Placing both propellers on a double shaft on the same axis has the disadvantage of decreasing the efficiency of the propellers because they are operating in each other's draft.
FIG. 47. A method of arranging two propellers on the same axis.
The first of these methods is undoubtedly the best construction. It is then possible to use the same rubber skein to drive both propellers. Also any possible difference in their speed will not so readily cause the machine to change its course as if the propellers were alongside of each other. When two propellers are used in this latter position it is a very good idea to fit them with two small pulleys and a connecting belt so that any tendency for a difference in speed between the two will be immediately equalized.
The power absorbed varies directly with the volume of air acted upon and the square of the speed with which it moves away. If the pitch of the propeller or its rate of revolution were doubled, four times the power previously required would be necessary. Vice versa, decreasing the rate of revolution or the pitch by one-half will make only one-fourth the power previously required necessary.
Doubling the speed and doubling the diameter requires eight times more power. Doubling the diameter, halving the pitch and halving the speed will give twice the thrust for the same power as in the first case.
CHAPTER VII. BEARINGS, THRUST BLOCKS AND GEARS.
Since the power available for driving the model is very limited it is obvious that every precaution should be taken to enable the propeller to absorb every last fraction of energy stored in the motor. With this end in view the bearing or thrust block in which the shaft of the propeller revolves should receive careful attention in order to remove as far as possible all causes which would result in friction.
FIG. 48. Simple bearings.