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Best KV for a prop size chart

rcspaceflight

creator of virtual planes
#1
I've only been in this hobby for a little over a year and I'll admit that I don't know everything. I may pretend, but I don't.

A fairly common noob question is about KV. I know as a general rule, the lower the KV, the bigger prop the motor can handle.

But I was wondering, is there a general rule chart that shows more specifically how much KV you should have with any given prop size? At least prop diameter?

It probably depends on motor size, but as far as I can tell a 2000-2200KV motor is good for 6x3 or 5x5 prop. 1300-1500KV is good for a 8x4 prop. And 1000-1300KV is good for a 9x5 prop. But is that only for "park fly" sized motors?

I'm considering doing the research to make such a chart. But I don't want to waste my time if one already exists. I did a quick search and didn't really find anything. It does sound like motor size matters, but I think that motor size is more about how many cells the motor can handle and may not have much to do with prop size.



So I'm thinking a chart that lists specific prop sizes with the recommended KV, number of cells, and then approximately the motor size.

To research this, I plan on looking at a ton of specs for different motors and use that as the raw data.



I think it would be incredibly useful, not only to me.
 

Craftydan

Hostage Taker of Quads
Staff member
Moderator
Mentor
#3
RCSF,

I don't think one exists, but even if you find it impossible to build something simple and meaningful, the exercise to understand the relationships will be worth while! Press on!!!

I'll add in one wrinkle (assuming you're not aware, apologies if you are). Props tend to have a maximum RPM, and better OEMs will spec it for their props (typically in RPM*in for a product line). This would make a upper limit to a KV*voltage -- no matter how much power you throw at it, a longer prop will fail. Adding blades both helps and hurts this (just one more relationship).

Quick search turned up APC and MAS's prop specs . . . but most of the other "off the top of my head" OEM's don't spec it:

http://www.apcprop.com/v/html/rpm_limits.html

http://masterairscrew.com/images/selecting_a_prop.pdf

Keep chatting about what you discover, I'm not the only one out here who are curious about what you might discover ;)
 

rcspaceflight

creator of virtual planes
#4
I was unaware of props having a max RPM. I remember some one saying something about that, but it hasn't crossed my mind. I'll probably add that to the chart.

I think such a chart would be very useful because you are supposed to start with the prop and go from there. I could probably even throw some static thrust calculations into the chart.

Lately when people have been asking about motors I've been trying to recommend a good prop size and KV, but it was only based on what I think I know. It would be nice to be able to more effectively answer people's question about what motor to choose. It is a daunting task to choose a motor.


I guess I have some work to do. Currently the plan is to simply go through all of the motors on Hobby King's website and hopefully other websites to get information on motors. Basically I'm going to look at the motor size / amp draw, the KV of the motor, and the recommended prop size. I'm going to assume that all of the info on websites are correct and after I collect a bunch of raw data, I'll figure out what's the most common sense and make a chart. But the chart is basically going to be an estimate and a guide line, not something that is supposed to be 100% right. All motors are a little different.

Well, when I have time to collect the raw data.
 
#5
RCSF,
The information exists. Go to "Experimental Airlines" and check out Ed's work. He has tested 4 or 5 of the motors he regularly uses, on the bench and provided the results on his web site
The second place to look is at RCPowers.com. To access the information you will need to sign up, log into the forum, go to "Master Parts list" and look for "Motor testing" by e3Scott. This is the most comprehensive list of motors, props, ESC timing options, actual thrust numbers that I have come across on the web. Scott is a retired air force tech so I thing his testing methods and results are accurate. Have a look, let us know what you think. WJ.
 
