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Position of the center of aerodynamic forces

Question asked by sebastien griveau on Jan 6, 2011
Latest reply on Jan 13, 2011 by sebastien griveau

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Hello,

I have a 30 meters long frame that resemble the chassis of a plane.

I simulated the effort due to a 100 km/h wind and got my results. I used variable pressure on my tubes and I did my study only on the front part for mesh timing purpose.

Then, I checked the reaction forces and moments list but I also need to find the exact point where that force and moment is going to be.

I have attached a print screen of this. I am perplexed on the fact that the biggest torque is around the X-axis when it should be around the Y-axis.

I have been looking for days, so I don't know if I am being very clear. If you think you can help me and need more details, don't hesitate to ask.

Thanks,

Best regards.

Seb.

Bill McEachern Jan 13, 2011 10:46 AM

Correct Answer

You could do a transient analysis though I doubt that the results would be any different than if you ran a series of static studies. My recommendation would be to run the series of static studies - easier to control and if you want you can explore the descretization errors under more controlled conditions. It will avoid issues of coordinating time steps to save out the data you want. On the other hand, if carefully done the transient might be quicker.

See the reply in context

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Daniel Ferrucci-Herzberg Jan 6, 2011 11:11 AM

Hi Sebastien,

I'm not too sure about the torque issue, but I wonder if it has to do with your reference coordinate system? Try defining your results using the default coordinate system, just to see if that fixes the torque direction. If it does, then you know that your reference systrem is the problem, and you can redefine it as needed.

You can approximate the Aerodynamic Center of your aircraft as a point one-quarter of the way along its length (known as the Quarter-Chord Point). Alternatively, the Center of Pressure may be another point of interest. "The center of pressure of an aircraft is the point where all of the aerodynamic pressure field may be represented by a single force vector with no moment." However, these points are based on analysis of an airfoil, not a fuselage lifting body, so it may not be very accurate. It may be more appropriate to use the aircraft's center of gravity, which is simple enought to find using SolidWorks' Mass Properties tool. However, since you've only modelled the front section of the aircraft, the the CG won't represent the entire airframe.

Hope this helps!

Dan

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sebastien griveau @ Daniel Ferrucci-Herzberg on Jan 7, 2011 3:49 AM

Hello Daniel,

Thanks for your answer which was helpful.

The point I am looking for is the Center of Pressure, which I believe depends on the velocity, the coefficients of pressure and the geometry which makes it impossible to calculate by hand.

I attached the complete frame. I need to find this center of pressure precisely because the batteries, which will be near the end of the structure, are going to be moving with the variation of speed of the balloon in order to counter that pressure.

I am now sure that SW Simulation is not able to do that but maybe Flow Simulation does ?

Thanks for you help again,

Sebastien

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Structure.JPG159.9 KB

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Loic Ancian @ sebastien griveau on Jan 11, 2011 8:30 AM

use flow simulation and a torque goal.

The torque value should be 0.

Loïc

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Bill McEachern @ sebastien griveau on Jan 11, 2011 9:42 AM

Build a model that allows you to restrain, I am assuming you are using shells here on the FEA side (if not put some in) and then adjust you model so you fix a single shell node as the restraint system. Get the reactions and since you know the location of the reactions and Sigma F =0, you solve (yeah by hand) for the location that provides the center of pressure - same as a center of gravity calc - sum of external moments are zero at Cp.

On the flow side you do the same thing - do a torque and force goal about some known location - then use that data and that location to compute the center of pressure.

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sebastien griveau @ Bill McEachern on Jan 13, 2011 8:12 AM

Hello Bill and Loïc,

Thank you for your answers. I am now able to find the aerodynamic center of my structure at 22.22 m/s using flow simulation.

However, I am now in need to find the position curve of that center when the velocity is varying from 0 to 22.22 m/s.

I am tempted to do multiple simulations for which only the velocity vary of 5 m/s and find my curve thanks to those points.

Or maybe I could use a dynamic simulation, but I have never used it before, so I'm guessing that won't be an easy task. but the results would be much ore accurate.

Have you guys ever used it before, and if so is that kind of simulation doable or it is just not worth it ?

Thanks again.

Best regards.

