Quite a number of devices have been tried, but the rubber string is by far the most efficient power yet discovered. Rubber has a great deal more power than an equal weight of steel in all ways that steel has been tried. The power of the rubber motor is dependent on the unwinding of the strands of rubber after having been wound up. The longer this unwinding may be delayed the further the little air craft may be propelled, providing there is force enough expended at any time to give the necessary momentum to the propeller. A long motor has more revolutions than a short one. Hence some advocate a long spined frame to the model aeroplane. Others prefer to cut the frame a little shorter and give great pitch to the propeller which demands more energy at a given time, hence heavier strands, or more strands of the smaller rubber string. With the greater pitch propellers, the model is propelled faster and so may cover as great or greater distance than one with a motor that gives more revolutions in unwinding, but it is possible to revolve so fast as not to propel at all. Many use the 1/16" square rubber string, others the 1/8" while many use ribbon rubber, say 1/32" to 1/16" thick by 3/16", 1/4", and 5/16" wide. For small models, rubber bands can be looped together.