You discover an old hollow steel propeller (in WWI, propeller blades were often sheet steel,
formed then welded into a continuous surface) at your local junkyard, which might be perfect to
install on your homemade aircraft.
After an examination, you determine it is a soft, mild steel like hot-rolled 1212. In order to
confirm it is strong enough for your application, you want to know its endurance limit.
For defining your modifying factors Ka, Kb, and Kc, assume your hot-rolled material is
subject to bending with chord length (max blade cross-sectional thickness) of 4.0” where the
highest stress occurs (hint: use this as a conservative equivalent diameter.)
For temperature, reliability, and miscellaneous factors, you apply your own factor of safety
of 4.0 (so Kd = Ke = Kf = 1.0 but you apply your own “modifying factor” of .25 to reduce the
idealized endurance limit Se' by a factor of 4.0.)