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Motor differences.......what would happen?

#1
Hello everyone! I hate asking newbie questions, but I'm a newbie, so...... I have been reading and reading and reading about motors, but there's just no replacement for hearing from people that have actual experience.

If I have a plane that has a 2218 1180KV motor, then replace said motor with another 2218, but 1800KV, are there going to be any noticeable differences? There has to be some sort of difference, even if it's not much. I'm curious what that might be. Flight characteristics changes, extra heat, etc?

Also, what if that same 2218 1180KV motor was removed and a 2223 2300KV motor was installed? Obviously more power, probably a bit more weight, but what about noticeable differences in performance? Or is that too big of a jump up?

I just don't have any experience with this sort of stuff. I would appreciate any info you all can throw my way to help with my edjumacation!
 
#3
Generally, higher kv equates to less torque. So spinning the same propeller on the 1800kv version as the 1180kv version will pull more current and likely lead to more heating of the motor. So in general, you would want to install a smaller diameter propeller which would lead to a faster airplane.

Same idea for the second comparison. The first motor is suited for an 8x4-9x5 prop, whereas the 2300kv motor is more suited for a 5x5-6x4 prop. You would install the first motor on something like a sport-style plane for aerobatics, and the second would be more suited for a parkjet where speed is desired
 

leaded50

Well-known member
#5
eg: (by shop specs)
Sunnysky-X2216-1400KV w/9047 prop, 3s= 700g thrust (128W)
Sunnysky-X2216- 900KV w/9047 prop 3s = 680g thrust (106W)
- not a big difference ..without in power consumption, where shows most.

(another brand racestar)
2216 2000KV 3s= 832 g thrust, 143 W (unknown prop)
 
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"Corpse"

Well-known member
#6
KV means that per volt it will spin a certain rpm. (1800kv will spin 1800rpm if you feed in 1volt).

If you run a higher kv motor on the same prop it draws more amps, (because you need more power to increase speed). Think of it as conservation of energy. To keep the same amp load, you can either spin a small prop faster, or a large prop slower. You pick a higher or lower kv motor to fit your prop.
 

Merv

Well-known member
#7
Kv is the number of RPM a motor makes per volt.
A 1180kv motor will spin at 11,800 RPM on 10v. An 1800kv motor will spin at 18,000 RPM on 10v. That's a 53% increase in RPM.

Larger diameter props will draw more amps at a given RPM. Higher pitch props will draw more amps at a given RPM. All props will draw more amps when they spin faster. If you increase Kv or the voltage, the prop size will need to be reduced to offset the higher amp draw.
 

"Corpse"

Well-known member
#8
Kv is the number of RPM a motor makes per volt.
A 1180kv motor will spin at 11,800 RPM on 10v. An 1800kv motor will spin at 18,000 RPM on 10v. That's a 53% increase in RPM.

Larger diameter props will draw more amps at a given RPM. Higher pitch props will draw more amps at a given RPM. All props will draw more amps when they spin faster. If you increase Kv or the voltage, the prop size will need to be reduced to offset the higher amp draw.
It's all conservation of energy, and if you think about all the energy, (including kinetic and potential) it all makes sense!
 
#9
I hear what you all are saying and I understand it, but I'm looking to see what would actually happen. How the plane would perform and "feel like" with the different motors (same prop). If I understand properly, it seems like the higher KV would make the plane a bit sluggish on take-off, be faster on the top end, and have a shorter flight time because of the higher current draw. That's just my take on what I think I've learned. Anyone have any actual experience with that scenario?
 

Monte.C

Well-known member
#10
Could y'all please make it simpler for dummies like myself?!

Please confirm:
I THINK it equates to higher kV = higher RPM and less torque.
Lower kV means more torque but lower RPM.
It's just like the gears on my bike.
So to really go fast you'll want higher kV but it'll take a moment to get up to speed.
Lower kV equals more torque, so you can drive a bigger bite-ier prop and have a quicker response, but it won't reach the highest top end.

I can't say for current draw. Seems to me if you're working a lower kV motor plenty hard with a suitably aggressive prop, like a lot of aerobatics, high-alpha and the like, you'll draw just as much off the battery as a higher kV motor with a prop suitable for that motor.
 

Monte.C

Well-known member
#11
Hello everyone! I hate asking newbie questions, but I'm a newbie, so...... I have been reading and reading and reading about motors, but there's just no replacement for hearing from people that have actual experience.

