Transition Prediction and Design Philosophy for Hybrid Laminar Flow Control for Military Aircraft

An investigation into the application of transition prediction methods for hybrid laminar flow control for military aircraft has been carried out. Linear stability theory and the "e" criterion are commonly used for predicting the onset of transition. Although a great deal of experience has been gained in their use over the years, there are still issues involved in the use of these methods which affect the accuracy of transition prediction The investigation has shown the importance of the inclusion of negatively oriented oblique waves in the calculation of the N-factors. The effect of these waves on the predicted N-factor values and hence transition onset has been shown to be significant and cannot be ignored. These effects are more marked for the combination of adverse pressure gradients and high angles of sweep which are relevant to military aircraft. A parametric study has been carried out to investigate the effect of surface suction or cooling on transition and to determine the appropriate suction quantities or cooling rates required to suppress the various instability modes in order to delay transition for military aircraft Trends in the possible extents of laminar flow achievable for different pressure distributions and flow conditions with various suction and/or cooling distributions are shown and their implications for design are discussed.