Now bend the tin on the two upright lines until the two sides are parallel. This support is fitted to the end of a motor base and secured by driving nails through the three holes at the base covering the wood. The end of the hook which holds the rubber strands of the motor should be passed through the opening at the end, bent over and fastened into position with a drop or two of solder. Such a support adds practically nothing to the weight of the frame, and obviously anchors the motor rigidly.
Model built by Rutledge Barry, winner of spectacular flight contest.
A model by Percy Pierce, winner of the indoor long-distance record.
The efficiency of a rubber-strand motor may be considerably increased by careful adjustment. If the strands first be wound rather loosely, as a rope is formed, and strung between the propeller hook and the motor anchorage, you will find that about thirty per cent. more rubber may be added without increasing the length and that a five to ten per cent. increase in the number of effective turns may be gained as well. By increasing the amount of rubber, you will, of course, add accordingly to the power of the motor. It is safe to say that the efficiency of your motor is increased upwards of twenty-five per cent. by this adjustment. The credit of this ingenious arrangement is due to Mr. W. Howell, Jr.
It will be well to experiment with a short-strand motor, using a single strand of rubber for the test. Let us assume that your motor is twelve inches in length, thus making a double strand twenty-four inches long. First knot this, string it between the two hooks and turn it, counting the revolutions until the first row of double knots begins to appear. Note the number of turns.
Now untie the strand and, holding one end, twist it until the lines of the edges make a continuous loose spiral throughout its entire length. The easiest way of twisting them is to lay them on a flat surface and rub with the palm of the hand.
Now bring the two ends together and let the strands twist and wriggle until they come to rest. Fasten the ends and measure the double twisted strand. You will find that it measures less than ten inches.
To prepare a strand for a twelve-inch motor, you must therefore begin with a piece of rubber fully thirty inches in length. It is clear, therefore, that the new plan enables you to gain considerably more rubber length for length. Now string your twisted rubber on the hooks of your motor so that to wind up you must turn against the twisted strand. You will find that a number of turns are required before the rubber strands are untwisted and lie parallel, which is pure gain. Count the number of turns up to the time the first line of the double knots appears, and you will find that it is about five per cent. greater than in the case of the single strands.