In the "Axial Fan" take any radius of the impeller leaf gate. Cascades imports, the fluid having a circumferential velocity 1u, relative velocity w1, absolute speed v1, exports have u2, w2 and v2, the velocity vector of the three formed a triangle and export speed. Absolute velocity may be decomposed into a circumferential direction component vu, and component axial direction va, this time, the axial direction of the axial differentials, it is represented by the symbol va. Axial flow impeller inlet and outlet fluid flow along the surface of the flow of the same radius, and thus the circumferential speed of the import and export of u1 and u2 are equal, that is u1 = u2 = u. In addition to the incompressible fluid, the fluid to boost small fan impeller Importers axial velocity can be regarded as equal, v1a = v2a = va.