Note on aerodynamic camber

Abstract

The following note has been written at the suggestion of the Chairman of the Stability and Control Sub-Committee. It is intended to explain the theoretical significance of camber derivatives, and to assess the various available methods of making experimental measurements with particular reference to the use of a curved-flow tunnel. The note amplifies the arguments put forward by the writer in Ref. 1 (1950), that there is particular need for sytematic information about the influence of curvature of flow on control hinge-moments as a step towards the understanding of three-dimensional viscous flow. After a definition of aerodynamic camber and a historical account of the development of the idea and its importance, the present state of knowledge of its aerodynamic derivatives is described. Camber derivatives are required for evaluating tunnel interference corrections and are useful for stimating corrections for aspect ratio and scale effect, in so far as the flow at a section of a finite wing can be represented as an equivalent two-dimensional flow. This quasi two-dimensional approach to the problem of control surfaces should be combined with experimental checks on the aerodynamic derivatives of various wings with flaps and also with a study of three-dimensional boundary layers. Formulae for the camber derivatives of lift and pitching moment need confirmation. The derivatives of lift at the stall and of hinge moments over the whole range of incidence are virtually unknown and in consequence the determination of (CL)max and CH is seriously limited. The significance of these two-dimensional camber derivatives is illustrated by the quantitative uncertainties that may arise. It is suggested that these might be removed by establishing formulae for the unknown derivatives from a series of tests of uneambered aerofoils with a range of flaps in the curved-flow tunnel at the Langley Aeronautical Laboratory, U.S.A., by simulating a uniform rate of change of pitch. The uncertain characteristics of the curved flow would make necessary a check between results so obtained and those deduced from tests in a straight tunnel of aerofoils with various amounts of parabolic camber. There appears to be no other satisfactory technique for measuring camber derivatives.