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Old 01-13-2010, 04:56 PM
  #43  
kyleservicetech
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Location: Wisconsin, USA
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Originally Posted by Larry3215 View Post
Where the rpm will help is when adjusting ESC's or comparing two similar setups to see which is "best".

Watts out is only part of the story.

Lets say for example you start out with a motor/battery/prop combo thats drawing 100 watts. Then you make changes to the timing or PWM rate or what ever on your esc and now your drawing 125 watts.

At first you might think - this is great! Now I have a more powerful setup.

But wait - what happened to the RPM when you made those changes? If the rpm didnt go up then all your extra 25 watts is doing is heating up your motor and reducing your flight time.

If those extra watts are not making extra RPM then they are wasted watts and in fact causing harm rather than good.

Lets say instead that the watts went down instead of up. You might think thats bad - but what if the rpm stayed the same or close to the same? That would actually be a good thing.

So, it important to know the rpm as well as the amps and volts.

The power required to spin a prop changes as the 3rd power of the change in rpm.

So to spin a prop at twice the rpm takes 2x2x2 = 8 times as much power. If you cut the rpm in half, it takes 1/8 as much power - all other details remaining unchanged.

You can use that formula to tell if the extra power your drawing is doing its job of spinning the prop or just making heat.
These are all good points! That's why increasing your batteries cell count by one cell can go from an underpowered dog, to a situation where the motor/batteries/ESC can burn up.

One thing that helps in all of this is a program such as www.motocalc.com that gives an indication on just how hard you are pushing your power equipment.

Something I've noticed on those "slow flyer" props is you don't want to run them at to high of power levels. I tried running an 8X4 "Slow Flyer prop" at 150 watts, that prop sounded like a paddle wheel at more than 70% power.
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