When a propeller revolves it sets in motion a cylinder of air. If the angle of the blades is uniform throughout their length the air in the centre of the cylinder will move much more slowly than that near the outside as shown by the arrow heads in A of Fig. 27. If the blades are given an increasing pitch, the air in all parts of the cylinder will move away from the propeller at the same speed.
From a diagram like this it is very easy to calculate the angle of a blade at any point to secure a certain pitch. Suppose that the problem in hand is to design a propeller eight inches in diameter and a pitch of twelve inches. On a sheet of paper draw a vertical line AM twelve inches long to represent the pitch. Draw a long horizontal line AN of indefinite length from the lower end of AM and at right angles to it. The diameter of the propeller being eight inches, the tips of the blades must travel in one revolution 8 x 3.1416 (the circumference of an eight inch circle in inches), a distance of 33.1 inches. Lay off on AN the distance AB which is 33.1 inches, draw the line MB. The angle MB forms with AN is the proper angle for the blades at the tips. To find the angle one inch from the tips lay off the distance AC, which is. 8 - 2 x 3.1416 or 24.8 inches. MC gives the right angle. The angle two inches from the tip would be shown by MD where AD is 8 - 4 x 3.1416 or 18.8 inches. Any other points can be located in the same manner.
FIG. 27. A propeller of the truly helical type delivers a cylinder of air in which all parts move at the same speed as at A. A propeller having blades of the same angle throughout their length throws the air as in B in which the centre of the cylinder moves more slowly than the outside.
FIG. 28. Templets for testing and carving a propeller.
If desirable, a number of small templets having the proper angle may be cut out of sheet tin and fastened to a board as shown in Fig 28. When making the propeller it can be frequently laid on the templets to see if the proper angle has been secured yet.
There are a great many other ways of making propellers for model aeroplanes, the simplest and best of which are described below.
Metal Propellers have advantages and disadvantages which may be summed up only to find that as far as efficiency is concerned the advantages outweigh the disadvantages.
