Aerodynamics for Engineers: IV.—Elliptic Loading—Channel Corrections—Downwash—Viscous Motion and Boundary‐Layer Theory

(1) Theoretical consideration of aerofoils of limited span led, in Article III, to a need for some assumption as to lift‐grading along the span, to give the span‐grading of circulation. Fig. 1 shows at (a) the experimental distribution of lift along a rectangular aerofoil of 7·6 per cent camber and aspect ratio 6, at an angle of incidence of 6 deg. At (b) elliptic loading is shown, which, from several points of view, may be regarded as ideal for the same lift coefficient. Assumption of the latter form permits of greatly simplified treatment. Even compared with rectangular aerofoil distribution, the assumption gives differences of little importance in many practical calculations, while these are still less with aeroplane wings, owing to the wash‐out usually introduced at their rounded wing‐tips. The following is, therefore, restricted to elliptic loading, but sufficient generality of presentation is preserved to allow, if desired, other forms to be substituted.