It has been shown that considerable improvement may be made in the performance of an aeroplane by the use of an airscrew, the pitch of which can be varied. The matter was discussed by Mr. H. A. Mettam in R. and M. 577, Part II. There are, however, certain pratical drawbacks to the use of a variable pitch airscrew. Such an airscrew is expensive to construct and probably more liable to breakage or derangement in flight than the ordinary type. It also imposes on the pilot the necessity of making constant and accurate use of the variable pitch control, if the full advantages of the system are to be obtained.

The present writer considered, therefore, that it might be worth while to inquire whether a similar improvement in performance could not be obtained by other and simpler means, namely, by the use of variable gearing between the engine and airscrew (the latter being of the usual fixed pitch type). The calculations have been made for the same machine as was used to illustrate R. and M. 577, the S.E.5 with Wolseley Viper engine. As in the variable pitch airscrew calculations, the weight has been increased from 1,976lbs. to 2,000lbs., thus allowing 24lbs. for the weight of the gearing.