The accompanying illustrations ([see page 43]) show the method of making bentwood and screws. [Fig. 51] is a view of a finished pair of propellers. To the left of this illustration is given the method of setting out the blank in terms of pitch and diameter relations. The maximum blade width should be located one-third of the radius from the screw tip, and should be about one-eighth the diameter. This latter, in turn, should be two-thirds of the pitch. Inversely, therefore, the pitch should be one and a half timed the diameter. With twin-screw machines this may be extended to twice the diameter, or even more, but should never exceed three times the diameter.
[Fig. 52] is a view of the Camm type of bentwood screw, which has a high thrust to power ratio. Birch should be used for bentwood screws, as this bends easily and yet has a tenacity which is lacking in other woods. Ash or hickory may be used as an alternative, but neither of these is as satisfactory as birch. Before bending, the blanks should be filled with gold size to keep the blade as rigid as possible, and prevent it from going back or flattening out after bending.
[Fig. 53] shows the Twining type of screw, which has long, narrow tapering blades and fine pitch. Under test this has given extremely satisfactory results, and can be recommended.
[Fig. 54] shows the method of attaching spindles to bentwood screws, a strap of tin being wrapped round the blank centre to which the shaft is soldered. Care should be taken to ensure that the shaft is quite central sectionally and diametrically.
A method of securing carved screw shafts is shown by [Fig. 55], and is self-explanatory. When the elastic skein is in tension it has a tendency to pull the hooks out straight, so releasing the skein, with sometimes painful consequences to the hand. The safety hook shown by [Fig. 56] has a brass-tube collar which slides over the end. All the hooks should be covered with valve tubing, to prevent the elastic cutting through.
[Fig. 57] gives the proportions of the Camm bentwood blank, and will require no explanation beyond the fact that it is bent along the dotted lines.
CHAPTER VI
Practical Construction:
Planes
There is little difference of opinion regarding the construction of the planes of a model aeroplane, and the methods of making can be classified under three headings—cane, wood, and wire.
There are advocates for each form of construction, and it is difficult to state definitely which is the best practice, each having equally good results. The wire plane, especially when steel wire of the music or piano variety is used, is much stronger, offers less resistance to the air, and has a neater appearance than the others, but it is slightly the heavier. A wooden plane can also be made extremely neat and light, although it is a little weak. Birch is the best wood to use for this purpose, as it is extremely tough and not too heavy. Where cane is used for the frame, pinning and gluing is out of the question, hence binding and gluing must be resorted to. A plane so made is very strong and flexible, and will withstand a great amount of rough usage. It is, however, not neat in appearance and hardly to be recommended, although many prizes have been won by models possessing such planes.
Yet another form which can be considered good practice consists of a combination of umbrella ribbing and piano wire. This gives a very rigid and almost unbreakable plane, but its weight for small machines is prohibitive. It should chiefly be used for power-driven machines, power in this instance meaning any form of motive power other than elastic.