Reduction
Why electric ppgs dont mount a reduction gear?
All gas engines have one.
Is there any tecnical reason?
I think that a smaller motor with reduction would consume less, make battery last longer and the total cost would be cheaper.
Maybe the cons would be the cost and weight of the reduction gear, and maintainance of the belt and the bearings…
Any thaughts?
As far as I know, many early electric paramotors had high kV motors, combined with 12-14S-ish batteries and belt reductions.
My guess is that reductions, either by belt or gears, lead to mechanical losses (maybe around 2% on geared ones). They add as well more parts (ie more weight and more things to buy and break). ICE have reductions because their power curves work best at high RPM, and because prop we use need a lot less of RPMs. In a way electric ppg with direct drives use some sort of reductions, coming from the winding of their motors (the kV numbers).
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Electric motors can be made to have high torque and low rpm. The M50 motor from the SP140 has only 34kv(34rpm/volt) that means it doesn’t run over 3400rpm at 100v(fully charged battery) so it can turn huge props to about 1800-2200rpm thus high eficiency. Ther’s no need in reduction there which is awsome!
Gas motors range from zero to 8.000-12.000rpm so in order to use high diameter high eficiency props they need reduction to boost the torque and lower the rpm.
The fact that an electric motor can be made to have a specific top rpm to the aplication needs, is one of the most important pros they have over the gas motors.
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A direct drive electric motor that can deliver the long-term power that an EPGP needs weighs at least 3.5 kilograms to actually function for a long time under all conditions. An electric motor that is supported by a belt transmission has a weight of at least 2.5 kilograms. In addition, the reduction weighs at least 1-1.5 kilograms. A version like this has been built around more then 20 times in various early versions around the world over the last 15 years. There is no advantage to it. The only disadvantage is that everything is more sensitive and a shrill noise is produced at full power. If you use a version with reduction, you often have a cooling problem when using the motor in cruise flight because the fan speed in the motor is too low.
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Great info – TY
Gas engines are cooled by the prop, and some also have fans.
I agree about the extra weight with reduction gear, but maybe it would be compensated using a smaller motor and a smaller ESC and with a smaller battery.
So I think that eppg manufacturers have chosen direct drive mostly due to easier build and less maintainance.
Many EPG manufacturers have learned from their misjudgments or from the poor results of other pioneers in the field of e-flight. If you use a gearbox, you need the same ESC size as well as the same batteries. The gearbox system is perhaps 2% more efficient at full throttle than a direct drive motor. But the gearbox completely negates the advantage… If EPGs worked with gearboxes, they would also be available commercially. The few such as U Turn, Skywalk etc. have determined through parallel tests with direct drives which system is more ideal. Maintenance or construction is not relevant here, as a gearbox with a belt is simple and requires no effort. In RC in the 5 to 10 kilogram class, acrobatics or glider propulsion area where only short power outputs are required, a gearbox is of course superior to a direct drive due to higher peaks at lower weight. But it is not scalable and comparable with manned projects.
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