Turnigy SK3 3542 1250 kv on 4s with 10x6 prop? - RC Groups
Thread Tools
Jul 24, 2017, 08:21 AM
Registered User
Question

Turnigy SK3 3542 1250 kv on 4s with 10x6 prop?


I am upgrading my Volantex Ranger EX to a better power system. Is a Turnigy SK3 3542 1250 kv motor under too much stress with a 10x6 prop on 4S? This motor seems to be common among people who have replaced their Ranger's stock motor. If so, are there better motor options out there?

Motor: https://hobbyking.com/en_us/turnigy-...r-motor-1.html
Sign up now
to remove ads between posts
Jul 24, 2017, 10:07 AM
Registered User
Fourdan's Avatar
Quote:
Originally Posted by uninc4life2010
I am upgrading my Volantex Ranger EX to a better power system. Is a Turnigy SK3 3542 1250 kv motor under too much stress with a 10x6 prop on 4S? This motor seems to be common among people who have replaced their Ranger's stock motor. If so, are there better motor options out there?

Motor: https://hobbyking.com/en_us/turnigy-...r-motor-1.html
Hi
Turnigy SK3 3542 1250 kv on 4S LiPo and APC 10x6E is too much
It is a Stator 2820 weight 167g
You can try Dualsky Calc v3.26 with the XM3542EA-4 that is similar in Kv - weight
Louis
Jul 24, 2017, 10:45 AM
Registered User
Quote:
Originally Posted by Fourdan
Hi
Turnigy SK3 3542 1250 kv on 4S LiPo and APC 10x6E is too much
It is a Stator 2820 weight 167g
You can try Dualsky Calc v3.26 with the XM3542EA-4 that is similar in Kv - weight
Louis
Just to clarify what you are trying to say, are you saying that the motor is too heavy, or that it is too high in KV?
Jul 24, 2017, 11:21 AM
Registered User
Fourdan's Avatar
Quote:
Originally Posted by uninc4life2010
Just to clarify what you are trying to say, are you saying that the motor is too heavy, or that it is too high in KV?
Too much load (power) for that motor
Go down prop or voltage or both
Louis
Jul 24, 2017, 12:44 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by uninc4life2010
Just to clarify what you are trying to say, are you saying that the motor is too heavy, or that it is too high in Kv?
Kv = 1250rpm/volt, 4volt/cell, would give no load rpm 20krpm high, giving an 14krpm loaded (70% rule of thumb). I.e. if motor could handle it, which it can't as Louis stated above.

Vriendelijke groeten Ron
Jul 24, 2017, 05:12 PM
Registered User
Quote:
Originally Posted by Ron van Sommeren
Kv = 1250rpm/volt, 4volt/cell, would give no load rpm 20krpm high, giving an 14krpm loaded (70% rule of thumb). I.e. if motor could handle it, which it can't as Louis stated above.

Vriendelijke groeten Ron
I'm sorry, I'm having a bit of trouble understanding the calculation you are using. Are you multiplying 1250 rpm/volt by 16 volts to get 20,000 RPM's? That part I understand, but I'm not sure how you are taking that figure and determining that it is too much for the motor to handle. I believe you, but I'm just not seeing how you are coming to that conclusion.

Would this motor (1050 KV, 50A max) be more appropriate if I spun a 10x6 or possibly 9x5 prop?



https://hobbyking.com/en_us/turnigy-...ner-motor.html
Jul 24, 2017, 06:17 PM
Registered User
scirocco's Avatar
The combination of voltage, prop size and Kv results in maximum current and power that is excessive for that motor. Note that the max power spec of 755W is very unrealistic for a 141g motor - 450-500W is more reasonable.

ECalc prediction on 4S 10x6 is 68A /900W - way beyond spec and reality. If you use 3S you can expect about 44A/450W, which is a realistic max power for the 3542-1250.

The 174g SK3-3548-1050 you linked should be OK with 10x5 on 4S; expect about 44A/600W max, which should be OK given the Ranger will be at part throttle most of the time. If you go this way, use a minimum 60A ESC

As a reality check, I run the slightly larger OS 3820-1200 EF-1 pylon racing motor on 4S with an 8x8 prop, which pulls about 700W max which is OK for short periods but is pretty much maxing out the motor. 10x6 is a higher load on a motor than 8x8; so 10x6 on 4S is definitely too much for the 3542
Jul 24, 2017, 06:29 PM
Registered User
Quote:
Originally Posted by scirocco
The combination of voltage, prop size and Kv results in maximum current and power that is excessive for that motor. Note that the max power spec of 755W is very unrealistic for a 141g motor - 450-500W is more reasonable.

ECalc prediction on 4S 10x6 is 68A /900W - way beyond spec and reality. If you use 3S you can expect about 44A/450W, which is a realistic max power for the 3542-1250.

