The most nearly indestructible propellers are the fabric screws. They are also doubtless the lightest form. The blades will, of course, be perfectly flat, making straight pitch propellers. You will need a small cylindrical piece of wood one half an inch in diameter, and one half an inch in height, of some tough, hard wood. The blades may be made of reed or cane, or, still better, of wire. Aluminum wire being very light is probably the best for the purpose. Bend the wire into the form of a triangle two inches in width and four inches in length. Determine at what angle you wish them to be set, and bore holes in the hub and fix wires of each frame firmly in them. Cover the frames neatly with cloth and mount it in the usual way. [Illustration: An efficient model, showing excellent construction, designed by John Caresi] [Illustration: One of the best minimum plane models of 1911] THE LANGELEY BLANK. Many model builders still retain the Langeley propeller. It is a very simple one to build. To prepare a blank secure a block, as before, eight by two inches and three-fourths of an inch in depth. Connect the four corners with diagonal lines. Parallel to the longer side draw two lines, one three-fourths of an inch inside and the second one-half inch below it. Cut away the block forming a double fan-shaped piece. Some prefer a wider center and the hub may be made a trifle broader if desired. In shaping the propeller cut away from opposite sides of the blank. The original Langeley is a flat blade propeller so that the modeling is very simple. You may use your own judgement as to the thickness of the blade, although about one-eighth of an inch is suggested. The Langeley is mounted in the usual way. To heighten the pitch of your propeller secure a thicker blank. CHAPTER VII ASSEMBLING THE MOTORS In the present stage of model aeroplane building, rubber strand motors satisfy every demand. Models have been flown for more than 2,500 feet by the force of these twisted strands, and doubtless their efficiency will be still further increased. Such motive power is besides very easily obtained and applied. Careful tests have shown that more energy may be stored up in twisted rubber strands than in the same weight of springs of steel or any other metal. In gauging the strength of your motor, much depends upon whether your model is to rise from the ground or be launched from the hand. In the model tournaments in England, the flying machines are almost invariably thrown across the starting line, while in America they are required to rise unaided. It is obviously unfair, therefore, to compare the distance records of the two countries. [Illustration: A Metal Motor Anchorage]