How does an inlet generate force? (ie how does an unbalanced force appear at or about the inlet components?). The purpose of the inlet is to slow down the air to a velocity that the compressor/fan can accept. In slowing down the air a (more or less) adiabatic compression takes place (ie a compression without any energy being added to the airflow). If the duct is just a cylinder, no unbalanced force will result; but if the duct has a profile that is narrower at the front (inlet) than the rear, a net forwards force will be generated.
The
NACA cowl used on radial engines is a good illustration of this. I once had occasion to ferry a vintage SE ship with the cowl missing. It cruised 15 kt slower than usual; the cowl contributed substantial thrust, as well as reducing external drag.
A more complete history of the NACA cowl see:
N.A.C.A. Cowling