Any way to calculate prop rpm?

[Hawth]

I saw this while looking on how to calculate thrust: https://www.flitetest.com/articles/propeller-static-dynamic-thrust-calculation

But I'm wondering if you can calculate how many rounds per minute the prop is going to spin by doing some math stuff with the kv of the motor and the width and pitch of the prop

I'm not a math wizard but I can copy and paste a few things into desmos

 

[JasonK]

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.

 

[LitterBug]

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

 

[Hawth]

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

 

[leaded50]

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

what size prop is recommended for the motor? (normally many producer do)

 

[JasonK]

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)

 

[LitterBug]

What motor is it that you are using? If you provide the manufacturer and model, we can help you track down the spec sheet for it.

 

[Homey]

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

Peter

 

[JasonK]

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)