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of the prop to maintain the engine (and prop) in the required RPM zone. (refer to Fig 47 for more about torque) This means the cage diameter will usually increase resulting in an aesthetically unappealing design for many pilots, as the trend seems to be small compact units. Ideally longish props over small coarse pitched are not a bad idea and provide the advantages of span efficiency that have governed the sail plains throughout the world for so long. Lets look at this a bit closer. Suppose we had a reduction ratio of 2.5, a propeller 115 by 30 pitch, giving us 2500RPM on the prop and 6200 on the engine. If we now took the reduction ratio to 2,55 with the same prop, we may obtain 6300 on the engine, however theoretically the prop RPM may stay constant. This is because as the torque is increased the engine and not the prop unloads it`s self, with this excess energy being absorbed by the prop. (Remember energy is never destroyed, it is always converted from one form to another). If we now increase the ratio to for example 2,7, what happens now is unloading again from the prop, resulting in a further climb in engine RPM. This is similar to driving a car up a hill in 4th gear. As one changes down (similar to increasing reduction ratios and increasing torque), we are able to retain the same car speed, however the engine RPM increases). When this happens on a paramotor prop, we obtain a concurrent increase in horse power and obviously torque. Because of the now much higher reduction ratio, the prop is not able to absorb all excess energy and thus NOW the props RPM starts to climb as well. Most props have an optimal operating range (Between 2400 -288), anything under this is out of the power band, whilst anything over this is inefficiency due to the fast blade tips. As the prop RPM climbs higher we increase the pitch, diameter, blade chord or a combination of all three. Effectively what this accomplishes is to increase the drag and maintain it within the power band. Just remember not all props are similar just because they have the same pitch. One 27 pitch prop may be completely dissimilar from another because of the chord and or diameter. When changing props make sure you are aware of all parameters. This may be easily accomplished by using a rev counter and modifying the pitch and or length until the desired engine and prop RPM is attained. (Refer to REV counters in Instruments & accessories.) |
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