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Newbies: Tips and Information Section of HF, specifically for Passing along info to newcomers to the hobby. Setup, tweaking, orientation practice, etc.


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Old 03-01-2016, 06:51 PM   #1 (permalink)
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Default KV and gearing calculations...

Ok, so I'm trying to figure out this whole KV crap and how it relates to head speed...
And I'm not sure I have a clue what I'm talking about, so I'll bounce it off you guys lol.

I'm informed that KV + Volts = RPM...

Gear is: Teeth on Main Gear / Teeth on Pinion Gear = Gear Ratio...

So, is this calculation correct..?

Blade 360 CFX
Main Gear 136 Teeth
Pinion Gear 12 Teeth
136 / 12 = 11.33 (recurring)

KV 1800 x 25.2 Volt (6 cell) = 45,360 RPM

45,360 RPM x 11.33 (gear ratio) = 4003.5

Is this right?
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Old 03-01-2016, 08:37 PM   #2 (permalink)
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KV is revolutions per volt (on the motor)

Lower teeth count on pinion, usually means lower rpm on the head and vice versa. I've messed around with pinions and motors and realized there seems to be a "magic number" somewhere between 11 and 13 T pinions.

Not familiar with a 360 heli as my smallest is a T rex 450 running at about 2500 to maybe 2800 rpm headsped. The 550 and 600 are governed at about 2200 to 2300 if I remember correctly. 4000 rpm headspeed seems very high, don't know how will the blades take it.

Perhaps others can contribute.

Hope this helps some.
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Old 03-01-2016, 09:15 PM   #3 (permalink)
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figure out the HS you want
you should know what cell count you want 3S = 11.1v, 4S = 14.8v or 6S = 22.2v
nominal cell voltage is 3.7v per cell


kv = HS x gear ratio / volts
? = 3000 x 11.3 = 33,900 motor rpm divide by pack voltage 22.2 = 1527 kv

HS = kv X V / gear ratio
?= 1800 kv x 22.2v = 39,960 divide 11.3
<3,536>
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Old 03-01-2016, 09:16 PM   #4 (permalink)
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Yes, it does seem high, but then I don't actually know what head speed I have. This is just a test of the simple math...
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Old 03-01-2016, 09:19 PM   #5 (permalink)
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Quote:
Originally Posted by Bladefx View Post
figure out the HS you want
you should know what cell count you want 3S,4S or 6S
find out your gear ratio (example 10:1)


kv = HS x gear ratio / volts
No, I'm not trying to get a specific head speed, I'm just trying to get a hang of the math.
Those are the number I currently have, I just need to know if I've worked it out right.

I'm running on a governor anyway, so I'm not saying I have a 4k headspeed.
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Old 03-01-2016, 09:47 PM   #6 (permalink)
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I could post all the formulas for any electric heli but they just get burried away.
1 more time

to find an unknown in your set up usualy you will know at 3 of the 4 items to consider using this does not include the esc.
The ESC is a different topic

the 4 items are motor kv, gear ratio, head speed, pack voltage

KV = hs x gear ratio/ pack voltage
HS = pack voltage X kv / gear ratio
PackV = kv X gear ratio / hs
gear ratio = PV X KV / hs

pack voltage and kv combine to make the "motor rpm" which is all that we're really concerned about

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Old 03-01-2016, 09:51 PM   #7 (permalink)
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So my math is correct then?
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Old 03-01-2016, 09:56 PM   #8 (permalink)
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yes
except maybe 25.2v VS 22.2v

but this is based on what I was told to use nominal voltage 3.7
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Old 03-01-2016, 10:43 PM   #9 (permalink)
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Oh?
So you're supposed to do the calculations on what is very nearly a storage charge? Not full charge?
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Old 03-13-2016, 07:10 PM   #10 (permalink)
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Default Head Speed Calculations

Head speed is calculated by:
HS = (V) x (Kv) x (PT) / (MGT)

Nominal battery voltage x Motor Kv x Pinion Tooth Count , all divided by the number of teeth on the main gear.

Battery nominal voltage=11.2v for a 3 cell lipo

So for a TRex 450 heli using a 3 cell lipo and a 3950Kv motor:

11.2 x 3950(Kv) x 12 (teeth)= 530880

divide that by 150 tooth main gear = 3539 RPM

Since you never get 100% efficiency, I use 87% of the calculated HS or

0.87 x 3539 = 3080 RPM.

This usually works out pretty close to actual.
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Old 03-13-2016, 10:12 PM   #11 (permalink)
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So...

Main Gear 136 Teeth
Pinion Gear 12 Teeth
136 / 12 = 11.33 (recurring)

KV 1800 x 22.4 Volt (6 cell) = 40,320 RPM

40,320 RPM / 11.33 (gear ratio) = 3558.7

3558.7 x 87% = 3096 (and doing the math your way round gives me 3095)

That works better... My Castle Talon lists (as default) the highest of my three governed RPMs as being 3009...
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Old 03-13-2016, 10:40 PM   #12 (permalink)
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Default More Arithmatic ("Math" makes it sound hard)

BTW, the calculated HS is assuming you're at full throttle when you tach the heli.

So in your case:

3559 x what% = 3009?

3009/3559 = .85 x 100% = 85% Pretty durn close!!

BTW, you don't have to calculate the gear ratio separately if you use the formula above
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Old 03-14-2016, 02:19 AM   #13 (permalink)
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Yeah, I know... I'm not very good at math so I've sometimes gotta do things a little differently lol.
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