The problems of aeroplane stability in flight have been studied very thoroughly, both practically and in theory. Extensive mathematical investigations have been carried out by such men as Bairstow, Bryan, etc., and it is largely due to this work that the numerous questions arising with regard to the stable flight and equilibrium of the aeroplane are so well understood.

Nevertheless, though the work of these investigators is undoubtedly of primary importance, a discussion of the more practical aspects of the subject will be more within the scope of the designer. It is proposed in the present paper to deal with but one form of stability, namely, longitudinal, and to discuss the requirements of practical design together with the methods of complying with these. The question, therefore, resolves itself into an investigation of the main points controlling the size and setting of the tailplane of the aeroplane.

In connection with gliding flight enough measurements of “ lift ” and “ drag ” have been made to enable us to calculate the conditions for success of an aeroplane fitted with wings of standard sections; but no attempt has been made, as far as the present writer is aware, to ascertain what would happen if a flying machine were fitted with wings of standard section and these were flapped in a rhythmical manner. Would it support and propel itself? Several authors, including M. F. Fitzgerald and Colonel J. D. Fullerton in the previous issue of this journal, have discussed various portions of this problem; but instead of appealing to wind-tunnel determinations the latter has used such expressions as pSUV or pSUV2 for the pressure on a wing of area S, where p is the density of the air and V, U are the component velocities along and at right angles to its surface.