Hi.
A model airplane (that flies) has an ~8 inch propeller. It is meant to spin at a specific direction.
One side is subject to cavitation or suction, the other under pushing pressure to produce a flow force.
Its sides have different curvatures, almost flat or concave on its pressure side; convex on its suction side.
Usually the leading edges are blunt, the trailing edges are sharper.

If that propeller is put to use as a windmill; which of its surfaces should face the incoming wind ?

 

Try it both ways and see which one works better.

 

The flat side should face the wind, but a propeller will not make a very good wind turbine. The pitch of the blades is wrong. It will create a lot of turbulence but not much torque. Do a bit of research on the effect of the angle of attack on airfoils.
How an Airfoil's Angle of Attack Creates Lift and Drag | System Analysis Blog | Cadence

 

The pitch of the blades is wrong.

Agreed. The pitch is designed for very high rotation rates, which you wouldn't get if used as a windmill .....even in Hurricane Milton !