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Prop size

Old 03-26-2020, 12:34 PM
  #1  
Volstruis
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Default Prop size

I have an RC Smart evo 833 motor, what is the recommended prop size.

Rotational Speed: 833 RPM/V
Continuous Current: 55A
Max. Current: 70A
Input Voltage: 8 - 23V
Max. Efficiency: 98%
No Load Current: 1.1A
Internal Resistance: 76m ohm
Motor Dimensions (Diameter x Length): 37.1mm x 44.5mm
Shaft Diameter: 4.98mm (Front and Back)
Shaft Length: 15.1mm (Front) 14.2mm (Back)
Input Battery Types: NiCd/ Nimh/ Li-po Battery
Recommend Model: Airplane
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Old 04-07-2020, 10:47 PM
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RideTheLightning
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It will depend on the batteries you want to use. Input voltage is one of the factors that dictates prop size as a higher voltage pack will spin the propellor faster and increase the current the motor is pulling.

There's a good calculator online called Ecalc (www.ecalc.ch) you can input your power system parameters into the calculator and adjust the variables (battery, prop, esc) until the motor is operating within specifications.
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Old 04-08-2020, 04:02 PM
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Volstruis
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I have 3 cell 2200mah 11.7v batteries and would prefer to use those. If I have to I will get a 4 cell (lipo batteries))
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Old 04-08-2020, 10:34 PM
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quorneng
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Volstruis
Your motor will draw 50 A on a 2 cell if you give it a big enough prop!
The prop clearance of a specific plane will determine the maximum prop diameter and that in turn will set the no of cells (voltage) you require to achieve the required level of power.
There is no 'best prop' just for a particular motor by itself but only when any limitations or characteristics of the plane are taken into account.
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Old 04-09-2020, 01:55 AM
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RideTheLightning
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The choice of 3 or 4 cells will be driven by prop size which will be driven by aircraft weight and ground clearance. To generate equivalent thrust, you will need a larger and/or higher pitch prop on 3 cells than you'd need on 4 cells. The current will also be higher on 3 cells and you may be pushing the max rating of the motor. What's the projected weight of the model?
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Old 04-10-2020, 10:11 AM
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Volstruis
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Thank you for your replies, I hope this info is enough.
the plane is a 1.6kg (3 cell battery and motor included) high wing trainer that flew with a .40 OS max using a 10*6 prop
wingspan 160cm by 35cm
Ground clearance to shaft is 17cm
I would prefer to use the 3 cell batteries if it will work, if not I will go to 4 cell.
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Old 04-10-2020, 12:06 PM
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quorneng
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The key figure with electric flying is Watts/lb. This list shows the sort of figures required.
  • Less than 50W/lb - very lightweight / low wing loading slow flyer.
  • 50 to 80 W/lb - light powered gliders, basic park flyers and trainers, classic biplanes and vintage ('Old Timer') type planes.
  • 80 to 120 W/lb - general sport flying and basic/intermediate aerobatics. Many scale (eg warbirds) planes suit this power band.
  • 120 to 180W/lb - more serious aerobatics, pattern flying, 3D and scale EDF jets.
  • 180 to 200+W/lb - faster jets and anything that requires cloud-punching power.
On a 3s at the maximum 55A your motor is capable of will be using about 550 W. For your plane at 1,9 kg (4.2 lb) that would be 130 W/lb which would put it into the 'serious aerobatic' category.
What you need is a Watt meter to measure the Watts the motor is actually drawing. Start with an 'electric' 10*6 prop. If the Watts/lb is more than 80 it would be advisable to do a flight test to see how it behaves.
If more performance is required then fit a bigger prop (say 11*6) as long as the full power amps does not exceed the limit for the motor and/or the ESC. The ESC in particular will simply fail at any significant over load.
I hope this helps
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Old 04-11-2020, 05:06 AM
  #8  
AEAJR
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Originally Posted by RideTheLightning View Post
It will depend on the batteries you want to use. Input voltage is one of the factors that dictates prop size as a higher voltage pack will spin the propellor faster and increase the current the motor is pulling.

There's a good calculator online called Ecalc (www.ecalc.ch) you can input your power system parameters into the calculator and adjust the variables (battery, prop, esc) until the motor is operating within specifications.
I would also recommend ecalc prop calculator. You can model battery/prop/motor combinations for your plane to ensure that you have the right ESC. It also predicts the plane's performance characteristics.
https://www.ecalc.ch/motorcalc.php?ecalc&lang=en


Highly recommended.


