PROPELLER OUTLINE.—While the marks thus given show the angles, and are designed to indicate the two faces of the blades, there is still another important element to be considered, and that is the final outline of the blades.
Fig. 82. Outlining.
It is obvious that the outline may be varied so that the entire width at 1, Fig. 82, may be used, or it may have an outline, as represented by the line 2, in this figure, so that the widest part will be at or near the dotted line 3, say two-thirds of the distance from the center of the blade.
This is the practice with most of the manufacturers at the present time, and some of them claim that this form produces the best results.
FOR HIGHER SPEEDS.—Fig. 83 shows a propeller cut from a blank, 4" x 6" in cross section, not laminated.
Fig. 83. Cut from a 4" x 6" Single Blank.
It should be borne in mind that for high speeds the blades must be narrow. A propeller seven feet in diameter with a six foot pitch, turning 950 revolutions per minute, will produce a pull of 350 pounds, if properly made.
Such a propeller can be readily handled by a forty horse power motor, such as are specially constructed for flying machine purposes.
INCREASING PROPELLER EFFICIENCY.—Some experiments have been made lately, which, it is claimed, largely increase the efficiency of propellers. The improvement is directed to the outline shape of the blade.
The typical propeller, such as we have illustrated, is one with the wide part of the blade at the extremity. The new type, as suggested, reverses this, and makes the wide part of the blade near the hub, so that it gradually tapers down to a narrow tip.