[uberjay]

It's kind of like a nutball with squared off wings and a hole in the middle

If you have an RPM meter / tach you could attach the motor shaft to a drill and measure the voltage out between two lead wires. When you hit 1 V output the RPM you read on your tach is you kv value. Give or take losses, etc. (Or so I've read - haven't actually done this myself ).

Cheers,
J

[OLD PRAIRIE RED NECK]

Originally Posted by uberjay

If you have an RPM meter / tach you could attach the motor shaft to a drill and measure the voltage out between two lead wires. When you hit 1 V output the RPM you read on your tach is you kv value. Give or take losses, etc. (Or so I've read - haven't actually done this myself ).

Cheers,
J

Interesting!

However the kv rating of the motor only tells me what rpm to expect out of a given number of cells. Not really very useful information in sizing a motor or choosing a prop.(yes it helps choose the correct pitch but you are still in the dark about diameter) The most important info in my opinion is the maximum watts a motor is rated for. That tells me how big the plane can be and also when I am approaching the maximum prop load.

Kv rating info only would be like buying a glow or gas engine based solely on the engine's max RPM rating.

" I have a 12,000 rpm engine for sale".

... Ok, so... what size is it?

Does that make sense, am I seeing this correctly?

[uberjay]

Oh, of course... totally agree. kv is no 'magic' number; however, it actually _does_ allow one to get an idea of prop size for a given input voltage (ie. cell count).

I wasn't trying to provide an exhaustive motor white paper .... just trying to add one way of getting at least one of the motor parameters. Wattage rating in itself is actually not a 100% conclusive metric either. Of course.

The watts / lb rule is obviously in play, but there's also maximum prop RPMs which reputable prop manufacturers provide (eg. APC, etc.). The rule of thumb I've always heard is _roughly_ a max RPM on a prop is ~ 120000.

So, if your kv turns out to be, say 1500 then:

- 3S (max V = 4.2 x 3 = 12.6V) x 1500 kv = 18900 rpm
-- take the max RPM / this number
-- 120000 / (V x kv) = 120000 / (18900) = 6.35 inches

So on a 3S pack I would shoot for a 6 inch prop'ish... typical pitch being 3inch so a 6x3 or 6030.

- 2S (max V = 4.2 x 2 = 8.4V) x 1500 kv = 12600
-- 120000 / 12600 = 9.52 inches
So on a 2S pack I would shoot for at most a 9 inch prop... most likely I'd land on an 8 inch, but only because I have a ton of 8x4.3's sitting here .

----
Now, if the motor kv turns out to be different here's the difference:

1300 kv -> 3S (120000 / (12.6x1300)) = 7.3" | 2S ~= 11"
1700 kv -> 3S = 5.6" | 2S ~= 8.4"

So, even given the really 'rough' rule of thumb, the kv does matter in prop size.

There are obviously other factors, Watts as you mentioned, winding resistance, etc. etc., but without knowing the kv you could be under or over using the motor.

Sorry... I'm rambling now. Gotta go tuck in the kiddos .

Cheers,
J

[uberjay]

I forgot to add that those are 'maximums' within which to pick a combo... there's also motor/prop efficiency metrics that help you know how long you'll get on a battery, etc.

One other thing I forgot to add in is this site here:

http://www.---------------------/RCc...r.php?a=4#Prop

It is basic info, but if you take the "Prop Noise" value with a grain of salt it can also steer you to a decent prop size to avoid over propping.

Hope this all makes sense and is helpful to the discussion?

Cheers,
J

[OLD PRAIRIE RED NECK]

Ok. I'm fairly new to the electric stuff.

Let me see how this works out for another motor that I have been flying for a couple of years.

Its an 870kv motor on 4 cells so...

4.2 x 4 = 16.8v 16.8v x 870kv = 14616

120000/14616 = 8.21

So according to that an 8 1/4" diameter prop is the right size.

I am running an APC 11 x 7 and the manufacturer recommends 11 x 8 to 14 x 7.

What did I do wrong here?

