I know the pressure distribution on an airfoil but not shear stress. How is shear stress distributed on an airfoil?
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Shear stress is proportional to the speed gradient at the wall. This means:
The shear acts along the local flow direction, so it is mostly front-to-back. Only near the wingtips, where lateral flow becomes non-negligible, will a lateral shear component become noticeable.
Friction drag over chord for an E502mod airfoil at 3° AoA. Blue: Top surface, Red: Bottom surface. Source: XFOIL 6.97. The transition points are clearly visible: It is where shear shoots up again at mid-chord. Note the separation bubble ahead of the transition point on the bottom side: It is marked by negative values of shear.