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Motor/Prop Match-up

Old 02-12-2021, 07:21 AM
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captglvr
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Default Motor/Prop Match-up

Question: I have a Himax brushless 2815-1220. Rated at 200 max watts and 20 max Amps. The recommended prop is 12x6 on 2S.

Is it safe to run a 10x4.7 on 3S? What are the disadvantages?

Thanks all.
Old 02-12-2021, 06:57 PM
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ron_van_sommeren
 
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Pitch speed will be higher, thrust???, I can't tell you how that prop and rpm will fly your plane.

Prop recommendations: always highest voltage with smallest load, lowest voltage with biggest load. Most suppliers mess this table up.

For calculations you can use e.g. www.ecalc.ch (several languages). Calculators get you in the ballpark, provided the input is ok. Therefore always measure/check current and power in a new or changed setup.
Watt-meter selection and usage instructions/help.

Without an e-flight calculator:
Going from 2s to 3s current wants to change by factor (3/2)²=2.25
Going from 12 to 10inch current wants to change by factor (10/12)⁴=0.48
Going form 6 to 4.7inch current wants to change by factor (4.7/6)¹=0.73
Total change in current drawn will be the product of those three numbers: 0.80, i.e. 20% less
Power = current×voltage, therefore power drawn will change by factor 0.8×3/2 = 1.2, 20% more.


The 'full' story.
Even a seemingly small change/error in setup can result in quite an unexpected increase.
Because current and power drawn are proportional to
  • pitch
  • #blades
  • voltage² respectively voltage³
  • kv³
  • diameter⁴
So even a small change/error in setup can have huge effects.
Increase in current with one or two cells added, simple table - RCG

Expensive examples.
Going from 2s to 3s, a 50% increase, would (3/2)²=2.2 fold current, more than double, 120% extra.
A 25% increase in velocity konstant kv³ would (1.25)³=2 fold current, 100% extra.
A mere 10% increase in diameter would (11/10)⁴=1.1⁴=1.4 fold current, 40% extra.

Extreme examples.
Doubling voltage would 2²=4 fold current.
Doubling velocity konstant kv³ would 2³=8 fold current.
Doubling diameter would 2⁴=16 fold current.


Prettig weekend en wees voorzichtig, Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC

Last edited by ron_van_sommeren; 02-12-2021 at 07:15 PM.
Old 02-15-2021, 04:15 PM
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captglvr
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Ron, what a thorough answer. Thanks! Some of it is a bit over my head but I get the gist.
Old 02-16-2021, 09:12 PM
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Thank you And the clippings add-on for Firefox as well
It can be over people's head, the first time, that's why I added the examples. They are good for getting the gist

Without a watt-meter you are in the dark.
Until something starts to glow Vriendelijke groeten en wees voorzichtig, Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC

Last edited by ron_van_sommeren; 02-16-2021 at 10:59 PM.

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