[uberjay]

Hey Mervin,

Does the manufacturer recommend 4S on the motor?

Often I'll see a list of specs like this on mfg sites:

- 2 - 4S lipo
- 8x4.3, 9x5, 11x7 props

They list the cells and the props in numerical order, but it does not mean you should be putting an 11x7 on for a 4S setup. Ideally they should list the prop with the cell count - something like this:

- 2S -> 11x7
- 3S -> 9x5
- 4S -> 8x4.3

Oh, and those numbers I just picked randomly to show the pattern. They aren't meant to be actual .

If you plug your motor and prop numbers into that link I posted (http://www.---------------------/RCc...lator.php#Prop) you'll see that the noise rating is "Med/High Noise Output". Also you'll see that the prop's "tip" speed is 659.3 km/h. As the tip goes faster and faster it can become unstable and vibrate.... mileage of different props will vary. As the prop gets closer to the 'threshold' it becomes dangerous and could fail catastrophically. This is why manufacturers rate their props for certain RPMs... for safety sake.

So, yes, one can actually get away with running larger props faster and enjoy tons more oomph, but it depends on the props and the motor's windings' ability to handle the heat.

Also, like I mentioned before, my numbers are maximums / extremes that I use to give me a starting point when looking at combos. I know that other site I've quoted for doing the calcs uses some other value for the cell voltage (you'll notice the RPM value lower than ours).

In your case, I'm guessing that flying style (not always on full throttle), airframe (good airflow), and motor structure (good windings, etc.) are what are allowing you to fly that prop.

I feel like I'm digging a hole here, but I hope what I'm typing makes sense? I'm no motor guru and am just thinking out loud with what I've learned so far. Always happy to entertain correction where I'm wrong .

Cheers,
J

[dr. crashalot]

What I do is figure out the max watt rating for a motor and use that together with the kV rating to determine prop size. Jays rule about max prop rpm is a good start but that 120 number is somehow flexible. But if you start there you can then use a watt meter and see where your watts and amps are at. You want to max out your watts without going over the motor max efficiency rating and without exceeding your ESC/battery amp limit.

[uberjay]

Yup... what Sal said.

I tend to go on at the mouth sometimes and cloud the issue or muddify the output .

The 120 thing is just a rule of thumb, starting point.

Cheers,
J

[OLD PRAIRIE RED NECK]

Yes Jason, the motor is rated for 3 or 4 cells. So the 14 x7 would be with 3 cells and the 11 x 8 with 4 cells.

Max watts is 600 and with the 11 x 7 I'm running it peaks at 450 watts on fresh batteries. I have never seen the motor anything but just slightly warm.

Here is another motor I'm running:

950kv on 3 cells. 4.2 x 3 = 12.6 volts x 950kv = 11970

120000/11970 = 10.02"

run the numbers for the same motor on 2 cells comes to 15.03"

The manufacturer recommends 10 x 5 on 3 cells and 12 x 6 on 2 cells. I am running a 10 x 7 on 3 cells and the motor temp is warm but not so warm that you can't keep your fingers on it.

Max watts here is 296 and I measure 275 watts on a fresh pack.

So the math is close on the 3 cell pack but doesn't work for 2 cells.

I'm not sure this is a dependable formula.

[uberjay]

Yeah, I don't think it's meant to be an accurate "here's your prop" magic formula, but just a guideline or somewhere to start. Other factors need to be brought into the mix like wattage and the effect of pitch (greater pitch on the same diameter will draw more current, etc.). Also, some motors have higher quality and/or different windings which can affect wattage ratings.

Lastly, I would hope that most manufacturers' recommended props would be for the most efficient combination where the watts/gram (or similar) of thrust was ideal.

There are a couple calculators online/downloadable that are great for doing more accurate calculations and/or predictions:

eCalc: http://www.ecalc.ch/motorcalc.php?neumotors&lang=en
(Tutorials: http://www.rcgroups.com/forums/showthread.php?t=2101447)

Scorpion eCalc: http://www.scorpionsystem.com/files/..._Calc_v358.zip

Cheers,
J