achieve longitudinal stability. The requirements for the moment coefficient of the airfoil is a direct result of the stabilizing mechanism.
Longitudinal stability is created solely by the airfoil.
A plank requires an airfoil with a positive moment coefficient. It is possible to use any airfoil, because longitudinal stability can always be achieved by selecting a suitable combination of sweep and twist. They need smaller amounts of twist, which results in a broader speed range without paying too much penalties off the design point. The moment coefficient is less important and it is possible to use traditional airfoils with negative moment coefficients. The position of the c.g. can be chosen to guarantee stability, but usually airfoils with medium moment coefficients are chosen to achieve higher penetration speeds and a wider speed range.