Longitudinal Stability in Aeroplanes: A Summary of Mathematical Theory taking Account of Interferences

THE mathematical theory of longitudinal stability appears to have been adequate to explain the salient features of the behaviour of aeroplanes in longitudinal motion. In general the provision of a stable slope to the static pitching moment curve has been found in practice to fulfil all requirements, and although increasing oscillations do on occasion occur, they are on the whole surprisingly rare. The reasons for this are fairly well recognised and are briefly indicated in what follows. There is little doubt, however, that the designers' principal difficulties centre round the complex interferences between the wings and the tailplane, particularly with the air‐screw running. The downwash from the centre section in many machines, even with no engine on, is quite unpredictable in the present state of knowledge, and the calculation of the downwash due to the slipstream has not yet been successfully made even in the simplest cases. Some attempt is here made to summarise the present position.