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Any way to calculate prop rpm?

JasonK

Elite member
#2
the kv of the motor * the voltage is the expected rotations per minute, however, prop loading/etc can limit that and cause it to spin slower the the expected rotation speed. The only way to get 100% sure info would be to put the motor on a stand with an optical rotation sensor and measure it.
 
#4
There are apps out there that will listen to the prop and calculate the RPM. The other method is to use an IR emitter/detector to count the number of times the IR bounces off the prop. These would be the more accurate methods.

There are these laser RPM detectors too:
https://www.amazon.com/dp/B004Q8L894/?tag=lstir-20
I'm just trying to find an equation that will find out how fast the prop will spin so that I know what prop to buy with the motor
 

JasonK

Elite member
#6
I'm just trying to find an equation that will find out how fast the prop will spin so that I know what prop to buy with the motor
That is the problem, the load of the prop will reduce the RPM from the theoretical (kv * voltage). The motor's torque ability, the prop's drag, pitch, size, etc all effect by how much.

I did some thrust calculations on the Flite Test A pack motor with a 6x3, 6x4.5, and 5x3 prop with 2s and 3s and not one of the combinations created the thrust that it would have given the kv * voltage RPM calculation, IIRC the 5x3 prop's thrust was a higher % of the 'ideal' thrust by the calculator then the 6x3 or 6x4.5 props.

To my knowledge, there isn't a formula that takes into account everything that could impact the RPM for you to just plug in numbers to do this with.

if your trying to figure out what prop to use with a motor, the motor manufacturer should have some recommended prop sizes for the motor, because if you over prop it, you can overheat/burn out the motor as it draws to much current trying to spin to big of a prop. if you undersize it to much, you just end up throwing away thrust (and likely efficency from what I found as, for a given amount of thrust, a larger slower prop tends to be more efficient then the smaller faster prop)
 

JasonK

Elite member
#9
I don't know where I got this, but in my notes: KVH x actual volts (12.6 for a 3s)

Peter
that is the theoretical RPM, however in practice that doesn't happen because of the load of the prop/etc (at least unless your loading the motor very lightly)
 
#12
It's usually gonna be 2 blades unless otherwise specified. If there are any more then it will be clear.
Would a 3 blade prop that creates the same thrust as the 5050 2 blade prop perform better, worse, or the same?

I heard that more blades help going around corners on fpv drones
 

quorneng

Elite member
#14
If you have sufficient motor power a 3, 4, 5 or even 8 bladed contra rotating prop will generate more thrust for a given diameter, but the efficiency of converting the motor power into thrust (thrust/Watt) falls away the more blades you use.

If you are chasing maximum thrust for a given power input a bigger diameter and finer pitch will always win but a static thrust figure does not determine how well a plane will fly but only its initial acceleration when starting from rest!;)
 
#16
That is the problem, the load of the prop will reduce the RPM from the theoretical (kv * voltage). The motor's torque ability, the prop's drag, pitch, size, etc all effect by how much.

I did some thrust calculations on the Flite Test A pack motor with a 6x3, 6x4.5, and 5x3 prop with 2s and 3s and not one of the combinations created the thrust that it would have given the kv * voltage RPM calculation, IIRC the 5x3 prop's thrust was a higher % of the 'ideal' thrust by the calculator then the 6x3 or 6x4.5 props.

To my knowledge, there isn't a formula that takes into account everything that could impact the RPM for you to just plug in numbers to do this with.

if your trying to figure out what prop to use with a motor, the motor manufacturer should have some recommended prop sizes for the motor, because if you over prop it, you can overheat/burn out the motor as it draws to much current trying to spin to big of a prop. if you undersize it to much, you just end up throwing away thrust (and likely efficency from what I found as, for a given amount of thrust, a larger slower prop tends to be more efficient then the smaller faster prop)
I only recently started studying engineering. And it is difficult for me with math, but I try to improve my knowledge. Recently I found this project https://plainmath.net where there are many answers to students' questions. It helped me a little, but still I would like to understand better in practice on their own.
I am also looking for a solution for such calculations.
 
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Piotrsko

Master member
#17
You could graph the published data from various motors using volts and amps for the axises and rpm for the line and see if the charts generate a predictable trend which could the be used to forecast performance on a different motor specs. However, motor manufacturers tend to over enthusiastically state their data.