In spite of the use of talcum powder, however, when a skein of rubber is twisted very tightly, the strands stick together, causing it to soon break up.
This nuisance may be somewhat alleviated if the strands are lubricated with pure redistilled glycerine free from grease, etc. Such a precaution will not always greatly lengthen the life of the rubber, but will increase the number of turns which it is possible to give the skein (and this is a very important advantage in model contests). Due to its sticky nature, however, the glycerine will cause the rubber to gather dust and particles of dirt which, if allowed to grind into the rubber, would soon weaken it. The skein should therefore be washed from time to time in warm soda and water and fresh glycerine applied. By all means, avoid all oils or substances of a greasy nature, such as lubricants. They quickly soften and rot the rubber.
The Amount of Elastic required for a model will vary considerably for propellers of the same pitch and diameter. There is always a tendency to use too much rather than too little and this fault should be carefully guarded against. In nine cases out of ten it is the cause of the unsatisfactory behavior of a model.
The motor should always be "stranded," that is, made up of a skein of bands. It is then possible to secure a larger number of turns than if a single strip were used.
Always start a new machine with a small number of strands and gradually add to the number until the proper amount of power is obtained. The distance between the propeller and the fixed hook should always be as great as possible so as to secure the maximum number of turns.
Doubling the Number of Elastic Strands increases the power of the motor but cuts down the number of turns which it is possible to give the propeller. That is to say, a certain skein composed of six strands of rubber will take perhaps two hundred and twenty-five turns while a twelve strand skein of he same sized rubber strands strands is only capable of less than half or about one hundred turns before it is wound tight.
Doubling the number of strands and at the same time keeping them the same length increases the torque more than three times but diminishes the number of turns from one-half to one-third.
Doubling the length of the strands does not materially reduce the torque for the first hundred turns. After two hundred turns have been reached, the torque is only about one-half as great as it would be in case the length were not doubled.
Doubling the length of the strands doubles the number of turns it is possible to give the skein. It is easy to see from this why it is always advisable to make the motor as long as possible and to compose it of the fewest number of strands if long flights are desirable.
By using several separate skeins geared together so as to apply their energy to one screw, it is possible to obtain a greatly increased number of turns. The weight of the gearing is very small and hardly a factor, considering the advantages derived therefrom. Since the skeins revolve in opposite directions the frame of the machine is relieved of the harmful twisting effect so often present in a single skein.