T N A RECENT COMMUNICATION, Yoshihara and Strand concluded A that the conditions for minimum drag in lifting surface theory are not valid for wings with singular edge forces. Such edge forces are normally incorporated in the surface integral for drag by a limiting process. In reference 1, however, the edge forces are given a separate expression in the form of a line integral taken around the edge. The line integral is then added to the expression for the direct drag, but is omitted in the expression of the interference drag—i.e., in the Ursell-Ward relation. This inconsistency evidently leads to the erroneous conclusion. The selection of a distribution of lift by the condition of minimum drag does not, in general, agree with the selection imposed by the Kut ta condition. The latter condition merely selects a flow as being physically probable in a viscous fluid. With plan forms pointed at the front or rear the optimum distributions of lift do, in fact, require suction peaks at either the leading or trailing edges. Thus, in the case of a slender triangular wing moving base foremost, such a peak appears along the trailing edge and a component of the minimum drag appears as a downstream edge force. Figure 1 illustrates two methods by which this singular mathematical model might be interpreted in a practical sense. (In actual practice boundary-layer control might be employed.) Clearly, our consideration of the sharp edge and the limiting process is only an effort to simplify the formulas. If the limiting calculation produced a result different from that obtained by integrating the nearby smooth pressure distributions, then we would find it difficult to attribute any physical significance to the result.