FIG. 24.
Skids of course weigh something and offer a certain amount of resistance, but the advantages more than outweigh those drawbacks.
Skids are usually made of piano wire, split bamboo or rattan. The skids should not be made any larger than is necessary to protect the machine. They do not usually take any special shape but are formed to fit each individual case.
CHAPTER V. MOTIVE POWER.
By far the simplest and most efficient form of power which could be installed in a model aeroplane to drive the propeller is a twisted skein of rubber. Nothing is lighter, or more easily handled and repaired.
The word elastic, in physics, is the name given to the tendency which a body exerts, when distorted, to return to its original shape. Rubber possesses more elasticity than any other material known, it being possible to stretch a piece of rubber cord to eight or nine times its original length without fracture. Rubber also possesses the added requisite of lightness and will store up more energy than any form of steel spring.
The Simplest Form of Motor is a single skein of elastic stretched between two hooks, one fixed and the other to which the propeller is attached, free to rotate. In some cases it is a decided advantage to divide the motor into one or more parts. One phase of this question has already been considered. The others will be discussed in the following chapter.
The type of Elastic which gives at once the longest life and the greatest power is the square rubber, preferably about 3/32 x 3/32 inches, and not the flat strip. When examined under the microscope the edges of the square rubber show to be cleaner and sharper and not so ragged as those of the flat strips. To be of any value for use in a model aeroplane, the rubber should be absolutely pure and fresh.
There are certain precautions which if observed will add greatly to the power and efficiency of a rubber band motor.
Always remove the elastic from the machine when the flights are over for the time being. Rubber spoils very quickly when kept under tension. It also deteriorates if warm, so keep in a cool place. Strong sunlight causes rubber to harden and lose its elasticity, due to the presence of the sulphur used in vulcanizing. If talcum powder or finely powdered soapstone is rubbed on the bands from time to time it will prevent them from sticking together. The strands will then run and slip more easily upon each other, making it possible to store up a greater number of propeller revolutions.