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Under sufficiently large wake deflections, the forward portion of the wake is found to flow forward along the floor leading to a vortex pattern which results in Rae's limits (Journal of Aircraft, May-June 1967). Although wind-tunnel data cannot normally be corrected successfully beyond these limits, it may be possible to obtain ground-effect data for conditions more severe than those implied by Rae.
A theory is developed which predicts the interference velocities and interference velocity gradients caused by the walls of the tunnel. Large wake deflections are allowed in both the lateral and vertical directions. The theory includes V/STOL and conventional wall-interference theories and ground effect as special cases. Symmetry and interchange relationships between the interference factors are developed and extensive numerical results are presented. Use of the interference factors to correct data depends upon the availability of detailed aerodynamic treatments in nonuniform flow of the model under test. In most tests the available aerodynamic treatments will be found either inadequate or too time consuming for rigorous routine correction of data relating to lateral-directional stability.
An approximate treatment of V/STOL wall interference in a circular tunnel indicates that the interference factors at the model for this tunnel should be of the same order of magnitude as those presently available for a square tunnel of equal cross-sectional area. There is a greater degree of uncertainty with respect to the lateral and longitudinal distributions of interference; however, the available results for the square tunnel should be reasonably close to those of the circular tunnel provided that the model is relatively small in comparison with the test section.