Motor/Prop Match-up
#1

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.
Is it safe to run a 10x4.7 on 3S? What are the disadvantages?
Thanks all.
#2

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
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 calculators • watt-meters • diy motor tips&tricks • Cumulus MFC •
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
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

• Without a watt-meter you're in the dark ... until something starts to glow •
• E-flight calculators • watt-meters • diy motor tips&tricks • Cumulus MFC •
Last edited by ron_van_sommeren; 02-12-2021 at 07:15 PM.
#4

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

en wees voorzichtig, Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
• E-flight calculators • watt-meters • diy motor tips&tricks • Cumulus MFC •


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


- To trust is good
- To calculate is better
Will get you to say within 10%, IF input ok.
e-flight calculators & propdata (compilation) - To measure is a must
Without a watt-meter you are in the dark. Until something starts to glow
A watt-meter will more than pay for itself, several times over, your battery, ESC and motor will love you for it. After calculating, always check current when you have a new/changed setup. Will also help you find optimal setup. And it's a great tool for debugging your power train. - Keep battery-/watt-/multi-meter wires short
too long wires batteryside will kill ESC over time: precautions, solutions & workarounds - Hyperion Emeter II, with optical and electrical tach, servo tester, local&remote logging - RCG
A very long clearance sale, originally +200$, now literally peanuts

• Without a watt-meter you're in the dark ... until something starts to glow •
• E-flight calculators • watt-meters • diy motor tips&tricks • Cumulus MFC •
Last edited by ron_van_sommeren; 02-16-2021 at 10:59 PM.