Making sense of Watts, RPM, Amps and how to calculate thrust?

[sgf323]

I've got an eLogger and I've been doing some playing around with it. With a nearly charged battery here are some readings I just got with my Park 450.

11x7E: 190 watts, 17 amps, 7800 rpm
10x7SF: 220 watts, 20 amps, 7300 rpm

The 20 amps is just above the max burst for the motor but, how would these two setups differ in the real world? Which would provide more thrust?

Since the 11x7E is pulling less amps the watts are lower but that doesn't mean it's a less powerful setup, right? Being that both are x7 props with the SF just having more prop area I'm assuming the 11x7E would be faster and provide more thrust since it is turning at a higher RPM. Is that right?

I though I read somewhere that you could calculate thrust based on several parameters, including RPM, but I can't find it anywhere. Anyone know how to do this?

[Dr Kiwi]

Here is a good way to get a ballpark estimate of thrust if you know your rpm: http://www.badcock.net/cgi-bin/powertrain/propconst.cgi

The other way is for me to check my spreadsheet with 8500 entries for motor/prop/rpm/thrust etc and match your rpm for each of those props.

Back in a minute!

11x7 APC E: My measurement from my thrust stand: 7770rpm = 1374g/48.52oz

From badcock - his figures tend to be just a fraction low: 7800rpm = 1334g/47.00oz

10x7 APC SF: I've never dared take one that high - safe rpm limit for that prop (as listed by the manufacturer) is only 6500rpm; from badcock: 7300rpm = 1268g/44.65oz

[Murocflyer]

Originally Posted by sgf323

I've got an eLogger and I've been doing some playing around with it. With a nearly charged battery here are some readings I just got with my Park 450.

11x7E: 190 watts, 17 amps, 7800 rpm
10x7SF: 220 watts, 20 amps, 7300 rpm

The 20 amps is just above the max burst for the motor but, how would these two setups differ in the real world? Which would provide more thrust?

Since the 11x7E is pulling less amps the watts are lower but that doesn't mean it's a less powerful setup, right? Being that both are x7 props with the SF just having more prop area I'm assuming the 11x7E would be faster and provide more thrust since it is turning at a higher RPM. Is that right?

I though I read somewhere that you could calculate thrust based on several parameters, including RPM, but I can't find it anywhere. Anyone know how to do this?

I gave up on trying to figure that out long ago.

Here is what I use when I need to know that: http://www.gobrushless.com/testing/t...calculator.php

I'm sure someone knows how to do that. Looking forward to a better response.

Frank

[Murocflyer]

Originally Posted by Dr Kiwi

Here is a good way to get a ballpark estimate of thrust if you know your rpm: http://www.badcock.net/cgi-bin/powertrain/propconst.cgi

The other way is for me to check my spreadsheet with 8500 entries for motor/prop/rpm/thrust etc and match your rpm for each of those props.

Back in a minute!

Cool! Another one for the reference book.

Thanks!

Frank

[sgf323]

Heh, I guess I never even thought about seeing if I was taking the SF prop too high!

I guess I'll try the 11x7E out for a while!

Thanks

[Larry3215]

This illustrates one other thing - diameter is king when it comes to thrust.

Even tho the 10x4.7 prop had more power and was turning at a significantly higher rpm, it developed less thrust than the 11" prop.

Thrust is a function of two things - prop diameter and prop rpm.

Thrust goes up as the 3rd power of RPM but it goes up as the 4th power of diameter. In other words - if you double the rpm you get 8 times the thrust 2x2x2=8. But if you double the diameter you get 16 times the thrust 2x2x2x2=16.

So if all your concerned about is thrust - larger props are your friend

Larry