figure is a partial front elevation view of fan 10 showing hub 11 and blade 13 blade 13 has root 17 where the blade meets and attaches to the hub and tip 18 which is the outer extremity of the blade blade 13 also has leading edge 20 and trailing edge 19 line 14 is the blade midchord line which is the locus of points that are circumferentially equidistant from leading edge 20 and trailing edge 19 blade 13 has span s the radial distance from hub 11 to tip 18 blade 13 can be divided into root portion 15 and tip portion 16 in root portion 15 of blade 13 midchord line 14 has a backward sweep with sweep angle a h at the hub at the transition from the root portion to the tip portion of the blade midchord line 14 has zero sweep a 0 at the tip of blade 13 midchord line 14 has a forward sweep with sweep angle a t midchord skew angle u03c3 is the angle between a radius of the swept disk of fan 10 that intersects root 17 at the same point as does midchord line 14 and another radius of the swept disk that intersects tip 18 at the same point as does midchord line 14 blade spacing angle u03c6 is the angular displacement between a fan radius passing through any given point on a blade and a.