The Effect of Turbulence: An Investigation of Maximum Lift Coefficient and Turbulence in Wind Tunnels and in Flight

THE extensive series of investigations to be discussed in the present paper has its origin in an attempt to clarify the reason for certain discrepancies, which have long plagued aerodynamicists, between the results of tests on similar aerofoils carried out in different wind tunnels. It has been well known for many years that the Reynolds number has an important influence on aerofoil characteristics. It is therefore highly desirable that aerofoil tests, to be useful for full‐scale predictions, be made at as large a value of the Reynolds number as possible. For several years the variable density wind tunnel of the National Advisory Committee for Aeronautics has been able to employ Reynolds numbers considerably higher than were attainable in any other wind tunnel. Its results have, therefore, very properly been generally accepted as furnishing the standard aerofoil data for aeroplane designers in the United States, and to some extent also in Europe. It appears from the present investigation that another factor, turbulence, may be of the same order of importance as the Reynolds number in determining certain aerofoil characteristics. In discussing the possible effects of this factor, it is desirable that as wide variations in its magnitude as can be obtained should be considered. The turbulence characteristics of the N.A.C.A. variable density tunnel and of the wind tunnel at the Guggenheim Aeronautics Laboratory at the California Institute of Technology (referred to in the figures as Galcit) are as different as those of any two large contemporary wind tunnels. It is for this reason that in the following discussion results obtained in these two tunnels are compared.