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Old 04-11-2020, 08:56 AM
  #9  
CHELLIE
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Originally Posted by Volstruis View Post
Thank you for your replies, I hope this info is enough.
the plane is a 1.6kg (3 cell battery and motor included) high wing trainer that flew with a .40 OS max using a 10*6 prop
wingspan 160cm by 35cm
Ground clearance to shaft is 17cm
I would prefer to use the 3 cell batteries if it will work, if not I will go to 4 cell.
Use a 10 x 6 prop with a 4 cell lipo
Use a 10 x 8 prop with a 3 cell lipo

your motor is similar to this motor

https://www.motionrc.com/collections...rushless-motor

Motor Type
Outrunner
kV Rating (RPM/Volt)
830kV
Maximum Watts
680W
Continuous Current
41A
Maximum Burst Current
45A, 10 seconds
Recommended Battery (Input Voltage)
4S (14.8V)
Recommended ESC
60A
Recommended Propeller / EDF
10x6x3
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Old 04-11-2020, 09:11 AM
  #10  
CHELLIE
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When looking for a prop for a motor, look at other motors with about the same KV and Can Size, Normally there will be prop spec for motors and and voltages of similar size motors, this is a very safe way to determine a prop size for a motor, also use a Wattmeter to be on the safe side.
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Old 04-11-2020, 03:45 PM
  #11  
AEAJR
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If you are just trying to set-up one airplane and will never do this again, then having someone give you the parts is great.

If you plan to continue in the hobby and will do this multiple times then you want to learn how to do this for yourself.

Sizing power systems for electric airplanes
https://www.wattflyer.com/forums/sho...95&postcount=3

An essential tool is a Wattmeter - Here is an example
https://hobbyking.com/en_us/turnigy-...-analyzer.html

Who needs a Wattmeter?
https://www.wattflyer.com/forums/sho...02&postcount=7

Everything You Wanted To Know About Electric Powered Flight

https://www.wattflyer.com/forums/showthread.php?t=31071


@CHELLIE provided a list of parts. Go to eCalc and put them in to see what the calculator spits out. Then try changing the prop, the battery, the Kv of the motor to see what that does.

Give a man a fish and you feed him for a day.
Teach him to fish and you feed him for a lifetime.
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Old 04-11-2020, 05:43 PM
  #12  
ron_van_sommeren
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Originally Posted by Volstruis View Post
... Max. Efficiency: 98% ...
Either they have Nobel Prize material there, or they are lying through their teeth.

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Old 04-11-2020, 05:47 PM
  #13  
ron_van_sommeren
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Note that the vitesse konstant Kv (in rpm/volt) says absolutely nothing about motor max.power, max.current, max.torque, efficiency. It is not a rating, not a figure of merit, not an achievement, just a matter of changing number of winds.
Kv only depends on desired rpm and battery voltage: Kv = RPM_noload / voltage.
It does effect current and power drawn greatly though, both are proportional to Kv cubed.


And below an excellent quote about motor selection.
From
brushless motors Kv?.
Originally Posted by scirocco View Post
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. On the other hand, 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, e.g. limited prop diameter, if it's a pusher configuration, or if you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS

Prettig weekend Ron
• Without a watt-meter you are in the dark ... until something starts to glow •
e-flight calculatorswatt-metershigh power motor tips&tricksCumulus MFC
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Old 04-16-2020, 12:42 PM
  #14  
Volstruis
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Thank you all for your replies, I tried a 11*10E prop , because I had one available, we are in lockdown so cannot get recommended prop size. Pulling power feels good, maybe a little low on power , ESC, battery and motor are not getting hot. I will use an amp meter to check.
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Old 04-16-2020, 07:31 PM
  #15  
ron_van_sommeren
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To give you some idea, current drawn is proportional to pitch, #blades and diameter⁴.
So even a small change in diameter will have huge effect.
Double diameter will 2⁴=16fold current drawn.
A more realistic example: a seemingly small increase in diameter by factor 1.1 (+10%) will 1.1⁴=1.5fold current, 50% more.

Ignoring the extra voltage losses in battery, wiring, controller and motor.
Assuming the battery, controller and motor can handle the extra current without going up in smoke.
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