If I have a plane that has a 2218 1180KV motor, then replace said motor with another 2218, but 1800KV, are there going to be any noticeable differences? There has to be some sort of difference, even if it's not much. I'm curious what that might be. Flight characteristics changes, extra heat, etc?

Also, what if that same 2218 1180KV motor was removed and a 2223 2300KV motor was installed? Obviously more power, probably a bit more weight, but what about noticeable differences in performance? Or is that too big of a jump up?

I just don't have any experience with this sort of stuff. I would appreciate any info you all can throw my way to help with my edjumacation!
So after my last post here (unless I got it backwards) I would want to say try it, you'll probably notice a difference that you'll either find desirable or not. When it comes down touch the motor to check if it's getting hotter than you want. And if the motor's getting real warm then you're working it harder, and that means you're drawing more from the battery. There - no math. And I'm good at math, but no need to complicate this.
 

Monte.C

Well-known member
#12
Hello everyone! I hate asking newbie questions, but I'm a newbie, so...... I have been reading and reading and reading about motors, but there's just no replacement for hearing from people that have actual experience.

If I have a plane that has a 2218 1180KV motor, then replace said motor with another 2218, but 1800KV, are there going to be any noticeable differences? There has to be some sort of difference, even if it's not much. I'm curious what that might be. Flight characteristics changes, extra heat, etc?

Also, what if that same 2218 1180KV motor was removed and a 2223 2300KV motor was installed? Obviously more power, probably a bit more weight, but what about noticeable differences in performance? Or is that too big of a jump up?

I just don't have any experience with this sort of stuff. I would appreciate any info you all can throw my way to help with my edjumacation!
What would you like to achieve?
If you had the 2218 1180KV and wanted to increase speed, try the 1800kV. Try it on the same prop and see if it gets hot, then downsize the prop if needed.
A 2223 is a bigger motor, but the 2300kV wants a higher RPM. Personally I think if I wanted more overall performance I might try that one with the same prop and see what it does, but I don't know how many kV is too many...

Also: It never hurts your motor or electronics if you use a prop diameter or pitch less than recommended for whatever reason. The "recommended" prop range is spec'd for optimal performance without burning anything. The only cost is a bit less power/weight ratio when you could be using a smaller motor to drive the same prop.
 
#13
Monte, I'm not trying to achieve anything specific. Just trying to understand motors better. I get the textbook explanation of size vs. KV, but i'm interested in hearing what it does in real life from someone that has done it. I'm curious about the actual changes to the overall system, whether it be an increase or decrease in flying performance, how it might change the "feel" of the plane when it's in the air, flight time increase or decrease, and any other physically noticeable changes, if any. That's all.....just curious and looking to learn. :) I appreciate everyone's input!
 

"Corpse"

Well-known member
#14
Could y'all please make it simpler for dummies like myself?!

Please confirm:
I THINK it equates to higher kV = higher RPM and less torque.
Lower kV means more torque but lower RPM.
It's just like the gears on my bike.
So to really go fast you'll want higher kV but it'll take a moment to get up to speed.
Lower kV equals more torque, so you can drive a bigger bite-ier prop and have a quicker response, but it won't reach the highest top end.

I can't say for current draw. Seems to me if you're working a lower kV motor plenty hard with a suitably aggressive prop, like a lot of aerobatics, high-alpha and the like, you'll draw just as much off the battery as a higher kV motor with a prop suitable for that motor.
Higher kv means more rpm, and less torque if you keep the amperage the same. It is like gears on a bike, but to keep the torque the same you have to push harder on the pedals (more amperage)
 

Merv

Well-known member
#15
If I understand properly, it seems like the higher KV would make the plane a bit sluggish on take-off, be faster on the top end, and have a shorter flight time because of the higher current draw.
Yes, Yes & Yes.
But not due to amp draw. The amp draw is dependent on the prop. The higher Kv motor could have the same or lower amp draw if you used a smaller prop. Also you could increase the amp draw on the lower Kv motor with a larger prop.

The difference in flight time has more to do with efficiently, expressed grams of thrust per watt of power. Larger prop spinning slowly are more efficient than small prop spinning fast. Smaller prop race quads will be 1-2 g/w. Larger props will be 8-10 g/w. I’ve seen some setups as high as 20 g/w.
 
