FIG. 36. Propeller blank (top). Carved propeller (bottom).
In order to make the second type the wood must first be bent into shape. It is steamed and bent along the dotted lines. It is attached to the shaft by means of a piece of sheet brass doubled over the edge and soldered to the end of a bicycle spoke. The only disadvantage of this form of propeller is that it is easily broken. It turns very easily with little expenditure of power.
FIG. 37. Langley type propeller (top). Wright type propeller (bottom).
Size of Propeller. One bad feature about most of the model aeroplanes offered for sale in toy shops is the propeller. In almost every case it is decidedly too small. In order for a model to fly really well the propeller must usually be out of all proportion to the rest of the machine. In fact its size will make the machine appear very awkward and unsightly.
FIG. 38. Quasi-helical propeller.
The enormous slip of small screw propellers when turning rapidly makes them very inefficient. The thrust of the propeller is dependent upon the volume of air sent backwards. A large propeller naturally deflects more air than a small one and so in order for the latter to equal the work of a large propeller it must either have an increased pitch or revolve more rapidly.
FIG. 39. Blanks for racing (top) and chauviere (bottom) propellers.