=Motors.= Quite a number of devices have been tried, but the rubber string is by far the most efficient power yet discovered. Rubber has a great deal more power than an equal weight of steel in all ways that steel has been tried. The power of the rubber motor is dependent on the unwinding of the strands of rubber after having been wound up. The longer this unwinding may be delayed the further the little air craft may be propelled, providing there is force enough expended at any time to give the necessary momentum to the propeller. A long motor has more revolutions than a short one. Hence some advocate a long spined frame to the model aeroplane. Others prefer to cut the frame a little shorter and give great pitch to the propeller which demands more energy at a given time, hence heavier strands, or more strands of the smaller rubber string. With the greater pitch propellers, the model is propelled faster and so may cover as great or greater distance than one with a motor that gives more revolutions in unwinding, but it is possible to revolve so fast as not to propel at all. Many use the 1/16” square rubber string, others the ⅛” while many use ribbon rubber, say 1/32” to 1/16” thick by 3/16”. ¼” and 5/16” wide. For small models, rubber bands can be looped together. =Gears.= Small, light weight gears can be made or bought. They are attached to propeller shafts and are geared back different pitches. Some one to two, others one to three, while some gears are one to one. It might seem that one to one is added friction and no gain in winding, but a hook is attached to each gear wheel which allows two rubber motors instead of one, and allows longer unwinding. If the rubber strands are divided they wind up many more turns than when combined in one bunch. If eight strands are twisted together you cannot turn as many times as with four strands. Fig. 260 shows a one to one gear and its connections, and Fig. 261 a gear with a greater ratio. A gear of one to one might be placed at the opposite end of the framework from the propeller, thus extending the number of revolutions in that manner, the second rope or motor extending back and below the first, Fig. 262. [Illustration: FIG. 262.] =Winding Devices.= It is tiresome to wind up the rubber motors by hand, so mechanical winding devices have been made. A drill with a hook in the place of a drill-bit is quite satisfactory. It is best to have a ring on the rubber motor where it is attached to the anchoring hook. This ring can be hooked on the hook of the drill. This winds but one at a time, so they should be changed about in the winding process, first on one and then on the other back and forth, until tight enough. A very good winder can be made of a revolving egg-beater. The egg-beating part is cut off, leaving two shafts instead of one. If there is a hook on each, both motors can be wound at once and as they should be wound in opposite directions, the device works all right. CHAPTER XVII.