The 174g SK3-3548-1050 you linked should be OK with 10x5 on 4S; expect about 44A/600W max, which should be OK given the Ranger will be at part throttle most of the time. If you go this way, use a minimum 60A ESC

As a reality check, I run the slightly larger OS 3820-1200 EF-1 pylon racing motor on 4S with an 8x8 prop, which pulls about 700W max which is OK for short periods but is pretty much maxing out the motor. 10x6 is a higher load on a motor than 8x8; so 10x6 on 4S is definitely too much for the 3542
I have an 85 amp ESC which I plan on using. I also purchased a watt meter so I can be sure. I also have a 9x4.5 prop incase the 10x6 is too much. The smaller diameter and less aggressive pitch should put me within a safe limit
Jul 24, 2017, 07:36 PM
Registered User
Fourdan's Avatar
Quote:
Originally Posted by uninc4life2010
I have an 85 amp ESC which I plan on using. I also purchased a watt meter so I can be sure. I also have a 9x4.5 prop incase the 10x6 is too much. The smaller diameter and less aggressive pitch should put me within a safe limit
Hi
Your reasoning is completely out
You have to start from the plane (AUW, Wing load, flight mission and style)
Then you select the prop and the rpm
Only after that, you can select the motor (Kv, weight) and the voltage (battery 3 or 4S or any) to provide the rpm and the power
At the end, the battery capacity (Ah) is giving your flight duration
Calculators like Scorpion Calc (wizard) or Dualsky Calc or eCalc are very useful to go on the good way (+ or - 10%)
Of course a wattmeter could be the final judge
Louis
Jul 24, 2017, 08:23 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by uninc4life2010
I'm sorry, I'm having a bit of trouble understanding the calculation you are using. Are you multiplying 1250 rpm/volt by 16 volts to get 20,000 RPM's? That part I understand, but I'm not sure how you are taking that figure and determining that it is too much for the motor to handle. ...
Motors can handle that kind of rpm, but it's far from optimal. Slower, and thus bigger, turning props are more efficient.

Quote:
Originally Posted by scirocco
Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, eg limited prop diameter if it's a pusher, or you already have a bunch of 3S packs and don't want to buy more, and so on.

While an absolutely critical part of the system ...
Kv is actually the item one should choose last.
  1. Decide your peak power requirement based on the weight of the model and how you want to fly it.
  2. Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
  3. Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate.
  4. Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.
  5. Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as Ecalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.
The reason I suggest picking Kv last is that prop choices have bounds - the diameter that will physically fit and the minimum size that can absorb the power you want. OTOH, combinations of voltage and Kv are much less constrained - at least before you purchase the components.

So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, eg limited prop diameter if it's a pusher, or you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS
Vriendelijke groeten Ron
Last edited by Ron van Sommeren; Aug 15, 2017 at 02:38 PM.
Jul 29, 2017, 02:49 AM
Registered User
Quote:
Originally Posted by uninc4life2010
I am upgrading my Volantex Ranger EX to a better power system. Is a Turnigy SK3 3542 1250 kv motor under too much stress with a 10x6 prop on 4S? This motor seems to be common among people who have replaced their Ranger's stock motor. If so, are there better motor options out there?

Motor: https://hobbyking.com/en_us/turnigy-...r-motor-1.html
3542 is a great motor. I run a 10x5 on 3s comfortably (4s 10x5 gets hot) or I run a 9x6 when using 4s.
Jul 29, 2017, 03:04 AM
Registered User
Quote:
Originally Posted by timeme
3542 is a great motor. I run a 10x5 on 3s comfortably (4s 10x5 gets hot) or I run a 9x6 when using 4s.
I ended up getting the 3548 1050 kv. I plan on running an 11x8 prop on 4S, so I think that the lower KV should be fine. I ordered a power meter just to be safe. If the 11x8 ends up drawing too much current, I can use the 10x6 or the 9x4.5 I have on hand .

https://hobbyking.com/en_us/turnigy-...ner-motor.html
Jul 29, 2017, 05:45 AM
Registered User
scirocco's Avatar
Quote:
Originally Posted by uninc4life2010
I ended up getting the 3548 1050 kv. I plan on running an 11x8 prop on 4S, so I think that the lower KV should be fine. I ordered a power meter just to be safe. If the 11x8 ends up drawing too much current, I can use the 10x6 or the 9x4.5 I have on hand .

https://hobbyking.com/en_us/turnigy-...ner-motor.html
I'm curious as to your rationale for choosing an 11x8 to start with.

11x8 on 4S at 1050 rpm/V is going to draw over 75A/1000W - way excessive for a 174g motor. The 10x5 I suggested previously will provide plenty of power while not over loading the motor, so if you have a 10x6 on hand why not start with that?
Jul 29, 2017, 06:25 AM
Registered User
Only 1000W? That's really disappointing. I may need to use a 12x9 or possibly 13x9. I didn't realize that my motor would only be putting out 1.3 horsepower with the 11x8 prop.
Last edited by uninc4life2010; Jul 29, 2017 at 06:43 AM.
Jul 29, 2017, 11:24 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by scirocco
I'm curious as to your rationale for choosing an 11x8 to start with.
... .... way excessive for a 174g motor. ...
Quote:
Originally Posted by uninc4life2010
... I may need to use a 12x9 or possibly 13x9. ...
Both motorcurrent and power are proportional to pitch¹ and proportional to diameter⁴.
Going from 8" to 9" pitch will give increase by factor 9/8 = 1.1
Going from 11" to 12" or even 13" diameter will give increase by another factor (12/11)⁴ = 1.4, respectively (13/11)⁴ = 1.9.

Vriendelijke groeten Ron


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Help with motor timing? Aerodrive sk3 3542 tware Power Systems 3 Mar 19, 2017 04:24 PM
Discussion Turnigy SK3-3542-1250kv Outrunner sako7mm Electric Motor Design and Construction 7 Oct 17, 2016 04:42 PM
Help! Turnigy Aerodrive SK3 - 3536-1050kv 12X6 prop on 4s? will it work? Edge540Pilot Power Systems 11 Jan 06, 2016 01:28 AM
Discussion Turnigy SK3 3542/Motor for a 90mph T-28 dragonshade Power Systems 2 Apr 24, 2012 03:15 PM
Question Turnigy SK 3542-1250 Magnet Question Tommy D Power Systems 4 Jun 11, 2010 04:14 PM