The airfoil design in xfoil MDES is such that it preserves the lift. Hence, L/D is increased by decrease of drag.
Drag types:
Vortex (Induced) drag (named after Prof. Veldhuis)
Interference drag
Viscous drag
Pressure
Skin Friction: pressusre loss
Bubbles/Separation/
Skin friction
Viscosity
∂u/∂y
Wave drag
For a problem, only viscous drag exists. To reduce viscous drag, laminar flow has lower friction, viscosity and ∂u/∂y but not separation.
Therefore to decrease viscous drag, we want laminar flow and we want it attached (since laminar flow separates easily). Therefore, to decrease viscous drag, we should delay transition and we should prevent bubbles and separation.
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