### Author Topic: questuon about calculation of propeller specific thrust !  (Read 485 times)

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#### ammjy

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##### questuon about calculation of propeller specific thrust !
« on: October 19, 2018, 03:26:11 am »
Hello ! always Thans you for your help, EEVblog peoples !

fPropeller spacific thrust (g/W) = thrust(g) / mechanical power(W)

mechanical power(W) = torque(N*m) × propeller speed(rad/s)

from this equations, i have a question.
(i got this equations from book Introduction to multicopter design and control/Quan Quan)

1. can i use just motor electrical power Watt for mechanical power(W) ?

2. Propeller speed(rad/s) is motors RPM ?

#### IanB

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##### Re: questuon about calculation of propeller specific thrust !
« Reply #1 on: October 19, 2018, 03:33:51 am »
1. The mechanical power output from the motor will be less than the electrical input power, depending on the efficiency. Output power = input power x efficiency.

It's hard to know efficiency unless you have the motor data sheet, but you might assume 80-90% as a rough estimate.

2. RPM is revolutions per minute. Rad/s is radians per second. There are 2π radians per revolution. From there you can do the conversion. Probably,  1 rad/s is 9.55 rpm.
I'm not an EE--what am I doing here?

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#### ammjy

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##### Re: questuon about calculation of propeller specific thrust !
« Reply #2 on: October 19, 2018, 04:22:01 am »
Thanks you so much IanB !

then, can i use values of Propeller speed and torque from  Motors ? not a propeller.

this is what i think now.

1. set the target RPM for both motor and propeller.

2. find a torque and propeller speed of Motor's at the target RPM in data sheet.

3. calculate Mechanical power(W).

4.  in data sheet, find a thrust at the target RPM.

5. calculate Propeller spacific thrust (g/W)

6. result and of course correct result should be get by real experiment.

#### IanB

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##### Re: questuon about calculation of propeller specific thrust !
« Reply #3 on: October 19, 2018, 04:42:14 am »
Usually you would start with the propeller and work back to the motor.

A given propeller will have a characteristic performance, usually a curve or a chart. For various speeds of rotation it will generate a particular amount of thrust and require a certain input power from the motor.

First you will choose a suitable propeller for your application, one which will produce the required thrust at an appropriate speed. From the performance map you will read off the necessary mechanical power to drive the propeller.

Given this information you will find or choose a motor that can produce the required power at the necessary speed.

If instead, you have the motor first, then you would have to select a propeller to match the motor. In this case you would find the optimal operating speed for the motor where it produces the maximum power output without overheating. Then you would choose a propeller that consumes the right amount of power at the speed of the motor. In this case you will get whatever thrust the propeller produces.
I'm not an EE--what am I doing here?

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#### ammjy

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##### Re: questuon about calculation of propeller specific thrust !
« Reply #4 on: October 19, 2018, 05:17:49 am »
Thanks you for detail reply, IanB !

you correctly pointed out my problem !

Thanks you again, IanB.

#### dmills

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##### Re: questuon about calculation of propeller specific thrust !
« Reply #5 on: October 19, 2018, 09:11:16 am »
Don't forget that the prop and motor combination has a moment of inertia, so the dynamics are not quite as trivial as the steady state case of thrust = f(power) because the thing takes time to change speed.

Regards, Dan.

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#### ammjy

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##### Re: questuon about calculation of propeller specific thrust !
« Reply #6 on: October 19, 2018, 10:44:30 am »
Thanks you for sharp advise, Dmills !

i'll not forgot ! Thanks you !

Smf