Gearing Ratio question regarding 540kv and 520kv motors - HeliFreak
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Old 12-22-2016, 06:07 PM   #1 (permalink)
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Default Gearing Ratio question regarding 540kv and 520kv motors

Dear All
A rather basic question for experts, but needs little dwelling to understand the use of 540kv motor in 700 class helis which is generally not recommended for following
a. Less battery friendly for mild to extreme 3D maneuvers in 700 class helis.
b. Less flight time on the cost of good torque.
My basic and novice understanding for this rigmarole which relates to Epicycloidal Gear train principles equally apply for the selection of right and suitable gearing and motor pinions in RC world. My aim of putting across this information is to understand the difference between power which is TORQUE and SPEED in a 700 class heli as like in my case Goblin 700 Black thunder. I being a mild 3D flier, had a disagreement with few of my friends on my question of installing an already held scorpion 4035-540kv LE motor in my upcoming Goblin black thunder 700 Green kit. Their plea was that motor is too heavy for this setup and besides providing an advantage of better torque, will eventually eat away the battery and would also give very less flight time as compare to any 520kv motor. They might as well be right about this principal and I also agree without doubt. But I would like to know that instead of buying a new motor in the presence of 540kv monster, I should put a bigger motor pinion and get the desired head speed, while still keeping the motor in calm limits. This way I might loose some torque and because I am a novice and mild 3D pilot, I may not even come to know the difference that I have lost power. I might be able to feel some loss of torque while attempting any maneuver like long Negatives etc. Secondly, at the end of flight, I still will have a cooler motor and longer battery life and time because the motor would not be consuming full power, yet providing a good desirable head speed. So to cut it short, kindly give your opinion if this idea supports me NOT to spend extra bucks on buying another motor which might not be necessary.
Thanks and Regards
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Old 12-23-2016, 05:44 PM   #2 (permalink)
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Join Date: Aug 2010
Location: AZ

All Kv is- is an electronic gear ratio. It allows you to alter your pack voltage without changing physical gearing, etc. But you are worried about a change from 540 to 520, which is about 3.5%. You see more than 5x that change in RPM just as a function of pack voltage reduction through a single flight (4.2 to 3.6 V). If your current setup with governor is adequate for a flight, then changing the Kv 3.5% will not be noticeable.

If the motor has different characteristics (different Rm, different efficiency, peak power etc), then of course there is potential to see a difference IF you use that power. If the motor has a significantly different mass, you WILL see a difference in performance for any given power level.

But Kv doesn’t imply those things- it says nothing about efficiency or power or mass.

Keep in mind, you lose a huge % of your system power to loss in the rotor as drag, and a significant amount to the motor as heat. If you are very concerned with power and with flight time, you can find far more gain there (optimized headpseed, better blades, better (more efficient) motor, lower mass packs/motor) than you will with optimizing Kv, when there is not optimal value given a pack voltage swing of 15% or more through a flight.

Just don’t misgear the motor- so that it is required to run at a too low an RPM at too high a power level- that is very inefficient and tends to kill motors/ESCs/packs.

"The problem with quotes found on the internet is you have no way of confirming their authenticity." - Abraham Lincoln
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