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Thread: How to Size an Electric motor for your airplane

  1. #1

    How to Size an Electric motor for your airplane

    Title says it all. Here is some info on doing just that.
    I'm working to catalog my Exceed motors and put them into a table for some prop/battery combos.
    Until then, I wrote this up so you can better understand the relationship of Battery/Prop/Motor for your plane.

    http://www.mycoolrc.com/SizingMotor.html

    This is a work in progress so expect more pictures and links to come.
    If you have a specific motor selection issue, you should start a thread on it in this section so people can answer your specific questions.

  2. #2
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    Re: How to Size an Electric motor for your airplane

    Nice job!

  3. #3
    Super Moderator dfarmercse's Avatar
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    Re: How to Size an Electric motor for your airplane

    Good Job Ditch,

    I have a corsair with a 11x3x3 prop. I want to go to a 2 bladed prop. What size could I use?

    Dave F
    Farmer's Flying Service

  4. #4

    Re: How to Size an Electric motor for your airplane

    what corsair? Any idea the Kv the motor and what battery/esc do you have?

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    Super Moderator dfarmercse's Avatar
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    Re: How to Size an Electric motor for your airplane

    Quote Originally Posted by dditch66
    what corsair? Any idea the Kv the motor and what battery/esc do you have?
    Art-tech, motor 4038 700kv, 20 amp esc. on the motor it says prop 9-11.

    I hand launch and belly land my planes, but not a good idea with a 3 bladed prop.

    Dave F
    Farmer's Flying Service

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    Re: How to Size an Electric motor for your airplane

    Quote Originally Posted by dfarmercse
    Good Job Ditch,

    I have a corsair with a 11x3x3 prop. I want to go to a 2 bladed prop. What size could I use?

    Dave F
    The formula for prop load is diameter cubed times pitch times square root (#blades -1). That's d*d*d*p*sqrt(b-1) = pl in algebraic form. Your prop yields about 5650. Any prop that produces at or about that number will draw about the same power. So, if you want to stay with an 11 inch prop, that would be an 11x4 and 10 inch prop would come out to 10x5.5; both would pull a bit less power than your current prop.

    BTW, if that prop isn't pulling the recommended power from your motor, you can determine which one will with that formula, too, as long as you don't change the number of cells in your battery. Just take the desired power, in watts, divide it by the current power, and multiply the prop load by the result to get the desired prop load, or DW/CW*pl = Pl. Reverse the above equation to get the new prop dimensions.

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    Super Moderator dfarmercse's Avatar
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    Re: How to Size an Electric motor for your airplane

    Thanks for the info.

    Dave F
    Farmer's Flying Service

  8. #8

    Re: How to Size an Electric motor for your airplane

    I have one 3 bladed prop in my P-47 but don't use one anywhere else. What I've been told for 2vs. 3 blade agrees with what Wintersol says about using an 11x4.
    Just know that larger diameter means low end thrust and vertical climb. Moving to a higher pitch and lower diameter but keeping the same wattage as per his formula yeilds more speed and less vertical climb. If you look at that adamone calculator I link to, it will show top speed, but that also is a function of the wind resistance of your plane. There is SOO much to play with that it gets overwhelming at times. Me, If I'm going 2:1 ratio of diameter to pitch, I'm expecting my speed not to be that great. Some MFG will give a high thrust/low speed prop so planes take off better and are more efficient at lower speeds.

  9. #9
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    Re: How to Size an Electric motor for your airplane

    Yeah, sometimes prop suggestions are really too high; if that motor can really turn a 9X11 two blade prop at 20 amps, it should pull an 11x4 three blade. From my limited experience, that 9X11 would load the motor above 20 amps with a battery that can provide more. That's why I use a wattmeter.

  10. #10
    Super Moderator dfarmercse's Avatar
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    Re: How to Size an Electric motor for your airplane

    Great info, I think I will go with a 11x4.

    Dave F
    Farmer's Flying Service

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