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-   -   How to measure thrust ? (http://www.cfd-online.com/Forums/cfx/71689-how-measure-thrust.html)

Attesz January 12, 2010 05:52

How to measure thrust ?
 
Hi all,

I'm simulating a quadrotor RC helicopters propeller:
http://www.cfd-online.com/Forums/cfx...imulation.html

The main goal is to find the best blade shape. To get this, I have to measure thrust in CFX post.

Now, I define a plane behind the rotor, and let the CFX calculate the normal force on it. The CFD forces are smaller than measured. I know, it can caused by a lot of things, but maybe I'm wrong in force calculation.

So my question is: how to measure the thrust of the propeller?

Thanks,
Attesz

ghorrocks January 12, 2010 18:04

Most people are more interested in the thrust from the rotor, not a plate underneath the rotor. Why don't you calculate the thrust on the rotor blades?

Attesz January 13, 2010 04:27

Hi Glenn,

I've calculated normal force on the rotor blades, but the values are lower than on the plane measured, so more lower than real. I don't know why?

I think, CFX calculates the force on a plane from the equation massflow*velocity, which is the general equation for thrust calculation. To this equation comes the pressure difference member: area*(pressure@outlet-pressure@environment), which has a low value in my work.

Thanks for your reply,
Attesz

ghorrocks January 13, 2010 06:50

So let me paraphrase you: When you put in a feature which has nothing to do with the rotors the force acting on it is closer to what you think it should be so therefore you will use this force rather than the force on the rotors which is actually what you want.

Pardon my sarcasm but I think this line of logic is not helpful. If the force you want seems wrong then you should look at the accuracy of your simulation rather than doing some fudge on the results to make it appear better than it is.

Calculate the force on the rotor using the forceX/Y/Z() function, and if the answer is wrong then you have an error in your simulation and you should fix it.

Attesz January 13, 2010 07:17

Thank you Glenn,

but I didn't want to cheat, I didn't know how to measure force simply. I tought measuring that is more difficult, than measure it on the blades.
However, I'll examine the mesh, but also the measuring of the real quadrotor can be inaccurate (ground effect, the effect of rotors to each other, and also the "measuring device": a kitchen balance :) . it's a student team project).

Best regards,
Attesz


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