Page "Aerodynamics" Paragraph 11

This equation is incorrect for the calculation of drag in most cases.

Drag on a flat plate is closer to being linear with the angle of inclination as opposed to acting quadratically at low angles.

The Newton formula can lead one to believe that flight is more difficult than it actually is, due to this overprediction of drag and thus required thrust, and it may have contributed to a delay in human flight.

However, it is more correct for a very slender plate when the angle becomes large and flow separation occurs, or if the flow speed is supersonic.