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Bill McEachern @ sebastien griveau on Jan 13, 2011 10:46 AM

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sebastien griveau @ Bill McEachern on Jan 13, 2011 11:13 AM

OK, I think I'll go with the static studies.

Thank you for your help.

Regards.

Seb

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7 Replies

Daniel Ferrucci-Herzberg Jan 6, 2011 11:11 AM
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Correct Answer
Hi Sebastien,

I'm not too sure about the torque issue, but I wonder if it has to do with your reference coordinate system? Try defining your results using the default coordinate system, just to see if that fixes the torque direction. If it does, then you know that your reference systrem is the problem, and you can redefine it as needed.

You can approximate the Aerodynamic Center of your aircraft as a point one-quarter of the way along its length (known as the Quarter-Chord Point). Alternatively, the Center of Pressure may be another point of interest. "The center of pressure of an aircraft is the point where all of the aerodynamic pressure field may be represented by a single force vector with no moment." However, these points are based on analysis of an airfoil, not a fuselage lifting body, so it may not be very accurate. It may be more appropriate to use the aircraft's center of gravity, which is simple enought to find using SolidWorks' Mass Properties tool. However, since you've only modelled the front section of the aircraft, the the CG won't represent the entire airframe.

Hope this helps!

Dan
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- sebastien griveau @ Daniel Ferrucci-Herzberg on Jan 7, 2011 3:49 AM
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  Hello Daniel,
  
  Thanks for your answer which was helpful.
  
  The point I am looking for is the Center of Pressure, which I believe depends on the velocity, the coefficients of pressure and the geometry which makes it impossible to calculate by hand.
  
  I attached the complete frame. I need to find this center of pressure precisely because the batteries, which will be near the end of the structure, are going to be moving with the variation of speed of the balloon in order to counter that pressure.
  
  I am now sure that SW Simulation is not able to do that but maybe Flow Simulation does ?
  
  Thanks for you help again,
  
  Sebastien
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  - Structure.JPG159.9 KB
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  - Loic Ancian @ sebastien griveau on Jan 11, 2011 8:30 AM
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    use flow simulation and a torque goal.
    The torque value should be 0.
    
    Loïc
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  - Bill McEachern @ sebastien griveau on Jan 11, 2011 9:42 AM
    Mark Correct
    Correct Answer
    Build a model that allows you to restrain, I am assuming you are using shells here on the FEA side (if not put some in) and then adjust you model so you fix a single shell node as the restraint system. Get the reactions and since you know the location of the reactions and Sigma F =0, you solve (yeah by hand) for the location that provides the center of pressure - same as a center of gravity calc - sum of external moments are zero at Cp.
    
    On the flow side you do the same thing - do a torque and force goal about some known location - then use that data and that location to compute the center of pressure.
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    - sebastien griveau @ Bill McEachern on Jan 13, 2011 8:12 AM
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      Hello Bill and Loïc,
      
      Thank you for your answers. I am now able to find the aerodynamic center of my structure at 22.22 m/s using flow simulation.
      
      However, I am now in need to find the position curve of that center when the velocity is varying from 0 to 22.22 m/s.
      
      I am tempted to do multiple simulations for which only the velocity vary of 5 m/s and find my curve thanks to those points.
      
      Or maybe I could use a dynamic simulation, but I have never used it before, so I'm guessing that won't be an easy task. but the results would be much ore accurate.
      
      Have you guys ever used it before, and if so is that kind of simulation doable or it is just not worth it ?
      
      Thanks again.
      
      Best regards.
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      - Bill McEachern @ sebastien griveau on Jan 13, 2011 10:46 AM
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        You could do a transient analysis though I doubt that the results would be any different than if you ran a series of static studies. My recommendation would be to run the series of static studies - easier to control and if you want you can explore the descretization errors under more controlled conditions. It will avoid issues of coordinating time steps to save out the data you want. On the other hand, if carefully done the transient might be quicker.
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        sebastien griveau @ Bill McEachern on Jan 13, 2011 11:13 AM
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        OK, I think I'll go with the static studies.
        
        Thank you for your help.
        
        Regards.
        
        Seb
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Position of the center of aerodynamic forces

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