At present, while we have but a vague conception of the form a practical helicopter is likely to take, it is useless to attempt to set out in detail the dynamics of helicopter flight or performance calculations. For the evolution of a practical machine of this type it seems to the author that it will be necessary first to study the stability equations neglecting everything but gravity and the screws themselves. This should show what stabilising surfaces are required, and it is with this object that the stability equations are given below. In the meanwhile it is perhaps worth while to obtain a broad view of the ground to be surveyed, and we, must expect the work to be more complicated than in the case of the aeroplane.