Flat plate aerodynamic profiles which have constant thicknesses were intensively used at the beginning of aviation, especially on canards and stabilizers. Today, the constant thickness is usually a necessity borne of the manufacturing process, because the blades are made out of steel plates, in most of the cases. These profiles are used on some domestic ventilation and air conditioning fans and as wing profiles for balsa wood rubber band model airplanes. A more efficient spin off of the flat plate profile, the curved plate, of constant thickness and curvature, has found wide application in fans for industrial use, automotive cooling systems, air conditioning and in carbon fiber propellers and rotors of Unmanned Air Vehicles, UAVs, especially electric mini-quadrotors.
However, there is enough evidence that combining the curved plate profile with flat plate profile does bring better results than the current days use of constant curvature profiles very intensively.
This paper shows that, for constant thickness blades, it pays off having a curved profile at the leading edge and further to mid-chord, but ending it with a diminishing curvature profile, until the trailing edge region, where the flat plate is more advisable than the curved profile.