Chapter 9

 

Chapter 9

Rubber Motors: Quantity, arrangement, turns.

We have considered this subject to a certain extent under the chapter dealing with gearboxes, so it is only proposed to give a general description of the sizes and amount of rubber most suitable to our scale model.

The amount of power we can store in a motor is directly proportional to its weight, that is to say, we can expect to get twice as much power from two ounces of rubber as we should from only one ounce. Hence it would appear that the more rubber we can store in a model the more power we shall be able to have.

But remembering our redistribution of weight and wing loading, it will readily be seen that there must be a limit to the weight of rubber we can carry, as we cannot move our wing about to obtain trim. In determining the amount required it is as well to start on the low side, always remembering our trim, and add a loop of rubber at a time until we have attained a steady level flight.

Rubber is made in different sizes and thicknesses, but probably the size most useful is 1/4 in. wide by 1/30 in. thick, and it is suggested as a guide with a 10 in. propeller on a 1 in. to 1 ft. scale, and a weight of about 6 oz., that, without a gearbox, six loops of the above rubber would make a good beginning, and with a gearbox of two gears, that three loops could be employed on each. It may be found that four loops on each will be better, but this must all be tried out on the flying field. When trying out, do at least find the softest stuff for the model to come down on, because we must remember that as we vary our motor so we must slightly alter our trim accordingly.

The length of our motor, too, is important, and, naturally, the longer we make it the more turns we can put on, and so a practice has arisen of using a motor actually longer than the distance between the rear hook and the hook on the propeller shaft. When a few turns have been put on it is quite taut, but the trouble starts when the motor is run out in the air, and, being loose inside the fuselage, it may flop about, and even bunch into knots, perhaps at the tail end, thus upsetting our weight and causing the model to stall and come down in a succession of dives. To overcome this, various methods of tension devices are used, such as explained in the chapter on construction dealing with nose-blocks.

Where, however, a gearbox is used, an extra loop of rubber can be put on to one particular skein and that skein given a few turns in the opposite direction before attaching to the motor hook.

This opposite winding is called "prewinding" and when the motor comes to rest it will attain a balance of so many prewound turns on one skein and so many unwound turns on the other skein, with a consequent even tautness on both and the slack of the motor taken up.

It must always be borne in mind, however, that the fewer strands we use the more turns we can put on, but, naturally, with fewer turns the power output will be less.

By a suitable arrangement of strands with a gearbox we can get a variable power output with the same weight of rubber and the same number of total strands and no deviation from trim.

As an example, the writer's Heston Phoenix, of 1 in. to 1 ft., with a wing area of 196 square inches, a total wing loading of 8 1/4 oz., and a gearbox with four gears, the following combinations were used: 1/4 in. x 1/20 in. rubber 27 in. long, driving a 10 in. dia. airscrew with 13 in. pitch was arranged in the form of four gears with three loops of rubber on each, i.e. 12 loops in all, and in still air this gave a duration of 25 sec. power and 5 sec. idling at the end, on 600 turns.

This was suitable for a quiet day, but on a roughish day, when more power was required, the motor was rearranged, using four loops on only three of the gears, thus still using the original 12 loops but differently arranged.

This combination on 600 turns gave a power output of 20 sec. and 5 sec. idling.

It will be noticed that although the same power is available in both cases, yet in the latter case the power runs out sooner, and so is greater during the effective time of motor run.

Six hundred turns were not the maximum turns available, but gives a good indication of what to expect from a motor.

You will notice the expression "idling" that is to say, when the motor is nearly out, although the airscrew continues to revolve, there is not sufficient power for sustained flight, and therefore these turns are practically useless.

The safe number of turns we can put on a motor is a variable factor, depending upon the freshness of the rubber, the way it is treated and lubrication used, but assuming everything to be in tip-top order we can take as a safe average the following for 1/30 in. thick rubber.

Table data.Table data.

Taking this table, a motor of four strands 3/16 in. wide could be wound 42 turns per inch of unstretched motor. Therefore if the motor was 20 inches long already made up in its six strands we should expect to be able to wind on 42 turns multiplied by 20 inches long, equals 840 turns.

Also, with a gearbox, we can take one individual skein or motor, and the number of turns we can put on this is the total for the whole gearbox.

Unfortunately this table does not cover all thicknesses and sizes of rubber. Then again, different makes of rubber sometimes vary slightly in size, so if in doubt, and we have the energy, there is a formula which can be used for working out the number of turns for any size of rubber.

It looks a bit awful at first, but we can sort it out fairly easily. The formula, then,

for the nurriber of turns that will just break the rubber is :

Formula 1.Formula 1.

We do not want to break the rubber though, and if we want the rubber to last a long time without breakages it is not safe to give more than 80 per cent of the breaking turns. At the same time, the last few of the possible turns increase the torque enormously, and if we do without them we shall have a much smoother power run. The formula, then, for best results all round is

Formula 2.Formula 2.

Now let us see how to apply this to the rubber. First we must measure its length. The length we want is that of the folded skein. We fold the rubber into the number of strands required for the motor, lay it out on the table and find its length. Then we want its weight, and we want to find the square-root of the weight in ounces. Now that is not very difficult, is it? The next part, L ^1.5, is practically as easy, since it is the square-root of L cubed. In other words, we find the square-root of L and multiply it by itself three times. L must be in inches.

The lubrication of rubber is very important, and the entire motor should be well covered, and any excess wiped off, great care being taken to see that no foreign particle is on the rubber, otherwise on winding this particle may cut into the rubber and break it.

The rubber should be kept in a cool, dark, air-tight container between use, and direct sunlight should be avoided as much as possible.

When tying the ends of the rubber together to complete a skein, it is not necessary to tie them into a knot, but get someone to overlap both ends and stretch them tight while you bind them together with a fine thread - say, six or eight turns will do - this will stand up to all the power you are likely to put on it.

One more point. When winding, stretch the motor as far as you can without putting any undue strain on the fuselage, and wind in this condition. A much more evenly wound motor is the result, with a corresponding more even running of the motor and a better flight.

 
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