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What effect does RPM Have

Old 12-21-2007, 10:18 PM
  #1  
lennyshotgun
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Default What effect does RPM Have

I was looking over a site and saw the same outruner ("Just like a 2826") with 3 options as to KV value.
Wiyhout need for the formula can someone tell me what the KV value on the same motor would mean to power or battery drain or prop size...
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Old 12-22-2007, 02:16 AM
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Kosh
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The higher the KV value the smaller the prop in most cases. Look at it this way, A 3D plane needs a larger prop with low pitch speed to hover well so a lower KV motor would be used here. A jet with a pusher motor needs a smaller prop with higher pitch that spins faster for speed and the higher KV motor works well on that. I always try to match a motor to a plane by what size prop that plane should have.

Most motors have a recommended prop range for the amp limits of that motor to stay within it range. Your friend the wattmeter is really the best bet to find what works best for your setup testing prop sizes and amp draw.

I'm sure someone can explain this better than my feeble attempt here.
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Old 12-22-2007, 02:35 AM
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diverdon
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Hey Kosh , That was a Great "Lesson" for me ... The prop sise recomended for each motor can help to pich which motor to use ... Good Tip ........

I am not a total idiot but I feel stupid asking this question ... KV .. Is that spec related to watts ... I thought it was till I hesrd that it has something to do with how fast? the prop spins .. so do you have an easy way to explain KV to me ?? Duh ?? ...

thanks again for being here , Don J.
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Old 12-22-2007, 03:14 AM
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Kosh
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Hi Don,

I'm not good at this part so bear with me here. There are just so many variables in inrunner, outrunner designs such as winding wire size, number of polls, magnets used that its hard to give the answer your looking for. KV of a given motor is hard to relate to watts because of different prop that can be used on it. Say motor X is spinning a 12x6 at 200 watts, What happens when the same motor spins a 10x4.7 prop? The amp draw and watts go down but by how much only your wattmeter knows for sure.

Below is a example of the same motor wound 3 different ways made by Little Screamers. This gives you a better idea how KV ratings are used to select the proper motor to fit your plane.

Little Screamer DeNovo:
1250 Kv (rpm/volt), 128 watts. Best for Parkflyers and 3D airplanes from 10-12 oz. flying weight. Use with 3 cell lipo battery capable of sustaining 11 amps. Prop range is from 6x5.5 to 9x5. With an 8x4 direct drive prop on 3 cells (11.1V) the motor will draw about 11 amps and deliver about 19 ounces of thrust.

Little Screamer Purple Peril:
1550 Kv (rpm/volt), 165 watts. Perfect for 3D airplanes to 14 oz. and Slowflyers to 25 oz. Use with 3 cell lipo battery capable of sustaining 18 amps. Prop range is from 4.75x4.75 to 9x5. With an 8x4 direct drive prop on 3 cells (11.1V) the motor will draw about 16 amps and deliver about 22 ounces of thrust.

Little Screamer Park Jet:
2800 Kv (rpm/volt), 185 watts. High speed high performance motor for small high speed flying wings and park jets. Prop range is limited due to the High Kv rating of this motor. Special care must be taken to make sure that enough cool air gets to the motor. Use with 3 cell lipo battery capable of sustaining 20 amps. Prop range is from 4x4 to 6x5.5. With a 6x5.5 direct drive prop on 3 cells (11.1V) the motor will draw about 18.5 amps.

Not sure I helped any here but did try.
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Old 12-22-2007, 06:15 AM
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Hi Kosh , That is some more good info but I guess what I'm asking is ... Can KV be converted to watts ?? From the look at the #'s posted there I'd say No as it does not look like the KV # has any divisabality or relationship to the watts posted .. Other than the higher the KV the higher the Wattage ..

