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Published online by Cambridge University Press: 14 September 2016
Introductory.—In the course of inquiries made recently by the Accidents Investigation Sub-Committee they were impressed by the fact that the various terms normally used to describe the characteristics of an aeroplane in regard to its stability or instability are incomplete and defective from a pilot's point of view. Not only did there appear to be differences in the meanings attached to the various stability terms by pilots and others, but in several cases the meanings were not in accordance with the definitions used in physical science generally. It appeared that a scheme of definitions could be drawn up which would be acceptable both to those interested practically and theoretically in the subject. Definitions are needed for discussions on stability and particularly in making more definite the information given by pilots when describing the behaviour of aeroplanes. It is felt that the definitions, if accepted on all hands, would lead to such observations during the life of an aeroplane as materially to assist the investigations of accidents.
Aeronautical Research Committee Reports and Memoranda, No. 745. June, 1921.
Note on page 585 * “ Trim ” has in the past sometimes been used as synonymous with “ balance.” It is recommended that the word “ trim ” be reserved for a “ rigging ” operation.
† The definition of a stable aeroplane as one which, whatever be done to it, the aeroplane comes back to straight flight on releasing or centralising the controls, is to be discouraged, since it is far less useful than the new proposal when it is desired to refer to the various qualities or behaviour of an aeroplane for variations of flight conditions.
Note on page 586 * When an even keel is maintained by means of the aileron and a constant angle of incidence by the elevator, the effect is to preclude any rotation except about the vertical axis.
† This has sometimes been described as “ lateral stability,” a term the true meaning of which is given below. It is recommended that such use be discontinued, as the term has another and more important use dealt with elsewhere.
‡ The motion has no connection with the “ aerobatic manceuvre ” called a “ roll.”
Note on page 585 * 587* This is probably a departure from usual practice, but is a possible motion.