THERE are reasons for believing that in certain states of working, the resultant aerodynamic force on a blade element of an airscrew is normal to a helicoidal surface. The object of the present inquiry was to see to what extent this is borne out by experimental results. It is found that between no‐thrust and maximum efficiency, the hypothesis is confirmed by experimental results for a certain set of helicoidal airscrews. In the course of the inquiry, a method of evaluating the torque coefficient! over the same range, is indicated—the method giving good accuracy.