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Direct Drive RPM Calculation

Old 08-01-2005, 05:01 PM
  #1  
jlindqui
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Default Direct Drive RPM Calculation

I'm looking to build a Speed 400 pylon racer for all out top speed and am looking for some ideas for a motor prop. I don't have much knowledge about brushless technology, so any recommendations are appreciated. So far, I've been reading about the Hacker B20-12L which is rated to turn ~4500 RPM per volt. If I were to run it at 11.1v, that's ~50k RPM which sounds way to good to be true, so I'm assuming that this is a no-load RPM.

First Question: If I were to stick a 5x5 prop on this, is there any way to calculate the loaded RPM? I'm hoping to get a prop speed around 35k-40k RPM range.

Next Question: Will I run into any heat issues? The hacker motor is 2cm in diameter and the fuselage has a ~3cm opening with 2 nacelles for cooling. Do they make any heat syncs that would fit in this application?

Last Question: What type of battery would you recommend? Li-po would obviously be lighter which is important on such a small plane, but would I run into any heat issues with this? Also, what is the minimum capacity that I would need for a decent (5-10) minute flight with a several (~10) full throttle runs?

Thanks?
Old 08-03-2005, 07:38 AM
  #2  
Matt Kirsch
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Default RE: Direct Drive RPM Calculation

It's unloaded RPM, no prop.

'calc programs like Pcalc at www.flydma.com, or Motocalc at www.motocalc.com, are useful tools for getting an idea of how a power system will behave under load. I ran the numbers on Pcalc, and there's no way that the motor will handle a 5x5 prop direct drive:

44 Amps [X(]

Prop RPM is also only about 26000, but pitch speed is 123MPH. Still, the motor will burn up in a few seconds with 44 Amps flowing through it. Heat won't be an issue because the windings will burn out long before the motor has a chance to get hot...
Old 08-04-2005, 09:51 PM
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jlindqui
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Default RE: Direct Drive RPM Calculation

Good Info. Good tools. I thought it sounded too good to be true. I'll have to play around with some different motor specs and see if there's any chance of spinning a decent pitch prop at 30k+ rpm.

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