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Help me understand motors

CrashRecovery

I'm a care bear...Really?
Mentor
#1
I've been reading and searching but having a hard time understand a few things.
First is size of a prop compared to motor. Why does a larger kv motor use a smaller prop? Why isn't it the other way? I would think it would take more power and torque to move a bigger prop. For instance the FT F-22 uses 800-1200 kv motor and around an 8" prop. You look at the RCPowers F-22, is bigger, heavier and they recommend using a 2200kv motor with a 6" prop.
Why is this?
 
#2
the kv stands for rotations per volt . so the higher the number the faster it spins and therefore the smaller the prop required for generating the same thrust . if you put a larger prop on a higher kv motor it will draw excessive current and burn the motor out . the smaller props on higher kv motors also allow for a faster change in speed compared to the larger ones as the carry less inertia . the lower kv motors also have a higher resolution when it comes to changing speeds . that is why they are used on multi rotors . they also usually have a greater torque ratio on lower rpm . that's why they can spin larger props and generate more thrust . the problem with larger props is that they cause drag when flying at high speed that's why the rcpowers one uses a smaller one and the ft f22 uses a large prop because its made to go slow and fly high alfa .

hope i helped
 

FlyingMonkey

Stuck in Sunny FL
Staff member
Admin
#4
Jamiedco seems to have answered it very well. But I'm still going to link a few semi relevant videos... you know, for posterity.

Choose your prop.


Measuring Thrust.


Connecting Electronics.

 

CrashRecovery

I'm a care bear...Really?
Mentor
#5
I just realized my bonehead move....... I looked at the wrong motor online...... Guess I need to buy a few more 9" props to go with the motors and 3cell batteries I'm running. F-22 and the osprey twin twirl might fly now. Ft flyer flys but that has the 9" prop on it. Guess I know what I'm doing soon
 

Ak Flyer

Fly the wings off
Mentor
#6
Every prop has a maximum speed that it can turn. Say that number is 10,000 rpm.
Using a 1S 3.7 battery, you need a 2700 kv motor. 2700 x 3.7 = 9990 rpm
2S battery needs a 1350 kv motor 1350 x 7.4 = 9990 rpm
3S battery needs a 900 kv motor 900 x 11.1 = 9990 rpm
4S battery needs a 675 kv motor 675 x 14.8 = 9990 rpm

This is pretty simplified of course, but take your really big motors and look at the voltage they are designed to run on. You will find that the end rpm is about the same. You have to wind the motor to run on higher voltage without over speeding your props.

If you tried to run a 2700kv motor on 4S, you would end up with 39,960 rpm. That would blow your prop apart. Chances are you wouldn't even be able to turn it fast enough because it would draw too much amperage and overheat. Same thing if you took a bigg 500kv motor and tried to run it on a 1S battery, you would only spin it at 1850 rpm. That's like a house fan. Your plane wouldn't even move.

Now, why do big motors need higher voltages? Because to do the same larger amount of work, you either need to increase the voltage or you need to increase the amperage. Either way you are getting more wattage. Volts x amps = watts. To get more watts to do more work you have to increase one or the other or both. If you simply draw more amperage instead of using more voltage then you end up with massive heat generated which is a huge loss of power. You are more efficient by increasing your voltage.

So take all this into account, plus prop designs and pitches and it gets very confusing. This is why most people find a motor they like and prop it according to the motor manufacturers recommendations.
 

CrashRecovery

I'm a care bear...Really?
Mentor
#7
Copy that! So really the best thing to do before I really understand this motor stuff is to stick to the motors I have adjust planes to fit the 9" prop since I'm using 3 cell batteries.
 

Ak Flyer

Fly the wings off
Mentor
#8
Well, there are options in props as well. Basically you are loading the motor with the prop to get the max wattage that the motor can provide without overheating or drawing too much amperage through your esc. You can run a steeper pitch for a plane that goes faster without overloading the motor providing the airframe is designed to go fast. If you have a high drag low speed plane then a steep pitch small diameter prop isn't going to perform well. If you have a larger diameter prop with a shallower pitch like a slow fly prop then you will be better matched to a slower airframe while still putting the max load on the motor and esc. I've never seen anyone match the prop perfectly the first time. Nitro and gas planes have the same issue. Trying to find the perfect prop to match the power and the plane. You also have to take tuning and altitude into account when really dialing it in.

An 8x4 on my cub is totally anemic, yet a 10x7 is almost too much power. The 10x7 puts a good load on the 480 I have on it, where the 8x4 just spins freely. I could run a 12x4 if I had clearance and it would hang better on the prop than the 10x7 but there would be more torque roll. If I had a 9x7 or 9x6 it would be a good compromise. Thing is I just need to try them all to find out which I like best.
 

CrashRecovery

I'm a care bear...Really?
Mentor
#9
I get ya. The ft flyer works great with the 9x5. I need to adjust the throws more. Even with 40/40 mix it's still to squirrely. I think the 22 will fly better