Theoretical Study of Boundary-Layer Control,

Inviscid flow past the upswept rear part of a fuselage is directed beneath the fuselage. This leads to an accumulation of boundary-layer material at the lower side of the fuselage which might be prone to separation. In this study the cross-sections of the rear end of a typical transport airplane fuselage were modified from the original circles to rounded triangles, in this way shifting downward the centers of gravity of the cross-sections. The side-view contour and the width of the fuselage remain unchanged the same holds roughly for the volume. Three different configurations without wing and tail unit were studied -- the original and two modified ones. The inviscid flow was computed with the MBB-panel method, the boundary-layer development with the integral method of Cousteix and Aupoix for three-dimensional turbulent, compressible flow. Results show that less boundary-layer material is transported under the rear part of the fuselage in the modified cases. The tendency of the skin-friction lines to converge is reduced. The whole flow pattern at the base is improved. In the frame of boundary-layer theory, however, no statement can be made about possible pressure drag reductions. Improvement of the elevator performance appears to be probable. The study shows that the upsweep of the fuselage is the main factor in the boundary-layer development. The addition of the wing and the tail unit will only modify the picture.