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Model Motors is VERY specific about their recommendations for a given motor and # of cells and the prop diamter and pitch. Sometimes they only recommend one diameter and pitch. How closely should one stick to this? If there some reason with the lack of gearing that allows you to damage the motor if you use too big a prop, etc. Normally, I play around with an engine in Motocalc and simply make sure I am not drawing too many amps.

I'm new to outrunners. Is there something else I need to consider besides total current draw or can I play with the best prop pitch combination for what I want to do so long as I stay within the current limitations?

Matt Kirsch

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You hit the nail right on the head. There aren't any "propeller police" lurking in the bushes to pounce on you and drag you off to a Siberian prison if you stray from the recommended combos As long as you stay within the motor's specifications, you can tweak the prop diameter and pitch. Exceeding the motor's specifcations generally results in damage to the motor.

Generally, you can extrapolate how a prop that isn't listed will behave from the props that are listed with a conceptual knowledge of how props scale. You won't be able to calculate exact Amps in your head, but you can tell if the next size bigger prop is a go or no-go. Remember that pitch scales linearly (2x pitch = 2x the Amps), while the prop diameter scales in a "square" (area) fashion (2x the diameter = 4x the Amps)

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Matt,

I always appreciate your feedback.

So it sounds like, as in any other electric motor, as long as you stay within the current limitiations of the battery, motor, and speed control you can use whatever prop combination makes sense for the model you are flying and the type of flying you are trying to do.

I have motocalc and a Medusa Power Analyser and have found so far that Motocalc is in the ballpark and then I verify with the Power Analyser. Motocalc tells me I should be able to use a larger prop than Model Motors was recommending, but somewhere I thought I remember seeing that there was some additional consideration for using smaller props. But, I guess that doesn't make sense because really with an electric motor current delivered equates to a specific torque.

Greg Covey

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Think of the Model Motor specifications like the traffic signs in Boston; they are merely suggestive.

In between the specs, you can add proper cooling, throttle management, and some common sense mixed with your own skill level. Unlike a glow engine, a brushless motor has a wide dynamic range of operation for supply and load. The final fit often depends upon the application or type of plane design.