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Motors - Prop Size - Why is it Important? Do NOT "OverProp"

Question: My brushless motor came with a label attached to the prop shaft or firewall that strongly warned me about the maximum propeller size that I should use with the motor and recommended a particular prop. The READ ME FIRST sheet that came with the motor also made reference to this. Why does the prop size matter?

Answer: Electric motors are very different than glow or gas engines. An electric motor will attempt to overcome any load. Rather than just stop, it will work very very hard to overcome any load and can draw massive amounts of current  from the battery, through the speed control and into motor while doing so. In the case of a totally locked motor for example, the current will become very high and we include a fuse in the line that will blow to protect the system.

This characteristic of electric motors is both good and bad. The good part is that an electric motor can produce a great deal of torque (power) from the first revolution whereas an internal combustion engine must get spooled up to a higher RPM level before it really begins to produce power. This is one of the main reasons combustion powered automobiles need a transmission... without a transmission we'd never be able to get our cars rolling in the first place.

The down side of this ability of electric motors to automatically attempt to overcome any load by drawing more current from the power source is that the motor will pull more and more current until the load is overcome or something blows. Sometimes the speed control will detect this and shut down, while at other times when the current really goes through the roof, a fuse in the line will pop. However, if the current draw is high but just below the level at which the ESC will shut down the motor will run and attempt to overcome the load. Massive amounts of current flow into the windings of the motor and this produces heat... lots of heat. In time this heat will melt the insulation on the windings and it's all down hill from there. Once the insulation begins to melt some of the windings will short out, more current flows, more heat, smoke appears and the end comes quick! In some cases this can happen in seconds... in other cases it can take 10 minutes or so.

OK... so what the heck does all this have to do with propeller size? Well, as far as the motor is concerned, the prop is the load on the motor. A prop with a big diameter induces more load than a small diameter prop. More pitch causes more of a load than low pitch. Bigger diameter props coupled with more pitch is double trouble if you exceed the propeller specs for the motor.

So, when we release a Power Module and/or Power Pack for one of our models, we have carefully matched the propeller, speed control, motor and battery to give you the best performance and best durability. If you change the prop to a larger prop or a prop with more pitch, you are changing the whole system and the part that suffers is the motor and sometimes the speed control. If you "over prop" a motor it will burn up quick. Sometimes in as little as 10 seconds. The worse scenario is over propping and then doing a run up of a cowled installation on the ground or test bench. This maximizes the load on the motor because the model is not moving and the prop cannot "unload". Static run ups also do not produce as much airflow into and out of the cowl to cool the motor and ESC. Static run ups on the ground or bench can gang up on the motor very quickly... they maximize the load on the motor while restricting the cooling air flow... ugly stuff!

Bottom line... pay strict attention to the maximum prop size specifications and use the prop we recommend. If someone tells you to do otherwise, make sure they buy you a second motor first because you are going to need it within anywhere from 10 seconds to 10 minutes later! You also might pick up a spare ESC while you are at it!

Article ID: 4821