A BASIC GUIDE TO SUCCESSFUL |
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| chapter three | |
Direct Drive vs Geared |
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| Consider that
gearing changes the size of the propeller not the size of
the motor, (thanks to Bob Kopski for putting it that way).
It does not matter to the motor if it is driving a small
diameter propeller at high revs. or a gearbox at high
revs. which happens to be driving in turn a large
propeller at low revs. More important to the success of the particular model is the matching of propeller to model. As a generalisation larger, slower flying models perform best with large, slow turning propellers (geared). Whether it be a powered glider, scale light plane or vintage model (pre 1950 model design). A small fast aerobatic or pylon racer type of model will perform best with a small, fast turning propeller (direct drive). The thrust from a large, slow propeller and a small, fast propeller could be the same but the speed of the airflow produced would be vastly different. Gearboxes. All low cost electric flight motors turn at very high revolutions per minute. Using a small propeller allows the motor to approach it's best operating speed. This is not a problem in a small fast flying model but in a powered glider there is a loss of efficiency. Due to drag the lightweight glider simply cannot travel through the air as quickly as the motor and propeller combination would like it to. Enter the gearbox, to provide a simple solution. Allowing the motor to spin quickly and efficiently yet driving a larger, more efficient propeller more slowly. The nett result is that more of the 'input energy' is converted into 'output energy' i.e. thrust. Once available it was found that the gearbox was not only suitable for slow flying glider types but could provide a more powerful power unit for almost all types of model except for very fast aerobatic or pylon racers. This extra 'output energy' can be used in a number of ways. For instance a powered glider can have a longer motor run from the same battery. An aerobatic model could have more thrust to gain a steeper climb angle or to fly larger loops. A cabin type sports or scale model could be a little larger or heavier and still fly with the same motor and battery. The reasons behind these improvements can be found by looking at the AMPS being consumed and the WATTS being produced, about which more later. There are very many gearboxes available for both the SPEED 400 and SPEED 600 size of motors, ranging from the cheap to the very expensive. Different gearboxes have different gear ratios and some are available with a choice of interchangeable ratios. Generally the higher the ratio, the larger the propeller the same motor can turn without exceeding the motor's current limit (AMPS). Ratios above 3:1 (3 rotations of the motor shaft to one rotation of the propeller) are most suited to powered glider use, but have been successfully used in faster models, below 3:1 can still be used in gliders of course and are particularly useful in "Vintage" models, trainers, scale, etc. Most gearboxes have the output shaft out of line with the motor shaft, this needs to be allowed for in the model design and can prove to be a difficulty when substituting one gearbox for another. In-line gearboxes such as the Graupner SPEEDGEAR series have the output and input shafts in line. Belt drives provide a further alternative, the most commonly available unit being the "Olympus" unit made by MFA for their own motor or the SPEED 600 size, the ratio is approximately 2:1. |
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