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National Advisory Committee for Aeronautics, Technical Notes - A Comparison of the Turbulent Boundary Layer Growth on an Unswept and a Swept Wing

In order to check the applicability of simple sweep theory to the
turbulent boundary—layer growth on swept wings, an experimental investi-
gation was undertaken in which measurements were mde of the turbulent.
boundary layer of a twa—dimensional unswept wing and a comparable wing
swept 11-50. The tests were conducted at a Reynolds number of about
1!- million based. on the component of velocity normal to the leading edge.

The experimental results indicate the applicability of simple sweep
theory for determining turbulent boundary—layer growth on the swept wing
for a lift coefficient of zero. For the wings at moderate lift coeffi—
cients, the growth of the bomdary—layer component normal to the leading
edge was more rapid on the swept wing than on the unswept wing. This
difference, however, is believed to be primarily the result of differ—
ences in the surface conditions of the two models. Support is given to
this belief by the fact tint the growth of the momentum thickness on
the swept wing calculated in accordance with simple sweep theory by
using the component of flow normal to the leading edge was in good agree-
ment with the measured growth.

A theory has been advanced to the effect that certain aerodynamic
characteristics of an infinite swept wing are determined solely by the
component of velocity normal to the leading edge. (See, for example, ref—
erence 1.) Various experimental investigations have shown that this idea,
commonly referred to as simple sweep theory, provides a satisfactory
explanation of many of the observed characteristics of swept wings. For
example, it is shown in references 2 and 3 that the chordwise distribu—
tions of pressure, and hence the lift coefficients, when based on the
component of velocity normal to the leading edge, were the same for a
straight wing and a swept wing. In reference 3, comparison is made of
two constant-chord wings, both of which completely spanned the wind
tunnel. It was found. that simple sweep theory, insofar as pressure dis—
tribution was concerned, applied to the central 1+0 percent of the span
of the swept wing.