Longitudinal Stability

Abstract

THE basic theory of stability has undergone no important modification since the publication of Professor G. H. Bryan's book on Stability in Aviation in 1911. The stability equations derived therein serve to‐day with the difference that axes and symbols have now been standardised and with the additional refinement of a non‐dimensional form of the stability equation introduced by H. Glauert. Due to the vastly increased knowledge of aerodrynamic characteristics, however, the stability derivatives are more readily assessable in any particular design case. This applies more particularly to longitudinal stability calculations which may, and indeed often arc, carried through with no wind tunnel tests available apart from a lift and drag curve for the aerofoil section used. There has also been some extension of the use of stability charts for deriving an approximate knowledge of the behaviour of the aeroplane when it receives a disturbance. These charts are exceedingly useful for obtaining periodic time and damping factor, but the assumptions on which they are based should be clearly realized.