The subject defined by the title of this paper is one which cannot be covered completely in the time at my disposal. I propose therefore to confine my remarks to-night almost entirely to the aerodynamic aspects of the problem and to put before you some possible advantages that may be gained by substituting a variable pitch airscrew for the normal fixed pitch airscrew, and in certain cases to compare the results with those given by the introduction of a variable gear airscrew. The mechanical side of the problem is a subject complete in itself and will not be dealt with here.