Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/344

§ 217 become visible by their empty cores, and may be seen as interlacing helices following in the track of the extremity of each blade, like adherent strings of sea-weed.

The vortices from the external extremities of the blades are all of the same "hand" and consequently tend to wind round one another; they may be conceived to break up into spiral groups and perhaps sub-groups, as they are left behind in the propeller race, after the manner indicated already in the case of the aerofoil (Fig. 86).

§ 218. On the Design of an Aerial Propeller.—A few simple rules may be formulated for the design of an aerial propeller; these rules will be applicable mutatis mutandis to the marine propeller.

(1) From the conditions, assess the probable value of $$\gamma$$ (usually about 10 degrees), and (Fig. 136) plot the efficiency curve from the equation (§ 204). Any arbitrary scale may be employed.

(2) Decide on "discard point"; that is, the minimum percentage of maximum available efficiency, and so determine blade length.

(3) Draw the thrust grading curve, $$b\ b\ b$$ (Fig. 136), as in § 213 (Fig. 133). At this point the designer has to exercise his judgment; it is perhaps best to draw a trial curve freehand, the object being a smooth curve beginning and ending at zero, but in general character to simulate the truncated wedge form based on the Newtonian theory; then let fall perpendiculars from the conjugate points of equal efficiency, and draw radial lines through the origin to suit the freehand curve as nearly as possible; then correct the freehand curve to pass through the intersections.

(4) From the thrust grading curve $$b\ b\ b$$ (Fig. 136) derive the load grading curve $$c\ c\ c ;$$ the ordinates being calculated by multiplying the thrust ordinates by the corresponding values of sec $$(\theta + \gamma)$$ (Fig. 136).