Since the driving torque exerted by the engine is a periodic function of the time, the speed of rotation must also be periodic, which implies that the airscrew must be subjected to inertia stresses as well as to the stresses due to air forces and centrifugal forces. In the case of engines having six or more cylinders these inertia stresses will not in general be important as the cyclic variations of torque will be comparatively small. But with engines having two or three cylinders these stresses must be considered, for.it will readily be seen that the bending moment due to inertia forces is such as to increase the stresses due to the air load.