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Underproping a motor

Old 08-12-2016, 03:35 AM
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Mesaflyer
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Default Underproping a motor

Just curious about something. I have several electrics, powered mostly by what came out of the box, or what was recommended by the ARF manufacturer. But I also have a couple twins that I set up. They fly just fine, but one of them has motors that are too big (too much power). So, is there any harm in putting on a smaller prop, or lesser pitch prop on a motor? I started with glow, then gassers, and finally added electrics, and just love them. I just don't want to harm the motor, but I don't really see how it would hurt them. I usually just fly around at half throttle.
Thanks in advance for the info.
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Old 08-12-2016, 03:48 AM
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kyleservicetech
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Originally Posted by Mesaflyer View Post
Just curious about something. I have several electrics, powered mostly by what came out of the box, or what was recommended by the ARF manufacturer. But I also have a couple twins that I set up. They fly just fine, but one of them has motors that are too big (too much power). So, is there any harm in putting on a smaller prop, or lesser pitch prop on a motor? I started with glow, then gassers, and finally added electrics, and just love them. I just don't want to harm the motor, but I don't really see how it would hurt them. I usually just fly around at half throttle.
Thanks in advance for the info.
You can put any size prop on an electric motor, up to one that pulls the maximum allowed Ampere rating of the motor.

Here is where a Wattmeter is mandatory to prevent putting to big of a prop on the motor. Doing so can fry the motor/ESC/battery.

To small of a prop, your model may not even taxi. To large a prop, your motor may leave a smoke trail from fried windings.
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Old 08-12-2016, 09:42 AM
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Simplified a bit...

Electric motors attempt to deliver the power required to turn the load at their designed rpm.
They never make it. They come very close sometimes.
If the motor has zero load other than friction from its own bearings and air inside the motor it turns pretty close to kV * voltage applied. Motors mostly will run at that high rpm for as long as you want to waste the power. High rpm means high "counter electromotive force" and that results in low current demand.
Most of the heat produced in a motor is from current * winding resistance = watts. Low current = low heat.
As load increases it turns a lower percentage of that rpm.
Depending on the motor, they are turning appx 80% to 85% of kV * voltage when delivering full rated power.

A common 110v 60 hz AC direct drive circular saw WANTS to turn 3600 rpm. Due to the squirrel cage rotor and bearing friction, when not cutting wood it will turn appx 3500 to 3550 rpm. It can run at that RPM for days without issues. As soon as it starts cutting wood, rpm goes down, current goes up and if you try to cut hard, thick wood too fast the motor will smoke.

The normal prop is like the circular saw cutting at maximum safe rate. The motor gets warm, its not turning much over 80% of its design rpm but its fine.
A larger (pitch and/or diameter resulting in higher load) prop is like feeding the circular saw thick hard wood at excess rate. It can do it for a while... not long.
A lighter prop is like feeding the saw butter.
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Old 08-12-2016, 10:19 AM
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JetPlaneFlyer
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As the others have said, there is no harm in proping down.. it's fitting too big a prop that can cause problems.

But there is an easier way.. simply use throttle management. The plane only has 'too much power' if you open the throttle too far. An excess of power can be useful sometimes to get you out of trouble.
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Old 08-12-2016, 11:59 AM
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Mesaflyer
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Thank you very much for all the explinations, especially the reference to the circular saw. That really helps putting things in a perspective that a guy like me can relate to.
I bought a pair of motors and ESC's brand new from a club member at a very good price. I knew they would be overkill, but it didn't think it would hurt anything, as I needed the nose weight for this particular plane. It came out perfect for the CG.

I did search on here and RCG for this very topic, but could not find a definite thread about this subject. Like I said, I was pretty certain I wouldn't hurt anything, but I was just looking for some reassurance from the experts.
Thanks again, and happy flying!
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Old 08-12-2016, 12:05 PM
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Oh, and FYI, the first thing I purchased when I started with electrics is a Watt meter. I have used it to check all my set ups, as I was afraid of over proping on a couple planes. It's a very useful tool.
I'm always trying to learn more, and web sites like this one are a very good source of information.

Well, most of the time.
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Old 08-12-2016, 09:25 PM
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ron_van_sommeren
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Originally Posted by Mesaflyer View Post
... So, is there any harm in putting on a smaller prop, or lesser pitch prop on a motor? ...
Electric motors try to maintain constant rpm, IC engines try to maintain constant torque.

Motor will never exceed rpm = Kv voltage
Current will go down (current is proportional to voltage squared, and Kv cubed).
Temperature will go down
Motor will operate closer to point of maximum efficiency

Originally Posted by Mesaflyer View Post
... I'm always trying to learn more ...
Some well-structured reading and handy e-tools for rainy/windy days. Will save you, and us , a lot of questions. Notably the 'what went wrong?' kind of questions
Will also prevent you from burning up several controllers and/or motors and/or battery:
E-flight primer and tools

Prettig weekend Ron
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Old 08-15-2016, 07:26 AM
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Do a couple of group discussion tests with the motors before hand when you are swapping around props.

I had two 450 motors, one was 900kv the other was 1100kv. As I flew, the 10x5 gave me less power on the 900kv setup, which required me to use more throttle and actually cut down on flying get times compared to the 11x5 I usually ran with it. I like to way overpower my planes, and I have a heavy foot so to speak. Your mileage will vary.

When I ran up the motor watching the watt meter, it was over 15 amps at half throttle and peaked at 19 amps under full load with the 11x5. The same motor with the 10x5 used about the same at half throttle and around 17 at peak, but also provided much less thrust, so I had to throttle up higher which actually shortened my flight times, if that made any sense.
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