CHAPTER VII. BEARINGS, THRUST BLOCKS AND GEARS.
Since the power available for driving the model is very limited it is obvious that every precaution should be taken to enable the propeller to absorb every last fraction of energy stored in the motor. With this end in view the bearing or thrust block in which the shaft of the propeller revolves should receive careful attention in order to remove as far as possible all causes which would result in friction.
FIG. 48. Simple bearings.
The simplest form of bearing is a simple piece of sheet brass or aluminum having a hole drilled through it and bent up at right angles so that it may be lasted to the frame as shown in Fig. 48 by A.
Single bearings of this type are employed on the model aeroplanes manufactured by toy makers whose only desire is to flood the department stores and toy shops around Christmas time with their impossible machines. Such single bearings are a decidedly poor and unsatisfactory construction. Unless the elastic is very short it soon begins to vibrate in unwinding. Since the rubber is directly connected to the propeller shaft, the propeller is set into vibration as shown by the dotted lines in the second part of Fig. 48. The long blades of the propeller considerably magnify the motion and there is a very appreciable loss of power due to the erratic path of the propeller and the increased friction at the bearing. The rubber skein also offers considerably more resistance to the forward travel of the machine than if it were not in vibration.
FIG. 49. Double bearings.
The advantage of a double bearing more than offsets the added weight. Such a bearing is formed out of a piece of sheet brass bent up at right angles at both ends as in Fig. 49. The third method of construction in the accompanying figure makes it possible to employ lighter sheet metal in the construction of the bearing and still resist the pull of the rubber and the thrust of the propeller successfully.
FIG. 50. Simple thrust bearing.
Friction is reduced and the thrust taken up by placing one or two glass beads between the propeller and the bearing. Only those beads which are flat, with parallel sides and have a round hole in the centre, should be used.
Four or five copper washers or rings may be made to serve the same purpose with equally good results.
Another method is to employ two washers separated by a small spiral spring. Such an arrangement is employed on some of the French models and might be called a "friction thrust." That is, when the rubber skein is wound up tight and the propeller is released the friction acts as a brake and reduces the speed, preventing the propeller from "racing." As the elastic unwinds, the tension, and with it also, the friction becomes less so that the propeller revolves more rapidly and maintains a somewhat even speed. The importance of preserving as far as possible an even propeller speed can hardly be overestimated and that is why such emphasis has been laid in several places upon the desirability of a model whose propeller is driven by a long skein of rubber composed of the fewest possible number of strands.
FIG. 51. Ball thrust bearing.
Ball bearing thrusts are by all means the most desirable, but not all models are large enough to accommodate their size and weight. Wherever it is possible, however, to use them it should be done. The increased amount of energy available for turning the propeller will make it possible to employ less rubber and so increase the number of turns and consequently the length of the flight.
FIG. 52. Hooks.
The hooks at either end of the rubber skein are apt to cut the rubber unless some precautions are taken to prevent it. This can be done by binding with cotton thread or slipping a piece of rubber tubing or aluminum over the hook as shown in Fig. 52.