A piece of wire is passed through the hole previously drilled, and the heavier blade carefully glasspapered down (with No. 00 glasspaper to finish) until the screw poises in a horizontal plane. [Fig. 47] shows the sort of brush to use for polishing, and [Fig. 48] the finished screw.
For models that require a good finish an excellent form of construction (incidentally it may be remarked that full-size screws are made in this way) is that shown by [Fig. 49], the laminated type. These laminated screws are exceedingly strong, as the grain, by virtue of the splayed blanks, follows the blade. Screws carved from the solid block are a trifle weak near the boss owing to cross grain. The laminæ could be alternate layers of whitewood and mahogany, which give a pleasing finish to the screw. A is an end view of a carved screw.
Fig. 49.—Laminated Air-screw
The method of obtaining the pitch angles at various points along a screw-blade is shown diagrammatically in [Fig. 50]. It will be obvious that the pitch of a screw should be constant along the whole length of blade, so that the air is deflected or driven back at a constant velocity. An efficient screw will deliver a solid cylinder of air, whereas an inefficient one delivers a tube of air.
If, for instance, the pitch at the propeller tip is 30 in., whilst at, say, 3 in. from the centre it is only 25 in., obviously the tip of the screw will be imparting a higher velocity to the air than the portion approaching the boss, and thus this latter would be acting as a drag upon the other portion.
Fig. 50.—Setting Out Pitch Angles
The method is to lay off a distance, equal to the pitch, to some convenient scale, and to erect another line vertically and to the same scale equivalent to the circumference of the disc swept by the propeller, which may be called the peripheral line. Subdividing this line into a convenient number of equidistant parts (three or four are sufficient for screws up to 14 in. in diameter), and connecting up the points so obtained to the right-hand end of the base line, gives the pitch angles at the corresponding points of the blade. It is the subtended angles which are required, as indicated by the arrows.
Templates should be cut to these angles (which, of course, are the angles made with the axis) with which to check the angles along the blade during construction. This checking is more necessary with bentwood screws than with carved ones.