PROPELLERS
Propellers may be cut from various kinds of wood, but the most suitable, from every standpoint, is white pine. The advantage of using this wood lies in the fact that the propellers may be cut more rapidly and when cut are lighter than those made from most other kinds of wood. When coated with the proper kind of varnish they are sufficiently strong for ordinary flying. Wood selected for propellers should be free from knots, holes and other imperfections and it is very desirable that it should be of perfectly straight grain.
A piece of such clear white pine 8″ long, 1″ wide and ³⁄₄″ thick should be selected and on one side marked Top. A tracing of the propeller similar in design to [Figure 1], should be laid on this piece of wood and an imprint of the propeller design drawn on the Top side.
Diagram 1
To find the center of the block two lines should be drawn from the opposite corners, their point of meeting being approximately in the center—near enough for all practical purposes to insure greater accuracy. Similar lines should be drawn from the corners on the Bottom side of the block of wood. A hole ³⁄₃₂ of an inch in diameter should be bored through the center thus obtained, through which the propeller shaft will be inserted when the propeller is finished. The two sections of the propeller blades drawn in diagrammatical form on the Top of the block, should be marked respectively Blade 1 and Blade 2, as shown in [diagram 1]. The block is then ready for the commencement of the actual cutting. In cutting out the propeller, Blade 1 should be held in the left hand and the knife in the other, with the blade of the knife on the straight edge of Blade 1. The cutting should be carried out very carefully with attention constantly paid to [Fig. 2], and should be stopped when the line shown in [Fig. 2] has been reached. The semi-blade should then be sandpapered until a small curve is obtained by which the propeller will be enabled to grip the air.
Diagram 2
To cut Blade 2, Blade 1 should be held in the left hand and Blade 2 cut until the line shown in [Fig. 3] is reached, after which the sandpapering process is carried out in the same manner as in the case of Blade 1. During all of the foregoing operations it must be clearly borne in mind that the Top of the blank propeller must always face upward, and the cutting should always be done on the Straight lines. Should the straight edge be cut on one edge of the blank propeller and the curved edge on the other, it would result in the blades of the finished propeller having a tendency to push in opposite directions and in consequence no propulsion of the model would be possible.
Attention should next be turned to the back of the propeller blank on which the manner of cutting is exactly like that suggested for the top side, with the exception that instead of cutting along the Straight lines, the cutting is done along the Curved lines. In this part of the work great care is to be exercised for by the time the necessary cutting has been done on the back of the propeller the entire structure is very fragile and one excessive stroke of the knife may result in destroying the entire propeller blade. By constantly holding the wood to the light it is possible to determine with a reasonable degree of accuracy the evenness of thickness. To complete the Bottom side of the propeller the blade should be sandpapered as was the top.
The method of cutting the second propeller is exactly that used in cutting the first propeller, only that the diagram shown in [Fig. 4] should be used. This will result in two propellers being made that will revolve in opposite directions in order to produce even and balanced propulsion. If both propellers revolved in the same direction the effect would be to overturn the model.
Diagram 3
In [diagram 1] the propellers are shown with the straight edge as the entering or cutting edge of the blade. Some of the model builders prefer the curved edge as the cutting edge ([diagram 2]). It is significant that Mr. Frank Schober, a well known model constructor, tested both designs on his compressed air driven model, and while both propellers were the same in weight, diameter and pitch, the one having the straight edge as the cutting edge was found one-third more efficient.
When the propellers have been given a light coat of shellac they should be laid aside until the assembling of the complete model.
By following the foregoing instructions a simple and effective set of propellers will be produced. But in order to vary the experimental practice of the constructor various other diagrams, [Nos. 3] and [4], illustrating suitable designs, are provided and can be made by applying the above general theory and using the diagrams herewith.