The gears should be of steel accurately cut and of no larger diameter than is necessary to separate the rubber skeins the requisite distances so that they will not rub.
Holes may be bored in the gears to lighten them. The gears are easily and conveniently cut out of steel pinion wire.
CHAPTER VI. SCREW PROPELLERS.
We might compare a propeller to an ordinary screw or bolt by likening the thread of the screw to the two blades of the propeller. If the screw penetrates wood or metal nut it will advance a certain distance known as the pitch which is always the same, namely, the distance separating two consecutive turns of the threads. The revolving blades of the propeller cut their way through the air in identically the same manner. But since air is a very thin medium as compared to wood or iron the propeller slips a little just like a screw going into an unsteady nut and does not advance the distance it theoretically should considering the angle of the blades. The distance lost in each revolution is called the slip. Thus a screw having a ten-foot pitch in actual operation perhaps only advances the aeroplane eight feet.
FIG. 25.
If a propeller blade had a uniform angle throughout its entire length the portions of the blade near the centre would not have as great a pitch as the extreme tips because the diameter of the circle they travel in one revolution is not as great as that at the tips. For this reason it is usual to give the blades an increasing angle as they approach the centre.
FIG. 26. Method of laying out a screw propeller, that is, determining the angle of the blades at different points.
Fig 26 shows a diagram illustrating the theoretical pitch of a screw, the angle of the blade varying inversely as its radial distance from the centre of the screw.