Multhopp's lifting surface equation is obtained after passing to the limit in the z-component of the fluid velocity. The difficulty occurs because the involved integrals become singular. Some authors either ignore this issue or they use, without any justification in their proofs, interchanges of limit and integration or differentiation operations. In this paper, we prove rigorously the validity of the limit in the frame of the distribution theory.

Key Words: lifting wing, singular integral, theory of distribution

2010 Mathematics Subject Classification: Primary: 76G25, Secondary: 45E05