In a recent article in “ Engineering ” the writer has given an account of the principles which govern the design of aeroplane control surfaces, making special reference to wing ailerons. Included in the information presented were data on the properties of wing ailerons of the ordinary unbalanced form, but varying in span and chord length and fitted to wing tips of different plan forms. From these data several conclusions of practical importance were drawn, some of them finding general application in the design of control surfaces, and it may be useful to give a résumé of these before proceeding to deal with the balancing of control surfaces.

(i) All ailerons and elevators become relatively inefficient for movements above about 15° to 20° from the normal position, owing to a sudden reduction in the slope of the curve relating moment on machine with angle of aileron or elevator. For movements up to about 15° or 20° the moment is nearly proportional to the angle.