PROPERTIES OF RUBBER.
Different additives are used in the manufacture of rubber depending on its intended use, consequently a ‘rubber band‘ will behave differently from ‘rubber strip‘ as used by modellers as a power source in models such as aeroplanes, boats etc.
Natural rubber’s main ingredient is Latex.
In its natural form latex ages very quickly and so becomes hard and disintegrates into a powder. This is caused by air and light.
Additives are mixed with latex to make it into usable rubber.
Carbon Black is added to rubber used for car tyres. This slows down the effect of sunlight ageing the tyre and helps the rubber withstand the heat generated when in use.
The additives in the rubber strip used by modellers improve the energy efficiency of the rubber (these additives are not present in a rubber band – a rubber band when held stretched will not return to its original length whereas rubber strip will).
Energy can be stored in rubber by pulling, squashing or twisting.
The normal method used in modelling is to twist the rubber. The rubber strip is usually made up into a motor that has two or more strands in it. For small models, rubber strip with the two ends knotted together to make a loop, makes the two strands.
When the twisted loop of rubber is released, having been attached to the hook of the propeller shaft at one end and anchored at the other end, the natural tendency of the rubber to untwist will cause the propeller to revolve.
The amount of energy stored in the rubber strip depends on the loop length and the rubber strip’s cross sectional area. See Fact Sheet 2 for Performance of Rubber in models.
Lubrication of the rubber motor will extend its life and improve its performance. A drop of liquid soap rubbed on the loop will help reduce damage to the rubber when it is twisted and will ensure that when the turns are released they do so smoothly.
It is essential that lubricant is only applied to made up motors(strip tied to make a loop) as any knots made in the motor will not hold if tied after lubrication.
Storage of rubber strip is important. From the time of manufacture rubber strip starts to deteriorate and to reduce this it should be stored in an air tight container in a cool dark place.
Performance of Rubber in Models.
To get the best performance of a rubber powered model the rubber motor size must match the model. A heavier model needing more lift . To produce more lift the model must fly faster and the rubber motor must produce more power. The opposite applies to a lighter model. The amount of power that the rubber motor produces is dependent on the cross sectional area of the rubber strip and the rubber motor loop length. As rubber strip is generally a uniform thickness, the cross sectional area is quoted as ‘rubber width’. The wider the strip the more power the motor will produce however the overall weight of the model will go up and the amount of safe turns that can be applied to the motor will be less so the performance of the model will be reduced.
The other way of increasing the power output of a rubber motor is to reduce its length, this reduces the overall weight of the model but also reduces to number of safe turns that can be applied to the motor reducing its performance. Keeping the model weight low is the key to good performance. The size of the rubber motors supplied in the Sky Hi Products kits are the best guess for that model and may not be the optimum for performance.
Once a rubber thickness and length has been decided performance can be improved in a number of ways. The first way is by lubrication. A motor unwinds smoother if lubricated. Rubber Lubricant is obtainable from Sky Hi Products (C002) or you can use washing up liquid. Use only a small amount. The best way to apply it is to put the made up motor in a small plastic bag, add a drop of rubber lube. to the bag and ‘scrunch’ up the bag, distributing the lube. on to the rubber. The second way is to stretch wind the rubber motor using a geared winder (T001, T002 & T003) with someone holding the propeller or use a foot stooge (E004). The technique of winding is to attach the winder to the knotted end of the motor and stretch the motor to about 3 to 4 times its original length. Now turn the winder to put turns on to the rubber motor.
The number of turns required can only really be determined by experience. To apply 500 turns, apply 400 then whilst reducing the length of the motor to its original length. apply a further 100 turns. Fly the model to see how it performs. If more turns are required then apply the turns as before. With this method the turns lie on the motor more evenly, there is less bunching of turns, and so the motor unwinds smoothly and you are able to store more energy (turns) in the motor and so the model flies longer.
Rubber Powered Helicopter Designs.
Rubber powered helicopters MUST be made light to give good performance. They work on the principal that the propeller powered by the rubber motor provides lift. As the propeller turns the torque reaction is taken up by the silhouette of the helicopter fuselage. Without the fuselage being there the motor stick would spin round and the propeller would not turn – no thrust so the models will not fly.
On ‘You Tube’ it shows the motor stick as being a lollipop stick, this is far too short to give good flights. A short motor stick will only hold a short rubber motor which means only a small number of turns of the rubber.
Also as a lollipop stick is a lot thinner than the hole in the propeller mounting bracket so the propeller will move about reducing efficiency.
I recommend a 9” long motor stick made out of balsa.
On ‘You Tube’ there is no indication of the size of the fuselage. To get the best results the size (area) of the fuselage is important. Too small and the motor stick will still spin round a lot, reducing the performance. If the fuselage is too large in area the overall model will be too heavy and the model will not lift off the ground. Be prepared to modify the fuselage area. It is best to mount the fuselage on to the motor stick so that the area is balanced front to rear. The fuselage needs to be light but stiff, Paper stiffened with a framework of balsa wood strips or artstraws work well. The fuselage can be of any shape not just helicopter fuselage shape.
For good performance a rubber powered helicopter needs a lot of turns to be applied to the motor. Usually about 200 to 300.
To successfully launch the model point the propeller upwards. Let go of the propeller and let it turn and then let go of the motor stick and the model should lift out of your hand. There is no need to throw the model and you should never launch the model with the propeller in a horizontal position.
Have FUN flying your model.