The development and applicability of the empirical Helmbold-Diederich equation, including the various improvements used in this app, are discussed in ESDU Technical Memorandum (TM) 169; that document also includes accuracy assessments based on comparisons with the predictions of the lifting-surface theory based method given in ESDU 70011 and wind-tunnel data. The relevant equations are summarised in Appendix B of ESDU 70011 and are developed in full in ESDU TM 169. The ranges and limits on geometric and flow parameters for which warnings are given and computations executed have been taken directly from the method of ESDU 70011.