If the blade element method is applied to helicopter rotors in forward flight it is necessary to consider the flapping motion of the blades as well as the longitudinal and lateral distribution of the induced velocity on the rotor disk.
12.
For free vortices and other bodies outside one body without bound vorticity and without vortex production, a generalized Lagally theorem holds, with which the forces are expressed as the products of strength of inner singularities ( image vortices, sources and doublets inside each body ) and the induced velocity at these singularities by all causes except those inside this body.
13.
Now, if the " change " in lift distribution is known at given lift section, it is possible to predict how that section influences the lift over its neighbors : the vertical induced velocity ( upwash or downwash, \ omega _ i ) can be quantified using the velocity distribution within a vortex, and related to a change in effective angle of attack over neighboring sections.