#6
Lord knows there are a gazillion different options out there for bushless power but I would dare say every RC guy has a top ten or in my case a top five. Point is out of those tons of options there are a handful of widely available, appropriately priced reliable motors that are popular across the board. For starters the 2826 2200kv is a huge one....the DT-750 another. If you, I would focus on these few popular options, perhaps start with a list of 10 commonly used motors and delve into the specificities of each. If and when possible I would pull real test data from forums rather than rely exclusively on vendor websites as they tend to over-rate numbers (badly). Search Dr. Kiwi and take a gander at his Fly Brushless website and prepare yourself to be amazed. Perhaps for each motor you could do something like a "general, speed and efficiency" category with different props and S count. Food for thought.
 

pgerts

Old age member
Mentor
#7
As said before - KV is not a good factor - unless
You have a specified battery voltage and a specified motor power.

A 9*6 prop might be all to big for a 150 KV motor - if you have a 12 cell LiPo.

I will always start with the plane.
What diameter size propeller fits the plane with gear without looking to stupid (scale look).
Next - how much power do i want for the plane (weight and speed).
Last - find the right KV to fit your propeller and power requirements.
Get that motor....
 

rcspaceflight

creator of virtual planes
#8
The second place to look is at RCPowers.com.
I couldn't find the thread that you're referring too.

If and when possible I would pull real test data from forums rather than rely exclusively on vendor websites.
Great idea. I am just looking for estimates, but real life data would be great to create estimates off of.

---

Although I was also considering simply using a static thrust calculator, typing in the most popular prop sizes, and making a chart based on that. All I'm looking for is a guide line. A starting point for selecting a motor for beginners. I know some thrust calculators include amp draw and horse power. I could use the amp draw and horse power to determine a motor size. And then make the chart simply park fly sized motors. You know, if an 8x4 prop with a 2200kv motor is drawing more amps than any park fly sized motor could handle, then clearly that isn't a viable option.

And then compare that chart with actual motors. Hopefully real life data and not manufactures / vendors. Then I'd know how accurate of an estimate it is.
 

rcspaceflight

creator of virtual planes
#11
So far all I got was confused. :confused:

*takes a deep breath*

Theoretically all you need to know to figure out thrust is prop size and RPM. (Ignoring air density and prop efficiency.)

Theoretically all you need to know to figure out amps is volts and drag / load created by the prop.

Thrust would tell you if it's enough power for the weight of your plane. Amps will tell you how powerful / big the motor needs to be (and efficiency).

Hopefully I can come to some conclusion about maximum efficiency when it comes to KV verses prop size.

------

The resistance R in ohms (Ω) is equal to the voltage V in volts (V) divided by the current I in amps (A): R = V/A

I think that equation would work to figure out the load of a prop because a load is nothing but extra resistance.

Hopefully by looking at raw data of motors I can come up with a constant "resistance" created by prop sizes. Of course the faster a prop spins the more resistance it creates so RPM would have to be considered when trying to figure out prop resistance.

Maybe I don't need to know thrust to know how efficient a prop is at a given RPM. The lower the resistance the prop creates, the more efficient it would be. I'm guessing that it's going to be a steep curve that dramatically increases at a certain RPM.

So, rethinking this, I think all I really have to do is to create graphs that show electrical resistance (created by props) versus RPM. So, one graph per prop size. And then come to some conclusion about the best KV for a certain voltage for each prop size. Which would only be a rule of thumb type of conclusion. The graph may look weird because for each prop the RPM would be increased by KV to a certain point, and then increased by volts.

-----

If I actually figure this out I'm going to have to create an article to share any conclusions that I come to. It should be useful information that isn't easily found anywhere. It would be nice to make it easily accessible.
 

Merv

Legendary member
#19
I agree with @pgert, a chart which only looks at Kv is a mistake. The battery voltage is just as important. Kv is rpm's per volt. The size of the motor ia another factor. A 2830 2200Kv may be very happy with a 6x4 on 3S. But a 6x4 would fry a 1506 2200Kv on 3S.

To be effective, you would need a separate Kv by prop chart for each motor size and battery voltage combination. Many vendors already supply such a chart for their motors.