The crudest way of taking account of the airframe superstructure in calculating undercarriage performance is to represent it by a single concentrated mass. Crude as this method is, it is often used by aircraft firms because the only alternative known to them, apparently, is something they hesitate to face. And no wonder, because it means having to represent the wing-fuselage system by a large number of lumped masses and springs, each of which means an extra degree of freedom. This complicated dynamic system has then to be integrated with the undercarriage system, itself complicated by its non-linear characteristics.

The natural approach that suggests itself is not to consider the w/f (wing-fuselage) structure ab initio but to make use of its (usually known) normal modes and frequencies. But this is just what cannot be done by existing techniques—hence the present impasse.