Good question.

The thing is in the velocity profiles for laminar and turbulent boundary layers. Lets look at the picture below. The profiles are little different. The turbulent profile is "fatter", or fuller, than the laminar profile. For the turbulent profile, from the outer edge to a point near the surface, the velocity remains reasonably close to the freestream velocity,then it rapidly decreases to zero at the surface. In contrast, the laminar velocity profile gradually decreases to zero from the outer edge to the surface.

The key now is in the wall shear stress. More shear stress results in higher skin friction drag. The wall shear stress is defined as a product of viscosity coefficient [mu] and the velocity gradient at the wall [(dV/dy) at y=0], which is the reciprocal of the slope of the curves at the surface. It is clear that velocity gradient near the surface for laminar flow is smaller than for the turbulent one, thus wall shear stress for the laminar flow is smaller than for the turbulent one.

This means that laminar flow has smaller skin friction drag than the turbulent flow due to faster velocities near the surface.

Good question.

The thing is in the velocity profiles for laminar and turbulent boundary layers. Lets look at the picture below. The profiles are little different. The turbulent profile is "fatter", or fuller, than the laminar profile. For the turbulent profile, from the outer edge to a point near the surface, the velocity remains reasonably close to the freestream velocity,then it rapidly decreases to zero at the surface. In contrast, the laminar velocity profile gradually decreases to zero from the outer edge to the surface.

The key now is in the wall shear stress. More shear stress results in higher skin friction drag. The wall shear stress is defined as a product of viscosity coefficient [mu] and the velocity gradient at the wall:

$$ \left.\frac{dV}{dy}\right|_{y=0} $$ Which is the reciprocal of the slope of the curves at the surface. It is clear that the velocity gradient near the surface for laminar flow is smaller than for the turbulent one, thus wall shear stress for the laminar flow is smaller than for the turbulent one.

This means that laminar flow has smaller skin friction drag than turbulent flow due to faster velocities near the surface.