An Annular Fairing can be used to control the flow through an airscrew, which is then generally referred to as a ducted fan. There are obvious affinities between such a fairing and the fairings used to control the flow through turbo-jet engines, but there are certain significant differences which have to be recognised when such fairings are designed. In particular, a fairing for a ducted fan is usually much shorter than one for a turbo-jet engine. Whereas it is usually convenient and justifiable to treat the internal and external flows of a turbo-jet nacelle separately and to consider the interactions between the intake and nozzle to be small, a short fairing must be treated as a whole. The simple calculations made in this note may give some indication of the order of fairing length where this becomes necessary.