Confusion has arisen regarding just what is meant by adjustable variablepitch and controllable-pitch propellers, and the matter should Be more clearly defined in the following way:—Fixed-pitch propellers to apply to both (a) the ordinary fixed-pitch propeller, made up with the hub and blades integral, and the material the same throughout, and (b) the detachable-blade or ground-adjustable type that will usually have the hub and blades of different materials, and in which the blades will be clamped in the hubs on the ground to an angle that is assumed, usually by experiment, to give the best general or some specially desired performance results.