Thanks again for trying to teach me .. I don't think I'm asking the right questions . Thanks anyhow , Don J.
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Old 12-22-2007, 07:54 AM
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RickAvery
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DiverDon,
Kv is the RPM/volt that a motor will try to spin. A 1000kv motor with 10 volts put to it will try to spin 10000prm. Put a small prop on it and it will use a certain amount of energy, amps to try and spin the prop at 10000rpm. put a really big prop on it and it's still going to try to spin it at 10000rpm, but it will now use a lot more energy, amps to try and do so. since amps X volts = watts, trying to spin the larger prop will over amp your motor. Watts = heat. Too many watts = too hot the motor. Get it extremely hot and you'll burn the windings and short out the motor. A motors' watt rating is in part based on the amount of heat it can dissipate. That's why it's very important to provide sufficient cooling air to your motor. That's also why a watt meter is such a necessary tool when setting up a power system. You can see the volts, amps & watts, and see the effect a different prop size has on the system. To answer you question, the Kv of a motor is not directly converted to watts, because watts is a product of variables. Those being the voltage being put to the motor, i.e. what battery your using, and the amount of work your asking the motor to perform, i.e. how big the prop is. Hope this helps.
Rick
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Old 12-22-2007, 08:34 AM
  #7  
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Yeah, What he said. :p

Well done Rick, That gets a thanks from me.
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Old 12-22-2007, 09:50 AM
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AHHHH !!! , NOW I Get it !! That is a great explanation .. Make it a "Sticky" somewhere .. You should be a teacher .. Thanks sooooo Mucn ..Don
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Old 12-22-2007, 11:44 AM
  #9  
jooNorway
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Hope you all will allow me to add more info/confusion to the kV-numbers?

Everything explained above is correct, but I also like if people understand the relationship to the voltage in an other way too. Especially since a lot of people mostly have planes flying on 3S LiPo. If you would like to build and fly a slightly bigger plane and aim for 6S LiPo then the kV-numbers for a motor acting equal have to be a bit different

Let me take an example: You have a sportplane, 1 meter WS, 1 kilo AUW and have good performance. Then your setup propably consumpt 300 Watt. Lets assume the RPM on the propeller is 7000RPM at WOT... In this case I know the theorethical kV-number of the motor is appr 1000, because in practice the motor, within its "best efficiency" range will spin at appr 70% of the no-load kV-number... In my example I could use the AXI 2814/16 which numbers suits. This motor have a kV of 1035.

Lets assume Santa have brought a bigger sportplane to you, something around 1,8 meter WS, and you need to find a setup for this one. AUW would propably be 3,5 kilo, and you need 1000Watt to get the performance you need. But you still want the propeller RPM to be appr 7000! This is because an E-propeller with proper size will give just this performance you need. IF you used a motor with 1000 as kV-number theoretically it would spin at 14000RPM, in practice it isn`t possible because you would have to use such a small propeller it wouldn`t suit the plane at all! So; the motor have to have a kV of... YES: 500! Remember your voltage is doubled because you run on 6S LiPo If you choosed an AXI the 4120/20, with kV appr 500 could have been a choice.
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Old 12-22-2007, 03:51 PM
  #10  
lennyshotgun
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So, let me try to make this question simpler
If I want a motor for a sport plane, lets sat 25 size and I'm looking at 3 motors all same manufacturer "like the axi 2826" with 3 differant KV ratings (800,900,1100) - which would be the best choice?
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Old 12-22-2007, 04:39 PM
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RickAvery
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Lenny,
I think you would need to know as least a rough estimate of the total flying weight to determine a motor battery combination. I don't know the numbers exactly, but there's a general set of values somewhere, that set the approximate watts/lb you would need to obtain a certain flight charicteristic, i.e. lazy flying, sport flying, high speed aerobatics or 3-D. It's something like, 50watts/lb for a cub to putt around, 75watts/lb to do some aerobatics, 100watts/lb to do most aerobatics & 125watts/lb and up to do 3-D. I would think that a 25 sized plane, 40-45" wing span? probably weighing 35-40oz and running a 10-11" prop?? would probably run fine on a 3s system. This helps keep it lighter with the smaller battery. It can become a bit of a balancing act. Most motor manufactures will give you a table which outlines the approximate current draw when using a certain battery and prop. They usually tell you the max watts the motor is rated for. These are very helpful when choosing a motor/battery combo. I usually shoot for 100watts/lb & can then fudge a little one way or the other and still be assured of a spirited flyer. Again, use the watt meter to check things out. Don't want to melt that nice new motor or puff your Lipos. Good luck.
Rick
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