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Monte.C

Well-known member
#16
Monte, I'm not trying to achieve anything specific. Just trying to understand motors better. I get the textbook explanation of size vs. KV, but i'm interested in hearing what it does in real life from someone that has done it. I'm curious about the actual changes to the overall system, whether it be an increase or decrease in flying performance, how it might change the "feel" of the plane when it's in the air, flight time increase or decrease, and any other physically noticeable changes, if any. That's all.....just curious and looking to learn. :) I appreciate everyone's input!
Me too, and me too.
And I still think if your motor's working too hard and getting too warm it's because you're drawing too many amps (anyone confirm?).

Well for sure, at the higher Kv you're less responsive - you don't get the instantaneous push of acceleration with increase in throttle, like my old '76 Monte Carlo with a 350 V-8 vs anything standard with less displacement. (chuckle :p)
Has anybody compared performance with two similar motors @ different Kv and noticed the difference?
 

Merv

Well-known member
#17
Could y'all please make it simpler for dummies like myself?!

Please confirm:
I THINK it equates to higher kV = higher RPM and less torque.
Lower kV means more torque but lower RPM.
It's just like the gears on my bike.
So to really go fast you'll want higher kV but it'll take a moment to get up to speed.
Lower kV equals more torque, so you can drive a bigger bite-ier prop and have a quicker response, but it won't reach the highest top end.

I can't say for current draw. Seems to me if you're working a lower kV motor plenty hard with a suitably aggressive prop, like a lot of aerobatics, high-alpha and the like, you'll draw just as much off the battery as a higher kV motor with a prop suitable for that motor.
The difference in performance between a high Kv & low Kv motor is all about the prop. Moving a large amount of air slowly takes the same power as moving a small amount of air fast. On the bench a lower Kv motor spinning a large prop will always have more thrust than a high Kv motor spinning a small prop. Even though they both have the same amp draw. Assuming you are using the optimum prop, that is a prop that is absorbing all of the power a motor can produce. To increase the diameter of a prop you will need to lower the pitch, more air at a lower speed. Likewise if you increase the pitch, you need to lower the diameter, less air at a higher speed.

A larger diameter low pitch prop will deliver its thrust at lower airspeed. A lower diameter higher pitch prop will continue to deliver its thrust at a higher airspeed. Similar to the gears on your bike.
 
#18
Monte, I'm not trying to achieve anything specific. Just trying to understand motors better. I get the textbook explanation of size vs. KV, but i'm interested in hearing what it does in real life from someone that has done it. I'm curious about the actual changes to the overall system, whether it be an increase or decrease in flying performance, how it might change the "feel" of the plane when it's in the air, flight time increase or decrease, and any other physically noticeable changes, if any. That's all.....just curious and looking to learn. :) I appreciate everyone's input!
To more specifically answer your question, you will never notice an appreciable difference between those motors in terms of "responsiveness" if they both have the same prop. You will probably get similar thrust, but the higher kv with a larger prop will have significantly more power demand and hence shorter flight times and maybe even a toasted motor.

Think of the prop as the deciding factor on your flight performance. Want speed? small prop high pitch. Want slow-fly floaty-ness? large prop smaller pitch. Sport/aerobatics? Large prop, medium/high pitch. Diameter is your "how much thrust do I want?" metric and pitch is your "how much 'grip' do I want?" metric, and you can interpolate in between a bit.

Once you've defined how you want your airplane to fly, you now need to select a motor that efficiently operates with the torque demands of that prop for your operating condition. The better the match you can get, the longer the flight times and the longer the life of the motor overall. The best resource is manufacturer thrust testing datasheets, the next best resource is product reviews.

As an example, I recently built a slow-flyer type aircraft in the ~300 gram range. I threw a small motor on with a 6x4 prop, and the thing barely flew because the thrust had much too high velocity due to the small, high pitch prop built for speed. Threw on a slightly different motor with an 8x4 and nearly the same static thrust, and the plane flew great. I actually did put the 8x4 on the smaller motor first and it flew great too, but because it was a little higher kv, it overheated, hence upgrading to the bigger motor 8x4 combo.
 

quorneng

Well-known member
#19
There is a tendency for people to think of kV as a measure of the power a motor can produce, it isn't. It only determines the RPM it can achieve for a given voltage.
An electric motor is designed to handle a maximum amps figure. Exceed that and the motor can be damaged. Exceed it by a lot and the motor will be wrecked.
This is why people are advised to get a Watt meter so they can check how close there are to the motors maximum amps.
Of course there is the speed controller (ESC) in between the battery and the motor so its volts & amps limit must not be exceeded either!
Within the max amps the motor can take you can play around with different props to give the performance you want.